HF AC DHT Filament Supplies

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Ray P
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#46

Post by Ray P »

Well, although Bruce Rozenblit has introduced a kit of his 300B OTL design that includes the 25Hz filament supply;

http://www.transcendentsound.com/Transc ... e_Amp.html

it would appear that he isn't going to make the filament supply circuit available to the DIY community to complement the amp circuit, which was published in his recent book. His reasoning seems to be that because other 300B/2A3 type amps would benefit from 25Hz filament supplies he would be helping his competitors if he made the circuit available (though I suspect it wouldn't be onorous for a competent circuit engineer to produce their own version anyway if they wanted).

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Paul Barker
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#47

Post by Paul Barker »

Well the race is on to come up with our own.

It isn't exactly a concept you could patent.

simply a matter of coming up with a oscilator and an amplifier.

I am just trying to picture the speakers wobbling at 25hz.
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#48

Post by Clive »

I realise that for Bruce this is a real breakthrough. For the rest of us there will be some skepticism, I'm sure it works fine in his 300B OTL but he hasn't compared it to a well designed DC heater supply as he can't use DC in his circuit. I can understand why it's better than 50Hz or 60Hz but he's having to use 25Hz out of necessity, it must still be noisy as Nick points out.
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#49

Post by Paul Barker »

I think I would sooner play with Stephies AC heating solution which is as old as the hills, and I still haven't tried it yet. At the very time or very soon after Stephie invented it, Gido was planning his dc modules and it looks like everyone went in the Dc direction, everybody forgot all about AC. Until now.
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#50

Post by Ray P »

Paul Barker wrote:I am just trying to picture the speakers wobbling at 25hz.
Paul, surely the signal will be very low level so unlikely to have this affect; 50Hz hum with conventional AC filament supplies is low level?

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#51

Post by Paul Barker »

I suppose so but you can hear it.
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#52

Post by Paul Barker »

I suppose so but you can hear it.
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#53

Post by Andrew »

Perhaps I'm being dense but I don't see why he can't use DC? How does the valve know its DC or AC?
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#54

Post by Ray P »

Andrew wrote:Perhaps I'm being dense but I don't see why he can't use DC? How does the valve know its DC or AC?
The valve doesn't but the current sink does.

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#55

Post by Paul Barker »

Bump :!:
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#56

Post by Ray P »

Not sure why Paul bumped this but I'll pick up on it; A DIY build of the Transcendent 300B SE-OTL is my current active project.

Anyway, having introduced the 25Hz filament supply there was an issue with woofer 'pumping' on some 16ohm Altec speakers. This has resulted in some amen dements to the original circuit that has reportedly fixed the issue, details here:

http://tubehifi.websitetoolbox.com/post ... t!-6383815

My build will use conventional AC filament supplies, at least to begin with.

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#57

Post by Alex Kitic »

I am not sure how 25Hz AC is going to be inaudible. If nulled, the 2nd harmonic remains at 50Hz which should be pretty audible unless filtered out by the inability of the amp and/or speakers to reproduce it.

HF AC on the other hand is definitely inaudible and if the 100Hz hum caused by 50Hz mains rectification is filtered, there is no frequency we can hear even with 20kHz AC, and most electronic transformers operate above 30kHz, most common today is between 40 and 60kHz! It is easy to null the first harmonic, and the 2nd is definitely inaudible.

The only real complaint for AC heaters, besides hum (100-120Hz), is IMD. The latter is easy to understand at 50/60Hz fundamental and 100/120Hz 2nd harmonic, but what about 30kHz fundamental? Just imagine the results measured by S. Bench for 1kHz tone and various levels of 120Hz... but 1kHz and 30 or 60KHz? Besides, this distortion seems to be caused by heating and cooling the filament at regular intervals (mains frequency): but what happens when the frequency is increased 500x or 1000x?

HF AC definitely has to overcome prejudice, that's how I see it.
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#58

Post by Mike H »

I believe Nick G mentioned earlier, one big problem with HF AC is the heterodyning effect, viz 30 kHz (say) mixing with upper treble or ultrasonic in the signal and producing an audible difference frequency.

