need help getting rid of clicks, pops, and hum in design

[tempus fugit]

Hey all;

I've built a switching unit for guitar effects, and I'm trying to fine tune
things by getting rid of the audible pops and clicks I get when switching.
THe setup is basically this:


1/6 of a 4049 inverter input gets grounded momentarily by switch, generating
a logic hi ==============>

logic hi goes to input of 74HC373 octal latch, sending a high to the output
=============>

this hi goes to a ULN2003 relay driver ==============>

this pulls 12v through an LED and the relay coil to ground.

The relay is SPDT, and the pole selects between the input of the effect
(when it's off) and the output (when it's on).

I also have 2 channels dedicated to amp functions (channel switching and
volume boost). These require 15v from the amp to be grounded to be
activated. Initially I did this with 2 separate relays, but, in an effort to
clean up the switching pops, I decided to try to replace the relays with
transistors. This time, I routed the 5v control signal to the base of a
transistor through a 1K resistor, with a 10k resistor from base to ground.
The 15v from the amp is connected to the collector, and the emitter is tied
directly to ground. Worked OK when I breadboarded it, but now that it's in
the system, the switching pops are way worse, and there is a noticeable hum
when the 15v is grounded through the transistor. I figured that the
switching noise must have been mechanical (from the contacts making and
breaking contact), but I pulled out the footswitch unit that came with the
amp (which uses heavy duty stomp switches), and it is dead quiet. Not a pop
or click to be heard.

I'm about to start experimenting with some analog switches to see if I can
get rid of the pops when switching effects in and out, but after this
experience with the amp functions, I'm having my doubts....

So, can anyone help? Where do the clicks and pops from switching come from
in the first place? Why is the transistor clicking when there are no
mechanical parts?

Any help would be appreciated - I'm getting to my wit's end here and running
out of ideas. I realize that my description isn't too exact, but I gave you
the basic idea for brevity's sake. If more info is needed, please let me
know and I'll supply all the gory details.

Thanks

 

[Eeyore]

So, can anyone help? Where do the clicks and pops from switching come from
in the first place? Why is the transistor clicking when there are no
mechanical parts?

Don't forget that a relay will cause 'fourier clicks'.

Clickless audio switching is done with jfets carefully timed to avoid this ( the
audio fades rather than cuts ).

Graham

 

[niftydog]

Where do the clicks and pops from switching come from
in the first place?

In my experience, one source of these sounds is DC leaking through input
or output caps and causing the two terminals that are to be connected
together to be at different DC levels. The sudden connecting of these
two terminals causes the DC conditions to suddenly change, which
manifests itself as a loud thump or pop in the audio path.

"True bypass" pedals all exhibit this behavior to some extent, so using
relays is going to cause the same thing. (You can find plenty of
semi-baked solutions to the true bypass popping problem with a quick
google search.) I guess this is why Boss and other manufacturers have
stuck to their FET switching arrangements throughout the true bypass craze.

I'm yet to find the perfect solution, and I'm beginning to doubt there
is one. As I'm about to embark on a similar project, I am very keen to
see what others suggest.

 

[John Fields]

Hey all;

I've built a switching unit for guitar effects, and I'm trying to fine tune
things by getting rid of the audible pops and clicks I get when switching.
THe setup is basically this:


1/6 of a 4049 inverter input gets grounded momentarily by switch, generating
a logic hi ==============>

logic hi goes to input of 74HC373 octal latch, sending a high to the output
=============>

this hi goes to a ULN2003 relay driver ==============>

this pulls 12v through an LED and the relay coil to ground.

The relay is SPDT, and the pole selects between the input of the effect
(when it's off) and the output (when it's on).

I also have 2 channels dedicated to amp functions (channel switching and
volume boost). These require 15v from the amp to be grounded to be
activated. Initially I did this with 2 separate relays, but, in an effort to
clean up the switching pops, I decided to try to replace the relays with
transistors. This time, I routed the 5v control signal to the base of a
transistor through a 1K resistor, with a 10k resistor from base to ground.
The 15v from the amp is connected to the collector, and the emitter is tied
directly to ground. Worked OK when I breadboarded it, but now that it's in
the system, the switching pops are way worse, and there is a noticeable hum
when the 15v is grounded through the transistor. I figured that the
switching noise must have been mechanical (from the contacts making and
breaking contact), but I pulled out the footswitch unit that came with the
amp (which uses heavy duty stomp switches), and it is dead quiet. Not a pop
or click to be heard.

I'm about to start experimenting with some analog switches to see if I can
get rid of the pops when switching effects in and out, but after this
experience with the amp functions, I'm having my doubts....

So, can anyone help? Where do the clicks and pops from switching come from
in the first place? Why is the transistor clicking when there are no
mechanical parts?

Any help would be appreciated - I'm getting to my wit's end here and running
out of ideas. I realize that my description isn't too exact, but I gave you
the basic idea for brevity's sake. If more info is needed, please let me
know and I'll supply all the gory details.

---
Hum could be coming from inadequate shielding and/or bad grounding,
or from power supply ripple getting into the circuitry.

Clicks and pops are the result of transients created when switching
is done abruptly.

Can you post a schematic of your circuitry?

 

[tempus fugit]

Thanks for the replies.

I wondered about the DC leakage issue myself, and so have connected 2M
resistors from hot to ground on each input and output, which (I think) would
drain away any DC that got away from the caps. Now, these are on the in/out
jacks, not on the pedals themselves. I don't know if that would make a
difference.

Please feel free to ask questions about your project nifty. I'm really happy
with my board. It can switch multiple fx and fx combinations in one stomp.
It really is cool, and these pops are the only drawback (they're not
terrible mind you, just kind of annoying and I want to get it perfect).

Thanks

 

[tempus fugit]

Hi John;

Thanks for the reply. I'll post the schematic in blocks if that's OK. I
don't have any schematic drawing software, and it's pretty extensive....

Thanks

 

[tempus fugit]

Here's the latest bug. I have these 2 2222s as replacements for relays to
switch +15v from the amp to ground for amp channel switching and volume
boost. With the relays I had in there before, there was a small click; now
it is a big click and there's an annoying hum now that wasn't there before -
only when the transistor switch is on (i.e., the 15v is grounded). Not quite
sure what's going on with that. Oh one other thing - the stomp switch unit
that came with the amp is totally clean and quiet.

Any ideas on this one?

Thanks



+15v from amp
+15v from amp
o
o
|
|
|
|
|
|
o
o
1K |
1K |
___ |/ +5v control
___ |/
----5v control -|___|------|
----|___|----o--| 2222
| | |> |
| |>
| .-. | |
..-. |
| | | o |
| | o
| 10k| | | |
10k| | |
| '-' | |
'-' |
| | | |
| |
|
o----o---------------------|------------o----o
| |
| |
| |
| |
| |
=== ===
| +5v |
GND GND
| | |
(amp)
| | |
__ | | __ |
-o| |o- | -o| |o- |
-o| |o- |--o| |o- |
-o| |o- ------o| |o-o |
-o|__|o- | |---o|__|o-| |
-o| |o- |-----o| |o-| |
-o| |o- ||| -o| |o-o |
-o| |o---||||--o| |o-| |
-o|__|o----||| -o|__|o-o |
|| | | |
74hc373 |o--o4049 | |
| | | |
| o-----o |
| | |
| === |
| GND |
| |
| |
-----------------------------------------------------|






(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

 

[niftydog]

Ideally, you should get a hold of the schematic for your amp. Have you
tested the factory switch with a multimeter to be sure of what it's
doing? Are all grounds connected together? How are you sourcing the 5VDC
for the logic?

Generally a simple transistor switch just has a 10k from the control
voltage in series with the base, no resistor to ground needed.

Are you aware of diystompboxes.com? They have travelled this road many
times before, a quick search of the archive might prove fruitful.

nifty

 

[tempus fugit]

Thanks Nifty

I do have the schem for the amp, and I know the switch is DPST controlling
LEDs and the switching to ground. The 10k from base to ground is there to
ensure a definite logic state in the absence of a control voltage. The 5VDC
for the logic is coming from the PS to an octal latch, through and inverter
and onto the transistor.

I'll check that link too.

 

[niftydog]

This is troubling me, tell me if I'm crazy. I think I'm misunderstanding
the amps switching arrangement. What is the amp, BTW?

I know the switch is DPST controlling
LEDs and the switching to ground.

The original switch has LEDs in it right? And it's only power source is
the 15V from the amp? If so, it can't be directly switching the 15V to
ground AND powering the LEDs at the same time right, it's gotta maintain
that 15V while it switches the control lines. Your switching arrangement
with the transistor is attempting to drag the 15V down to the saturation
voltage of the transistor, the way it's depicted in the schem you posted.

The 5VDC
for the logic is coming from the PS to an octal latch, through and inverter
and onto the transistor.

So it's sourced from a seperate power supply? Is it's ground connected
to the amps ground?

 

[tempus fugit]

The amp is a Carvin X100B. I'll try to explain as best I can. The amp has 3
switching possibilities - reverb, channel, and volume boost. I leave the rev
on all the time, so I just left it unconnected. All the functions are
switched by connecting +15v (from the amp) to ground. So to activate the
lead channel, there is a phone plug with +15v on the tip, and the amp's
ground on the sleeve. When I stomp a switch (from my pedalboard or the
supplied footswitch), the 15v is connected directly to ground, thus
switching the effect in or out. The LEDs are also controlled by the same
DPST switch. There is a 1k resistor for each LED in the amp, and 15v travels
through it to groun when activated, or just floats when it isn't. So in the
case of the suppled footswitch, the DPST has 1 pole switching the amp's 15v
for the fx switching, and the other pole switches the +!5v for the LED.

