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
Looks interesting, Pete. One question, any reason you chose the 7903 over the 1674 or even 1544a?
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
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
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:
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..
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 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
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!
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 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
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.
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.
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 😀
Congratulation Pete,good job.Here is my Tube Microfone preamp with Condenser Catridge(with integrated Fet)Mic.I use this Mic preamp for Radio Tube Transmitter long time ago.Never let me down,work extremly stable,quality depends from Condenser Catridge Mic only.Good Luck to All
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I can understand why you chose the components you chose. I like Lundahl's stuff, they make quality components. I'm considering building one of your pre-amps but have a few questions before I drop the bucks on components.
How did the final product turn out?
Has anyone here built/heard it, and can anyone objectively vouch for it's sonic integrity/quality?
(I don't mean any disrespect in that regard, however, sometimes designs don't sound as good as they do on paper or in numbers I have experienced this all too many times!)
Sonic quality I meant more in musicality terms, not necessarily numbers as we already have proof of numbers from the inventor himself. (Is the preamp musical?)
Thanks All!
How did the final product turn out?
Has anyone here built/heard it, and can anyone objectively vouch for it's sonic integrity/quality?
(I don't mean any disrespect in that regard, however, sometimes designs don't sound as good as they do on paper or in numbers I have experienced this all too many times!)
Sonic quality I meant more in musicality terms, not necessarily numbers as we already have proof of numbers from the inventor himself. (Is the preamp musical?)
Thanks All!
pmillett, any updates to this design since you released it. What do you think about switching to LED biasing?
No, no updates. LED bias would probably be fine, though I sort of like the symmetry on the second stage using equal resistors.
To be honest, since I'm not a recording guy, I can't really comment on the sound of this preamp vs. others. There have been quite a few built, but I have not heard a lot of feedback from builders. Not sure of that's a good thing or a bad thing...
Pete
..Hi, banat, i tried to make a pre like your long ago..back electret on a 6dj8 catode..and it sounded very good :warm voice and little natural compression..the only shortcoming was the excessive gain ..
..With 12ax7 two stage you probably get over one hundred volts output
..With 12ax7 two stage you probably get over one hundred volts output
..Hi, banat, i tried to make a pre like your long ago..back electret on a 6dj8 catode..and it sounded very good :warm voice and little natural compression..the only shortcoming was the excessive gain ..
..With 12ax7 two stage you probably get over one hundred volts output
Hi 0feedback !
Yes I know that this my design work OK ,
- Hart of this my simple but effective tube Mic pre. is the grounded grid triode where cathode supply DC for FET condenser capsule but in the same time same cathode is modulated with AF from the same Mic capsule .(fig- a) .
-(Fig- b) represent just one variation of this Mic pre. where second triode is CF ,DC connected to first triode anode , so in this configuration you can get lower output voltage swing but in the same time with much lower output impedance.
-Many other variations is possible depending from what you need , but Hart of this circuit will be allways that simple&effective circuit from fig-a .
For example that circuit from my post #11 was actually designed to directly feed phase inverter of tube Modulator for my Pirate AM / MW tube transmitter , it was work perfect without any problem caused from RF interference 🙂, contrary to others dynamic microphones used there .
And yes for any further information please open new thread ! , I will be there , since I Don`t Want to hi-jack this thread originally started from Mr.P.Millet .
Best Regards !
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Clever, I'm assuming these capsules have an internal njfet amplifier? (Actually obvious since you marked it in your schematic.) 😀
Yes electret mic capsule must to have internally integrated NJFET unit ,
but this design is pretty universal and allow to be used Any electret capsule , from cheap capsule chopped from some old tape deck unit up to HQ studio electret capsules .
Best Regards !
This is a project is something I would like to build one day, I just wish I could find a comparison with some other tube mic preamps out there. If I didn't have about 3 projects started and a baby on the way, I would be building this right now.
"The power supply is, uh, unconventional. What? Who? Me? Yup. A switching supply, generating 6.3V filament voltage and +200V B+ from a 48VDC input. The beauty of switching supplies is that the noise they generate - and yes, they do generate some noise - is at high enough frequency that it's easy to filter, and above the audible range. For a low-level preamp like this I'd rather have a few millivolts of 100kHz than a few millivolts of 60Hz!"
I would love to see more people use SMPS with tube amplifiers. I appreciate you taking time to explain the benefits of SMPS and moving the switching frequency, and thus noise, outside the audible range. I would love to see this utilized more in tube amps. Is there any chance you have a S/N measurement of this circuit? I didn't see one in your write up.
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