DIY Audio Home
Tube Microphone Preamp
a while back a friend that does some home recording asked me about a tube microphone preamp. something i hadn't designed yet... so i designed one.
This particular design is pretty high-end. I started with the best microphone input transformer I could find (at least without having something custom made). So this is not an inexpensive preamp... the input transformer alone sells for $135! The total cost for everything including the case and PCB is right around $400.
I'm selling the PCBs for $30 each on .
The circuit topology uses an input transformer, followed by a 6922/6DJ8/ECC88 cascode stage. The output uses another ECC88, in SRPP, with some feedback around it. The feedback has two settings, giving two gain selections.
Click here for a full PDF schematic. You can download the BOM either in PDF or XLS双色球模拟选号器 format.
Input is via a Neutrik combo 1/4"/XLR jack. The signal passes through a Lundahl LL7903 transformer. Provisions for phantom power as well as a polarity flip switch is included.
双色球模拟选号器the amp stage proper: cascode followed by srpp. i know some people hate srpp - it's too trendy, i guess. but here is seems to me the best choice, able to drive a 600 ohm load. note some nfb around the final stage, two different amounts selected by a switch. the output - unbalanced, no output transformer - is supplied via another 1/4"/xlr combo jack. yes, the xlr is the wrong sex, but you can't get a male combo jack, and front panel space was limited.
双色球模拟选号器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!
双色球模拟选号器here's the input and filament buck:
双色球模拟选号器...and here's the b+ boost:
双色球模拟选号器i powered this from a desktop-type switching 48v 1a power supply.
|Parameter||Gain setting||HiZ load||600 ohm load|
10mV in 1kHz
Maximum output voltage 1kHz
|Low||20V RMS||9V RMS|
|High||42V RMS||9V RMS|
10mV in max gain
双色球模拟选号器10mv in 1v out 1khz
no LP filter
freq response 20hz - 50khz
10mV in 1V out
....and some graphs (all taken with a high-z load):
frequency response, low gain setting, 10mv in and 1v out:
Same at high gain:
双色球模拟选号器...the peaking at hf can be adjusted by changing the rc loading on the transformer secondary.
双色球模拟选号器thd+n vs. output voltage, low gain setting:
...and high gain:
thd+n vs. frequency, 10mv in 1v out, boring:
...and finally an FFT, 10mV in, 1V out:
i designed a pcb to contain the amp and fit inside an off-the-shelf enclosure.
Here's what the PCB looks like (click for a full-size image):
...and the assembled pcb:
双色球模拟选号器the enclosure i put this in is an lmb-heeger eas-500 extruded box. i had front and rear panels made by front panel express to fit the box.
You can download a zip file with my FPE front and rear files here.
i'm waiting to get the front and rear panels back from being laser-engraved. when i get them back i'll post photos of the whole thing put together. but really the guts are more interesting anyway :)
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