op amp-based effects loop circuit for guitar amplifier

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I need help with a circuit design for an effects loop in my tube guitar
amp. Because I don't have space for another tube, I'd like to use an
op amp based circuit. However, my knowledge re: op amp circuit design
is limited - I'm a tube guy....

After poking around I found this schematic:

http://www.drtube.com/schematics/marshall/pcbp-00119-60-02.pdf

Is this a reasonably decent design to clone?

I think it would work well for me since it was designed for an
amplifier very similar to mine and doesn't require an additional power
transformer; just step down the 380VDC rail (B+) voltage with a power
resistor and let the regulators do the rest.

Are there any easy improvements that could be made to this design? Am
I right to think it would be improved by using 7815 regulators over
Zener diodes and, by using an improved op amp (Analog Devices OP249)?
Any help is appreciated!

Eric

FWIW, an effects loop allows one to insert a low impedance signal
processor into an optimal section of the amp's high impedance signal
path.

 

[martin griffith]

On 17 Jul 2005 10:20:56 -0700, in sci.electronics.design

I need help with a circuit design for an effects loop in my tube guitar
amp. Because I don't have space for another tube, I'd like to use an
op amp based circuit. However, my knowledge re: op amp circuit design
is limited - I'm a tube guy....

After poking around I found this schematic:

http://www.drtube.com/schematics/marshall/pcbp-00119-60-02.pdf

Is this a reasonably decent design to clone?

I think it would work well for me since it was designed for an
amplifier very similar to mine and doesn't require an additional power
transformer; just step down the 380VDC rail (B+) voltage with a power
resistor and let the regulators do the rest.

Are there any easy improvements that could be made to this design? Am
I right to think it would be improved by using 7815 regulators over
Zener diodes and, by using an improved op amp (Analog Devices OP249)?
Any help is appreciated!

Eric

FWIW, an effects loop allows one to insert a low impedance signal
processor into an optimal section of the amp's high impedance signal
path.

Yep, looks reasonable at first sight.
But you are totally wrong in replacing the zeners. These are here to
prevent high voltage signals from zapping the opamps.

I'd just use 8 pin ic sockets on a homebrew, so you can easily change
the opamp. you may blow up a few prototyping. Stick with the TL072
until you have it working.

What the circuit doesn't show is the power supply. what are you going
to use?


martin

 

[martin griffith]

Yep, looks reasonable at first sight.
But you are totally wrong in replacing the zeners. These are here to
prevent high voltage signals from zapping the opamps.

I'd just use 8 pin ic sockets on a homebrew, so you can easily change
the opamp. you may blow up a few prototyping. Stick with the TL072
until you have it working.

What the circuit doesn't show is the power supply. what are you going
to use?

Oops misread your post about the power supply.

I would personally not use 380V dropped down. This circuit needs
split rail supplies, say +12V and -12V. Also there is a safety factor
while testing to be considered. try www.epanorama.net for suggestions
or
http://sound.westhost.com/project05.htm


martin

 

[Eric]

Thanks for the reply and comments, Martin.

Power is taken from the point W1 on this schematic:

http://www.drtube.com/schematics/marshall/1959-01-60-02.pdf

Filtration & regulation circuit are on shown on the first .pdf above
the main schematic. It's the zener's in that circuit (D5 & D6) I was
thinking about replacing with regulators. D1-D4 prevent the opamps
from getting zapped, correct?

Also, I will stick with a TL072 before trying the Analog Devices chip;
no sense blowing-up a $3 op amp!

Regards,
Eric

 

[Eric]

To adapt this circuit for a split supply, +15 would go to pin 8, -15
to pin 4, and VREF would essentially become ground, correct? If so,
are C10 and C3 necessary?

 

[YD]

To adapt this circuit for a split supply, +15 would go to pin 8, -15
to pin 4, and VREF would essentially become ground, correct? If so,
are C10 and C3 necessary?

Yes, that's correct. In case you take this route, recommended, you can
replace all capacitors by shorts, they're there only to block DC
levels from getting in each other's way. VREF and GRND should be
connected together. It might be an idea to keep C1 and C8 to block
external DC levels from getting into the circuit.

Whatever you do, DO NOT try to use the +B supply for this circuit.

- YD.

 

[Eric]

Martin & YD:
Great! Thanks very much for the help!
Regards,
Eric

 

[Joerg]

Hello Eric,

Just be careful and mind situations like when one of the supplies fails
but not the other. After all, you don't want to fry any expensive gear
that is connected. Maybe you could at least leave the output caps in.

Regards, Joerg

 

[Eric]

I am trying to avoid electrolytic caps in the signal path your point
about safety is valid.
Out of curiousity, is it common for op amps to leak DC when they fail?
Regards,
Eric.

 

[Joerg]

Hello Eric,

I am trying to avoid electrolytic caps in the signal path your point
about safety is valid.

Take film caps. Worth the few extra pennies.

Out of curiousity, is it common for op amps to leak DC when they fail?

They contain output transistors on the chip. One to the positive and one
to the negative rail. If one decides to short out then yes, they can
dump a whole lot of current into whatever is connected. Or whatever the
current limit on the power rail provides.

