How can i use an Accutronics 4AB3C1B in a GGG Stage Center Reverb?

[gigimarga]

Hello,

Can i use an Accutronics 4AB3C1B (which has a 8 ohms input impedance) in a GGG Stage Center Reverb (which requires a 310 ohms input impedance), eventually making some minor modifications?

Thx a lot!

[gigimarga]

nobody?

[birt]

with a transformer 1:40 (or something close to that) i think

[guitarman89]

I have a similar problem. I've a little cheap amp (a stinger) that has an unknow reverb tank. I do not know anything about it and a i'm thinking to build a spring reverb rack-sized...so can i use it in the GGG stage center reverb?

Thanks in advance!:)

[George Giblet]

> Can i use an Accutronics 4AB3C1B (which has a 8 ohms input impedance)


The drive level from the opamp isn't enough to drive the 8 ohm input. 

A transformer can match the impedance but it loses a whole lot of voltage swing so it might not work too well - basically the opamp hasn't got enough output power to drive the coil.

I see a number of options:

- replace the coil drive opamp ("to reverb") with a power amplifier suited to drive 8 ohms loads.  Amplifier must be inverting.

- add a power amplifier after the "to reverb" point.  Amplifier must be non inverting.

- Add a current boost stage to the opamp using transistors.  For example see fig 11c

http://www.headwize.com/projects/opamp_prj.htm

You need to reduce the 100R resistors but be careful of biasing.  The easiest solution is to short the 100Rs and short one of the diodes.

Here's another example which uses 2ohm resistors, which may or may not cause an excessive bias current:

http://www.accutronicsreverb.com/drive3.pdf

Often you will need to add a small capacitor, say 22pF to 100pF, between the opamp output and the opamp -ve terminal.

What I'm suggesting here is to add the circuit at the output of the opamps in the example circuits and retrofit these to the Stage Center driver opamp - not to copy these circuits.

[gigimarga]

Thx a lot for your answer George...i think that's too hard for me...i am not an electronist - only a hobbist 
Anyway, i will read what you said and i will try...but i have serious doubts about the result 

[George Giblet]

I will draw something up for you.  It's not too hard when you see it.   The tricky thing with these circuits is tuning the bias point.  (I will give you a class B design with the option to go class AB.)

I actually prefer a current drive circuit like the accutronics circuit but the stage center is a voltage drive circuit, so I will give you a voltage drive circuit.

[George Giblet]

This should work,

http://www.geocities.com/george_giblet/effects/stage_centre_reverb_8ohm_v1.0.png

[gigimarga]

WOW...man you're great!!!

Thx a lot for your kindness!!!

I will try next week and i will tell you about the result!

[George Giblet]

No problem.

If you have any problems with oscillations try pulling C2 (220pF) and moving it to where C51 (22p) is (and removing the 22pF).

[joefox]

Hello.
I wish to thank Mr. Giblet for his enlightened solution.
It worked for my Stage Center with a  9AB3C1B 10 ohm tank.
I only would like to know what difference there could be
in the sound compared with the original project.

[petemoore]

I wish to thank Mr. Giblet for his enlightened solution.
It worked for my Stage Center with a  9AB3C1B 10 ohm tank.
  It looks like an elegant SS solution to current draw/voltage swing problems.
I only would like to know what difference there could be
in the sound compared with the original project.
  I would say mostly in the tanks used, especailly since you can't just swap tanks and A/B them without changing the driver circuit.
  The impedance driver circuit output probably greatly resembles it's input, ie it just adds drive capability to the input AC waveform.
 

[joefox]

Thank you for the info.
My tank has a perhaps too long delay, but I realized that padding somehow
the coils with some sponge would make it much more usable.

Regards