Tube Microphone Preamp

[Mike F1]

My son has approached me on scratch building a Tube Mic Pre for some vocal recordings him and his friend want to do.

He found the Altec 1566A circuit that many people have cloned.
I remember reading that a Seduction can be re-wired as a Dynamic microphones preamp if you remove the RIAA EQ circuit.

The Altec uses 12AX7's so I suspect the gain will be more than the Seduction based Pre and possibly a bit hard to tame the noise.

A long time ago (if I recall correctly) Doc (or PB) make (or rewiring) a Mic pre for Jacqui Naylor. Curious about that circuit.

Just wondering if anyone has made something like this. Not looking for studio quality.. mostly for Garageband and Youtube.

Thanks

Mike

 

[Paul Joppa]

That's a simple, straightforward circuit. It does not have much more gain than a Seduction/Reduction with C4S loads, and the 6922 can drive the Altec 15095 output transformer without the cathode follower.. It looks quite do-able.

 

[Paul Birkeland]

This is such a hard project to start.  Yes, it would be awesome to clone a 1566A, but the parts cost will add up.  You'll have to figure out what you want to spend and what features you need, then see if the 1566A fits what you're looking for.

 

[Mike F1]

Thank you very much for the feedback.

If I went with a modded Seduction, I'm assuming you would need line matching transformers as well?
Since a lot of Dynamic mics are 150Ohm output, the Seduction likes to see a input impedance around 50K, correct?
For the output, most preamps like around 500-600ohm, correct?

As for the Altec 1566A, I have most of the parts already. I would just have to source the line transformers.

Either way, I'm just sourcing transformers.

As for options, I'm basically looking for something I can adjust the gain and have low and high inputs/outputs (we will be recording from Guitar amps to vocals)
Perhaps at a later date I can add some phantom power to it as well.

Thanks

Mike

 

[Paul Joppa]

Yes, you'll need an input transformers to get enough gain for most dynamic mics, and an output transformers if you want balanced outs to drive most studio gear (an unbalanced direct line out won't need the transformer). The 4722 input mic transformer has become popular as a moving-coil phono cartridge and has gotten kind of pricey lately, but CineMag makes an equivalent that is well thought of, and there are many other options.

 

[Mike F1]

Thanks again.

This all makes sense now and where the stumbling blocks are.
I didn't realize those little Step-Ups are that pricey and a bit hard to source for quality ones. Doubles the cost of the project right there!

Everything else seem very doable.

Mike

 

[Doc B]

I suspect the mic pre you are thinking of is an Altec 342B mixer/amp I converted many years ago into a mixer that was shunt regulated. It used 4722s. It's long gone, and I have a 250SU now, with nicer transformers. I haven't looked but I imagine the 1566A circuit is more or less the same one used for the mic pres in the 342B. You would most likely need a transformer for just about any tube mic pre you come up with.

 

[Mike F1]

Thanks for the feedback. I really appreciate it.

I've decided to try the basic Altec 1566A circuit with some mods to get the circuit as quiet as possible.
Some of my parts arrived today. l went with Jenson transformers (JT 115K-E & JT-10K61-1M) since the Peerless ones are hard to find and also cost a fortune! (as already stated)

Stay tuned!