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Old 7th August 2012, 07:20 PM   #11
20to20 is offline 20to20  United States
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Quote:
Originally Posted by SemperFi View Post
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...
It would be doubled into each primary because they are still independant primaries but each now sees the added secondary in series reflected back.

Also, why run them in AB? Would there ever be a reason to run SE that way unless the current was overloading the PS
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Old 8th August 2012, 06:08 AM   #12
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Originally Posted by tubelab.com View Post
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.
Which is why you must turn your screen around so he can't see which report you are working on ;-D
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Old 8th August 2012, 06:11 AM   #13
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Originally Posted by 20to20 View Post
Also, why run them in AB? Would there ever be a reason to run SE that way unless the current was overloading the PS
Just to get more power out. Not that there is a huge difference between 20W and 30W, but it's enough to be noticable.
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Old 8th August 2012, 04:43 PM   #14
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Originally Posted by SemperFi View Post
Just to get more power out. Not that there is a huge difference between 20W and 30W, but it's enough to be noticable.
That equates to 1.5 db difference from the speaker. If getting higher db without clipping amp is the goal, higher efficiency speakers (up to a point) may a better route.
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Old 9th August 2012, 07:05 AM   #15
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Originally Posted by Evenharmonics View Post
That equates to 1.5 db difference from the speaker. If getting higher db without clipping amp is the goal, higher efficiency speakers (up to a point) may a better route.
Yep, thats right.

I am building this for guitar btw. The 'typical' amp I seem to build is 15-20watt, but that little extra headroom can be nice in certain settings, and this time I want to build something > 30watts. But my original idea of class-AB (I was even thinking class-B) using two SE OPT was not a good one. Thanks to this thread I don't have to build it to find that out.

There is still one benefit tho, and that is better PSRR thru the balanced topology of a bridged amp. Since guitar amps sound like crap with well filtered and stiff PS, that PSRR is a great advantage. So I will likely build this one as a bridged SE amp, but running class-A, contra PSE. Same output power, better PSRR.
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Old 9th August 2012, 09:14 AM   #16
Ceglar is offline Ceglar  Australia
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Quote:
Originally Posted by SemperFi View Post

There is still one benefit tho, and that is better PSRR thru the balanced topology of a bridged amp. Since guitar amps sound like crap with well filtered and stiff PS, that PSRR is a great advantage. So I will likely build this one as a bridged SE amp, but running class-A, contra PSE. Same output power, better PSRR.
I'm thinking the same with regards to PSRR, also hum rejection with AC heating on the output pair (direct heated tubes) and any bias resistance in the output filament circuit wont add to the Rp.

Thought so far with regard to the filament circuit is to use a common current sink sitting on-top of two pots wired in parallel, wipers feeding the grid returns of the secondary windings of a phase splitting interstage transformer. Would give some means for DC balance, and nice thing about not having a fixed CT in a single OPT (ala PP) is that a centre zero meter can easily be added to balance the currents.

Ordering back issues of the following for some better understanding of at least why the author of the patent link (see original thread), decided it was worthwhile;

Linear Output Stage (Cohen) - Glass Audio Vol.7, #6, 1995
Direct Single Ended Amplifier (Cohen) - Glass Audio Vol.8, #3, 1996

The OPT in his custom offerings were apparently a little different (no iron??) and details should be in the article;

Transmission Line Output Transformers - Glass Audio Vol.7, #5, 1995

Should make for an interesting read.
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Old 9th August 2012, 11:48 AM   #17
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guys would you like me to merge the technical content from the original thread here? The thread will end up being owned by Ceglar again with his first post in the other thread becoming the first post here. No problem if you would prefer not to. I'm easy either way.

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Old 9th August 2012, 02:43 PM   #18
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That'd be great.
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Old 10th August 2012, 08:21 AM   #19
Ceglar is offline Ceglar  Australia
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yes, please do.
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Old 24th November 2012, 06:20 AM   #20
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Default The Dual Single Ended Article

After quite some time I have managed to locate the original article. Hopefully this can shed some further light on the topology, the article is well written and documented. The distortion comparison is also thoughtful in that one can open one set of curves in a seperate window and they are virtually superimposed over the other, which makes for an easy comparison.

