convert MC board into mic preamp?

[jpk]

Looking at the NA323 schematic I wondered if it could be converted into a good mic preamp by just taking out the RIAA filter parts and adding a gain pot like this:

The input from the mic I would take from a transformer:

I don't know if that's a good idea, so what do you think? Please share your suggestions!

 

[colasblue]

If it requires power then presumably your mic already has a suitable preamp built into it and gives a line out level?

Either way a Mic is likely to have an output way higher than a typical MC cartridge so a normal gain circuit is a more probable candidate.

 

[jpk]

Mics (no inbuilt PSU / pre) need gain between 20 and 80dB, most I would use between 40 and 60 - so the MC stage (minus RIAA) would be in the right range...?

 

[Arkless Electronics]

That circuit would not be a good idea no. A balanced version without the input transformer and you will be getting much nearer!

 

[jpk]

Just did a short try with a modded 323 board: it sounded great, but there were problems with the input mis matched to the transformer, some RF pickup, a ~10kHz oscillation and the gain pot did cause heavy distortion in some positions...

 

[RJohan]

In the olden days, 1960's, a couple of Swedish music studios used Quad valve pre amps in custom built mixing consoles. One for each channel. DON'T know what went before the Quads when mic's where connected, a guestimate is just transformers when using condenser mic's and, as suggested here, a revamped phono stage for dynamics. Maybe Quad did mic input board?

 

[jpk]

Good to know! I added 5p6 to C-B of Q8 and also played around with the input network: refitting the original 6n8 and replacing the 470R with 12k cured the RF. 12k is the recommended load resistor for the transformer I used. But the transformer works best with 1000p instead of the 6n8: I need to test if lowering C9 to 1n is enough to surpress RF. A 2k pot in series with the original 220R worked well as a gain pot. What else can be done? Any further advice is most welcome!

 

[Yank]

https://www.technicalaudio.com/neve/neve_pdf/1073-fullpak.pdf

 

[jpk]

Many thanks! Initially the Neve was what I planned to do, but after I compared the NA323 based preamp to a very good 1073 clone I preferred the Naim

 

[jpk]

I completed the project. The 323 based mic preamp sounded very good, but...

That circuit would not be a good idea no. A balanced version without the input transformer and you will be getting much nearer!

Following Jez advice I replaced the transformer with a balanced circuit as shown by Rod Elliot and used the Naim circuit only as a gain stage (C3/4 are Wimas and C12 is a 3u3 SMR):

Here is a rendering of the PCB (it can be cut into pieces so that each curcuit can be used seperately):

The phantom power section (also taken from Rod Elliot):

The regulators for the audio circuits (I used zeners instead of the trim pots):

I am amazed how good the sound quality is, also it seems to work well with my large-diaphragm Nevaton mic.

 

[Yank]

I'd like to see some metering, or at least a peak LED to help in setting the input trim.

 

[jpk]

Yes, metering would be nice: Rod Elliot has some solutions on his website, I will look into them. My scope showed clipping only at insane levels (starting from 20Vpp), so adjusting by ear and looking at the meter of the recorder works well for now. Here the scope shots - first the input signal (blue trace, 10kHz @ 0.003Vpp):

Output (yellow trace, 20Vpp):

The test board: