Maker Pro
Maker Pro

audio pre-amp design (long-ish)

M

Mac

Jan 1, 1970
0
So, I've been working on an audio pre-amp design for a while in my spare
time. Since my wife and I just had a baby, I haven't had all that much
spare time, but I thought I would outline my approach here to see if you
all can keep me from making a(ny) stupid mistake(s). I have a nearly
complete schematic, but I hesitate to post it, since going through a
schematic is a lot to ask. I have a fair amount of experience, but not
with audio, and not with mains-powered stuff.

The idea is to build a simple pre-amp with four stereo inputs, a volume
control, and a balance control. Some other parameters are that there must
not be any programmable parts (ROM's, uControllers, PLD's, etc.) and parts
should not be too expensive or difficult to obtain. Ideally, everything
would be stocked by Digikey. I would like the entire design to fit on a
single circuit board with no wires running to any panel mount stuff.

If the design is any good at all, I'll post all the design info on the web
somewhere so other people can build it if they want. I'm trying to avoid
programmable parts to keep the entry barrier low. Plus, I don't have any
kind of programmers at home, so it is convenient for me, too.

The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too? There will be a mechanical pushbutton switch in series with
the hot lead, also.

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey TE70034-ND).
I've got a bridge rectifier, and then an LM317 and an LM337 both in
SOT223. I've selected resistors to generate +/- 15 V. There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431) from
the +/- 15V.

I don't need caps across the bridge for such a small power supply, right?

Also, is this the right Voltage secondary for a +/- 15 V supply?

I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

Would you put a fuse on the secondary side for this kind of thing?

There are four stereo inputs and a volume control. There is one output. I
am aiming for a max gain of 1V/V.

Should I provide more or is that reasonable for consumer CD and DVD
sources and typical amplifiers? This is not critical since I can easily
add gain later by changing resistors.

The inputs are buffered with OPA2134's in the non-inverting configuration.
There is a shunt resistor (10k ?) to set the input impedance. After the
OPA2134's, I go to a 74HC4052 analog mux which is followed by another
OPA2134 which then goes to a digital pot and finally to another OPA2134.

This seems like way too many op-amps. My rationale is that I want to keep
the 4052 sandwiched between a low impedance source and a high impedance
load. Also, the digital pot I am using, the AD5220BR10, can't stand input
Voltages higher or lower than its rails (+/- 2.5) So I am putting a string
of four diodes to and from ground on the digital pot input to keep the
Voltage from exceeding the rails. The digital pot cannot withstand +/-15 V
rails, so connecting it up there is not an option. Because of this low
clipping level, I am keeping the pot sandwiched between op-amps, too, so I
can attenuate the signal before the pot, and amplify it back up after, if
necessary.

Channel select is done via a counter which is clocked by a user pushbutton
(with debounce and Schmitt trigger). So the user can't directly select a
single channel, but has to go through them sequentially. But hey, there
are only four channels.

Volume up/down and balance left right is done with pushbuttons, too. The
balance logic increments the digital potentiometer on one channel and
decrements the other. The volume just increments or decrements both
channels.

There are a lot of things about this design I don't like. The biggest two
issues are:
1) I don't like having +/- 2.5V rails for the logic (I'd rather just drive
it from 5V to ground.)
2) There are too many op-amps.

So, I am thinking of re-doing the volume. I could use the SSM2164. I would
use the same potentiometers, followed by buffers to drive the control
Voltage inputs on the SSM2164. I'd have to work out some way of doing
balance, too, but I think that can be easily done so that one SSM2164 can
do volume and balance for both channels. This would allow me to run from
+5V to gnd, instead of having split +/- 2.5V rails, get rid of the
protection diodes, and lose a few op-amps in the signal path.

The other option for driving the SSM2164 would be to use a free running
clock connected to a counter, connected to a DAC. The DAC output would
drive the control input on the SSM2164. The volume up pushbutton would
enable the counter to count up, and volume down would enable it to count
down. Balance would count up on one channel and down on the other. If this
is a good idea, I'd love to have a short list of inexpensive DAC's to look
at. I could also build a "DAC" using 1% resistors directly from the
counter outputs. (You know, 8 resistors in series from the lsb, 7 from the
next bit, ... up to 1 from the msb). Resistors are very cheap, and I
don't need great accuracy, although monotonicity would be nice to have for
volume, and I doubt I could rely on monotonicity with such a set-up. I
guess I could just do the math and see how bad such a setup could be.

And there would have to be some kind of logic to keep the counters from
wrapping around, since the user would find that non-intuitive. (The
digital pot already does this internally.) I should be able to handle the
logic, but if anybody has some recommendation for a counter that makes it
particularly easy, that would be great.

Thanks for reading my longish post.

--Mac
 
J

John Popelish

Jan 1, 1970
0
Mac said:
So, I've been working on an audio pre-amp design for a while in my spare
time. Since my wife and I just had a baby, I haven't had all that much
spare time, but I thought I would outline my approach here to see if you
all can keep me from making a(ny) stupid mistake(s). I have a nearly
complete schematic, but I hesitate to post it, since going through a
schematic is a lot to ask. I have a fair amount of experience, but not
with audio, and not with mains-powered stuff.

