Reverb

[jammybstard]

Hi All
I'm Building My first Stomp box, a Reverb unit based on the ESP http://sound.westhost.com/project34.htm circuit.

I've never really used Electrolytics in audio circuits before, so I thought I'd ask the experts; would you say the capacitor C4 in the bottom left of this circuit is in the wrong way round?

[Spidermonkey]

The negative side of polarized caps goes to ground. So yes, that looks to be the wrong way around. If I'm wrong, the worst that could happen is you'll get no sound. Good luck!

[jkokura]

I did think that the negative is supposed to go to ground...

But that doesn't mean I'm wrong. I'm a relative newb when it comes to these things and have only been able to absorb a little about these things.

Jacob

[CynicalMan]

The negative side of polarized caps goes to ground. So yes, that looks to be the wrong way around. If I'm wrong, the worst that could happen is you'll get no sound. Good luck!

Or it might blow up in your face...

This circuit looks like it's supposed to be powered off of a +/-15V bipolar supply. That means that, compared to ground, there will be both positive and negative voltages present in the circuit. So, the schematic is probably right. I doubt that there is an error, but if you're not sure, I would suggest breadboarding the circuit, replacing the capacitor in question with a non-polarized capacitor, and testing the voltages on either side. Then, replace it with an electrolytic capacitor, placing the positive side towards whichever side had a more positive voltage.

[Paul Marossy]

Uh, are you aware that this is not a reverb circuit? That's just a solid state send & return driver for a spring reverb tank.

Never mind what I said if that's what you are actually building (ie spring reverb stompbox).

[jammybstard]

This circuit looks like it's supposed to be powered off of a +/-15V bipolar supply.

Doh!, Of course! I'll try it the way it's shown in the diagram (and not have my face to close when I turn it on)

Uh, are you aware that this is not a reverb circuit? That's just a solid state send & return driver for a spring reverb tank.

Never mind what I said if that's what you are actually building (ie spring reverb stompbox).

Yea its to drive a Tank! I think I'm going to call it the TANK DRIVER!

[petemoore]

  "Low side down" is how I think of it to not RP capacitors.
  The + side shuold never see a lower voltage than the - side.
  In _ Gnd circuits that's easy, if the cap - is at ground, there's only up from there.
  But in split supply circuits, ground is 'in th emiddle', so you really have to watch the -rail in a different way than a single end +rail circuit.
  Paul is correct, and choosing a reverb tank suitable [so the driver circuit can control the transducer] for it is recommended before commiting to PCB or other build.

[Paul Marossy]

Cool, what kind of tank will it be driving? I always liked the old WWII Tiger, Sherman and the later M48 & M60 tanks.  

[anchovie]

I've built this with the negative terminal to ground and it works fine. Didn't try it as per the schematic, just altered it as it didn't look how I was used to.

I'm not sure the polarity matters as the op-amp input is biased to ground anyway.

[jammybstard]

I've built this with the negative terminal to ground and it works fine. Didn't try it as per the schematic, just altered it as it didn't look how I was used to.

I'm not sure the polarity matters as the op-amp input is biased to ground anyway.

I built it as per the diagram and it seems to be working just fine; I've added a tone control for the return signal from the tank which is a nice feature and also the a pot to vary the gain of the output stage so I can get about 10db of boost out of it , its a little noisy at the moment, I'll see what it's like when I get it into a metal case and post some pics

Cool, what kind of tank will it be driving? I always liked the old WWII Tiger, Sherman and the later M48 & M60 tanks. 

It's an Accutronics tank!
The "Sherman" would make a good name for the Pedal, or perhaps the "Tommy Cooker"!

[Paul Marossy]

Cool, what kind of tank will it be driving? I always liked the old WWII Tiger, Sherman and the later M48 & M60 tanks. 

It's an Accutronics tank!
The "Sherman" would make a good name for the Pedal, or perhaps the "Tommy Cooker"!

