I'm getting towards the completion of my Rozenblit 300B OTL amp builds. Initially I will use conventional AC supplies to the filaments but, as I've mentioned before, the commercial Transcendent kit includes a 25Hz filament supply that feedback suggests has two benefits, hum becomes much less of an issue but also, interestingly, sound quality apparently improves.
I would like to try DIY 25Hz supplies at some time but not sure of the best way to tackle it. Thinking thus far is along the lines of an oscillator feeding a sine wave to an amplifier.
In the 300B OTL amp the load would be four 300Bs per channel, arranged as two parallel pairs (i.e. 10V @ 2.5A).
Any ideas/suggestions?
Ray
Ray
25Hz DHT Filament Supply - How Would You Do It?
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Dave the bass
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#2
A local generator? Drive it mechanically from the mains but design the windings on the local generator rotor for 25Hz.
Or.
A 25Hz sinewave from a sig gen feeding A very powerful amplifier running into a transformer. Adjust the output of the amp to achieve 10V.
Thats what I'd do.
DTB
Or.
A 25Hz sinewave from a sig gen feeding A very powerful amplifier running into a transformer. Adjust the output of the amp to achieve 10V.
Thats what I'd do.
DTB
"The fat bourgeois and his doppelganger"
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IslandPink
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#3
?Dave the bass wrote:A local generator? Drive it mechanically from the mains but design the windings on the local generator rotor for 25Hz.
Or.
A 25Hz sinewave from a sig gen feeding A very powerful amplifier running into a transformer. Adjust the output of the amp to achieve 10V.
Thats what I'd do.
DTB
"Once you find out ... the Circumstances ; then you can go out"
#5
Mr Bass, I've been looking at the disc that goes around in the electricity meter; I thought if I could put a brake on it to halve it's speed.....Dave the bass wrote:A local generator? Drive it mechanically from the mains but design the windings on the local generator rotor for 25Hz.
Or.
A 25Hz sinewave from a sig gen feeding A very powerful amplifier running into a transformer. Adjust the output of the amp to achieve 10V.
Thats what I'd do.
DTB
More practically, current thinking is been along the lines of using an amplifier but, of course, the 300Bs will present a low impedance load so will be demanding. I guess that's why you've considered a transformer? I know of one guy who has gone this route feeding a sine wave into a meaty PA amp and he reports that it works and does make things better.
Ray
#6
Only a low impedance at start-up 5/1.5 = 3.3 well within the capacity of a small (its only 8W after all) amp. A chip amp would do. But I guess Dave assumed a transformer because there is some off isolation needed hence the near mystical requirement for AC heating.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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Dave the bass
- Amstrad Tower of Power
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- Location: NW Kent, Darn Sarf innit.
#7
Mystical Transformers, Shpongle album title in the making.
I was thinking a TX yes for isolation but also because we don't know if the 'generator amp' has a rail ref'd to 0V or earth, I was thinking (possibly incorrectly) that the 300B filaments would need isolating galvanically (correct word? get me eh) from generator amp itself leaving a floating 25Hz sine developing across the TX secondary.
DTB
I was thinking a TX yes for isolation but also because we don't know if the 'generator amp' has a rail ref'd to 0V or earth, I was thinking (possibly incorrectly) that the 300B filaments would need isolating galvanically (correct word? get me eh) from generator amp itself leaving a floating 25Hz sine developing across the TX secondary.
DTB
"The fat bourgeois and his doppelganger"
#8
It strikes me that we ought to examine why the 25Hz sounds better than 50Hz supply and see where that takes us...
We know from the measurements we have taken that one of the results of 50Hz ac heating is that we get 50Hz intermodulation throughout the audio spectrum. SO with 25Hz we would get 25Hz intermodulation instead. As the sidebands so generated would be closer to the fundamental frequencies in the audio signal, we might expect 25Hz to sound a little cleaner and tonally purer than a 50Hz supply. And as the 25Hz would be regenerated we might expect that the lack of mains borne noise and hash getting through to the filaments would be less too....
Assuming this is correct, where does this lead us? To my mind it leads to Andrews DHT heaters as they are, more or less, pure dc heating and so cause no intermodulation products with the audio signal and have exceedingly low noise and hash... We know how difficult it is to get dc heating sounding good and Owston showed that Andrews regs are at least as good as ac and, probably to a majority, better than ac heating. The really interesting thing is that they sound better than ac heating in almost exactly the same way as OTL sounds better than OPT outputs - for those that believe in OTL that is...
SO I conclude that if you like OTL then you will like Andrews dc filament heaters and that they are a better investment that 25Hz filament heating would be...
Of course the real thing to do is to build and try both!
Fully agree that the filament supplies need to be fully floating - maybe easier to float the supply to the filament sig gen and power amp as a 25Hz capable OPT is going to be a beast - although the bandwidth requirement is small!
J
J
We know from the measurements we have taken that one of the results of 50Hz ac heating is that we get 50Hz intermodulation throughout the audio spectrum. SO with 25Hz we would get 25Hz intermodulation instead. As the sidebands so generated would be closer to the fundamental frequencies in the audio signal, we might expect 25Hz to sound a little cleaner and tonally purer than a 50Hz supply. And as the 25Hz would be regenerated we might expect that the lack of mains borne noise and hash getting through to the filaments would be less too....
Assuming this is correct, where does this lead us? To my mind it leads to Andrews DHT heaters as they are, more or less, pure dc heating and so cause no intermodulation products with the audio signal and have exceedingly low noise and hash... We know how difficult it is to get dc heating sounding good and Owston showed that Andrews regs are at least as good as ac and, probably to a majority, better than ac heating. The really interesting thing is that they sound better than ac heating in almost exactly the same way as OTL sounds better than OPT outputs - for those that believe in OTL that is...
SO I conclude that if you like OTL then you will like Andrews dc filament heaters and that they are a better investment that 25Hz filament heating would be...
Of course the real thing to do is to build and try both!
Fully agree that the filament supplies need to be fully floating - maybe easier to float the supply to the filament sig gen and power amp as a 25Hz capable OPT is going to be a beast - although the bandwidth requirement is small!
J
J
#9
I cannot disagree that Andrew's DHT supplies would be the way to go, the problem is that the topology of the circuit precludes the use of DC filament supplies. The 300B cathodes are connected to the output and a current sink arrangement is used on the output.
Ray
Ray
#10
Hi Ray,
I'm not sure why the 25Hz supply will work but a fully floating dc supply won't and I understand why you can't post the circuit...
I think the chipamp suggestion in a transformer is a good one FWIW.
James
I'm not sure why the 25Hz supply will work but a fully floating dc supply won't and I understand why you can't post the circuit...
I think the chipamp suggestion in a transformer is a good one FWIW.
James
#13
No, I don't get it either, if the supply floats the only obstacle to DC would be a cap, and there wouldn't be one on a filament, would there? I must be missing something...JamesD wrote:Hi Ray,
I'm not sure why the 25Hz supply will work but a fully floating dc supply won't...
James
Andrew
Analogue, the lost world that lies between 0 and 1.
#14
Gents, attached are some pictures of my 300B-OTL output stage that I previously posted on the Transcendent website. The twisted black/red wires are for the heater supply connection (I've built the amp to have an external heater supply arrangement to facilitate experimentation). I've yet to connect the 300B cathodes to the output.
Ray
Ray