Constructing A Klein + Hummel Mic Preamp - Can Anyone Help?

Hi Samuel,

Thanks for writing back. I finished the circuit straight away using the OPA604 design.

I did not know what you meant about "DC issues" - did you mean offset voltage or bias current?

Anyway, I have not actually got the circuit to work yet. I thought I would test it with some different opamps first (the OPA604s cost me £5). So, I decided to plug in two spare OPA2228 opamps - very stupid - I forgot that they are dual opamps - not single.

I am using a 2 X 9v battery supply to provide +9v and -9v and polarised capacitors (I will get bipolar soon). After plugging the OPA2228 opamps in, they got very hot, so I pulled them out.

I thought I should check the voltages on the IC socket pins with no opamps connected.

I get -9v on my -supply pin which is good. But, my +supply pin reads 4v which is increasing slowly (4.1, 4.2, 4.3, etc.) - I think this is because I have drawn so much current from the batteries and they are recovering.

My inverting input (-) and my output both read -0.2v. Is this ok?

I don't want to plug in my new OPA604 opamps and burn them - should I plug in some other cheap opamps first to test it? I could buy some cheap ones.

The OPA627s are £15 each! That's a lot of money!

What is C1 for anyway? Why would an OPA627 not need C1?

Thanks for your help!

Roddy

Hi again,

Everything seems ok now. The mic preamp seems to work very well (it has a lot of gain with my Beyer 1:7 transformer). It even has a high output with my ribbon microphones!

I have not put it in an enclosure yet, but will do that soon.

I will calculate the noise difference between the Philips NE5534 and the OPA604 and also test if one sounds better than the other.

I have also not investigated the frequency response yet and I will see if a Zobel network is required.

One thing I did find slightly strange was that it did not like driving my Sennheiser 64 Ohm headphones. It made a repeating noise .... thud....thud.....thud.....thud... when I got to a certain gain level. Also, it distorted quite early.

Is this because I am having to drive a small (64 Ohm) load? I thought the NE5534 would be able to handle it ok? Also, I am only using a +9, -9 supply - maybe that is the problem.

I will try to make a recording and give results and photos when it is completely finished.

Roddy