Regards, Joerg

 

[Eric]

Joerg,
Thanks very much for your help. I doubt a 47uf film cap (C10) exists
that will fit in the space available so Black Gate electrolytics will
have to do...
Eric

 

[martin griffith]

I am trying to avoid electrolytic caps in the signal path your point
about safety is valid.
Out of curiousity, is it common for op amps to leak DC when they fail?
Regards,
Eric.

try the pansonic or nitai brands of bipolar caps. they are designed
for AC coupling.


martin

 

[Joerg]

Hello Eric,

Thanks very much for your help. I doubt a 47uf film cap (C10) exists
that will fit in the space available so Black Gate electrolytics will
have to do...

If it really needs to be this high you won't find a film cap. But I try
not to use electrolytics of any kind in high-end audio designs.
Considering that there is a 470 Ohms in series I don't know why 47uF is
needed, even at low frequencies.

Regards, Joerg

 

[Eric]

Now that you mention it....

A 47uf coupling cap results in a low frequency response of 1.4 Hz with
a 10kohm load (average for studio rack gear), based on this calculator:

http://www.duncanamps.com/zips/coupling.xls

Given the lowest note on a guitar is about 70Hz, a 0.47uf (film) cap
would be fine since low frequency response would be around 44hz with
the same load... or am I missing somthing?

Eric

 

[Joerg]

Hello Eric,

Given the lowest note on a guitar is about 70Hz, a 0.47uf (film) cap
would be fine since low frequency response would be around 44hz with
the same load... or am I missing somthing?

The Z of a 0.47uF cap at 70Hz is around 5kOhm. You might want to shoot
for a larger cap here. I am not a musician but AFAIK guitars generate
lower frequency resonances.

You can get film caps up to several uF. Except for SMT where
realistically the limit would be around 1uF.

Regards, Joerg

 

[Pooh Bear]

I need help with a circuit design for an effects loop in my tube guitar
amp. Because I don't have space for another tube, I'd like to use an
op amp based circuit. However, my knowledge re: op amp circuit design
is limited - I'm a tube guy....

After poking around I found this schematic:

http://www.drtube.com/schematics/marshall/pcbp-00119-60-02.pdf

Is this a reasonably decent design to clone?

It's kinda OK.

I think it would work well for me since it was designed for an
amplifier very similar to mine and doesn't require an additional power
transformer; just step down the 380VDC rail (B+) voltage with a power
resistor and let the regulators do the rest.

Are there any easy improvements that could be made to this design? Am
I right to think it would be improved by using 7815 regulators over
Zener diodes and, by using an improved op amp (Analog Devices OP249)?
Any help is appreciated!

The use of zeners as shown to provide the supply volts and centre tap is a
shunt regulator configuration. You can't use a 78xx type device that way.
It's not stunning but I'd leave well alone.

Graham

 

[Pooh Bear]

Whatever you do, DO NOT try to use the +B supply for this circuit.

Why do you say that ? It's good enough for Marshall and if there's no
other volts readily available inside the box......

Graham

 

[Pooh Bear]

Hello Eric,


If it really needs to be this high you won't find a film cap. But I try
not to use electrolytics of any kind in high-end audio designs.
Considering that there is a 470 Ohms in series I don't know why 47uF is
needed, even at low frequencies.

I'd forget about the audiophool caps and simply use a standard 100uF
electrolytic coupling cap.

To Joerg, the reason large caps are used like this is that any
non-linearities they generate are minimised as the Zc component becomes an
increasingly tiny fraction of the load.

My 'standard' audio output coupling cap is 100uF and 10uF for inputs and
internal ac coupling. Even high end mixing consoles do it this way.

You can also forget about any LF -3dB point issues.

Graham

 

[Joerg]

Hello Graham,

My 'standard' audio output coupling cap is 100uF and 10uF for inputs and
internal ac coupling. Even high end mixing consoles do it this way.

Yes, but ... I have repaired a few over the years. When this stuff is
used outdoors a lot it gets hot. Really hot (it is about 107F here right
now). Some of the electrolytics I have seen in there were in deplorable
shape. Dried gunk clumps on the outside, others with an oily goo around
the bottom. In one unit I found about five of those. Plus a few
cigarette butts.

From a performance point of view electrolytics are probably ok. But
film caps aren't that expensive anymore and they last a long time.

Regards, Joerg

 

[Pooh Bear]

Hello Graham,


Yes, but ... I have repaired a few over the years. When this stuff is
used outdoors a lot it gets hot. Really hot (it is about 107F here right
now). Some of the electrolytics I have seen in there were in deplorable
shape. Dried gunk clumps on the outside, others with an oily goo around
the bottom. In one unit I found about five of those. Plus a few
cigarette butts.

From a performance point of view electrolytics are probably ok. But
film caps aren't that expensive anymore and they last a long time.

Well, I'd use film caps if they were available at larger values and didn't
cost so much.

I'm surprised at the dried out electrolytics. The normal types are specced at
85C. I do know of electrolytics drying out in one specific audio console that
ran really warm though. The standard fix is to substitute 105C parts.

Never had an issue with dried out electros in any gear I've designed btw.

Graham