--------------------------------------------------------------------------------

Any comment or futher on topic discussion is very welcome and I have a couple of questions that someone might be able to answer

1) 2H cancellation with 'PP' only occurs when driving an output transformer?
2) If a SE to PP IT was used between a single driver and output(s), would you expect the typical SE distortion from that transformer loaded driver, even with zero grid current in the secondary windings?
3) Any issues with using a single bias resistor (unbypassed) that is common to both output stages?

--------------------------------------------------------------------------------

The excerpts here have been presented with kimd permission of the entity that holds the copyrights to the published article which appeared in Glass Audio Magazine Vol. 8 No. 3, 1996. Thanks to Mr. Barraclough and AA/CC/Elektor USA

Supplements to this article might include;
Transmission Line Output Transformers - Glass Audio Vol. 7 No. 5, 1995
Linear Output Stages - Glass Audio Vol. 7 No. 7, 1995



THE DUAL SINGLE ENDED AMPLIFIER

"The revival of the SE ampflifier is becoming more appreciated. By combining SE ampflifiers in the correct way, and driven correctly a new amplifier has been developed. Distortion figures lower than both the SE and PP types have been obtained. Listening tests have confirmed the sonic attributes of the Dual Single Ended amplifier."


"A revival of the operation of a single output device, in particular the triode valve, has emerged again for serious audiophile listening. The second harmonic distortion that can be present with SE operation is either desired, accepted or ignored in the quest for high quality amplifiers. Many engineers disregard valve amplifiers, particularly the SE type".

"By using two SE amplifiers per channel, driven correctly and combined in a such a way that the total power is added while reducing the harmonic distortion, a new SE amplifier techique has been developed.

Two identical SE output stages with their own output transformers are driven in a differential manner. The combined output is added, in series or parallel, at the secondaries of the output transformer.

Each output stage operates in Class A with the normal high standing current and high flux in each transformer in true SE operation. As the flux density is increased in one output transformer, it is reduced in the other transformer, with both the second and third harmonic distortion products being reduced".

Push-Pull

"High third order distortion can be present. Symetrical saturation reduces second order products but not necessarily third order products.

Any unbalance of the PP primary currents can cause high levels of distortion (Ref. 1). Typically, ten percent unbalance of DC current can cause several percent harmonic distortion due to change of flux density (Fig. 10).

Large changes of inductance and permeability are present due to the operating flux varying from zero to maximum each half cycle (Fig. 8, Ref. 2,3 & 4). Changes of up to 5 to 1 are typical with a flux density peak followed by a reduction of flux at high levels. The effect is compounded at the speaker resonance frequencies."

Single Ended

"The high standing current requires a large output transformer with an air gap. This results in low transformer core distortion and minimum inductance changes with changing flux density. The flux lines are in one direction only as they do not go to zero over any portion of the Class A operation (Fig. 9).

Valves are usually used today in SE operation, and due to their low output impedance the transformer distortion is less. If the triode is operated in a linear manner with low distortion, then the transformer distortion dominates and this can be minimised with good design.

For a low distortion valve stage, the transformer distortion dominates due to flux saturation at high levels. At low frequencies the change in inductance with flux density also modifies the affect of the above conditions."

Output Valves

"when a triode valve is used, the transformer is droven from a low impedance, which produces a linear voltage across the transformer, with the transformer current having the major distortion component.

If a Pentode or Tetrode valve is employed, the transformer is driven from a high impedance producing a linear current through the transformer, and the major distortion is the voltage across the transformer (Ref.1, as above).

A Pentode or Tetrode valve connected as a partial triodes (sometimes referred to as Ultra Linear, or Distributed Loading_, has the screen grid of the valcve connected to a tap on the transformer primary winding. The relative high efficiency of these valves is retained with some of the benefits of triode valves being obtained. This confuguration has a medium output impedance and the transformer distortion is a combination of voltage and current. This can result in a lower distortion power match to the loudspeaker load.

