OPA2228 Based MicPre

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Samuel Groner

Well-known member
Joined
Aug 19, 2004
Messages
2,940
Location
Zürich, Switzerland
Hi

In the course of the discussion with a Lab member about another micpreamp design of mine I suggested two new designs specifically designed to be used with the OPA2228. In case there's interest I thought I might share the schematics: [removed]

The second revision skips the interstage coupling capacitor by trimming the residual offset of the first amplifier. The attenuator between the two gain stages may look crude at first but there's in fact not much you can do otherwise if you don't want to resort to excentric external compensation methods (the OPA2228 is not unity gain stable).

[Edit final revision: OPA2228_micpre_r3.pdf]

Samuel
 
But I would still not DC couple to a pot wiper, even at 10 nA.
So what would be your maximum figure for bias current in this application?

Personally I've used a 10k pot even with a NE5534 (200 nA typ.)--yes, you hear that after a year or two, but with a high quality pot and a perhaps 50x lower typ. value..?

Samuel
 
Thanks for your suggestions! I think I'll include them in revision 3.

BTW, anybody knows how good the OPA2228 is in driving low loads? The distortion graph is for 10k only, which makes me somewhat suspicious and that's why I chose the feedback impedances to be rather high.

Samuel
 
[quote author="Samuel Groner"]
But I would still not DC couple to a pot wiper, even at 10 nA.
So what would be your maximum figure for bias current in this application?

Personally I've used a 10k pot even with a NE5534 (200 nA typ.)--yes, you hear that after a year or two, but with a high quality pot and a perhaps 50x lower typ. value..?

Samuel[/quote]

I was made hypersensitive to this issue when doing consumer stuff, where the potentiometer was used all the time as a level control, and basically resolved to a.c. couple unless Ib was in JFET territory (say about 100pA). But when the gain adjust is used less frequently it will be less of a problem, although aging will still take place with or without wear.

There is Murphy to contend with though: the likelihood of equipment to malfunction is directly proportional to the criticality of the recording session or concert.

Also, even if caps are used we need to be mindful of the piss-poor max specs on 'lytics, depsite most being far better than this. It helps when the votage across is low, but then you get into the debates about forming voltages and so forth.
 
[quote author="mediatechnology"]

I remember the Lexicon PCM-XX* having a pot wiper tied directly to an op amp input so the wiper provided the bias return path. After awhile they did get noisy rotationally-speaking. Saw a lot of them this way.
[/quote]

me too. I fixed an LXP-1 just the other day with this same problem. I added bias return resistors and some blocking caps after replacing the pots. eventually you can even end up with dead spots. "weird noises" indeed.

mike
 
Resistors provide alternate current path.

toa_in_6.gif
 
Hey Samuel, nice to see more of your designs.

Out of interest, what made you choose the OPA2228? I've used this opamp myself, but never listened to it critically.

Roddy
 
Resistors offer an alternate current path for both AC and DC but only if the impedence/resistance is lower than that offered by the pot itself.
 
[quote author="Svart"]Resistors offer an alternate current path for both AC and DC but only if the impedence/resistance is lower than that offered by the pot itself.[/quote]

Is it from The Bible? :roll:

We don't speak of pot's resistance, we speak of resistance between sliding contact and the resistive surface when it is too high. And a constant resistor with resistance 2-6 times more than resistance of the pot helps a lot to reduce noise because no matter how big it's resistance is it is less than resistance of the lost contact, so voltage drop variations because of input opamp's current will be not so fatal. The picture is taken from console with 4558 opamps, noise was horribly loud before soldering 51K resistors in parallel with 10K pots. Now it is barely audible.
 
Are the trimmer caps to tweak the square wave response?
Yes they are. With unity-gain stable opamps I'd just throw enough capacity in to make things stable and robust, but with the given amplifier the point of best stability will depend on the layout.

There is Murphy to contend with though: the likelihood of equipment to malfunction is directly proportional to the criticality of the recording session or concert.
I thought that once it was empirically proven that it is proportional to the square of the criticality? :green:

Out of interest, what made you choose the OPA2228?
Well, someone asked me if he could build a "shared gain preamp" with these, and as this design needs a unity-gain stable amplifier I suggested something new.

So here we go with revision 3, including two more resistors:
OPA2228_micpre_r3.pdf
I left the pot in, if someone needs highest security or best gain tracking he might better be using a switched attenuator instead.

Samuel
 
I was the one who asked samuel about the opa2228, just because I had some lying around... I had forgotten its the 227's that are unity gain stable, so he cooked this up! How awesome is that? :razz: I'll get some of these put together and post some pics as soon as I get some xformers...
 
[quote author="rodabod"]Hey Samuel, nice to see more of your designs.

Out of interest, what made you choose the OPA2228? I've used this opamp myself, but never listened to it critically.

Roddy[/quote]

availability of samples? :wink:
 
Back
Top