There's lots of tank names you that you could use. Pershing, Sherman, the M-3 Grant, T-34, KV-1, Panzer, Panther, Tiger, King Tiger, Crocodile, Matilda, etc. And then there's the self-propelled anti-tank guns like the M-10 Tank Destroyer, the Marder II, the Sturmgeschutz, Jagdpanther, Howitzer, etc. Or the even bigger guns like the "Big Bertha". 

[jammybstard]

Still very much in Beta, but working:





Still a bit noisy though, a low, but not quite low enough white noise leval from the tank; anchovie do you get much noise back from the tank on your circuit?

[Paul Marossy]

Cool, nice job on that. So how does it sound?

[jammybstard]

It sounds good; I have an amp to build tomorrow hopflully (if the parts arive!) I can record you some when I get that working!

[Paul Marossy]

Building amps is fun. I haven't built a tube amp in a long time.

[jammybstard]

I've previously built a Tube HIFI amp, but this is my first Guitar amp, I went for the Ampmaker 5W kit, http://www.ampmaker.com/se-5a-5w-british-overdrive-amp-1126-0.html shouldn't take two mins to make ! I've had a 1970 Celestion Greenback sat around doing nothing for years, I cant wait to hook it up!

[Paul Marossy]

Looks like a cool little amp. All three of the amps I have built so far have been completely scratchbuilt. I have wanted to get one of the kits offered at www.allenamps.com for years now. Maybe one day...

[PRR]

> would you say the capacitor C4 in the bottom left of this circuit is in the wrong way round?

U1B "+" input is biased to ground. By inference, it runs on +/- power, and the "-" input is also at ground.

The cap could get + or - voltage. Ideally you want a cap which does not mind this. A film cap is fine, except R7=1K suggests 10uFd for full bass, and that's a huge film-cap. In electrolytics, you can get "bipolar" which are rated for either +/ or - voltage.

However unless you abuse the input with subsonics, cap voltage will always be small. Much less than 1 Volt. Ordinary (polar) electros work fine in such applications. In this case, you can install either way round.

> Or it might blow up in your face...

In Power Supplies, a reversed electrolytic will soon try to suck infinite current at full voltage, and burst.

In this case, worst-case would be VR2 at zero, C5 shorted, and a large DC voltage at "From" jack (already an unlikely case). Then C4 gets wrong-way voltage, but through R7=1K. Assuming +/-15V supply, there will be 13V into R7, and maximum 13V/1K= 13mA to ground. Actually the situation will be about 10V across R7, 3V across C4, 10mA flowing, and 3V*10mA= 30 milliWatts in C4. The usual 10uFd electro will not explode, probably won't even be warm enough to notice.

So this is the case which slips through all the loopholes in "Always Observe Electrolytic Polarity!" In normal operation it does not matter, and in abnormal operation nothing exciting happens.

> negative side of polarized caps goes to ground

Only true with negative ground (positive hot) systems. True for most tube amps, true for simple NPN amps. Those two plans are very popular 'round here.

However the bias-supply on a large tube amp, or the main supply on a simple PNP amp, or the wiring on many older cars, will NOT be "neg to gnd".

FWIW: when in doubt, build without caps. Many circuits will come-alive without caps, they just won't pass signal. Put your meter where a cap goes, keep track of red/black +/- polarity, then insert caps accordingly. In this case jammy would find nearly zero voltage, probably +0.005V (+5mV).

[jammybstard]

Thanks PRR, the cap seems to be holding up ok, i could wedge two 4.7u poly props in there but I didnt want to take up half the case!

I did some audio samples through revalver this morning:

http://soundcloud.com/jammybstard/sets/reverb-stompbox-tests
(Used an Epiphone Riviera with P94 Pups and a simulated Peavey Classic 30 set cleanish with 1x 12" Celestion Greenback, only reverb pedals settings changed between samples)

Any comments welcome. (exept about my dodgy playing!)

[anchovie]

anchovie do you get much noise back from the tank on your circuit?

I was getting hiss at my testing/debugging stage because I was using a cheap switch-mode PSU (all I had at the time above 9 volts) - the PSU noise was being amplified by the LM386 before being sent through the tank. Once I switched to a nice clean rectified/regulated PSU there was hardly any background noise.