Pretty nifty (no pun intended) eh?

 

[niftydog]

Is this your amps schem? http://www.carvinmuseum.com/pdf/amps/xamp-revk.pdf
That seems quite different to what you're describing.

 

[tempus fugit]

No, this is a later revision of my amp. I can send you the schem if you're
interested.

Here's some other info that may or may not be helpful here. I disconnected
all the relays and LEDs last nite and there was no detectable noise from
switching (obviously I couldn't hear this directly because the relays
weren't actually switching, but I have an NS2 that has an LED that displays
when the noise gate is active. As soon as any sound goes through the gate
the LED goes off, and it stayed lit the whole time). With the pops that I
was getting from the transistor switches, I was starting to wonder if they
may have been originating somewhere in the switching logic, or maybe the
power supply, but that is looking less likely.

You mentioned the 10k resistor to ground at the base as being possibly
problematic. Do you think that this could be the cause of either the
switching noise or the hum?

Thanks again

 

[tempus fugit]

Woops sorry nifty I just noticed the rest of this message.


" Your switching arrangement

with the transistor is attempting to drag the 15V down to the saturation
voltage of the transistor, the way it's depicted in the schem you posted.

Yes, if I understand you correctly. Is that what it should be doing?

So it's sourced from a seperate power supply? Is it's ground connected
to the amps ground?

Yes. The 5vDC is coming from the pedalboard's PS, the 15v comes directly
from the amp. Its ground is connected to the amp's ground, by necessity. I
tried like mad to figure out a way to keep the grounds isolated, but it's
impossible without doing some revisions to the amp. The amp's ground will be
connected to the pedalboard's ground as soon as I connect the pedalboard out
to the amp in. This wasn't a problem before I replaced the relays with the
transistors, btw.

Thanks

 

[tempus fugit]

Here's what my relay to pedal connections look like. Is there anything here
that may be contributing to pops?

Thanks


Relay to pedal Connections



FX1 FX2 FX3 FX4

Gtr in--------------In Out In Out In Out In Out
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
o /o ---------o /o --------o /o --------o /o
/ | / | / | /
Relay1 / | Relay2 / |Relay3 / |Relay4 /
o | o | o | o
| | | | | | |
| | | | |
--------------- Amp out
| | | | | |
--------| -------| ------|





























(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

 

[niftydog]

The first thing I'd say is that it's considered bad practice to have the
inputs of the effects constantly in the chain. Even when an effect is
bypassed it's still in the circuit and affecting impedances. The pops
you're hearing is due to the slight difference in DC conditions between
the input and the output of an effect, not because of the mechanical
switching. However, even with a true bypass arrangement, the pops are
still a problem, so...

As for the DC leakage resistors, trial and error works for me, I haven't
sat down and worked out exactly what the solution is, but different
value resistors work for different FX pedals.

To the amp channel switching; The transistors can switch very quickly,
in fact faster than the relays. You might try slowing down the rise time
of the control signal with an RC network, perhaps try replacing the 10k
from base to ground with a largish capacitor so the transistor turns on
relatively slowly. I don't think the 10k is to blame, but while you're
mucking around, try taking it out of circuit.

Can you post voltage readings of the transistors when switched on and
off? That might give us something to go on.

With the relay amp channel switching, the 5V power supply is
mechanically isolated from the amps 15V. With the transitors, it's not,
it's coupled via the base/collector junction. This might be something to
look at, just a stab in the dark.

 

[tempus fugit]

Thanks nifty;

Good suggestions. The inputs in the chain (although considered bad
practice), is, surprisingly, not an issue (unless it is in fact the reason
for the pops). I tested this thoroughly before even beginning to build the
unit, and there is no audible difference between the effects unit in bypass
(i.e., all inputs connected and in the chain) and the guitar plugged
directly into the amp. I think this is one of those "tone sucking" paranoias
that has been blown out of proportion. I myself was leery of setting things
up this way too, and my bass player, who is something of an electronics
whiz, also thinks that this would affect the tone, but I can hear no
difference. I have considered switching the relays to DPDT and going true
bypass, but I thought I'd give the analog switch route a go first. Of
course, I'd like to ascertain where the pops are coming from before I do any
more work (this is cutting into my practice time too much already!).

So you're suggesting putting resistors across the input and output caps of
each effect right? I do have 2M resistors across each in/out JACK where the
pedals connect to the board. Is this the same thing or do they need to be at
the pedal itself?

