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Old 31st July 2012, 09:56 AM   #1
Ceglar is offline Ceglar  Australia
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Default Dual Single Ended Amplifier? - (not PSE)

Hi,

Wondering if anyone has seen, built, or heard anything like this..

Two SE output stages per channel, signal at each grid out of phase WRT the other. Two SE output transformers per channel, one of which has its secondary winding reversed so that the waveforms are recombined in phase.

There are some interesting things that supposedly occur in that there is a combined function of the two output transformers.

All of which is outlined in the diagrams and description at;
Dual single ended amplifier by Cohen, Graeme John ? AU1996042023

I'm interested in pretty much anything anyone cares to offer in response.


Thanks,
Shane
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Old 31st July 2012, 01:30 PM   #2
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Hi Shane,

yes, I did experiment with this setup. I built the prototype, basically a PP amplifier, with 2 single ended transformers per channel and swapt in a PP transformer for comparison. Indeed the 2 single ended transformers performed much better, sonic wise. Buy may also be due to the quality of the iron: the SE transformers were Silver Rock, while the other was a German 'Engel' (plain EI core)

Of course that for the same amount of power a PP transformer is much smaller than 2 SE transformers, but I had no problems with space in my hobby room.

Regards, Erik
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Old 31st July 2012, 05:20 PM   #3
DF96 is offline DF96  England
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The only advantage I can see is that the OPTs will not have to do a flux zero crossing, which may or may not create distortion in a PP OPT.
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Old 31st July 2012, 06:10 PM   #4
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Try PP with such size iron, you may like it even more.
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Old 31st July 2012, 09:59 PM   #5
Bigun is online now Bigun  Canada
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Quote:
Originally Posted by DF96 View Post
The only advantage I can see is that the OPTs will not have to do a flux zero crossing, which may or may not create distortion in a PP OPT.
this may be a nice approach. I wonder if the benefits of this approach are best suited for low power, both because the amp spends more time near the zero crossing and because the SE trafo's would be huge if high power were needed.

some imbalance between the two SE trafo's would be likely, so maybe this would play more like a SE at lowest power.
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Old 1st August 2012, 12:50 AM   #6
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I have done this from another perspective. Small SE OPT's are relatively common and cheap. So are tubes and circuits for them in the less than 10 watts per amp range. SE transformers and tubes in the 30+ watt range are not common or cheap. Why couldn't we wire 4 10 watt SE amps in parallel to make a 40 watt amp. Well, it works, and if you wire the two amps out of phase as you suggest.....it IS a push pull amp. Now granted it may have less than perfect coupling between the two halves, but it is a class A push pull amp. I noticed that when the 4 amps were paralleled in phase (two stereo SSE's) the maximum power output was roughly equal to 4 times a single channel. When two of the amps are wired out of phase and driven with a phase splitting transformer, I could push the amps harder and get nearly 6 times the output power. Why? Because it is a push pull amp and you can now drive one side into cutoff without overall clipping.

So, why couldn't I wire push pull amps in parallel....well that works too. I have a good supply of a certain P-P OPT rated at 80 guitar amp watts. I have seen 500 watts come out of 4 P-P amps running sweep tubes through these OPTs wired in parallel.

When you parallel 2 amps you need to drop the load impedance by half. If you have an 8 ohm speaker drive it with two 16 ohm taps in parallel. You can series parallel OPT's but pure series doesn't work as well since the coupling between the two amps isn't direct since they are connected only through the speaker.
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Old 1st August 2012, 01:45 AM   #7
Bigun is online now Bigun  Canada
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so you could take a stereo SET and wire the outputs in parallel and use it as a mono, or invert one channel and put them in parallel and use it as PP mono
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Old 1st August 2012, 03:35 AM   #8
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If each side is only required to swing one side of the voltage, does it mean that no biasing is needed beyond the phase splitter stage?
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Old 1st August 2012, 01:27 PM   #9
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Quote:
so you could take a stereo SET and wire the outputs in parallel and use it as a mono, or invert one channel and put them in parallel and use it as PP mono
Yes, I and a few builders have taken my SSE board which is a two channel SE amp, driven the channels out of phase, connected a normal P-P OPT and created a push pull amp. An SSE usually makes 12 to 14 WPC when run hard using KT88's at 450+ volts in UL mode. Using the same power supply I get 60+ watts mono from the same KT88's wired in pentode with a P-P OPT (no UL taps). No changes are needed to the SSE amp other than the OPT and a differential drive source (my guitar preamp). Makes a little terror of a guitar amp.

Quote:
does it mean that no biasing is needed beyond the phase splitter stage
Not sure what you mean by the words "no biasing". Most tubes must have some kind of negative bias applied to its grid to control the tube current, whether it has signal voltage applied or not.

I assume that you are reffering to operating the output tubes where the idle current is zero, or very near zero. Zero current operation would be true class B operation which yields high efficiency, high power output, and high distortion especially at low volumes. It was used for high power PA amplifiers where 10% distortion was acceptable.

If tubes, mosfets, BJT's or whatever were completely linear devices all the way down to zero current, this would be a perfectly valid approach. Unfortunately, they are not. Linearity is best in the middle or their operating range and degrades at each end of the curve. To avoid high distortion near the region where the signal transitions from one device to the other (known as crossover distortion) in a push pull pair, we apply enough bias to move the devices out of the region of worst linearity. This results in an overlap region where both devices are operating.

There has been talk, and even a few commercial products, of building "screen driven amplifiers" using TV sweep tubes (I have built a few). In this case the tubes can be operated very close to true class B with idle currents as low as 3 mA. This is because a TV horizontal (line) output tube was designed for use as a grid (G1) controlled CCS with the screen (G2) as the current controlling element. The curves of G2 voltage VS tube current are very linear and hold flat over a wide plate voltage range, down to very low currents. Screen drive has other issues like high drive requirements, tube arcs and spurious oscillations if high power levels are attempted, probably preventing mainstream popularity.
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Old 1st August 2012, 03:19 PM   #10
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so you could take a stereo SET and wire the outputs in parallel and use it as a mono, or invert one channel and put them in parallel and use it as PP mono
I think the correct answer is yes and no. Two signals in phase will be additive in parallel. Two signals, with one out of phase, will cancel each other. A push-pull signal becomes additive in the primary of the output transformer by the way it is constructed. This doesn't happen if the SE secondaries are simply paralleled. You would have to series them and refigure output impedance.
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