For some people, going through a schematic is a lot easier than going
through so many words.

(snip)
The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too?
No.

There will be a mechanical pushbutton switch in series with
the hot lead, also.

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey TE70034-ND).
I've got a bridge rectifier, and then an LM317 and an LM337 both in
SOT223. I've selected resistors to generate +/- 15 V. There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431) from
the +/- 15V.

I don't need caps across the bridge for such a small power supply, right?

Are you talking about small ceramic caps across each diode? They are
a good idea, because diodes can generate high frequency noise when
they switch from on to off. You may not hear it in the audio,
directly, but it might get into a nearby receiver. It is cheap insurance.
Also, is this the right Voltage secondary for a +/- 15 V supply?

You could use a lower voltage transformer, say, the dual 15 volt one
(and probably drop a size) that produces about 21 volts peak and 25
volt capacitors. This would lower the temperature of the regulators.
The regulators just need a minimum of about 17 or 18 volts to
accurately regulate 15, so 21 volts peak allows a ripple to dip 3 or 4
volts, including low line voltage and diode drops, before you drop out
of regulation. The 12 volt version of the transformer will produce
about 16, peak, but that isn't quite enough, unless you simplify to an
unregulated +- 15 volt supply (which, depending on the schematic, is a
might be a reasonable possibility). You should also probably put a
..1uF film capacitor from each of the regulator outputs to ground to
improve the high frequency regulation of the 317 and 337.
I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

A bit light, perhaps, depending on the actual current drawn by the
loads. You also need a pair of capacitors, one from each side of the
bridge output to ground.
With your transformer, I might use something more like a pair of 1000
uF @ 35 volts
With a dual 15 volt transformer you could use 25 volt capacitors. 10%
high line voltage would still drive them to only 23 volts peak.
Would you put a fuse on the secondary side for this kind of thing?

I think I would put a pair of positive temperature coefficient
resistors (that switch from a low resistance when cool to a very high
resistance when hot) like MF-R020:
http://rocky.digikey.com/WebLib/Bourns/Web Data/MF-R Series.pdf
between transformer and bridge. These will save your regulators from
overheating if something gets shorted to ground, and they reset when
they cool off.
There are four stereo inputs and a volume control. There is one output. I
am aiming for a max gain of 1V/V.

Should I provide more or is that reasonable for consumer CD and DVD
sources and typical amplifiers? This is not critical since I can easily
add gain later by changing resistors.

Since various signal sources can vary a lot, I think you might need a
gain of .1 to 10 for each input (or ganged for pairs of input channels).
The inputs are buffered with OPA2134's in the non-inverting configuration.
There is a shunt resistor (10k ?) to set the input impedance. After the
OPA2134's, I go to a 74HC4052 analog mux which is followed by another
OPA2134 which then goes to a digital pot and finally to another OPA2134.

I'm not going to think at that level of detail without seeing the
schematic.
This seems like way too many op-amps. My rationale is that I want to keep
the 4052 sandwiched between a low impedance source and a high impedance
load. Also, the digital pot I am using, the AD5220BR10, can't stand input
Voltages higher or lower than its rails (+/- 2.5) So I am putting a string
of four diodes to and from ground on the digital pot input to keep the
Voltage from exceeding the rails. The digital pot cannot withstand +/-15 V
rails, so connecting it up there is not an option. Because of this low
clipping level, I am keeping the pot sandwiched between op-amps, too, so I
can attenuate the signal before the pot, and amplify it back up after, if
necessary.

Channel select is done via a counter which is clocked by a user pushbutton
(with debounce and Schmitt trigger). So the user can't directly select a
single channel, but has to go through them sequentially. But hey, there
are only four channels.

Volume up/down and balance left right is done with pushbuttons, too. The
balance logic increments the digital potentiometer on one channel and
decrements the other. The volume just increments or decrements both
channels.

There are a lot of things about this design I don't like. The biggest two
issues are:
1) I don't like having +/- 2.5V rails for the logic (I'd rather just drive
it from 5V to ground.)

Why? And why 5 volts , total, since this limits the audio peaks to +-
2.5 volts, max. The CMOS chips can work with quite a bit more voltage
than that. +- 5 or +-7.5 would seem more useful.
 
K

Ken Smith

Jan 1, 1970
0
The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too?[/QUOTE]

No, never fuse the return line. In the US, the white and green wires are
both at ground. The black is hot. You never want a failure mode that
leaves the hot connected but opens the ground.

You want the fuse to be right at the entry point.
I don't need caps across the bridge for such a small power supply, right?

The regulators need capacitors on their inputs. These are effectively
across the bridge so you end up with having put capacitors across the
bridge.

You may also want to add some resistance between the bridge and the
capacitors. This serves two purposes. It limits the peak current of the
current from the bridge to the capacitors.

It also can reduce the risk of the regulators oscillating. If your wiring
is a bit long and sloppy, it makes a whole bunch of Ls and Cs that can
make tuned circuits with fairly high Q. If the regulator sees one of
those it really likes, it will oscillate at that frequency. Adding the
resistors lowers the Q. You could just use good wiring practice too to
prevent the issue from coming up.

