Help debugging reverb please

[thom]

Hello all,

Having built a stage center reverb but being unhapping with the sound, I've taken on another one.

It's the first time I've worked from a schematic only, so I would appreciate if someone could check my layout (i'm sure it could be much better arranged, but it's my first one ).

When the guitar is plugged into the circuit (with Accutronics 4DB2C1D, 250 ohms input, 2250 ohms output), I get a clean sound.

Here's what the pots do:

VR1 changes the tone
VR3 does nothing
VR2 acts as a volume pot (no sound when dialed towards C9), whereas it should control the amount of reverb (I think)

Here are the voltages of the LM358 (is 7 OK?):

1: 10.73v

2: 5.42v

3: 5.95v

4: 0v

5: 5.95v

6: 0.14v

7: 10.74v  -> 0v when input (reverb) plugged

8: 11.98v

I've been checking the circuit against the schematic and the layout for two days, and I'm at a loss.
Any help would be greatly appreciated.

[PRR]

Where did you get that schematic?

It clearly will not work as drawn.

Disconnect the wires to the Wipers of RV1 and RV3. Do you get the proper DC voltages on the opamps, 6V at each of the six signal-pins?

[thom]

Hi Paul,

Thanks for your reply.
I got the schematic from this French site: http://www.sonelec-musique.com/electronique_realisations_reverb_ressort_001.html
It's the second one.

Here are the new voltages with everything plugged in and RV1 and RV3 wipers disconnected , pin 6 seems a liitle low:

1: 5.98v

2: 5.99v

3: 5.95v

4: 0      

5: 5.95v

6: 3.39v

7: 5.94v

8: 11.98v

[PRR]

> I got the schematic from this French site

My French is very bad, but "L'alimentation doit être de type symétrique" looks something like "food {power} must be symmetrical"; indeed it links to a + and - 15V supply plan.

It doesn't have to be full 15V. A second 9V battery should power it up just fine.

> It's the second one.

Ah, that says "Le schéma est un peu modifié pour tenir compte de l'absence d'une tension négative", but I don't see how it can possibly work without a negative supply.

Hmmmmm.... if you put coupling-caps at RV1 and RV3 _wipers_ it should bias-up. Perhaps a drawing mistake.

[thom]

That's right, the second schematic is a variation of the first one that's supposed to work without a negative supply.

I've donne as you said, put coupling caps (0.022uf) at RV1 and RV3 wipers. The voltages are similar to the ones obtained after disconnecting the wipers (about 6v).

On top of the clean sound, there's now a loud "shhh" noise that gets louder as RV2 is dialed and drowns a hint of reverb.

[PRR]

> there's now a loud "shhh" noise

I'm not too surprised. This is a very strange circuit. I would not do a reverb this way.

[thom]

I think it's propbably best to give up on that one, then...

One more thing, if you don't mind.
I'm still keen to drive my reverb tank, and I'd like to try one of the circuits on the following site (figures 3 & 4): http://sound.westhost.com/project34.htm.

A couple of questions (sorry if they sound a bit stupid, I'm still new here):

1. Do you reckon figure 3 can be powered with negative at ground (although it says it should use a dual supply).

2. Actually, I'm not sure what dual supply realy means. Is the negative from the PSU separated from ground? In that case, where would you put the negaive on those schematics?

3. Would a 12v power supply be enough fot figure 3?

Thanks again for your help.

[PRR]

[thom]

Thank you so much Paul, I'll put that into practice and let you know how I get on (third time lucky, maybe?).

Forgive my ignorance, but I have few more questions about the schematic if you don't mind:

What happens to pins 1, 7 and 8 of the LM386?
And 4 and 8 of the NE5532 (is 4 to -ve/ground and 8 to +9v?)?
In your added mod (on the right), zero v = ground?

Out of personal interest, was the polarity of C4 correct in the original schematic (+ve at ground)?

I think that's all for now 

[PRR]

> was the polarity of C4 correct in the original schematic

In Rod's plan, using + and - supplies, C4 has zero(*) DC voltage. Also no large signal. It is customary to use a polar electrolytic either/which way.

(*) Not quite dead-zero. '5532 has NPN inputs. They pull current in the 100K resistor. The input tends negative. The typ/max input voltage will be 0.020V/0.1V. This is so small that e-cap polarity can be neglected (it takes more like 1V backward to hurt an e-cap). However Rod had it right.

With the single + supply, the "top" of C4 will be 4.5V positive of the grounded end, so must be poled as I show.

The extra pins ought to be Googled.