Having played with tape recorders a fair bit, including making my own, there is the exact same problem with the HF (ultrasonic) HF AC bias signal.

As an experiment I tried putting ultrasonic from my sig gen into record input and could produce any audio tone I wanted as a product of this difference signal with the bias, which was 50 kHz if memory serves, AND therefore could be easily recorded onto the tape.

As far as tape recorders go this is normally avoided by a combination of treble roll-off filters in the record channels and making the bias freq. high to get as far away from the audio band of interest as is practicable. I say practicable because higher freq. for the bias has its own problems because the erase and record heads are inductors, the higher the freq., the higher the impedance so the bigger the Voltage needed to achieve the necessary coil currents.

Cassette machines tend to have 50 - 60 kHz bias as their AF bandwidth is not great, typically up to 10 - 15 kHz or somesuch, whereas my Akai reel-reel as a different example uses 120 kHz. Note that the basic rule of thumb for tape machines is to make the bias freq. four times the highest freq. you want to record.

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#59

Post by Nick »

Given Mike has answered, just to correct another misfact.
Besides, this distortion seems to be caused by heating and cooling the filament at regular intervals (mains frequency)
No, it isn't, the thermal inertia of the filament is far higher than would allow this to happen.

If heating effect was the cause, then it would not be possible to affect it by the use of hum balancing pot.
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#60

Post by Alex Kitic »

Mike H wrote:I believe Nick G mentioned earlier, one big problem with HF AC is the heterodyning effect, viz 30 kHz (say) mixing with upper treble or ultrasonic in the signal and producing an audible difference frequency.
I assume you are referring to Heterodyne?
http://en.wikipedia.org/wiki/Heterodyne

There is (of course) an exact mathematic calculation to perform the get the results you are implying.
Heterodyning is based on the trigonometric identity:

\sin \theta \sin \varphi = \frac{1}{2}\cos(\theta - \varphi) - \frac{1}{2}\cos(\theta + \varphi)

The product on the left hand side represents the multiplication ("mixing") of a sine wave with another sine wave.

The result is the sum of two sinusoidal signals, one at the sum f1 + f2 and one at the difference f1 - f2 of the original frequencies
The problem here is that:
1) the input signal needs to be relevant in "dimension" i.e. power or voltage to the signal we are mixing it with;
2) we are mixing whichever audio signal with a signal that exceeds the audible band, thus neither sum of audio f with (lowest possible value) 40kHz nor difference of audio f with 40kHz is an audible signal?!
Mike H wrote: Having played with tape recorders a fair bit, including making my own, there is the exact same problem with the HF (ultrasonic) HF AC bias signal.

As an experiment I tried putting ultrasonic from my sig gen into record input and could produce any audio tone I wanted as a product of this difference signal with the bias, which was 50 kHz if memory serves, AND therefore could be easily recorded onto the tape.
Just as mentioned above, the signals need to be comparable in voltage or power terms, and you are using a 50kHz signal (tape bias) to add (not audible or recordable, of course) or subtract -- which can be audible if you are subtracting let's say 40kHz from 50kHz and getting 10kHz as a result.

But we are talking here about audio signals (the music) mixing with a signal which is in the worst case scenario 40kHz (2nd harmonic of the worse possible HF AC) while in most cases we are talking about at least 60kHz (cheap Chinese unit) and very often 120kHz (modern unit operating at around 60kHz, thus 2nd harmonic is 120kHz).

It must also be noted that the mentioned frequencies as 2nd harmonics are actually residuals and thus rather low as value... nevertheless, their mixing with audio frequencies are not something we can hear, and in the case of tube amplifiers with output transformers, the resulting frequencies are too high to be reproduced without dramatic loss (if at all).

Post Scriptum:
The HF AC I am proposing is not even a sine wave signal... it's an imperfect square wave... unlike your generated sine wave signal.
Last edited by Alex Kitic on Sun Jul 27, 2014 10:47 am, edited 1 time in total.
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