Microphone preamp with PP output stage

[Conviction]

Hi all,

I've got four J/S (plate-line) output transformers lying around that I'd like to build a couple of microphone preamps around. Thing is, they're push-pull and I haven't seen that many PP preamp schematics floating around. Any suggestions? I believe the quality of these are great and it's, really, a shame that they're just collecting dust.

Any input appreciated.

/Olle

 

[Bo Hansen]

Olle,

What is the type number on the JS/Jorgen Schou transformers you have.

Maybe from old "Disa" equipment owned by Swedish radio from the old days.

http://groupdiy.com/index.php?topic=32014.0
http://gyraf.dk/schematics/Disa_91_b21_micamp.gif
http://gyraf.dk/schematics/Disa91n_02_limiter.gif

--Bo

 

[Conviction]

Hej Bosse!

All four read 4623/2 (type 5MK can) and these are the specs:

1-2/3-4: 20K PP
5-6/7-8: 500R CT
5/7-6/8: 125R

The DISA schematic looks very interesting!

/Olle

 

[DaveP]

Conviction,

This one comes to mind.
I've not made one but the design looks sound to me.
Some people don't like ECC83's so you could sub in 12AY7's with a few changes.



There is a great selection in the technical docs section too, check out the Altec files.
good luck
DaveP

 

[Conviction]

Thanks Dave!

Speaking of Altecs, I just found your Altec 458A build thread. Looks very interesting! But how about the input transformer - 150K secondary? Is that, erm, inescapable in that kind of circuit? Got me a few 100K secondaries here. Please enlighten me

 

[DaveP]

I don't think its necessary, I used two transformers to get the gain and because the ones I had did not have a centre tap on the secondary.  I had to deal with some unexpected resonances, so I wouldn't recommend it.

My advice is to make a temporary lashup and see how it performs.  You are not trying to make an exact clone of anything so everythings up for grabs.

Best
DaveP

 

[Conviction]

Good idea. I'll do a temporary lashup, using the lab rectifier PSU.

Did you ignore the VU meter/switches?

I'm thinking of making two dual preamps, but with just one VU meter per unit. Perhaps these speculations should come later on... ;-)

 

[DaveP]

I have never put a VU meter on a mic pre, most of the classic tube pre's don't either, the meter is usually on the following compressor or desk.

A small 6v transformer can be used for the heaters, try to match the VA to the load or else put a resistor in series to get the right voltage.  Make sure the heaters are sweet before putting on the HT.
best
DaveP

 

[Bo Hansen]

Olle,

I checked my lab transformers collection, and actually had one of the JS-4623.
Have done a note that it behaved well as line-transformer driven by 12AU7/ECC82 or 12BH7.
You can also use the extra 125 ohms winding in the feedback loop, to get lower THD.

If you have find them here in Sweden, I think they may have come from any old Lyrec equipment or Swedish Radio's own made portable broadcast gear.

(if you not get any use for them, you might want to switch to anything else from me that you have use for instead)

--Bo