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Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
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In Reply to: Re: OTL and Parallel tubes posted by tottman2 on March 1, 2002 at 06:50:42:
Tom:You are correct - output transformerless.
Are Senn 580s 600 ohm? My 545s are 150 ohm. One of the things about a DIY headphone amp that is cool is that you can design it for specific impedance levels of phones. 600 ohm is such an easy load compared to 32 ohm Grados that your design flexibility would be very great. Most published designs and certainly commercial gear try to take care of the 32 ohm Grados as well as higher Z phones, which is more of a design task.
I would share my design, but there is a problem. I started with a back of the envelope shematic, changed things before I ever got that built on the breadboard, and have rebuilt it several times since (it is still on the breadboard, and is a truly frightening sight). The first idea was to build a cheap but great headphone driving equivalent to the Foreplay, using octal tubes instead of miniatures. The heart of the cheap approach was a $6.00 (still available) power transformer, a Rat Shack (Tamura built) PL 56-20 which pumps out 56VA. It is wound only for 10-0-10v output, 5.6 amps across the whole thing. But, it has dual primaries for 115v (one-half) or 230v (both halves), so I wired one of the primaries as a secondary, into a bridge rectifier into a cap-choke-cap-resistor-cap and so on filter. Out comes 150v DC.
Well, I never was sure how hard I should push the transformer so I used two (another $6.00 shot). This was mostly because my output tube had become the 6BX7, and I wanted to run a Ronan regulated DC through the heater, which needs 1.5 Amps at 6.3 volts. I use C4S loads on the plate of the driver tube and the cathode of the output tube (very cost effective).
Anyhow, I lost interest in cheap and decided I wanted to taste test tubes. So I wired up three sockets for the input side, with 12 and 6 volt regulated DC supplies available, so that I can have any of the following as driver tubes: 6SN7, 12SN7, 12SX7, 12J5, 6J5, 6F8G. I tried all of these, in many varieties. 6F8G (VT-99) is the sleeper here, because it is not used in any commercial amp, and therefore is not bid up by audiophools. Often microphonic, but some real gorgeous sounds come from 1940's versions.
Last week I added two 5-pin sockets, and now I am listening to #37 tubes (both of the 37s from the mid 1930s, and they sound terrific and glow brightly too). Wasted a lot of time and money buying tubes, but never paid much for any one, and it has been great fun.
As for the design, most of what I needed to know I learned first by building the Foreplay, studying the C4S manual, reading Valve (back issues are a goldmine) and Tubecad. For my basic design, I studied the article and adapted the schematic found at the following link:
http://www.glass-ware.com/tubecircuits/Tube_Headphone_Amplifier.html
The article is worth studying closely, as it analyzes most of the design issues. Skip the negative feedback loop (it made no audible difference), and you are looking at a topology very close to the Foreplay.
As I mentioned before, I abandoned the paralleled output tubes in favor of a single 6BX7, which made a great improvement in detail. Oh, I also used 47uF 600v Angela (SCR) capacitors rather than an electrolytics on the output. The capacitor coupling may be the biggest weakness in the design (sonically), which has me looking at transformers now.
I would be glad to share more detail on my amp, parts etc., but this gives you an idea of what it is.
Perseo
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- Re: OTL and Parallel tubes - Perseo 03/2/0205:15:04 03/2/02 (0)
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