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12AX7 Tube Preamplifier Kit - "Beauty"

 Bruce Heran USA Flag   To email Bruce, type out the email address.

Page 1: Introduction, Enclosure and Construction
Page 2: Oscilloscope, Impressions


Introduction - 12AX7 Tube Preamp Kit

The "Beauty" is a 12AX7 valve preamp. I called it "Beauty" as it sweetened the sound of my LM3875 Chip Amp. This preamplifier kit uses an internal DC to DC inverter to provide 12 and 250 volts DC. Being a valve preamp it does use dangerously high voltages. If you are not comfortable building and using equipment with potentially lethal voltages, you may not want to try this project.

LM3875 Chip Amplifier with 12AX7 Tube preamp
Photograph 01 - The Beauty and The Beast - 12AX7 Tube preamp and LM3875 Chip Amplifier

For the schematics of this kit, see Mark's 12AX7 Tube Preamplifier Kit project page.


Enclosure and Construction

Construction of the preamp is fairly straight forward. I studied the plans and schematic for a few days with an eye to improvements. The first problem I ran into was that since the original preamp kit was designed as a mono guitar preamp, my kit didn't have the parts to convert it to hi-fi use. That really wasn't a problem as the first thing I generally do is replace all the coupling capacitors with premium Auricaps. In this instance I wanted to try some capacitors from other sources just to see how good or not so good they might be. I really like the sound of equipment with Auricaps, but if there are more than a few it can turn a cheap DIY project into a fairly costly one. The capacitors I wanted to try are made by Jantzen and carried by Parts Express. The price is right $1-2US each and they are relatively small in size. This is a significant factor in this project. I also had to shave approximately half a millimeter off one side of each preamp circuit board so they would fit inside the case. From the photos you can see most of the modifications. All are pretty straight forward.

Printed Circuit Boards (PCB) for 12AX7 Preamplifier Kit
Photograph 02 - Printed Circuit Boards (PCB) for 12AX7 Preamplifier

I wanted the tubes to be visible as I really like to see them glowing. In order to get the preamps to fit under the lid and still have the tubes stick through I had to make a few changes. The first was to mount the B+ filter capacitor on its side. I used some silicon adhesive to make sure it stayed put. Then I added a B+ snubber (a 1uF Solen) to each board and a 100nf snubber across each heater. The 100nf capacitors are on the bottom of the boards. I didn't like the original design that had the volume control at the back end of the preamp. This is a good location for maximum signal to noise. Since the preamp only has a max voltage swing (with out having harmonic distortion going into orbit) of about 8 volts it isn't ideal for use with high level signal inputs. I like a larger amount of headroom and am willing to sacrifice a bit of S/N to get it. You can size up your system and put it at either end. An additional factor is that the companion amp (Beast) has level controls on the inputs. So for Beauty, I put a dual 100 K log pot out front.

As I mentioned earlier the B+ and heaters are powered by an internal DC to DC inverter. This went together fairly well with the exception of winding the coil. I don't know about you, but counting turns and trying to get the turns neatly spaced is not something I'm good at. In fact, I'm horrible at it. After two tries to be neat, I gave up and just scramble wound the secondaries. No one was more surprised than I was when it worked perfectly. So my hint here is to be careful in counting the turns, but don't worry how they look. I mounted the inverter in its plastic case and fastened it to the inside bottom of the bigger case. I added a 1uf Solen capacitor as a B+ snubber for good measure, but otherwise left it alone. I grounded the lid for the inverter to the preamp ground and in turn to the case top. The preamp is not "earthed". Also of note is that the chip amp PS is earthed, but the chip amp itself is not. I have had problems with ground loops in the past and wanted to avoid them in this project if I could (I was successful).

Switch Mode Power Supply for 12AX7 Preamp Kit
Photograph 03 - DC Power Inverter

The power for the DC to DC inverter needs to be supplied externally from a 15-17 volt DC source. It can run on 12 VDC source if you bypass the heater regulator circuit. Now if your parts bins are like mine, there are umpteen zillion wall transformers hiding in the back corners. Every one of them is the wrong voltage or current. That's probably why they are in the back of the bins. I must have 30 rated at 9 volts and half an amp. I did however find a switch mode power supply (SMPS) for a long defunct laptop computer. It was rated a 16.5 volts DC and up to 3 amps continuously. Pay dirt! It worked like a charm. It even had the proper power connector. The possibility of the two power supplies fighting over the regulation issue never appeared. Others have suggested that having one SMPS drive a second SMPS could have disastrous results. Maybe I was lucky, but they worked together perfectly.

12AX7 Tube Preamp Enclosure
Photograph 04 - 12AX7 Tube Preamplifier Enclosure

So I strung all the parts together, fire everything up and look for smoke and fire. Good there wasn't any. Hook up the cables and see if anything works. Hummm... music, not too bad. Overall not nearly as bright as the chip amp alone, but then again not nearly as mellow as the K-12 Tube Amp. I let everything break in for a week and then listened again. Pretty nice, it was something I might be able to live with. Crisp bass to the limits of my MTMs, very quiet, zilch noise and hum, plenty of drive. I switched in my dual 15 inch subs using an electronic crossover (low pass only at 50 HZ with a 24 dB/Octave slope) and listened some more. Very satisfying bottom end. There was a wide sound stage and good imaging. But something didn't seem quite right. I couldn't figure out what, but something was causing it to be fatiguing to listen to. So I hauled everything to my work bench and fired up my scope.


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Oscilloscope, Impressions