I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

The peak to peak ripple is:

V = I/(120 C)

where:
V is the peak to peak voltage (almost exactly)
I is the load current in Amps
C is the capacitance of a good capacitor in Farads.


You can figure out the ripple from that. If the lowest voltage into the
regulator is enough to keep it working, you can use the "ripple rejection"
value for the regulator to see what the output side ripple will be.

There are four stereo inputs and a volume control. There is one output. I
am aiming for a max gain of 1V/V.

I'd go for a max gain of maybe 3.
This seems like way too many op-amps.

It may not be enough op-amps. What are you hoping this thing will do?
1) I don't like having +/- 2.5V rails for the logic (I'd rather just drive
it from 5V to ground.)

So why didn't you do that and shift the DC point of the signals as needed?

And there would have to be some kind of logic to keep the counters from
wrapping around, since the user would find that non-intuitive. (The
digital pot already does this internally.) I should be able to handle the
logic, but if anybody has some recommendation for a counter that makes it
particularly easy, that would be great.


Many of the up-down counters have a carry output that will work to
indicate when another increment is a bad thing.
 
M

martin griffith

Jan 1, 1970
0
Let's consider the PSU first.

Yes, a fuse in the ac live is essential. Neutral does not need fusing.

Secondary fusing is required by safety regs if the transformer isn't
intrinsically safe.

You're going for 15V supplies, so the reservoir cap voltage mustn't drop below
~18V ( including ripple ) to ensure the regulation doesn't fail.

How do you plan to provide +/->18V to the regulators without using a centre
tapped transformer ? I'll assume you mean 18-0-18 in that case.

Practical experience of various transformers shows that it is not easy to
accurately predict the reservoir cap voltage. One of the larger issue tends to
be how transformer regulation affects the final DC voltage. Some transformers
have much higher copper losses than others. 18-0-18 sounds about right for
starters though. Remember that the reservoir cap voltage also includes ripple
voltage so you'll need more than 18V DC as measured on a meter.

The capacitance of the reservoir cap will influence both the pre-regulator
ripple voltage and the ripple after the regulators.

Ripple voltage is determined by load current. What's your load current ?

Ripple voltage on 50Hz supplies is ~ 8.10^-3 x Iload / Cres. Substitute around
7.10^-3 for 60Hz . This from deltaV= t x I / C

Why use a 63V cap for the reservoir ? Use 35V and double the capacitance for
the same can size / price.

Unless your load current is quite low I expect the SOT223 regulators will
simply fry ! Why not use TO-220 ?

Graham
dont forget to put 100nF caps accross the bridge rect diodes, Gets rid
of a potential source of RF nasties


martin
 
P

Pooh Bear

Jan 1, 1970
0
Mac said:
So, I've been working on an audio pre-amp design for a while in my spare
time. Since my wife and I just had a baby, I haven't had all that much
spare time, but I thought I would outline my approach here to see if you
all can keep me from making a(ny) stupid mistake(s). I have a nearly
complete schematic, but I hesitate to post it, since going through a
schematic is a lot to ask. I have a fair amount of experience, but not
with audio, and not with mains-powered stuff.

The idea is to build a simple pre-amp with four stereo inputs, a volume
control, and a balance control. Some other parameters are that there must
not be any programmable parts (ROM's, uControllers, PLD's, etc.) and parts
should not be too expensive or difficult to obtain. Ideally, everything
would be stocked by Digikey. I would like the entire design to fit on a
single circuit board with no wires running to any panel mount stuff.

If the design is any good at all, I'll post all the design info on the web
somewhere so other people can build it if they want. I'm trying to avoid
programmable parts to keep the entry barrier low. Plus, I don't have any
kind of programmers at home, so it is convenient for me, too.

The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too? There will be a mechanical pushbutton switch in series with
the hot lead, also.

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey TE70034-ND).
I've got a bridge rectifier, and then an LM317 and an LM337 both in
SOT223. I've selected resistors to generate +/- 15 V. There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431) from
the +/- 15V.

I don't need caps across the bridge for such a small power supply, right?

Also, is this the right Voltage secondary for a +/- 15 V supply?

I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

Would you put a fuse on the secondary side for this kind of thing?

Let's consider the PSU first.

Yes, a fuse in the ac live is essential. Neutral does not need fusing.

Secondary fusing is required by safety regs if the transformer isn't
intrinsically safe.

You're going for 15V supplies, so the reservoir cap voltage mustn't drop below
~18V ( including ripple ) to ensure the regulation doesn't fail.

How do you plan to provide +/->18V to the regulators without using a centre
tapped transformer ? I'll assume you mean 18-0-18 in that case.

Practical experience of various transformers shows that it is not easy to
accurately predict the reservoir cap voltage. One of the larger issue tends to
be how transformer regulation affects the final DC voltage. Some transformers
have much higher copper losses than others. 18-0-18 sounds about right for
starters though. Remember that the reservoir cap voltage also includes ripple
voltage so you'll need more than 18V DC as measured on a meter.

The capacitance of the reservoir cap will influence both the pre-regulator
ripple voltage and the ripple after the regulators.

Ripple voltage is determined by load current. What's your load current ?

Ripple voltage on 50Hz supplies is ~ 8.10^-3 x Iload / Cres. Substitute around
7.10^-3 for 60Hz . This from deltaV= t x I / C

Why use a 63V cap for the reservoir ? Use 35V and double the capacitance for
the same can size / price.

Unless your load current is quite low I expect the SOT223 regulators will
simply fry ! Why not use TO-220 ?

Graham
 
P

Pooh Bear

Jan 1, 1970
0
martin said:
dont forget to put 100nF caps accross the bridge rect diodes, Gets rid
of a potential source of RF nasties

martin

I reckon the issue is overstated. 1N400x diodes are so slow that it's not really a
huge source of RF.

I do actually spec caps there now but I do it more as belt and braces for EMI
conformity measurements as opposed to any intrusive audio effect.

Forgot to ask the OP if that's a toroidal transformer he's looking at btw.

Graham
 
B

Ban

Jan 1, 1970
0
Mac said:
The power supply will use a standard AC receptacle (I'm in the US).
I'll put a 5x20 mm fuse holder in series with the hot lead. Should I
fuse neutral, too? There will be a mechanical pushbutton switch in
series with the hot lead, also.
The best will be to not have any mains AC on the board, it facilitates the
handling immensly especially when you are not too experienced. Look at my
light sculpture post on ABSE how to make a bipolar power supply with an AC
adapter (in your case 15 or 18Vac single line).

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey
TE70034-ND). I've got a bridge rectifier, and then an LM317 and an
LM337 both in SOT223. I've selected resistors to generate +/- 15 V.
There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431)
from the +/- 15V.

I don't need caps across the bridge for such a small power supply,
right?

Also, is this the right Voltage secondary for a +/- 15 V supply?

I've got 220 uF 63V electrolytic capacitors after the bridge. Does
that sound right?

Would you put a fuse on the secondary side for this kind of thing?

There are four stereo inputs and a volume control. There is one
output. I am aiming for a max gain of 1V/V.

Should I provide more or is that reasonable for consumer CD and DVD
sources and typical amplifiers? This is not critical since I can
easily add gain later by changing resistors.

The inputs are buffered with OPA2134's in the non-inverting
configuration. There is a shunt resistor (10k ?) to set the input
impedance. After the OPA2134's, I go to a 74HC4052 analog mux which
is followed by another OPA2134 which then goes to a digital pot and
finally to another OPA2134.
You can just drop those buffers and directly switch the inputs with the mux,
because the impedance change on the input is irrelevant since only one
channel is active. Put 47k to gnd on each channel input, so the coupling
caps are discharged and don't make pops. Put also a series resistor (1k is
good) and then some diodes to the rails for protection.
This digital pot is not good here, either you get a +/-15V version or make
it with the ssm2164.
This seems like way too many op-amps. My rationale is that I want to
keep the 4052 sandwiched between a low impedance source and a high
impedance load. Also, the digital pot I am using, the AD5220BR10,
can't stand input Voltages higher or lower than its rails (+/- 2.5)
So I am putting a string of four diodes to and from ground on the
digital pot input to keep the Voltage from exceeding the rails. The
digital pot cannot withstand +/-15 V rails, so connecting it up there
is not an option. Because of this low clipping level, I am keeping
the pot sandwiched between op-amps, too, so I can attenuate the
signal before the pot, and amplify it back up after, if necessary.

Channel select is done via a counter which is clocked by a user
pushbutton (with debounce and Schmitt trigger). So the user can't
directly select a single channel, but has to go through them
sequentially. But hey, there are only four channels.

Volume up/down and balance left right is done with pushbuttons, too.
The balance logic increments the digital potentiometer on one channel
and decrements the other. The volume just increments or decrements
both channels.

There are a lot of things about this design I don't like. The biggest
two issues are:
1) I don't like having +/- 2.5V rails for the logic (I'd rather just
drive it from 5V to ground.)
2) There are too many op-amps.

So, I am thinking of re-doing the volume. I could use the SSM2164. I
would use the same potentiometers, followed by buffers to drive the
control Voltage inputs on the SSM2164. I'd have to work out some way
of doing balance, too, but I think that can be easily done so that
one SSM2164 can do volume and balance for both channels. This would
allow me to run from +5V to gnd, instead of having split +/- 2.5V
rails, get rid of the protection diodes, and lose a few op-amps in
the signal path.

