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12AX7 Tube Preamp Kit - "Beauty"

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

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

Oscilloscope Measurements

Sine wave response was flat from 25 Hz to 100 kHz (limit of the signal generator I was using). There was a rising slope starting at 25 Hz. It was close to +3 at 20, stayed elevated to about 10 Hz and then dropped off. This rise might be fine for some applications and cover up bass deficiencies in the rest of the food chain, but in my case it could become a problem with LPs. I may have to employ a subsonic filter to prevent record warps from getting to the subs.

100 kHz Sine Wave Response
Photograph 05 - 100 kHz Sine Wave Response (1V output)

Now back to the why things didn't sound quite right. If you look at the low level 70 kHz scope trace (Photograph 06), you will see right away something that shouldn't be there. It is a beautiful train of sharp spikes. They are just perfect to "modulate" high frequency audio. With a power amp that can easily reach into the low radio frequency range, the various sums and differences of frequencies would crud up the top of the audio band. At first I blamed the internal SMPS. It was supposed to run at about 30 kHz and be well filtered. Maybe I bunged up and it was "doubling" and the spikes were getting through. Not so, it turns out. Remember the other SMPS? It was running at 70 kHz. It was not earthed and the ground leg of the output was not filtered. It was passing spikes into the preamp and behaving sort of like an extremely high frequency ground loop. This is an easy fix. I fabricated a "line filter" for the laptop output. Two 100 uH chokes and two 100 nF caps and bye bye spikes.

Scope pattern at 70 kHz with no input signal
Photograph 06 - Scope pattern at 70 kHz with no input signal

Photograph 06 shows the Scope pattern at 70 kHz with no input signal. The lower trace is the output from the preamp. Spike peaks are approximately 40 milivolts peak to peak.

Impressions - 12AX7 Tube Preamp Kit

So I hauled everything back to the listening area. This time the music sounded much better, no top end oddities. Clean, clear and mellow. Just what I would expect. How does the combination sound? In short excellent. It is very easy to listen to.

Front view of LM3875 Chipamp and 12AX7 Preamp
Photograph 07 - Front View of Beauty and Beast

What's next? I can't leave things alone (never happens, just ask my spouse). First I plan on building a second preamp, but replacing the capacitors with Auricaps. The Jantzens are rather good, but I want to see how the stack up against the ones I really like. I'll update this project after I build the Auricap version. It could be that there won't be much change, if so then the Jantzens will have to be the bargain of the year as they cost about 1/10 the price of the Auricaps.

Rear view of LM3875 Chipamp and 12AX7 Preamp
Photograph 08 - Rear View of Beauty and Beast

What did the preamp cost? The basic kits were slightly over $100US, but came through another DIYer (thanks Mark) and he might not wish to go into the export business. The circuit boards seem to be available from the manufacturer at the links oultined in the 12AX7 Tube Preamplifier Kit. Of course, experienced DIYers can make their own boards. Other costs were about $25US for the case, connectors and such. What you get for about $100-150US is a very quiet preamp that can run with ones that cost a bunch more. I can easily envision a preamp with relay switched inputs and possibly a motorized volume control. If you haven't guessed, that's probably my next project. I already have the enclosures.

Good listening

UPDATE - December 2008
Unfortunately the 12AX7 tube preamplifier kit is no longer available from the supplier. Note that this is a fairly simple tube preamp and once can easily build this using point-to-point construction and with conventional linear power supply. For the latest update on this project, see the 12AX7 Tube Preamplifier Project on the forum where you can ask questions.

 UPDATE  - December 2013
Take a look at the 4S Universal Tube Preamplifier (12A*7) Project if you are looking for a simple tube preamplifier circuit.

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