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 :)