Denyer's "Guitar Handbook" Opamp Preamp Build Report/Quirky Problem

[Paul Marossy]

I've wanted to build this little opamp preamp circuit for many years, before I got into electronics really. It was a dismal failure. Anyhow, I was looking at that book the other day and thought, "Heck, I could build that", and so I did. I used a Burr Brown OP134A opamp and I was surprised - it's actually a pretty good sounding little circuit. I followed the veroboard layout in the book exactly, but did it on perfboard instead. Instead of the 50K trimpot, I am going with a 50K outboard pot for easily variable gain.

Here's my layout: http://www.diyguitarist.com/PDF_Files/OpampGuitarPreamp.pdf - I think the schematic is correct, but a second look is always welcome...

OK, now for the quirky problem. As I said, it sounds great, but when I switch it from bypass mode to preamp mode, it completely cuts out, and then about one second later, it comes back in. It only does then when going from bypass mode to preamp mode. Any ideas on why the heck it does that?! I'm thinking that perhaps one of my 10uF caps is not holding a charge and the delay is how long it takes for it to charge up again or something. Does that sound reasonable or should I look elsewhere? 

EDIT: I forgot to mention that it might make a nice little booster pedal, too. 

[gez]

I'm thinking that perhaps one of my 10uF caps is not holding a charge and the delay is how long it takes for it to charge up again or something. Does that sound reasonable or should I look elsewhere? 

Well, the 10u output cap (C4) is the wrong way round so if it's leaky then something like that might be happening.  Check for DC on the output with a meter...unless that's just a mistake made when drawing up the schematic?

[Paul Marossy]

Well, the 10u output cap (C4) is the wrong way round so if it's leaky then something like that might be happening.

Doh! That's a mistake on the schematic.  The layout is how it's shown in the book.

[George Giblet]

You might want to add a large resistor from input at C1 to ground to stop "bangs" when you switch the unit in and out.

The biasing scheme with R1 and R2 could be made better - checkout the MXR distortion + for example.

I'm always suspicous of circuit that use NE5534 without compensation capacitors, generally somewhere between 3p3 and 22pF err. on larger - see datasheet.   You supposed to be able to use these without caps for gains > 3 but it often make the behaviour very layout dependent.

[MartyMart]

I'm about 99.9% sure that an NE5534 is compensated as stock !

MM.

[George Giblet]

>  NE5534 is compensated

It is compensated for gains greater than 3.

Many opamps are compensated for gains down to one (unity gain compensated).  The reason manufactures make opamps which are compensated for gains greater than unity is to make the opamp faster.  When you unity gain compensate an opamp it is slow for life, the external compensation lets you unity gain compensate it if you want but also gives you the option of more bandwidth in higher gain ckts.

The common audio opamps are very forgiving regarding layout because they have fairly low bandwdths.    An uncompensated NE5534 rides around the boarderline for layout induced issues (and to ask for problems there's no supply decoupling on that circuit).

(Ed" One thing I didn't mention is that using a feedback cap like C3 usually requires the opamp to be unity gain compensated because the gain at high frequencies is unity).

[Paul Marossy]

Someone suggested this to me:

In you are in bypass mode, C1 is uncharged. When you switch to preamp, this capacitor has to charge to 4.5 volts through the volume control and the 470 K resistor, producting a voltage at the input of the preamp. I do not know if this overloads the op amp, or perhaps the next preamp in the chain, but if this is the trouble it can be cured by connecting a 10 meg ohm resistor from the left side of C1 to ground. Then C1 is always charged after you apply power to the circuit.

I think this is probably what is going on. I'm going to try adding a pulldown resistor and see what happens. I'll keep y'all posted... 

[MartyMart]

I spotted that, but wont a "standard" 1M - 2M2 do it ?

MM.

[Paul Marossy]

I spotted that, but wont a "standard" 1M - 2M2 do it ?

Oh, it probably would. I added the 10M resistor but I haven't had a chance to try it out yet, Mommy and big sister have been sick for a week now (since Christmas day), and I have to take of them and my little boy. Not off to a very good start this year...

[Paul Marossy]

Just an update - the 10M resistor to ground at the input cap does the trick - no more delay before you hear sound, it's instantaneous now.