Spring Reverb Units

[TGRANT]

Two general and basic questions about spring reverb units.

I am building a solid state guitar amp, and I bought a new Acutronics spring reverb unit.

First question. When I build the amp, where and how does one hook the reverb unit into the circuit? I assume somewhere in the preamp stage, but where exactly is the best place to put it? I’ve seen tube amp schematics that show various configurations, but I haven’t been able to put my hands on a solid state preamp that shows how to incorporate a reverb unit.

Second, when scope testing the unit hooked into a driver and recovery circuit, what kind of wave form should I see so I know that the reverb circuit it is doing what it is supposed to do? I breadboarded the driver and recovery circuits exactly as the Acutronics web site described, and hooked in the unit. I ran a sine wave through the circuit. The input and output wave forms looked exactly the same. Does that mean it’s not working? What does a ‘reverbed’ sine wave look like? I expected to see at least some disturbance of the wave, but it was reproduced almost exactly.

[ceharden]

Try testing it with something other than a sine wave. The reverb tank can only delay the signal. If you take a single frequency sine wave, delay it, then add it back to the original, the output is just the same wave with the amplitude and phase changed.

A pulse signal should show the effect of the spring reverb.

[llwhtt]

With your test rig up and running monitor the output with the scope and then vary the test frequency. The output should go all "goofy" if it's working correctly. Varying the amplitude will get some "goofiness" but not nearly as much as the frequency.

Craig

[TGRANT]

Many thanks, that worked. I tested it with a square wave and as I increased the reverb level, the square wave sloped down. Actually, the rise level increased, rang a bit, then sloped down. Interesting. I guess I forgot what I knew about sine waves - add two sine waves and you get a sine wave. I could attach a phot of the scope if anyone is interested.

As you would expect, the shape of the wave changed as the frequency varied. Hope this info helps someone else.

By the way, I have the reverb located between two op amps in a two-stage tone control for this particuylar test.

[audiohead]

Craig:

Thanks 4 your scientific response.

Art

[llwhtt]

Art,

You're welcome.

Craig

[sakis]

Originally posted by TGRANT
Many thanks, that worked. I tested it with a square wave and as I increased the reverb level, the square wave sloped down. Actually, the rise level increased, rang a bit, then sloped down. Interesting. I guess I forgot what I knew about sine waves - add two sine waves and you get a sine wave. I could attach a phot of the scope if anyone is interested.

As you would expect, the shape of the wave changed as the frequency varied. Hope this info helps someone else.

By the way, I have the reverb located between two op amps in a two-stage tone control for this particuylar test.

i am mostly wondering what causes what you described as ringing ..... i ve seen it while repairing some transistor amps and i wasnt able to detect what causes that in some cases

thanks

[TGRANT]

I’m not an expert, but some ringing is caused by resonance that is then dampened, for example the damped oscillation of an output transistor into a reactive load. I’m thinking that’s to some extent what a reverb circuit is. The springs are certainly reactive, but of course the load shouldn’t be (the load in my test circuit was a simple non-inverting op amp). I’d be curious if any of the more knowledgeable people have any comments on this one.

[llwhtt]

There are two transformers also, one at each end of the spring.

Craig

[TGRANT]

What determines the load impedence of the reverb circuit - the transformers, or the input impedence of the recovery circuit? The recovery circuit on the Acutronics web site appears to be a straight forward non-inverting op amp that should have a very high input impedence and should not be reactive. Is that correct reasoning?