Stage center reverb problems (and how I overcame them)

[Stevpo]

So, I thought I'd post my experience with building the Stage Center Reverb from GGG in case other future builders might learn from my experience. Once it was all sorted, I have found this to be a really nice sounding pedal.

My first issue I experienced was one of excessive hiss. This was solved by putting a buffered pedal in front. There is some interaction between the guitar pickups and the input circuitry of this pedal that leads to a lot of noise being generated.

The next big thing was what seemed like a huge amount of reverb tank drive. My trimmer setting the gain on IC1b would be set to almost the minimum, and I still needed to set the Dwell control extremely low to have usable reverb. I noticed that the reverb signal tended to be very boomy, so to cut bass, I changed C4 to a 1nF cap. This seemed to solve most of the sound issues I was having. This leads me to think that maybe C4 is mislabelled on that schematic.

A couple other changes I made were to use a 20k/27k voltage divider in place of the Mix control to set the dry signal to a fixed unity gain level when the pedal is engaged. I then made the Dwell control the Reverb Level control, and took the 500k trimmer out and replaced it with a 500k pot that becomes a true Dwell control.

With these changes, this is a very nice sounding reverb pedal. Hope someone finds this useful!

[Kipper4]

Please share a link to the schematic so we can look.
I'm guessing it's a belton brick verb.
Depending on which brick you bought will have a bearing on how much dwell/verb.

Glad you got it sorted.
Have fun
Rich

[Mark Hammer]

Forty-some years back, I purchased a late 1960s solid-state mid-powered Gibson amp.  The spring reverb was awful.  I located the bass-limiting series capacitor in the signal driving the pan and reduced its value.  Improved the resulting reverb sound 200%.  Couldn't tell you what the model number was, because I ended up selling it shortly after (it came as part of a package with what I really wanted - a Schaller wah-wah/yoy-yoy pedal.)  Sometimes, it's the little things, and sometimes it's littler things.

[PRR]

Please share a link to the schematic so we can look.
I'm guessing it's a belton brick verb.

Google "Stage Center Reverb" brings up a lot of things, Anderton's original --

https://experimentalistsanonymous.com/diy/Schematics/Reverb/Stage%20Center%20Reverb.pdf

AND an "improved" version from almost everybody with a pencil. (All I saw were spring reverbs; I think the original predates $19 digital reverbs.)

Nothing on the Anderton plan would go hissy without an input buffer unless you picked a very unsuited opamp.

The Anderton plan I found has no component designations "C4" so I can't know what cap you mean.

[Rob Strand]

Link to schematic:
https://generalguitargadgets.com/pdf/ggg_sc_verb_sc.pdf

The next big thing was what seemed like a huge amount of reverb tank drive. My trimmer setting the gain on IC1b would be set to almost the minimum, and I still needed to set the Dwell control extremely low to have usable reverb. I noticed that the reverb signal tended to be very boomy, so to cut bass, I changed C4 to a 1nF cap. This seemed to solve most of the sound issues I was having. This leads me to think that maybe C4 is mislabelled on that schematic.

The parts on the schematic are correct.   In many respects, the Stage Center Reverb isn't the way to do a reverb at all!  The reason you need to use such a small cap is because the Stage Center Reverb uses voltage to drive the reverb coil.   To do that correctly you need a high-pass filter across the whole audio band on the drive circuit, which the  Stage Center Reverb doesn't do.  As a result it is muddy.

Reverb springs have a flat response when they are driven by a current source not the conventional opamp output which is a voltage source.   When driven by a current source the voltage a cross the reverb drive coil increases with frequency because the coil is inductive.   The Stage Center Reverb drives the spring with a voltage source which means it's very muddy because the drive *current* is decreasing with frequency.  A way to get the correct drive with a voltage source is to create a high-pass filter where the cut-off region is across the whole audio band.    That's why your mod works.   With the 1nF cap the high-pass cut-off frequency is fc = 1/(2*pi*R4*C4) = 1/(2*pi*22k*1n) = 7.2kHz.    So the drive voltage will increase across the whole audio band then flatten off when you kit 7.2kHz.   That's a good approximation to the current drive case.   It's also why the reverb response sounds right and not muddy.

The 220pf and 470k resistors cause the drive voltage to roll-off above fc = 1/(2*pi*R7*C1) = 1/(2*pi*470k*220p) = 1.54kHz.   That's a little low but these things can be to taste to some degree.   The main point though is putting the HF foll-off on the drive IC IC1b isn't the way do it.   It is better to put the HF roll-off on IC1d.  You can do this with a cap across R9.   To keep the 1.54kHz HF roll-off you would need a 47pF cap,  fc = 1/(2*pi*R9*C) = 1/(2*pi*2.2M*47p) = 1.54kHz.    The reason fore putting the roll-off here is it filters the noise from IC1b and IC1d and it also filters any high frequency interference and noise from the reverb spring.    With this mod you would C1 small so the cut-off it above audio, for example C1 = 6.8pF then fc = 1/(2*pi*R7*C1) = 1/(2*pi*470k*6.8p) = 50kHZ.

Using a inverting stage isn't the best choice for noise for a reverb recovery amp (IC1d).    However if you wired this stage as a non-inverting amp, and used lower resistor values for lower noise, then you would need to wire IC1b as a non-inverting amp.   That's going to require too many mods, it's basically a redesign.   If you look at some commercial amplifier circuits you will see they use non-inverting stages.  In most cases they also use current drive, in which case you don't need a high-pass filter at 7.2kHz you can put one in the 100Hz to 300Hz zone, depending on taste.

Example,

This one has extra filtering and a rumble filter.
https://www.drtube.com/schematics/marshall/vs65-60-02-1.gif

This one has booster transistors on the reverb drive opamp for driving a low impedance tank,
http://4tubes.com/2-SCHEMATICS/Music-amps/Marshall/5275.gif


FYI, something which greatly helps reduce noise on reverb circuit, both electronic and mechanical, is to drive the reverb spring as hard as possible (within specs) then use a lower gain for the recovery preamp.