No, use a DAC or a simple linear pot for the ctrl voltage. Balance can be
done by adding or subtracting a second voltage from another DAC or pot to
the control voltage of right and the reverse to left, so only 2 sections of
the 2164 are used. You could use the remaining sections to allow to mix two
sources, fading from one source to another.
The other option for driving the SSM2164 would be to use a free
running clock connected to a counter, connected to a DAC. The DAC
output would drive the control input on the SSM2164. The volume up
pushbutton would enable the counter to count up, and volume down
would enable it to count down. Balance would count up on one channel
and down on the other. If this is a good idea, I'd love to have a
short list of inexpensive DAC's to look at. I could also build a
"DAC" using 1% resistors directly from the counter outputs. (You
know, 8 resistors in series from the lsb, 7 from the next bit, ... up
to 1 from the msb). Resistors are very cheap, and I don't need great
accuracy, although monotonicity would be nice to have for volume, and
I doubt I could rely on monotonicity with such a set-up. I guess I
could just do the math and see how bad such a setup could be.

And there would have to be some kind of logic to keep the counters
from wrapping around, since the user would find that non-intuitive.
(The digital pot already does this internally.) I should be able to
handle the logic, but if anybody has some recommendation for a
counter that makes it particularly easy, that would be great.

Thanks for reading my longish post.

A circuit diagram would have been better than your life story.
 
D

DaveM

Jan 1, 1970
0
Mac said:
So, I've been working on an audio pre-amp design for a while in my spare
time. Since my wife and I just had a baby, I haven't had all that much
spare time, but I thought I would outline my approach here to see if you
all can keep me from making a(ny) stupid mistake(s). I have a nearly
complete schematic, but I hesitate to post it, since going through a
schematic is a lot to ask. I have a fair amount of experience, but not
with audio, and not with mains-powered stuff.

The idea is to build a simple pre-amp with four stereo inputs, a volume
control, and a balance control. Some other parameters are that there must
not be any programmable parts (ROM's, uControllers, PLD's, etc.) and parts
should not be too expensive or difficult to obtain. Ideally, everything
would be stocked by Digikey. I would like the entire design to fit on a
single circuit board with no wires running to any panel mount stuff.

If the design is any good at all, I'll post all the design info on the web
somewhere so other people can build it if they want. I'm trying to avoid
programmable parts to keep the entry barrier low. Plus, I don't have any
kind of programmers at home, so it is convenient for me, too.

The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too? There will be a mechanical pushbutton switch in series with
the hot lead, also.

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey TE70034-ND).
I've got a bridge rectifier, and then an LM317 and an LM337 both in
SOT223. I've selected resistors to generate +/- 15 V. There are also +/-
2.5V rails for logic. These are created from shunt regulators (TL431) from
the +/- 15V.

I don't need caps across the bridge for such a small power supply, right?

Also, is this the right Voltage secondary for a +/- 15 V supply?

I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

Would you put a fuse on the secondary side for this kind of thing?


Thanks for reading my longish post.

--Mac

After reading some of the replies to your post, I would like to recommend
that you stick to the 18VAC transformer in your power supply. The reason is
simple... to accommodate low line voltage conditions. If you use the 15VAC
transformer, consider this:
At nominal line voltage and rated load, the transformer output is 15 VAC.
If the line droops 10%, that means the secondary voltage also droops by 10%,
to 13.5VAC.
At the secondary, the peak is 13.5 * 1.414, or 19.1 VDC. Allowing a drop of
1.5 across the rectifier (assuming a FWB), the capacitor charges to a peak
of 19.1 - 1.5 = 17.6 VDC.
Allowing for 1 volt of ripple, that leaves a minimum voltage available to
the regulator of 16.6 volts. The regulator needs a minimum of 17.5 volts
for regulation. Result... the regulator output is full of ripple because its
source doesn't meet the minimum requirements. You could use very large
filter capacitors after the rectifier, but then you run into high surge
currents, and if the PSU is fused properly, you risk blowing the fuse at the
most inconvenient times.
Stay with the 18 VAC transformer... it will give the regulator all it
needs, even at low line voltage, and you avoid the blown fuse problem.

Cheers!!!!
--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!
 
M

Mac

Jan 1, 1970
0
The best will be to not have any mains AC on the board, it facilitates the
handling immensly especially when you are not too experienced. Look at my
light sculpture post on ABSE how to make a bipolar power supply with an AC
adapter (in your case 15 or 18Vac single line).

I hate wall warts. They are so big that they always take up more than one
plug. And I have built small AC power-supplies before, but only for myself
or for little projects back when I was in college. Since I am
contemplating publicizing this design eventually I wanted to make sure
that it was exposed to more expert scrutiny.
[snip]
The inputs are buffered with OPA2134's in the non-inverting
configuration. There is a shunt resistor (10k ?) to set the input
impedance. After the OPA2134's, I go to a 74HC4052 analog mux which
is followed by another OPA2134 which then goes to a digital pot and
finally to another OPA2134.
You can just drop those buffers and directly switch the inputs with the mux,
because the impedance change on the input is irrelevant since only one
channel is active.

I'm not sure if I understand this comment. The point of the buffers is to
minimize the dependence of the switch resistance on the input Voltage (it
is a CMOS switch). I must have mistakenly given you another impression.
Put 47k to gnd on each channel input, so the coupling
caps are discharged and don't make pops. Put also a series resistor (1k
is good) and then some diodes to the rails for protection. This digital
pot is not good here, either you get a +/-15V version or make it with
the ssm2164.

I was planning on going DC-coupled.

