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Old 9th Sep 2010, 12:20 pm   #1
Skywave
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Question

I recently came across a method of producing two-channel stereo but using only one pair of output valves in push-pull: see the attached cct. diagram. (Please excuse the free-hand sketch - it's rough, but adequate for this Thread )

Can anyone provide a detailed analysis of how this works, please - in terms of signal amplitudes, respective signal phases, ratios of transformer windings, currents & voltages, etc.?

Thank you.

Al. / Skywave.
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Old 9th Sep 2010, 1:33 pm   #2
G8HQP Dave
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Default Re: Two-valve push-pull stereo

I can't really see the point of that circuit, unless it is intended to support a rather spurious marketing claim for push-pull. It is P-P for the L-R difference signal (usually small) and parallel single-ended for the larger L+R mono signal. The two transformers would need to be fairly well matched over the audio band, even though they would need to be quite different in construction (P-P vs. SE i.e. no gap vs. gapped core). Any imbalance would introduce interchannel crossstalk and phase errors.

I would be interested to hear what the originator of the circuit thought he was achieving!
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Old 9th Sep 2010, 4:27 pm   #3
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Default Re: Two-valve push-pull stereo

Whisky Tango Hotel

The lower transformer secondary produces the sum of the LH and RH channels across its ends. The upper transformer primary carries the difference between the LH and RH channels. This is then re-injected back into the centre-tapped secondary of the lower transformer.

Since two transformers are required anyway, they are both carrying DC in their windings and there are more opportunities than usual for things to go wrong, the whole thing seems monumentally pointless.

AMI juke boxes used to reverse the connections on one of the cartridge windings (to reverse the phase in that channel) and one of the speakers (to correct the phase), and then wire a mono subwoofer between the two channels so as to get L - (-R) = L + R. But they were still conventional amplifier circuits (with separate transformers per channel, when using valves).
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Old 9th Sep 2010, 5:26 pm   #4
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Default Re: Two-valve push-pull stereo

Other way round: lower transformer is L-R, upper is L+R. This assumes grid inputs are L and R, as shown. Inverting one input would reverse this.

The lower transformer (P-P) at least has DC cancellation.
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Old 9th Sep 2010, 5:32 pm   #5
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Default Re: Two-valve push-pull stereo

Ah, yes, I forgot there's no phase-inverter feeding the bottom valve.

The lower transformer only has partial DC cancellation, as the current flowing up the lower half of the primary will not necessarily be equal to the current flowing down the upper half.
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Old 9th Sep 2010, 5:33 pm   #6
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Default Re: Two-valve push-pull stereo

Quote:
Originally Posted by G8HQP Dave View Post
The lower transformer (P-P) at least has DC cancellation.
But the upper one carries twice the DC current of a single valve...
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Old 9th Sep 2010, 5:56 pm   #7
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Arrow Re: Two-valve push-pull stereo

Quote:
Originally Posted by ajs_derby View Post
Since two transformers are required anyway; they are both carrying DC in their windings and there are more opportunities than usual for things to go wrong; the whole thing seems monumentally pointless.
Except, perhaps, that it uses two valves instead of four?

Al.
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Old 9th Sep 2010, 6:39 pm   #8
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Default Re: Two-valve push-pull stereo

Well, not really - if you use the same two valves as single-ended outputs you'd get exactly the same total power. And you'd have two transformers, each carrying the DC magnetising current on one valve (instead of one transformer with zero and one with twice).

The 'push-pull' idea is really a red herring - if there is distortion, then the current in the lower transformer's centre-tap will carry the distortion components. These will pass through the upper transformer and will then be introduced into the loudspeakers via the secondary connection matrixing.
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Old 9th Sep 2010, 6:55 pm   #9
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Default Re: Two-valve push-pull stereo

No: it can only ever give (at best) the same result as two separate single-ended amplifiers; so it's using two valves and two transformers instead of two valves and two transformers.
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Old 9th Sep 2010, 7:03 pm   #10
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Default Re: Two-valve push-pull stereo

It uses two identical valves and two different transformers, instead of two identical valves and two identical transformers. The latter would be cheaper and give better channel separation. So where did it come from?

Was the original designer also doing a stereo decoder and got the two circuits mixed up?
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Old 9th Sep 2010, 7:53 pm   #11
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Arrow Re: Two-valve push-pull stereo

The source of this circuit is the Radio and Television Engineers' Reference Book, fourth edition, 1963; editor Pat Hawker, G3VA; advisory editor W. Pannet, A.M.I.E.E.

