Wednesday, July 19, 2006

The Johnson Power!

The question itself is: Can I substitute a TIP47 transistor for the MJE340s that Conrad Johnson likes to put in the power supply regulator stages?

Answer: Yes, you can. I tried it, works great. Should consider a TIP48 or better though, the Vce is a little too close to the limit with the TIP47. The TIP50 is even better.

My CJ Preamp is back in business, so my future postings should be a little more on topic, more or less.



Sunday, July 16, 2006

Delays, delays

I know last time I posted I was all gung-ho to get started on the AC probe design. Then a few things happened. First, Bill Whitlock sent me some of the slides he uses to teach a class on power quality and I suddenly had a bunch of questions for him, which I haven't gotten around to ask, especially about balanced/isolation transformers. This was going to be my weekend for reviewing his slides and really getting down to understanding his presentation.

However, instead, my pride and joy, my Conrad Johnson PV-10AL blew a gasket. Not really a gasket, UncleStu at Audio Asulym helped me diagnose the problem. It blew the voltage regulating transistor that fed the 340V filaments (is that the right word? ). With that out, the full 470Volts of the power supply were apearing there, and I was getting massive 120Hz hum on both outputs. If you look at the picture to the right, it's in the middle of the top, attached to the black heat sink.

I spent a little time trying to look for an "upgrade" for the MJE340 which blew. I think I found one, a TIPS47, which has almost the same or better specs. 350 Vce, 1 A maximum current, and a DC current gain of around 30. This gives me 50 more volts and doubles the current capacity of the MJE340, and has the same gain. The negative point is that it has a substantially higher collector current when off (1mA vs. 0.01mA). It's not a concern, because I doubt seriously that it will ever be drawing less than 5mA, and the cut off is 1mA. I also looked around for some heat sinks for the two TO-92s that are on the board. In any case, I bought the "right" kind and the upgrade kind just in case.

I also spent some time at aParts Connexion looking through their 9 pin tube sockets. Next paycheck I'm going to get a pair of the Azuma's to replace the plastic one's. Now, I know that a lot of people think the brown plastic sockets are really the best, but I've looked in the pins and felt the tubes going in and out, and I have to tell you, they just don't look or feel fabulous. The pins on the tubes and the sockets look oxidized, and the feel of inserting tubes is that of scraping rust off the pins as I insert them. So, call me a poseur, if you must, but I'm changing them. :)

One thing to notice about the right side of that picture. See all that bare space? That's where Conrad Johnson would put a phono preamp if you bought the right version. Seems a damn shame to waste all that space, doesn't it? I think I'll have to re-think my comments about not building a headphone amplifier. I think that I could put in either a headphone amp or a USB DAC in there depending on how ambitious I was. Stay tuned!