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Old 6th June 2011, 07:28 PM   #1
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Default Preamp improvements/suggestions

Hi everyone,

I've been designing and prototyping the attached guitar preamp circuit. Although it's very simple, I've tried to incorporate as many "best practices" as possible. Before I go and finally build this circuit I would like to hear of any improvements people have to suggest (components too big/small etc).

Overall the aims are:
  • to provide a suitable buffer for a guitar
  • to provide a suitable output for driving a cable or interfacing with other equipment

Just a few comments to clarify what my intentions for some of the components are:

C1 is to filter out RF

R1 is a stopper resistor which can apparently help remove RF if placed very close to the pin of the fet

R2, R3, R4 and fet provide a buffer for the guitar

C3 is again to remove RF - not sure if this is entirely necessary

R7 and opamp provides a nice low output impedance 50ohms suitable for driving cables and interfacing with equipment

R8 helps clean up power line by lowering the cutoff frequency of C5


I'm interested to hear what people think!
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Old 7th June 2011, 12:54 PM   #2
Mooly is online now Mooly  United Kingdom
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16 views and no comments... so just some thoughts...

The only real problem is C3 and R7.
C3, now that could actually make the opamp unstable. Many opamps (more particularly high speed ones) actually need protecting from capacitive loading. So I would remove that completely. RF pickup is not an issue with the opamp.

R7. The opamp output impedance is very low to begin when used like this. All R7 is doing is literally killing the available voltage swing that is available. Even "proper" audio opamps are not suitable for loads below 600 ohm.

What you actually need to do is fit R7 in series with the opamp output and C4 while keeping the 47K which is good practice as it defines and references the output of C4 to the zero or ground line. That would give a 50 ohm output impedance. Not quite sure why you want 50 ohm

C1 I am not not sure about in practice... its effect depends on the impedance of the source driving it.

The opamp is in many ways a logical choice for a battery powered device, yet I question how suitable it is for high quality audio, even though as you say it bass guitar. The high frequency performance is very poor by normal standards. At least its easy to try others later. The TLO71 is still a good all round choice providing it can give sufficient output from 9 volts.
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Old 8th June 2011, 04:53 PM   #3
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Hi Mooly,

Thanks for your comments! I have to make one correction, the op-amp I'm actually using is a lm741 - I know it's budget but if I can get the circuit working well with that then anything else is going to be an improvement right?

I wasn't aware of the capacitive loading issue for opamps - consider C3 gone!

I agree with you comment on the placement of R7 - it should be placed in series.


The opamp arrangement is purely there to convert the 1K5 output impedance of the fet to something a bit more suitable. R7 could be eliminated entirely, because in this arrangement (voltage follower) an ideal opamp would generate an output impedance of 0 - which would be the most ideal. However, I assume you have to have some form of resistance in case you short circuit the output?

Also I've realised that between the fet and opamp I decouple the signal simply to re-couple it instantly. Couldn't I eliminate R5, R6 and C2 altogether? :s
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Old 8th June 2011, 05:47 PM   #4
Mooly is online now Mooly  United Kingdom
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Well the 741 is a classic (I'm trying to be nice to it), but yes, its fine to experiment with. The TL071 is a drop in replacement and far better if you want to try that. And cheap.

All the common opamps are fully short circuit proof... no worries there at all.

You could elimnate the components you mention as long as the operating point of the FET was such that the voltage at the FET drain was at "half supply" or 4.5 volts with a 9 volt battery. Probably better as it is originally tbh.
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Old 8th June 2011, 09:16 PM   #5
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Haha yeah, slight contradiction with my paragraph saying about using best practices and then I go use a 741 :P

There was a comment you made previously which I forgot to ask you about earlier, you say,

"yet I question how suitable it is for high quality audio, even though as you say it bass guitar. The high frequency performance is very poor by normal standards".

I'm surprised - I thought op-amps had good frequency response as long as you keep the gain low? What alternative would you recommend? I'm trying to find something on google which isn't a balanced line driver, I know these are good for driving cables since it can help to eliminate hum, but I don't what to get into that yet (plus most amp inputs are unbalanced). Most also seem to be opamp based which goes against what you said. Something low output impedance preferably < 100 ohms and low distortion...

Finally, "The TLO71 is still a good all round choice providing it can give sufficient output from 9 volts". I found this circuit which I thought was a nice simple solution, might be of interest: AMZ Super Buffer for Guitars
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Old 8th June 2011, 09:37 PM   #6
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I found this site which gives a pretty in depth explanation of opamp driving capacitive loads for anyone reading this thread: http://www.st.com/internet/com/TECHN...CD00176008.pdf
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Old 9th June 2011, 04:54 AM   #7
Bonsai is offline Bonsai  Taiwan
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dwalker,
looking at your circuit, I agree with mooly that C3 should come out. However, its good practive to insert a resistor in series with the op-amps output (after the point where feedback is taken from) to isolate it from capacitive loads like cables. This will prevent the op-amps possibly breaking into oscillation.
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Old 9th June 2011, 07:30 AM   #8
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by dwalker0044 View Post
"yet I question how suitable it is for high quality audio, even though as you say it bass guitar. The high frequency performance is very poor by normal standards".

I'm surprised - I thought op-amps had good frequency response as long as you keep the gain low? What alternative would you recommend?
It's all relative.
The 741 has a very low "slew rate". That is the rate by which the output can change its voltage relative to time. Even used as a follower (gain of 1) its marginal in that it can only just manage a full voltage swing at 20 khz. If you saw its square wave performance then you might be shocked

Alternatives... TL071 for experimenting with. Its stable and docile to work with.

Quote:
Originally Posted by dwalker0044 View Post
Most also seem to be opamp based which goes against what you said. Something low output impedance preferably < 100 ohms and low distortion...

Finally, "The TLO71 is still a good all round choice providing it can give sufficient output from 9 volts". I found this circuit which I thought was a nice simple solution, might be of interest: AMZ Super Buffer for Guitars
Paralleling opamps to give more current drive! If you do need to drive many metres of cable then its a method of doing so.

If you don't then its more of a solution looking for a problem
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Old 9th June 2011, 07:44 AM   #9
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Thanks everyone for your input so far, I'll try and post an updated schematic tonight.

Just a final thought - rather than an opamp could an emitter follower be used instead for the output stage? I'm not sure what the trade offs would be but that could provide more than enough current gain and have low output impedance....?
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Old 9th June 2011, 07:47 AM   #10
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by dwalker0044 View Post
Thanks everyone for your input so far, I'll try and post an updated schematic tonight.

Just a final thought - rather than an opamp could an emitter follower be used instead for the output stage? I'm not sure what the trade offs would be but that could provide more than enough current gain and have low output impedance....?
Yes it could...

tradeoffs... current consumption, important if battery powered.
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