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SemperFi 7th August 2012 07:02 AM

Dual SE amplifier? - (not PSE) - Take two!
 
Sorry Shane, but your thread got messed up somewhere along the line. I find your original suggestion interesting, so hope you don't mind we try again...

There were many good replies untill all the 'my amp is better than your amp' started coming...

Please folks, absolutely no such 'I find SE sound worse than PP and OTL is really the best...etc etc'

Lets keep it to the factual design issues eh?

Ceglar wrote:
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
-------------------------------------------------------------------------------

The problem with the above is the inverted secondary of one of the SE OPT. The response of a transformer when the 'dots' are connected in phase is not the same as when connected out of phase. Must be b/c OPT are not wound 100% symmetrical, and Pri-Sec leakeage capacitances and inductances are exagerated when the windings swing opposite of each other.
(Kind of like how miller capacitance is actually the static capacitance times the gain(Cgd in FETs or Cgp in tubes - u get the picture)).

How ever, if connecting the outputs in a bridged config, hot positive and hot negative phase, then it should work just fine. Impedances must of course still match, but most OPT have 4,8, and 16ohm taps, so shouldn't be the greatest problem.

Bel Canto made an amp like this some 15years ago: Bel Canto SET 80 monoblock power amplifier | Stereophile.com

I am almost done with a guitar amplifier using two SE OPT b/c I ran out of PP OPT and have tinkering about this idea for years. So this Summer I desided to try it out. Hope to have it finished in a few weekends...

I am building it as a class-AB amp, which means I will let each phase run at about 15watt idle (5881 tubes) and one phase will cut-off while the other goes 'more-on'. That way I will get more power compared to simply running two pure class-A SE in bridged mode.

SemperFi 7th August 2012 07:09 AM

Attention Positron and EvenHarmonics! PLEASE discuss your discussion under the original thread, dont mess this one up as well. Thnx.

Ceglar 7th August 2012 09:43 AM

Quote:

Originally Posted by SemperFi (Post 3117453)
Sorry Shane, but your thread got messed up somewhere along the line. I find your original suggestion interesting, so hope you don't mind we try again...

The problem with the above is the inverted secondary of one of the SE OPT. The response of a transformer when the 'dots' are connected in phase is not the same as when connected out of phase. Must be b/c OPT are not wound 100% symmetrical, and Pri-Sec leakeage capacitances and inductances are exagerated when the windings swing opposite of each other.
(Kind of like how miller capacitance is actually the static capacitance times the gain(Cgd in FETs or Cgp in tubes - u get the picture)).

How ever, if connecting the outputs in a bridged config, hot positive and hot negative phase, then it should work just fine. Impedances must of course still match, but most OPT have 4,8, and 16ohm taps, so shouldn't be the greatest problem.

No problems at all, thank you infact. So you're saying to have the two secondaries in series with each other (the belcanto schem at the link) and not rely on the double phase inversion?.

I think the next step might be to get hands on his Glass Audio Article outlining the idea (Cohen) or a copy of his paper published in 1995 to the AES. AES is probably easier, more clear cut and cost effective.

I'm interested in his take on why he did it the way he did, of course.. and to hold that up to the light of the on-goings here (hopefully).

I'll order the paper and have a read - will get back with details.

Interested in your build progress, along with Tubelabs for the obvious reasons.

Sincerely,
Shane

SemperFi 7th August 2012 11:12 AM

1 Attachment(s)
Yes the secondaries in series, like this:

tubelab.com 7th August 2012 01:39 PM

Quote:

Interested in your build progress, along with Tubelabs for the obvious reasons.
Some people just like to argue and prove a point. I am not one of them, I would rather play with parts and make stuff.

My lab is rather messed up right now since we had to clean out our rented warehouse that contained 20+ years of collected "stuff" on rather short notice. The building was sold. There is "stuff" stashed in every room of the house.

I only have one PC running right now, so I don't have an FFT box for looking at individual harmonics. It will take a while before everything returns to normal.

In my mind there were two valid discussions in the other thread before it went astray.

One involved driving two SE amp channels out of phase and connecting the OPT secondaries together in series or parallel. Comparisons to be drawn between those two connections, an ordinary P-P OPT, and the SE amp as designed with the two channels paralleled in phase.

The other involved wiring a pair of SE OPT's into a P-P amp since this should emulate the two SE amps.

I had a Tubelab Simple P-P running most of the weekend to listen to the streaming Lollapalooza concert, so I played with some transformers. I immediately discovered the flaw in my thinking that the P-P amp could emulate the pair of SE amps. The Simple P-P is designed to operate in class AB1, usually in pentode mode. The OPT used is 6600 ohms which is 1650 ohms per side. The transition from A to AB occurs at a few watts with this load. Wiring the amp for triode mode and connecting a pair of 3000 ohm SE OPT's will still allow about 2 watts in class A with reduced B+. Pentode mode allowed only a little more power since the load wasn't optimum.

