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Old 18th January 2010, 05:40 PM   #1
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Default Basic opamp based preamp

I'm basing this design heavily on Rod Elliots P88. I would just use his design, but I don't care for his board design with the switches in the middle of the board for gain selection. Nifty, but it doesn't look like a finished project. Since I'm not using his board, I figured I'd change the design a little too.

Signal path:
Buffer > Vol control > 6db gain stage

Click the image to open in full size.

The buffer is there to drive the volume control, and then a small gain stage. I am wondering if the order should be reversed, gain stage > vol control > buffer. And, I might go with a less gain, 3db instead of 6db, but thats a minor detail. I may eventualy use a better opamp, LM4562 or similar.

Is there a simple way to make sure there is zero output dc offset, to get rid of the output cap? Can an Rc be used? I've already started working on the board layout and will be posting progress soon.

A balanced xlr output stage will be added after this, but I'm pretty much going to use Rod's design for that. Power supply will just be LM317/337 +/-15Vdc.

Comments?
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Last edited by Stormrider; 18th January 2010 at 05:44 PM.
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Old 18th January 2010, 05:48 PM   #2
Mooly is offline Mooly  United Kingdom
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Using a FET opamp will eliminate offset... using a bipolar as you show will cause significant offset due to unmatched impedances on the - and + inputs.

It's never a good idea to amplify, and then throw that gain away, and amplify again as the noise (all be it small for line level) adds up.

What's an RC Remote control ? Or a resistor/cap network somewhere ?
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Old 18th January 2010, 05:58 PM   #3
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Suggestions on which opamp to use?

Sorry, Rc is an input offset compensation resistor. It equalizes the +/- input impedances. With non-inverted stages it is calculated by Rc = (Rf//R1)-rs and it is put in series with the + input. I was going to include it, but with the varying source resistance from the volume control pot I didn't know how it should be calculated.
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Old 18th January 2010, 06:05 PM   #4
wahab is offline wahab  Algeria
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it will work very well...
and considering the reduced gain, the NE5532
will produce no more than 0.0006% distorsion in this
kind of application..
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Old 18th January 2010, 06:11 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Stormrider View Post
Suggestions on which opamp to use?

Sorry, Rc is an input offset compensation resistor. It equalizes the +/- input impedances. With non-inverted stages it is calculated by Rc = (Rf//R1)-rs and it is put in series with the + input. I was going to include it, but with the varying source resistance from the volume control pot I didn't know how it should be calculated.
As you say, the varying impedance means you can't optimise the design for zero offset.
Opamps... Well I keep coming back to the OPA2604 however there are many good devices out there. Why not initially fit a socket and try different ones ?
But it must be FET input to avoid DC offset issues.
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Old 18th January 2010, 06:15 PM   #6
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What about the balanced output driver stage? Rod's design shows no output coupling. Is dc offset canceled out in the receiver end?
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Old 18th January 2010, 06:15 PM   #7
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Also decouple the opamp across it's supply pins, preferably on the device itself with a small (0.1uf) cap, and perhaps also a low ESR electroylitic in parallel nearby on the PCB.
Don't decouple rail to ground as a general guide as that introduces PSU noise into the signal ground unless you opt for separate signal and PSU grounds which on a preamp isn't needed really.
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Old 18th January 2010, 06:20 PM   #8
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Quote:
Originally Posted by Stormrider View Post
What about the balanced output driver stage? Rod's design shows no output coupling. Is dc offset canceled out in the receiver end?
Project 88
Fig 2 is a FET opamp so no problem DC wise.
Fig 4 also FET opamp so again no DC problem. The caps are there because as a designer you have no way of knowing what someone may try and connect to it. It's just a safegaurd and good commercial practice...

http://sound.westhost.com/project88.htm
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Old 18th January 2010, 06:27 PM   #9
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Quote:
Originally Posted by Mooly View Post
Also decouple the opamp across it's supply pins, preferably on the device itself with a small (0.1uf) cap, and perhaps also a low ESR electroylitic in parallel nearby on the PCB.
Don't decouple rail to ground as a general guide as that introduces PSU noise into the signal ground unless you opt for separate signal and PSU grounds which on a preamp isn't needed really.
Thats good to know. Rod decouples the power pins to ground. Going from + to - pins will save some space on the board. I hadn't put them on the schematic, but I planned on doing it the "normal" way. I'll probably go your way. And, I may just the OPA2134 and drop the output cap.
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Old 18th January 2010, 06:28 PM   #10
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Quote:
Originally Posted by Stormrider View Post
What about the balanced output driver stage? Rod's design shows no output coupling. Is dc offset canceled out in the receiver end?
Gotta go, but what do you mean "balanced output driver stage "

it's all single ended, not balanced... where are you looking ?
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