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"guitar Handbook" Opamp Preamp


Paul Marossy
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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? :D

Edited by Paul Marossy
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If C4 is leaking some dc, it can wreak havoc with the input of the amp that this pre-amp is driving. A similar problem could occur if R6 is open or badly connected.

Disconnect the pre-amp from your amp and check the pre-amp output with a digital voltmeter. It should read a constant 0vdc even when you're switching it from BYPASS to PREAMP and vice versa.

BTW, if it's driving a guitar amp which would typically have an input impedance above 470k, 10uF for C4 is overkill since it would give you a low cutoff frequency of around 0.2Hz. You could easily get away with a 1uF and have a 2Hz low cutoff frequency which is well below the 20Hz needed to stay within the full audio spectrum (20Hz - 20KHz). Then you wouldn't be limited to using electrolytics or tantalums. In fact, I just noticed that with the values of C1, R1, R2, the circuit's low cutoff is 30Hz which can still be acceptable for a guitar circuit.

Edited by Saber
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Nice one Paul

I always liked that book... :D

I had intended making this as a preamp for the Sustainer Circuit BTW. Your problem does sound like a capacitor and I'd be interested to see if a change in capacitors (especially if it's going to make the thing smaller) would help.

I'm not familiar with the op amp...maybe there is something inside to avoid a pop in that IC...don't know. Did you use an IC socket? It would be interesting to compare differest IC's (741,071,081,etc) for tone...I hear they are all slighty different in flavour.

Another mod would be to add 2 diodes like the black ice in the feedback loop to turn it into an overdrive. These could be switched in perhaps for versatility or faded in with a tone knob...just a thought.

This mod was something I was also considering for the sustainer to add compression, but would make a neat onboard overdrive option two.

You could even put a gain control in the feedback loop to control the gain rather than the front end....i think.

I had heard somewhere that people didn't like this circuit...not sure why...try searching out Aron's if you haven't already to see if others have suggested anything.

good to see it made...post a pic if you can :D

psw

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Nice one Paul

You could even put a gain control in the feedback loop to control the gain rather than the front end....i think.

I had heard somewhere that people didn't like this circuit...not sure why...try searching out Aron's if you haven't already to see if others have suggested anything.

psw

The gain control is in the negative feedback circuit.

Maybe what turned people off was the input impedance. The input ac signal sees R1 and R2 as being in parallel giving it a 235K-Ohm input impedance. I think they could be replaced with 1M resistors to give it a 500K-Ohm input imp.

Edited by Saber
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The gain control is in the negative feedback circuit.

Ooops...quite right... :D

Yeah...not an expert but it looks like a fairly standard kind of opamp circuit. A lot of people seem just not to like opamps....I wonder if that's it...you don't see that many for guitar inbuilds.

psw

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The problem, maybe: When 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.

If you are using a FET input op amp, you can make R1 and R2 much bigger if you want to, for a higher input impedance.

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The problem, maybe: When 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 was thinking something along those same lines - a capacitor having to charge. I think I'll try the 10M resistor idea. I tested the caps and can't find anything wrong with them.

If you are using a FET input op amp, you can make R1 and R2 much bigger if you want to, for a higher input impedance.

Actually, the book recommends a TL071, which is an FET opamp.

I'm not familiar with the op amp...maybe there is something inside to avoid a pop in that IC...don't know. Did you use an IC socket? It would be interesting to compare differest IC's (741,071,081,etc) for tone...I hear they are all slighty different in flavour.

I don't remember all the ones I tried, but most of them wouldn't really work - you had to really slam the strings just to get a faint, misbiased kind of sound. I also had the completed circuit connected to my breadboard for testing - perhaps the connections weren't as good as they needed to be. And my battery wasn't brand new, it had a few miles on it... :D

I'll keep y'all posted on my progress. :D

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  • 1 year later...
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. cool.gif

i would like to built this preamp...

besides on adding a 10M resistor, are there any changes to be done on the circuit?

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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...

Well... one thing is wrong with the schematic - the orientation of C4 is incorrect. Since the only source of positive voltage is towards the opamp, the positive side should be towards the opamp, not away from it.

Electrolytics can take a bit of reverse voltage, depending on the voltage rating, but I would flip it around.

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It would be interesting to compare differest IC's (741,071,081,etc) for tone...I hear they are all slighty different in flavour.

if you want really really crunchy distortion try two 741 op-amps one running off 9v the second off 15v the amp i made with that has amazingly crunchy distortion.

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Well... one thing is wrong with the schematic - the orientation of C4 is incorrect. Since the only source of positive voltage is towards the opamp, the positive side should be towards the opamp, not away from it.

Oops. Well, the layout is correct at least. :D

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