[snip]
No, use a DAC or a simple linear pot for the ctrl voltage. Balance can be
done by adding or subtracting a second voltage from another DAC or pot to
the control voltage of right and the reverse to left, so only 2 sections of
the 2164 are used. You could use the remaining sections to allow to mix two
sources, fading from one source to another.

I will think about this.
A circuit diagram would have been better than your life story.

I was afraid some people might not go to the trouble. But taking the time
to visualize my description is troublesome, also, I guess. Anyway, thanks
again.

--Mac
 
P

Pooh Bear

Jan 1, 1970
0
Mac said:
I'm not sure if I understand this comment. The point of the buffers is to
minimize the dependence of the switch resistance on the input Voltage (it
is a CMOS switch). I must have mistakenly given you another impression.

How will a buffer make any difference to the switch resistance ? The applied
voltage will be the same with or without a buffer ( ignoring minimal loading
effects ).

Graham

p.s. a schematic would be appreciated
 
R

Roger Lascelles

Jan 1, 1970
0
John Popelish said:
You could use a lower voltage transformer, say, the dual 15 volt one
(and probably drop a size) that produces about 21 volts peak and 25
volt capacitors. This would lower the temperature of the regulators.

I have often seen the 15V transformer turn out to be marginal, so I'd stay
with the 18V.
A bit light, perhaps, depending on the actual current drawn by the
loads.

The size of the (presently 220u) capacitor depends entirely on the load
current. At 50mA load, 220uF is fine. I'd incrase it proportionately with
load current - eg at 100mA use 470uF etc.

Roger Lascelles
 
W

Walter Harley

Jan 1, 1970
0
Mac said:
So, I've been working on an audio pre-amp design [...]

The idea is to build a simple pre-amp with four stereo inputs, a volume
control, and a balance control.


....and then you go on to talk about DACs, digital pots, and whatnot.

How come?

What's wrong with the many time-proven simple designs based around analog
components? Why are you reinventing the wheel?

I think you're making your life a lot harder than it needs to. If there are
some sort of additional requirements that you didn't list, like a desire to
learn new technologies or like a need to control it from a PC, it might be
nice to know what they are.

I don't mean to be rude - I'm just suggesting that maybe there is a simpler
answer than you're looking at.
 
M

Mac

Jan 1, 1970
0
The power supply will use a standard AC receptacle (I'm in the US). I'll
put a 5x20 mm fuse holder in series with the hot lead. Should I fuse
neutral, too?

No, never fuse the return line. In the US, the white and green wires are
both at ground. The black is hot. You never want a failure mode that
leaves the hot connected but opens the ground.
[/QUOTE]

I know what the various colored wires are (in the US). Are you saying
never fuse the white wire? I do know not to fuse the green wire (but I
appreciate you emphasizing it).
You want the fuse to be right at the entry point.


The regulators need capacitors on their inputs. These are effectively
across the bridge so you end up with having put capacitors across the
bridge.

Oh, yeah, I should have been more specific. I mean small anti-EMI caps
across the bridge diodes.
You may also want to add some resistance between the bridge and the
capacitors. This serves two purposes. It limits the peak current of the
current from the bridge to the capacitors.

Hmm. For some reason I was thinking that such a small transformer wouldn't
need it. I don't think the short-circuit current is specified, but I could
check it. Or I could just add some "opt" resistors in series with the
secondaries.
It also can reduce the risk of the regulators oscillating. If your wiring
is a bit long and sloppy, it makes a whole bunch of Ls and Cs that can
make tuned circuits with fairly high Q. If the regulator sees one of
those it really likes, it will oscillate at that frequency. Adding the
resistors lowers the Q. You could just use good wiring practice too to
prevent the issue from coming up.

I don't think there's going to be any stability problem.
The peak to peak ripple is:

V = I/(120 C)

In the US. People in 50 Hz environments need to use 100 C.
where:
V is the peak to peak voltage (almost exactly)
I is the load current in Amps
C is the capacitance of a good capacitor in Farads.


You can figure out the ripple from that. If the lowest voltage into the
regulator is enough to keep it working, you can use the "ripple rejection"
value for the regulator to see what the output side ripple will be.

Well, I went through some such calculations in a spreadsheet. But
depending on your assumptions about transformer regulation and best/worst
case mains Voltage, it is not trivial (for me, at least) to get everything
right.
I'd go for a max gain of maybe 3.

Thanks.


It may not be enough op-amps. What are you hoping this thing will do?


So why didn't you do that and shift the DC point of the signals as needed?

Oh, you mean level shift using the op-amps. Duh. Thanks. I was thinking
that I would have to shift by using a blocking cap (which I don't want to
do), but that is not necessary. I can easily level-shift with the op-amps.
I don't know why I didn't think of that. ;-)
Many of the up-down counters have a carry output that will work to
indicate when another increment is a bad thing.

--

Yeah. I just need to make sure I don't accidentally build a state machine
where once max or min volume is attained the volume is no-longer
adjustable. That wouldn't be too intuitive for the user, either. ;-)

--Mac
 
K

Ken Smith

Jan 1, 1970
0
I know what the various colored wires are (in the US). Are you saying
never fuse the white wire? I do know not to fuse the green wire (but I
appreciate you emphasizing it).