Any attempt to operate the amp above this level resulted in audible distortion. The connection with the two OPT secondaries paralleled worked the best. The series (bridged) connection mentioned in the above schematics was the worst. The problem is occurring because the Simple P-P was not designed to operate in class A and attempting to increase the bias enough for class A required lowering the B+ too much to avoid red plating. The distortion occurs when one tube cuts off and the coupling in the two seperate OPT's isn't good enough to smooth out the "kink" created when the amps output impedance changes. I looked at the output with a scope, but did not hook up any test equipment or make any detailed measurements. The audible results did not warrant it!

So, as discussed in the original posting, we need to stick with class A amps here. The SPP wasn't intended to be a class A amp, so no further experiments are planned using it. I am hesitant to experiment on my two working Simple Single Ended amps since the OPT secondaries are grounded. Modification would require amp dissasembly.

I have a dusty SSE board, plenty of OPT's and several regulated bench power supplies. I will have to hook it up and get it working and preform those experiments. Don't know exactly when though.

SemperFi 7th August 2012 02:07 PM

Interesting Tubelab. In theory though the OPT coupling shouldn't produce more or less of the xover kink...? I am really curious how mine turns out.
Regarding testing the concept with your Simple SE I don't see the problem trying bridging them, since the speaker is connected to each hot output and the secondaries can be grounded. No need to open anything up and mod, just feed the two channels with opposite phases and crank it. (see the attached drawing in my previous post).

20to20 7th August 2012 02:46 PM

Quote:

Originally Posted by SemperFi (Post 3117790)
Interesting Tubelab. In theory though the OPT coupling shouldn't produce more or less of the xover kink...? I am really curious how mine turns out.
Regarding testing the concept with your Simple SE I don't see the problem trying bridging them, since the speaker is connected to each hot output and the secondaries can be grounded. No need to open anything up and mod, just feed the two channels with opposite phases and crank it. (see the attached drawing in my previous post).

Semper,

By your drawing with the OPT's in series there would be a doubling of the output Z, like adding a series resistor to accomodate a 4R speaker on an 8R terminal. So that would have to be figured into the reflected Z. Yes?

20

SemperFi 7th August 2012 05:28 PM

Yes the reflected impedance is halfed.

I just realized something Tubelab has been trying to say. As one tube goes off (as in class-AB) it represents an open circuit to the OPT primary on that 'off-phase'. At that moment the secondary of the other phase, the one going more on, will rather swing the high-Z secondary of the off OPT than the load. So his statement this only works in class-A must be correct. Unless one connects the secondaries in parallel, but then we are back to the problem of one OPT with switched phases...

tubelab.com 7th August 2012 05:41 PM

Quote:

Interesting Tubelab. In theory though the OPT coupling shouldn't produce more or less of the xover kink...?......By your drawing with the OPT's in series there would be a doubling of the output Z
I believe the series connection was the worst case because they were in series, which causes the output Z to be the sum of the output Z for each channel. What is the output Z of a channel when the output tube is cut off? So I have one channel pushing its output into a speaker through a high impedance (and not flat across frequency) network (the OPT connected to a cut off tube. I think the parallel connection could work if the coupling was perfect (which it never will be). It did work better with my admittedly crude setup.

Back when I did the first test (maybe 5 years ago) I also tried taking two SE OPT's apart removing the "I" core pieces, and clamping the "E" pieces (with coils) together. It seemed to work, but again a crude and somewhat uncontrolled experiment.

Quote:

No need to open anything up and mod, just feed the two channels with opposite phases and crank it.
This is true for the bridged case, and for the true paralleled case, the parallelled out of phase case can not be tested without mod.

I figure that If I am going to do this again, I want to observe every possibility, take some real measurements, and write it all down this time. I also want to try several different types of OPT's and tubes to see how that affects the outcome. I have big and little Edcors, Hammonds 123CSE and 1628SEA and some Transcendars, probably a few more that I don't remember right now. My previous test was only done with the small Edcors. The SSE will fit 6V6GT, 6L6GC, EL34, KT88, and several others with only a twist of the power supply knob.

I have an old SSE board that has been used for experimentation for the past 7 years. It has even been run through the dishwasher once to clean off the goo from an exploded electrolytic. It just keeps on working. I'll set it up once I clean off a spot on the bench and get my second computer running for FFT spectrum plots.

tubelab.com 7th August 2012 05:44 PM

SemperFi, I guess we are on the same page here. I was typing the previous post when the boss walked in. It took a while to get back to it and I didn't see your post.

The parallel connection may or may not be an issue, depending on the OPT. Thats why I want to try several.


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