I have thought of the RC thing also, and have tried something, but I've been
messing around so much I can't remember where I put the cap. I'm pretty sure
I tried one from base to collector (got that idea from the geofex site) but
it did nothing. I don't see any reason why the transistors should be causing
such a problem, but then I don't know all there is to know either. I still
marvel that the actual switch that came with the amp is so dead quiet when
it switches, and I would've thought the transistors would be as quiet,
but...

Your point about the isolation of PS voltages when using relays is also
well-taken. I'm sure that the hum problem is a result of that, and
discovered the ground loop I was creating last night. The grounds of the amp
and pedalboard are going to be connected no matter what, because as soon as
I plug the pedalboard into the amp, or the plug to control amp functions,
I've connected the grounds. That being said, When I used the relays, there
was only 1 connection to the amp's ground - from the output of the
pedalboard to the amp's input. The amp control functions was isolated. With
the transistors, though, the amp control function ground is connected
directly to the pedalboard ground, and I've got a loop. If I keep the
control function ground floating, the hum is gone, but it only works if I
have the pedalboard out connected to the amp's input, which is a problem for
some of the things I do. I'm going to try a 100 ohm resistor between the 2
grounds and see if that solves that problem. If it doesn't I may have to go
back to relays.


There is one other thing I should mention. I am using the relay's coil as a
resistor for an LED indicator light for each effect. So when the relay
engages, it also pulls the LED on. I wonder if that might be causing some of
the clicks as well, or contributing.

Do you want me to post any more schems of my wiring? I think I have ruled
out the logic section of the board as causing any problems, so I'm going to
concentrate on the functional side.

Thanks again

 

[niftydog]

I think you can rule out the logic too, I'm most sus about the power
supplies interacting via the transistors (possibly causing the hum) and
the DC leakage issue (pops). The only thing I would say is, perhaps you
should decouple the logic chips power supply. Small value caps placed as
close as possible to the logic chips power pins help reduce the
transient loads on the supply.

If you're cool with the non true bypass, that's all that matters. I was
just making the point in case you weren't aware of the whole debate.

I think this is one of those "tone sucking" paranoias
that has been blown out of proportion.

I agree. True bypass seems to cause more problems than it solves. In the
end, there's buffering and FET switching going on inside of most guitar
amplifiers and nobody is worried about that! But, use it in a pedal,
even after a high quality buffer, and it's the devil incarnate!

I make it a point to suggest to these people that perhaps in all this
searching for "transparency" they're forgetting that one good buffer up
front might actually IMPROVE their tone. They seem to think that any
change in tone from a direct connection between guitar and amp is bad.

For instance, the buffer I use has a 10M input impedance for passive
basses. Compared to a direct connection to my amp my basses have much
more top end and punch.

So you're suggesting putting resistors across the input and output caps of
each effect right? I do have 2M resistors across each in/out JACK where the
pedals connect to the board. Is this the same thing or do they need to be at
the pedal itself?

Provided they're always in circuit with the pedal I don't see a problem
with that. As I said, trial a few different values.


Try that cap from base to emitter (in place of the 10k) leaving the 1k
in place.

There is one other thing I should mention. I am using the relay's coil as a
resistor for an LED indicator light for each effect.

LEDs have been suspected in the past of doing strange things like that.
The fix is the same as what we're talking about with the RC time
constant creating a slow turn on.

FX guru RG Keen's article has an excellent discussion of some of these
phenomena.
http://www.geofex.com/Article_Folders/bypass/bypass.htm
Another article I stumbled across the other day says it more concisely
http://www.muzique.com/lab/led.htm

keep me posted on progress!

nifty

 

[tempus fugit]

Hi Nifty;

Thanks again for your reply. I just tried a 3.3uF cap to ground, with no
improvement. Maybe tomorrow I'll try a 10 and see if that helps. I do have
decoupling caps at the logic chips (not all of them mind you - they are
pretty close together on the board and I would think that 1 set of
decouplers would handle a couple ICs - correct me if I'm wrong).

 

[niftydog]

I would think that 1 set of
decouplers would handle a couple ICs - correct me if I'm wrong).

Well, some people are pretty strict about it, but I'm just thinking it's
one less thing to worry about if they're all over the place!

You know how to calculate the RC time constant?

time (to reach 63% of full charge) = R x C

So, a 1k resistor and a 3.3µF cap is only 3.3ms, but it starts to turn
on the transistor once it gets past 0.6V which is only 12% of 5V. You
would start to notice the delay at around 10ms so you might be able to
go as high as 47µF before it's too noticeable. (alternatively, bump the
1k up to a 10k and use a smaller cap.)

nifty