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

[rodabod]

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

 

[rodabod]

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