The circuit - as shown above - appears on page 14-47. At that point, it is an extract from the full circuit of a complete radio (aerial to speakers) of what is described as an Export Stereophonic Radiogramophone: reference pages 14 - 42 and 14 - 43 in that book.

This book, in its day, was a well-known and much respected source in its field. Therefore, it seems unusual that a circuit should appear therein which is of questionable design - especially considering the "it features in this model" statement - but I do note that the chassis / model / manufacturer of the radio are not stated.

In the book, a reference is given to the original design source: B.B. Bauer, J. Hollywood and G. Maerkle: first appearing in the magazine "Audio Engineering", October 1958, titled "A Two-Way Stereophonic Amplifier".

Al.
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Old 10th Sep 2010, 11:59 am   #12
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Default Re: Two-valve push-pull stereo

I have learnt a lot from reading Pat Hawker (mainly in RadCom) for many years, but occasionally he includes daft circuits without spotting that they are daft. Given his wide range this is almost inevitable.

The circuit would make slightly more sense if one of the inputs is inverted. At least then the main mono signal would be P-P so lower distortion, leaving the SE part to handle the smaller L-R signal. The speaker phasing (not shown) would also have to be swapped. Second-order distortion from the P-P would go through the SE transformer but would be applied antiphase to the two speakers so might partially cancel acoustically.
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Old 10th Sep 2010, 2:07 pm   #13
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Default Re: Two-valve push-pull stereo

I'd agree that the only sensible way to use it would be with one channel inverted before the input. If the amplifier is intended for a low-cost radiogram, the channel separation of the cartridge would probably be poor and hence the difference signal low and inferior at source. The SE performance, even perhaps with a transformer of lower rating than the PP one, might suffice for it. The cartridge would be wired to provide the necessary phase, avoiding the need for an actual inverter stage.

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Old 10th Sep 2010, 2:25 pm   #14
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Default Re: Two-valve push-pull stereo

I'd almost taken it for granted that one input would be inverted (and phase corrected by connecting the relevant loudspeaker appropriately).

This would allow the L+R signal to be amplified in push-pull - although distortion would be reintroduced as I noted above, but G8HQP Dave notes that this could cancel acoustically (though the speakers would need to be fairly close together for this, not really what you want in a good stereo setup).

Lucian suggests a neat way to do this. However, in a radiogram, I'm thinking how it would work when in 'radio' mode - from a mono detector, there would still need to be a phase inverter stage. A possible advantage of the circuit is the 'upper' transformer could be merely short-circuited in mono mode, giving true push-pull operation with all its advantages.
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Old 10th Sep 2010, 8:01 pm   #15
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Default Re: Two-valve push-pull stereo

Hi.
Either I am stupid or this is circuit is way off the mark. T2 has one core, Correct? so how on earth can there be any marked stereo seperation?
Any subtraction of the signal will in this circuit cause cancellation (no voices on stereo) or Addition thus all middle
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Old 10th Sep 2010, 8:47 pm   #16
G8HQP Dave
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Default Re: Two-valve push-pull stereo

Far be it from me to comment on another forum member's intelligence, even when invited!

If fed with one channel inverted, then T2 handles the sum signal i.e. mono. T1 then handles the difference L-R. The two signals are then demultiplexed at the speakers, if wired correctly. At left you get L+R + L-R = 2L, at right you get L+R - L-R = 2R.

The trouble is that this circuit is like a trade union statement from the 1970's - all double and triple negatives!
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Old 10th Sep 2010, 9:23 pm   #17
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Default Re: Two-valve push-pull stereo

I think it works by all the magic smoke !!!
one circuit I DONT think I will try

Joe
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Old 10th Sep 2010, 9:27 pm   #18
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Default Re: Two-valve push-pull stereo

T1 primary is fed from HT to T2 centre tap of its primary effectively null, look at the secondary same scenario, but we all seem to forget about the core what is induced from one valve will be seen by both speakers and and vice versa. I cannot honestly see how this can possibly work.

Amp Mangler I agree, perhaps its a subtle way of proving that theory is "theory" and not always correct!
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Old 10th Sep 2010, 9:40 pm   #19
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Default Re: Two-valve push-pull stereo

http://greygum.net/sbench/sbench101/#THE MATRIX

**Link doesn't seem to work correctly, click and scroll up a bit.

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Old 10th Sep 2010, 10:13 pm   #20
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Default Re: Two-valve push-pull stereo

Audiofools!!!!!!

I am sorry if this amp worked it would have been universal in it's use as most manufacturers would go nuts to save 1 penny per unit!
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