Mic output transformer

Sallivan · Nov 16, 2010

Hi, I'm doing a 8 channel of Elliot Sound project 66 mic pre, and I have decided to put an output trafo.
Being a low cost project, I would use the OEP A262A6E.
Anyone has ever tried it?

abbey road d enfer · Nov 16, 2010

This xfmr will introduce significant LF THD at nominal +4dBu. Do you want to drive A/D converters directly? In that case distortion may be higher, because of the general tendency of driving A/D converters at somewhat elevated level.
There are some tricks to reduce distortion using active cancellation (overall NFB or negative-impedance drive), but they don't do miracles...
For less complexity, you could do an active balanced circuit.
Why do you want output xfmr's?

Sallivan · Nov 16, 2010

The signal will go directly into the A/D converter.
I want to built this 8 channel mic pres in a small box, and for reasons of space I had chosen as balancing solution a small output trafo instead of the opamp circuit (the pcb will be very small).
I had read that the OEP have a high LF distortion, but I thought not excessively influenced the converter.
After what you told me, I will try to build the preamp using the two different solutions.
Then analyze what is the best choice, and I will set it accordingly!
Really thanks for your prompt answer.

mad.ax · Nov 17, 2010

If real estate is an issue, you might consider the THAT 1606. This little 8 pin SMD chip doesn't even need elcos coupling...

Axel

rodabod · Nov 17, 2010

What (circuit wise) would you be driving the primary with? I find the LF distortion generally tolerable at line-level, but possibly more concerning is the ~10H primary inductance.

abbey road d enfer · Nov 17, 2010

rodabod said:
What (circuit wise) would you be driving the primary with? I find the LF distortion generally tolerable at line-level, but possibly more concerning is the ~10H primary inductance.

i've checked on the OEP website; the spec sheet says 125mH per winding. I find this hard to believe; the LF response would go down to 200Hz only. I suspect it's the leakage inductance.
OTOH, your figure of 10H seems a little optimistic; is it what you actually measured? Indeed, I would not be worried regarding driving 40H (with two coils in series); this goes as 5kohms at 20Hz. I have a feeling the actual value is about half that.