|
|
Subject: July 1999's email section
First of all, I would to thank you for such an informative website and I hope that you keep up the good work. Looking back in July 1999's email section on David's inquiry about non-inverting headphone amp, I am wondering if he or anybody had actually built the amp and posted a result. I would really be interested to know how it went. By looking at the circuit, it looks awesome and I'm planning to build it as soon as I get all the tubes I need. I have a couple of flat pack power transformers with double primary and secondary windings (115/230 volts to 12/6 volts, 48VA) that I'd like to use. I intend to use the secondary windings as the primary and hook it via wall wart with equivalent rating of course. I also would like to try a 6X5 tube rectifier in the power supply. I don't have enough data for the diode to see if I get about 170V to supply the amp. This is merely a design idea which needs further examinations. My intention is to power your designed circuit with tube rectification using the materials readily available. I would really appreciate it if you could give me some feedback on this. I am physics major student but I've always loved tinkering with electronics and appreciates tube sound. Thanks.
C.A.
No one yet has written about his results. If anyone has results, please let us know about them.
Finding a 48VA wall wart transformer will not be easy. So might have to use another flat-pack transformer boxed in a remote enclosure in place of the wall wart. Effectively the backwards flat-pack transformer becomes a 230 VAC center-tapped transformer, which would work well with 6X5 rectifier. This rectifier differs from most in that its heater is separate from the cathode, which means that separate heater winding is not necessary, as long as the voltage differential between cathode and heater does not exceed 450 volts. Unfortunately, I do not think you will achieve the full 170 volts, as the rectifier has a plate impedance of about 150 ohms per plate, which will result in about a 10 to 15 volt drop in the potential B+ voltage. My suggestion is to make do with whatever B+ voltage you might wring from the power supply and compensate elsewhere. This means that the plate resistor values will have to be scaled down a bit and with them the noise canceling resistor values. Please keep us posted with your results, as many reader are on the same path.
|
|
|
|
Subject: Small OTL amp
First of all: thanks for a inspiring webzine with lots of good ideas. If I only had time to try it all...
Then the questions: I'm in the planning stage of a small OTL amp for my midrange horns, so I do only need 2-3 watts in 8 ohms, and I want to optimize it for the 200Hz and up range. But I'm not sure what output stage topology to use. Fist I thought maybe an SRPP stage could be used, but the current plan is to use the cathode follower driven circlotron from the December issue, and use the common-mode choke coupled version. I plan to use 6SN7 as cathode follower. Will this require great changes in the component values? And how much drive do I need, if I use 6AS7 (how many do I need)? Since this is a midrange amp, I can use capacitor coupling at the output. I want to use as little global feedback as possible, and preferably class A, or maybe slightly into class AB above 2W. What is the output impedance of this circuit?
Subject: Small OTL amp, correction
I have to correct my last mail: I wrote "...to use the cathode follower driven circlotron...", but of course I see that it's not cathode follower driven.
Another question: If I use a 110+110:110+110 volts transformer as common mode chokes, is this good enough? Will the sound quality be worse with the transformer?
Bjørn
Norway
The last question first: the 110+110:110+110 volts transformer might not have a 1:1 winding ratio, as the ration in isolation transformers are often skewed to compensate for voltage losses in the transformer. I once bought a transformer with a 115VAC primary and a 108VAC secondary. Its winding ratio was one to one, but after being loaded, the secondary voltage lost 7 volts. Test the transformer without loading it.
Now for the big first question: How to design a 4 watt OTL amplifier? First of all, I think the world could certainly use a great 4 watt OTL amplifier. Many of friends are running highly efficient horn load speakers with 3 watt single-ended amplifiers and adding the choice of an OTL amplifier would be for the good. But before embarking on this trip, check to see if anyone has gone there before you. Julius Futterman first amplifier is a good start.
|
|