Don't fuse the white wire. The odds of you getting a 3 prong outlet with
the white and black inverted are very low. This is the only situation in
which a fuse in the white would be an advantage. Normally, if the fuse on
the white opens, the unit appears to be unpowered but in fact has deadly
voltages inside.

Oh, yeah, I should have been more specific. I mean small anti-EMI caps
across the bridge diodes.

They usually cause no harm so you can include them. An EMI filtering power
entry module is also a good idea from that point of view. The down side
of those entry modules is that they connect capacitors to the chassis
ground. This is really a bad idea but has become standard practice.
Hmm. For some reason I was thinking that such a small transformer wouldn't
need it. I don't think the short-circuit current is specified, but I could
check it. Or I could just add some "opt" resistors in series with the
secondaries.

You can measure the winding resistance with an ohmmeter and get an over
estimate of the peak current. If you have an inductance bridge you can
measure the leakage inductance. If not you could try a signal generator
and scope. Short the secondary and measure the primary's impedance.

You need to consider the case when the switch is closed at the peak of the
sine wave.

[...]
I don't think there's going to be any stability problem.

Confidence: The feeling you have before you understand the problem :)
Well, I went through some such calculations in a spreadsheet. But
depending on your assumptions about transformer regulation and best/worst
case mains Voltage, it is not trivial (for me, at least) to get everything
right.

Assume the worst number you came up with, if you are fairly sure there is
no way to make it even worse. Remember an overdesign will work.
 
J

John Smith

Jan 1, 1970
0
currents, and if the PSU is fused properly, you risk blowing the fuse at
the
most inconvenient times.

If it is fused properly they should not blow all the time...
BTW, fuses blow when its most inconvenient, by definition ;o)

-- John
 
M

Mac

Jan 1, 1970
0
news:p[email protected]... [snip]
The power supply will use a standard AC receptacle (I'm in the US).
I'll put a 5x20 mm fuse holder in series with the hot lead. Should I
fuse neutral, too? There will be a mechanical pushbutton switch in
series with the hot lead, also.

I'm using an 18 V 390 mA transformer (amvecco 70034, digikey
TE70034-ND). I've got a bridge rectifier, and then an LM317 and an
LM337 both in SOT223. I've selected resistors to generate +/- 15 V.
There are also +/- 2.5V rails for logic. These are created from shunt
regulators (TL431) from the +/- 15V.

I don't need caps across the bridge for such a small power supply,
right?

Also, is this the right Voltage secondary for a +/- 15 V supply?

I've got 220 uF 63V electrolytic capacitors after the bridge. Does that
sound right?

Would you put a fuse on the secondary side for this kind of thing?


Thanks for reading my longish post.

--Mac

After reading some of the replies to your post, I would like to
recommend that you stick to the 18VAC transformer in your power supply.
The reason is simple... to accommodate low line voltage conditions. If
you use the 15VAC transformer, consider this: At nominal line voltage
and rated load, the transformer output is 15 VAC. If the line droops
10%, that means the secondary voltage also droops by 10%, to 13.5VAC. At
the secondary, the peak is 13.5 * 1.414, or 19.1 VDC. Allowing a drop
of 1.5 across the rectifier (assuming a FWB), the capacitor charges to a
peak of 19.1 - 1.5 = 17.6 VDC.
Allowing for 1 volt of ripple, that leaves a minimum voltage available
to the regulator of 16.6 volts. The regulator needs a minimum of 17.5
volts for regulation. Result... the regulator output is full of ripple
because its source doesn't meet the minimum requirements. You could use
very large filter capacitors after the rectifier, but then you run into
high surge currents, and if the PSU is fused properly, you risk blowing
the fuse at the most inconvenient times. Stay with the 18 VAC
transformer... it will give the regulator all it needs, even at low
line voltage, and you avoid the blown fuse problem.

Cheers!!!!

I'm finding this to be pretty convincing. So I think I will take this
advice. However, I will certainly check it out after it is built. I
believe the 15 V transformer is pin-compatible, so I could always change
it later. ;-)

--Mac
 
J

John Popelish

Jan 1, 1970
0
Mac said:
I'm finding this to be pretty convincing. So I think I will take this
advice. However, I will certainly check it out after it is built. I
believe the 15 V transformer is pin-compatible, so I could always change
it later. ;-)

A much better question that you might answer first, is why are you
using + and - 15 volts for the opamps and only +- 2.5 volts for the
logic and CMOS switches? The while supply requirement is pretty
arbitrary. You might do just fine running the opamps on +- 5, 7.5, 10
or 12 volts, depending on the rest of the design. As you say, you can
pick a transformer later. You have more important questions to
address first.
 
M

Mac

Jan 1, 1970
0
Mac wrote: [snip]
I have a nearly
complete schematic, but I hesitate to post it, since going through a
schematic is a lot to ask. I have a fair amount of experience, but not
with audio, and not with mains-powered stuff.