If a low amount, or zero overall feedback is desired, then triodes need to be used to obtain an inherently low output impedance, and hence an adequate damping factor. For zero feedback operation, inherently linear output stages using triode valves and wide bandwidth low distortion transformers need to be employed (Ref.5 & 6)."

Curvature Distortion

"If Class A PP is considered, it may be assumed to be the circuit topology which gives the lowest distortion. The major benefit of PP is to reduce the second order distortion for a given power output per anode dissipation, and compared to a SE stage for the same dissipation this may be the case.

However, Class A PP can have considerable third order (third and fifth harmonic etc) due to each half cycle rounding, due to either the output valve or the PP transformer. That is, each half cycle, can have severe rounding which can cancel for second harmonics, but can have a severe third harmonic distortion component.

The SE amplifier can have both second and third order components.

However, when two SE output stages are correctly added, each half cycle has cancellation of both second AND third order components, that is, the rounding of one stage due to and increase of both valve and transformer currents tends to be cancelled by the decrease of current in the other stage. As both stages are identical, but operating in the opposite phase, its is possible that all forms of distortion are reduced during each half cycle, and that the second harmonic distortion is not dependant on the next half cycle.

The Dual Single Ended concept therefore tends to reduce all distortion components during each half cycle for both valve and transformer distortion

In Figures 11, 12 & 13 are shown transformer distortion due to flux changes (B) with magnetizing current (H) for PP, SE and Dual SE respectively.

The PP case, (Fig. 11), shows a high level of third harmonic distortion (n.b. a square wave has low second but high third harmonic distortion).

The SE case (Fig. 12), has a considerable level of both second and third harmonic distortion but has other attributes

The Dual SE amplifier concept, (Fig. 13), using two SE stages driven out of phase bt added to give maximum power, tends to cancel distortion and this reduces both second and third harmonic distortion. One Single Ended stage curve is drawn updside down to show the ideal resultant when combined.

Demonstration

"To show the difference is distortion products between PP, SE and Dual SE output stage, a low distortion amplifier was developed. It was designed to have a high output impedance in contrast to the normal desirable low output impedance amplifiers. The reason is to achieve a linear current through the transformer primary winding so that the primary voltage distortion could be readily observed on an oscilloscope. The general form of this test ampflifier is shown in Figure 14.

The output valves had high value cathode resistors with feedback from their cathodes back to the input stage. A linear current drive to the output transformer resulted, with no feedback from the transformer itself.

A Trimax transformer TA1044 was used for the PP tests and two SE transformers Trimax TA851A were used for the Dual SE tests. The two SE transformer secondaries were combines to give maximum output power. The amplifier was driven in a balanced differential manner whith both of its halves being independant, identical SE designs.

The results are shown in Photographs 1 to 6 for a 20Hz input signal. The outputs were taken to an optimum resistive load. The total output power was 5 watts which was less than half of the transformers' rated power and well inside the linear operation of the current output Class A valves. The anode voltage waveforms were taken by inverting one channel of a two channel oscilloscope."

"In Photograph 4 is shown the Dual SE anode waveforms across the two individual SE transformers. Again, one oscilloscope channel is inverted. The typical SE rounding is seen on each waveform. However, when the two anode voltage waveforms are added in the oscilloscope, the two seperate SE wave-forms are combined to show the Dual SE waveform in Photograph 5. Note the absence of the kink as in the PP case. In Photograph 6 is the combined secondary of the two SE transformers. The waveforms visually appear better than the PP case without the kind present, also the wave analysers measurements show less distortion overall and in particular less third harmonic distortion

The above tests may not represent all cases of comparison but do show the mechanism for what might be considered high distortion in each SE output, and in fact has less distortion than a similar power PP case, when the two independent SE stages are combined in the Dual SE configuration".


---------------------
Regards,
Shane
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Last edited by Ceglar; 24th November 2012 at 06:23 AM.
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