Posted tube microphone preamp design

[pmillett]

A while back a friend told me he wanted a good tube mic preamp to do home recording. I designed one... and have finally gotten around to posting it on the website.

It uses a Lundahl LL7903 input transformer, followerd by a ECC88 cascode then an ECC88 SRPP output.

Details are here:

Tube Microphone Preamp

Pete

[dsavitsk]

Looks interesting, Pete. One question, any reason you chose the 7903 over the 1674 or even 1544a?

[Steve Eddy]

Originally Posted by dsavitsk

One question, any reason you chose the 7903 over the 1674 or even 1544a?

Or Lundahl over CineMag for that matter.

se

[pmillett]

Good questions.

I like Lundahl in general, and the 7903 looked to be the best from their line on paper. After all, it *is* the most expensive one I also wanted PCB mounting.

I haven't used Cinemag, maybe they are as good or better?

Seems like I looked at Sowter too but don't remember what finally made me pick Lundahl over them.

Pete

[kevinkr]

Hi Pete,
Interesting and cool design, one thing that seems counter-intuitive to me is that the high gain settings have consistently lower distortion than the low gain settings. I'm wondering whether you have the answer to my odd questions:

• Could the lower distortion in the high gain position be due to distortion canceling in the second stage and that in fact the first stage dominates distortion wise?
• What is the distortion distribution in the first and second stages, and what effect does increasing the value of R6 at the output have on THD in the high load impedance test scenario?
• I like Lundahl transformers too, but am curious about the choice - was it simply because it mechanically fit, is extremely well shielded, and sounds very good or ?

The whole thing is very cool, and I wish I had not given up on tape recording before this came along, but I did.. And as always the PCB design and indeed the electrical design show a lot of thought..

[Steve Eddy]

Originally Posted by pmillett

Good questions.

I like Lundahl in general, and the 7903 looked to be the best from their line on paper. After all, it *is* the most expensive one I also wanted PCB mounting.

I haven't used Cinemag, maybe they are as good or better?

I think they're every bit as good and in some respects, even better.

I've never cared much for their can design. Or their Swiss Army knife practice of giving you a bunch of windings and forcing you to strap them externally in order to get what you want (this just lends itself to greater noise pickup).

Check out their CMMI-10C. Excellent performance and cost is nearly half that of the 7903.

se

[DualTriode]

Hello Pete,
Another fine job, congratulations.
My wheels are turning! Have you or do you plan to use this mic amplifier with your sound card interface for testing loudspeakers or headphones?
DT
All just for fun!

[pmillett]

Originally Posted by kevinkr

Hi Pete,
Interesting and cool design, one thing that seems counter-intuitive to me is that the high gain settings have consistently lower distortion than the low gain settings. I'm wondering whether you have the answer to my odd questions:

• Could the lower distortion in the high gain position be due to distortion canceling in the second stage and that in fact the first stage dominates distortion wise?
• What is the distortion distribution in the first and second stages, and what effect does increasing the value of R6 at the output have on THD in the high load impedance test scenario?
• I like Lundahl transformers too, but am curious about the choice - was it simply because it mechanically fit, is extremely well shielded, and sounds very good or ?

The whole thing is very cool, and I wish I had not given up on tape recording before this came along, but I did.. And as always the PCB design and indeed the electrical design show a lot of thought..

I also found it a little surprising that the high gain setting had lower distortion. I have to believe it is because of distortion cancellation. I actually didn't measure the two stages seperately... that would confirm it.

I don't think R6 would make much difference - I was surprised that at 1V out, the THD was the same at 100k and 600 ohms load!

As for the transformer - I confess I didn't put that much thought into it! I basically succombed to Lundah's sales pitch: "This is the ultimate microphone input transformer, designed for high quality tube equipment. "

Pete

[kevinkr]

Hi Pete,
I like Lundahl transformers too, and find them to be consistent performers.. They're also well made, and more than competently engineered - so I can't fault you on that choice.

I've heard good things about Cinemag for SUTs in general, but have yet to try a pair.

[pmillett]

Originally Posted by DualTriode

Hello Pete,
Another fine job, congratulations.
My wheels are turning! Have you or do you plan to use this mic amplifier with your sound card interface for testing loudspeakers or headphones?
DT
All just for fun!

I've actually never done any loudspeaker testing... though I guess if I were going to I'be tempted to use a DC-to-light-mega-low-noise solid-state mic preamp