op amp-based effects loop circuit for guitar amplifier

[YD]

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

Graham

Well, of course it can be done, but unless you're sure of what your
doing can be a bit dangerous. If any of the bias zeners happen to fail
open there'll be much merriment.

To Eric (OP): If you want to try it, you'll need to feed the supply
circuit with about 6 mA, the TL072 draws 4 and the rest goes through
the zeners. You'll need to drop 350 volts from the +B over a 56k 5W
resistor connected between +B and the +30V point of the supply.

- YD.

 

[Eric]

"Good enough for Marshall" is what I'm afraid of... Their recent
products (1990 and newer) are not known for their reliability hence the
original question about the quality of the design. Furthermore, B+
drops >5% at times (when cranked) and no voltage regulator is perfect.
I'll stick with a separate, split supply.

 

[Meindert Sprang]

Well, of course it can be done, but unless you're sure of what your
doing can be a bit dangerous. If any of the bias zeners happen to fail
open there'll be much merriment.

I remember reading about a nice feature of zeners: when they fail, they'll
short circuit. But that was long ago...

Meindert

 

[Meindert Sprang]

"Good enough for Marshall" is what I'm afraid of... Their recent
products (1990 and newer) are not known for their reliability hence the
original question about the quality of the design. Furthermore, B+
drops >5% at times (when cranked) and no voltage regulator is perfect.

Since this amp creates it's own "ground" with the zeners, a 5% drop of B+
would be a 5% drop of both supply voltages of the opamps and hence not
noticeable on the signal.

I once designed a high quality class A headphone amplifier which appeared to
have about 1Vpp ripple on *one* of the supply rails (bipolar supply). This
wasn't noticeable on the outputs at all because the feedback of the opamps
cancelled it.

Meindert

 

[Joerg]

Hello Meindert,

I remember reading about a nice feature of zeners: when they fail, they'll
short circuit. But that was long ago...

The ones in the metal housing tried to become spacecraft after shorting.

Regards, Joerg

 

[Pooh Bear]

Since this amp creates it's own "ground" with the zeners, a 5% drop of B+
would be a 5% drop of both supply voltages of the opamps and hence not
noticeable on the signal.

I once designed a high quality class A headphone amplifier which appeared to
have about 1Vpp ripple on *one* of the supply rails (bipolar supply). This
wasn't noticeable on the outputs at all because the feedback of the opamps
cancelled it.

Not to mention the supply voltage rejection ratio.

Graham

 

[Pooh Bear]

Well, of course it can be done, but unless you're sure of what your
doing can be a bit dangerous. If any of the bias zeners happen to fail
open there'll be much merriment.

Why would they fail ?

To Eric (OP): If you want to try it, you'll need to feed the supply
circuit with about 6 mA, the TL072 draws 4 and the rest goes through
the zeners. You'll need to drop 350 volts from the +B over a 56k 5W
resistor connected between +B and the +30V point of the supply.

Entirely off the top of my head, I recall the TL072 draws 2.8mA typical.

I reckon a 68k 3W ( runnung at 1.5W dissipation ) should do the job
myself.

Graham

 

[Joerg]

Hello Graham,

Entirely off the top of my head, I recall the TL072 draws 2.8mA typical.

Typically, yes. But the data sheet says 2.5mA per channel max so that
would be 5mA total.

Regards, Joerg