Jarod_C

This is the first time I've designed a pre-amp myself and I wanted a few experienced people to look it over if they could. I have one question too. Do the inputs need to have something for shortcircuit protection? I might just go ahead and do both channels. I wanted some pretty good quality from this. Think it will do ok?

LOL, why did I put in a switch. Just noticed that.

Pre-Amp Schematic

Nigel Goodwin

It's missing vital components, check the opamp 'sticky' at the top of the general electonics forum.

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audioguru

Your opamp is missing an input resistor for its input DC bias current (0V reference).
What is the input device?

You have a left input shorted to a right input instead of using a simple mixer circuit. Many outputs are destroyed when they are shorted together like that.

You have R10 at 1k ohms and C14 at only 1uF. Then the output will rolloff below 160Hz and won't have any deep bass frequencies.

The lousy old LM358 and LM324 opamps are the worst ones for a preamp:
1) They are very noisy (hissss).
2) They have up to 3% of crossover distortion because they are the first "low power" opamps ever made.
3) Their max output is to only a few kHz.
4) The response drops above 10kHz when it has a gain of 100.

I use TL071 single, TL072 dual and TL074 quad low noise, low distortion and wideband opamps for audio. The OPA134, OPA2134 and OPA4134 opamps are even better.

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Uncle $crooge

Optikon

In addition to the advice already given, you probably don't need those 10pF sprinkled all over the place. If you are trying to bypass inputs, 10pF is likely way too small of a value to be of any use. You will probably get that much parasitic from your parts & PCB (or breadboard)

mneary

The circuit looks very much like it would have -12V on the op amp's pin 4. Otherwise, why C15 and C16?

Jarod_C

C15, and 16 was an accident, oops I'll take a look at the sticky, I couldn't find it the other day. Thanks for the input on this. I think I'll keep building the circuit other people have made.

Can anyone recommend any good op-amps. I was going to buy some. By the sound of audioguru it's a good thing I didn't, I was going to buy like 50 LM358's. Obviously I don't know how to use them yet, but hopefully I will. If not I kind of like that hot electronics smell anyway.

Torben

LM358s are probably OK to test things with if you're just trying something and are worried you're going to smoke it. Otherwise the TL071 is good for general audio use. The TL072 is a dual TL071 and I bet you can guess what a TL074 is.

OPA chips are quite nice. Lower distortion and I have some OPA2124s which work at nice low voltages.

I've also used a 5534 in a preamp and liked the sound.

If you want one reviewer's opinion on various higher-end audio op amps you can check out Notes on Audio Op-Amps


Torben

Optikon

TL07x and the like are toys... real designs use OPA627 or discrete low noise JFET front ends..


I'm kidding of course, I've got hundreds of TL071,72 &74's laying around waiting to get smoked..

audioguru

I have used many TL071, TL072 and TL074 opamps in audio circuits. They all sound fine.
I designed an equalizer and had tens of thousands manufactured and each one had a detailed test. Only two failed. One had its TL074 IC installed backwards and another had a shorted electrolytic supply bypass capacitor.

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Uncle $crooge

Torben

Don't forget the thousand-dollar speaker cables.

I know what you're saying though. A TL07x usually works OK for me but the OPAs have better specs. I don't think you'll see a lot of TL07xs in Neve mixers, for instance. But I suspect there are lots in everyday consumer audio applications. Some of the guys with repair experience could confirm or rebut my suspicion.


Torben

Nigel Goodwin

Never seen a Neve mixer, but I think you may well be surprised - using expensive opamps in place of TL07x's only makes a difference with some of the devices in a mixer, most would make no difference at all.

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audioguru

The distortion of a TL07x opamp is only 0.003%. You might be able to hear distortion that is 100 times higher if you could find a speaker with distortion that is lower than about 1%.
The distortion of an OPA2134 opamp is only 0.00008% which is much lower than you can hear.

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Uncle $crooge

Torben

Of course both Nigel and audioguru are right but audiophiles are weird people and neither facts nor logic seem to appeal to them. They'll swear up and down that they can hear differences which are so small as to be unmeasurable.

I just did a little reading and it appears that there are in fact a lot of TL07xs in older Harrison and some other higher-end boards. I've recorded on an old Harrison before and it sounded fantastic. I never pulled a strip to see what was in it.


Torben

CynicalMan

My guitar amp has TL07xs in it and it sounds great!

They're everywhere!

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Optikon

Yes of course, but you'll never convince the audiophiles of that. These are the people who think more expensive cables sound better and no name capacitor companies make better sounding electrolytics.. (bah!) I work with a guy who says he can hear 0.001% which, puts it right in striking distance of a TL07x.

Besides, some amount of distortion sounds *GOOD* to some people...

It is like arguing over why the color red is "better" than the color blue..
It can be amusing to say the least..