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OPT distortion cancellation w 2x SE OPT per channel?





Hi,

I'm interested in any comments regarding the following. I cant seem to add multiple images, so I guess adding one at a time via a response is perhaps my best option. Appologies for the confusion.. and I also have some questions regarding the topology which appear after the article.

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The excerpts here have been presented with kind 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."

"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".

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1) Does 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?

Thanks,
Shane



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Topic - OPT distortion cancellation w 2x SE OPT per channel? - shane. 05:16:55 12/29/12 (64)

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