For some people, going through a schematic is a lot easier than going
through so many words.

Point taken.
Are you talking about small ceramic caps across each diode? They are
a good idea, because diodes can generate high frequency noise when
they switch from on to off. You may not hear it in the audio,
directly, but it might get into a nearby receiver. It is cheap insurance.

Yeah, OK. I'll put them in the schematic and layout. I may or may not
install them. ;-)
You could use a lower voltage transformer, say, the dual 15 volt one
(and probably drop a size) that produces about 21 volts peak and 25
volt capacitors. This would lower the temperature of the regulators.
The regulators just need a minimum of about 17 or 18 volts to
accurately regulate 15, so 21 volts peak allows a ripple to dip 3 or 4
volts, including low line voltage and diode drops, before you drop out
of regulation. The 12 volt version of the transformer will produce
about 16, peak, but that isn't quite enough, unless you simplify to an
unregulated +- 15 volt supply (which, depending on the schematic, is a
might be a reasonable possibility). You should also probably put a
.1uF film capacitor from each of the regulator outputs to ground to
improve the high frequency regulation of the 317 and 337.

Thank you. I am undecided on the 18 vs 15 Volt thing. I actually put 1uF
electrolytic caps at the regulator inputs in the schematic but forgot to
mention them (Another reason to post a schematic instead of describing
the circuit). I could change them to 0.1 uF film capacitors. I will
re-read the datasheets for the 317/337 and see what they say.
A bit light, perhaps, depending on the actual current drawn by the
loads. You also need a pair of capacitors, one from each side of the
bridge output to ground.

That's where these are.
With your transformer, I might use something more like a pair of 1000
uF @ 35 volts
With a dual 15 volt transformer you could use 25 volt capacitors. 10%
high line voltage would still drive them to only 23 volts peak.

Interesting. I think I will stick with the 18 Volts and see how it goes. I
usually select capacitors with double the Voltage rating for the expected
DC Voltage. That is why I'm using 63 V caps. Not sure where I picked that
bit of baggage up.

I will double-check my calculations regarding capacitor size, too.
I think I would put a pair of positive temperature coefficient
resistors (that switch from a low resistance when cool to a very high
resistance when hot) like MF-R020:
http://rocky.digikey.com/WebLib/Bourns/Web Data/MF-R Series.pdf
between transformer and bridge. These will save your regulators from
overheating if something gets shorted to ground, and they reset when
they cool off.

That seems a good idea. Thanks.
Since various signal sources can vary a lot, I think you might need a
gain of .1 to 10 for each input (or ganged for pairs of input channels).

Wow. 10. OK. I'll use that as my starting point, and unless it seems
preposterously loud with the gear I have, I'll keep it.
I'm not going to think at that level of detail without seeing the
schematic.

Fair enough. I'll post an ASCII art signal flow diagram somewhere in
this thread. If you haven't lost interest, you can take a look. ;-)

[snip]
Why? And why 5 volts , total, since this limits the audio peaks to +-
2.5 volts, max. The CMOS chips can work with quite a bit more voltage
than that. +- 5 or +-7.5 would seem more useful.

[snip]

The digital pot can only take 6 Volts (abs max) across its rails. I have
pretty much decided not to use it directly for volume. I may use it
instead to generate a control Voltage for a VCA (ssm2164).

I very much appreciate your detailed comments.

--Mac
 
M

Mac

Jan 1, 1970
0
How will a buffer make any difference to the switch resistance ? The applied
voltage will be the same with or without a buffer ( ignoring minimal loading
effects ).

Graham

p.s. a schematic would be appreciated

I now regret not just putting up a schematic.

Let me try to ASCII art the signal flow of the configuration as is (which
I don't like), omitting numerous details:


input
buffer A B
|\ | | |\
| \ -+-+-- | \
| \ | | | \
in---+----|A=1+------------|mux |----|A=1+-------+
| | / | | | / | |\
\ | / ------ | / \ | \
R / |/ 74HC4052 |/ / | \
TERM \ \<---|A=1+--out
/ digital / | /
| pot \ | /
GND | |/
|
GND


Since the analog switch resistance varies with Voltage, I am trying to
follow it with a very high impedance load. But I don't want to present
such a high impedance load to the outside world. So I put R TERM before
the buffer, and then I have the mux, and I leave the following op-amp with
a very high impedance. I do actually have a shunt resistor on the buffer
after the mux, but I plan to either leave it out, or use the highest
possible value.

I think what Ban was saying is that I can just ditch R TERM, and the first
buffer, and then use a high-value termination resistor on the second
buffer. This would get rid of 6 op-amps in three packages in a hurry.

Probably this is right. I may do some analysis to justify it to myself.

Thanks for your comments.

--Mac
 
M

Mac

Jan 1, 1970
0
Confidence: The feeling you have before you understand the problem :)

Heh, heh. OK, I'll take another look at the datasheets and see if I can
find any app notes.
Assume the worst number you came up with, if you are fairly sure there is
no way to make it even worse. Remember an overdesign will work.

That's true. Nothing exceeds like excess!

Thanks again.

--Mac
 
Top