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 eBay.

The Circuit

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
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.

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.

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!

Here's the input and filament buck:

...and here's the B+ boost:

I powered this from a desktop-type switching 48V 1A power supply.

Performance

Some measurements:

Parameter

Gain setting

HiZ load

600 ohm load

Maximum gain

10mV in 1kHz

Low

48dB

46dB

High

58dB

55dB

Maximum output voltage 1kHz

Low

20V RMS

9V RMS

High

42V RMS

9V RMS

THD+N

10mV in max gain

Low

0.6%

0.65%

High

0.47%

0.5%

THD+N

10mV in 1V out 1kHz

no LP filter

Low

0.25%

0.25%

High

0.15%

0.15%

Freq Response 20Hz -
50kHz

10mV in 1V out

Low

+/-1.5dB

+/-2dB

High

+/-1.5dB

+/-2.5dB

....and some graphs (all taken with a high-Z load):

Frequency response, low gain setting, 10mV in and 1V out:

Same
at high gain:

...the peaking at HF can be adjusted by changing the RC loading on the
transformer secondary.

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:

...nicely
tubey!

Implementation

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:

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 :)