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1970s Design Indulgence

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    Posted: 6 hours 31 minutes ago at 8:07am
Measurably can't fault it...

S/N: 67dB (20Hz - 20kHz CCIR) - some people add the amp's gain which makes it 94dB

Crosstalk: -70dB @ 10kHz

Max out: 42 watts both channels driven (0.1% THD at 1kHz)

THD+N at 40 watts: 0.05% (1kHz)

THD+N at 1 watt: 0.07% (1kHz)

Frequency response: 20Hz - 20kHz -1dB

Note: there is no rising distortion at frequency extremes
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 7 hours 32 minutes ago at 7:06am
Single Power Supply

Yesterday was spent changing to a single power supply: secondary's wired in parallel to a 12A bridge rectifier; then 2 x 4700uF smoothers in parallel (all I had).

All "grounds" taken to the same "star earth" - a very short spur as Doug Self makes clear.

Each amp has 4 x 0 volt (ground) wires: output; power stage; decoupler; input stage (then onto chassis at input).

Positive rail starred-off from positive output end of the two smoothers to each amp's HT fuse.

All pure textbook stuff. What could go wrong?

Oscillation! That's what. It became clear that the 100n ceramic cap bypassing the decouplers was making a tuned circuit, so they were removed.

The result? Silence.

So here goes again: listening and measurement tests for another week.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 8:29am
The dual secondary windings of the transformer can be seen as an AC common connection of the two star "earths" which also have a common connection at the inputs.

This AC common is not stiff and will contain impedance differences along with frequency, as more current is drawn from one winding than the other. The magnitude of these impedances is hard (perhaps impossible) to calculate.

However, it enables us to see that the star earths of each amplifier are not going to be equal at all times, and that they will diverge in sympathy with left-right signal differentials.

Any divergence will result in some proportion of output current from one channel flowing in the other's input ground.

By adding some small resistance in series with both ends of each winding might help even things out as these are outside the magnetic circuit.

It is clear that this alone cannot solve all the "false" input ground current, and that some "stereo loop loading" resistance is required.

In the absence of a known method of calculation, and the absence of analytical tests, the values required will have to be decided empirically by subjective testing.

But because of component "burn-in" such testing relies on memorising the subjective effects from one test to another, removed by long periods of time.

The reason for using separated power supplies was to enable each channel to reach transients which it would otherwise not do, due to the loading of a single supply by the opposite channel.

However, this is fraught with the above mentioned difficulties, and so these have to be weighed up from the point of the objective versus the subjective.

The stereo distortion created by doing nothing, in this split supply configuration, is odious. I have seen in other circuits numerous "fixes" which simply try to cure the symptoms, and for quite some time I have tried to do the same.

It is only through direct experience, where no other explanation exists (or none can be found after serious research) that such lessons are learnt.

I trust this lengthy, nearly one hundred pages log or diary has gone much further to explain these difficulties than has been elsewhere.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 5:55am
If components didn't require a burn-in period, this job could have been condensed into a few weeks rather than more than a year.

The secret of analogue audio design is being accustomed to the mechanisms which result in particular shades of distortion.

However, those "shades" take time to reveal themselves, so nobody can claim a particular performance level on a 9 'till 5 basis.

I truly wish it were not like this, and I can seriously understand how damaging to the bottom line the admission of the existence of "burn-in" would be. But it's simple; what sounds like an improvement might not sound like an improvement given sufficient time.

Like all designs, this one has been no different, except it's easily the "longest-distance" one I've done to date. They usually take time, but it’s easier to find the mechanisms, or levers, which move it back to convergence.

Self on decoupler distortion explains that you can do everything else that's possible and get nowhere until decoupler distortion is solved.

I will now state that you can do everything that's possible and get nowhere until "ground" fluctuations are solved. This might have to be qualified by saying "in the application of a single rail supply".

I owe it to Dinsdale for providing the clues. Others took him to task suggesting a degree of incompetence, but those others never explained the mechanisms responsible for grounding distortion in such an understandable way. Do it their (now) textbook way and spend the rest of your life wondering why it's not quite as expected.

A big clue was the way in which he later defended himself by proclaiming this wasn't such a big problem with valve amplifiers, that being due to the much lower currents involved, and that is true. I had already sussed that one, but was pleased to see it confirmed in print.

Transistor amps are by comparison high current - low impedance devices, so impedance matters, and the textbook grounding scheme might not yield the best results. Parasitics describes the infection of a device by parasites, and there are a lot of them. They are due to components which will never be perfect. Every passive is also two other types of passive - take your pick from L, C and R. L has C and R; R has C and L; and C has L and R.

(L = inductance; C = capacitance; R = resistance)

Every active device, such as a transistor, has all three. Wire is L and R; PCB copper traces likewise. L and C have time functions, and the current at the hard work end is in amps, not milli-amps.

AC; alternating current, and whatever its frequency at any particular time, cross connects where you might not think it can; because basic understanding treats connections as being real-world "wired". In AC the imaginary factor j is found (as in jw). What it is saying is use your imagination - test all things - hold fast what is good - then keep doing it!

This adventure has looked at every conceivable permutation possible using a six transistor single rail amplifier configuration of the type found in thousands of 1960s to 1970s amplifiers and receivers; but at the end of the day, the circuit common has been found to be the Achilles heel.

Assuming the negative end of the smoother is a fixed reference might not be the correct assumption. OK, it looks like a reference point, but is it?

Everything can be all over the place but at that star point's convergence, all that's connected to it is still, which it will be. The problem is that star point connects to at least one other.

This is where it should dawn on us that separate power supplies are the problem.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 2019 at 5:21pm
The problem with the above is there is no true a.c. earth because of smoother ESR.

(Edit: my assumption here is wrong and corrected near the end of my next post)


Edited by Graham Slee - Yesterday at 7:49am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 2019 at 5:18pm
After a lot of cross-referencing and thanks to http://www.douglas-self.com/ampins/library/ampartew.htm, I eventually found the Dinsdale ground correction presented by C Artus in Letters to the Editor WW Feb 1965.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 2019 at 11:42am
The left-right differences need to be assessed further. My last conclusion seems a little silly.

The main difference between two monoblocks and a stereo power amplifier are now obvious, and that is the apparent lack of stereo-loop distortion in the monoblocks.

The stereo amplifier has to be made as near to two monoblocks in one case as possible.

There are practical limits however, one being the cost. Go the whole hog and it becomes an unreachable dream. The idea from my point of view, is to make the best available to all.

Currently we have two transformer secondary's and each amplifier has its own bridge rectifier and smoothers.

The unavoidable stereo loop joins at the transformer secondary's, and at the signal input grounds, and we need to see further into what's happening.

One thing I have not dealt with so far is the interaction between the bridge rectifiers: pull more current through a diode and its forward voltage (Vf) increases.

This might give some benign assistance to the stereo loop for an nth of the time; then at other nth of the time, it might be harmful.

Either way, it makes the case for separating the secondary's, and shows why the two transformer solution worked.

But let's say I want to keep it as a single transformer, what option do I have of unsticking the secondary's from each other?

It has to be non-invasive, and dual secondary's cannot really be separated, so I can only see the use of series resistance as being the way.

A single resistor on one end of each secondary can't really do the job, because, in this theory, the ends are just as stuck together as any other place in the windings.

How much voltage can we afford to waste? Will 1% do it? The price paid will be in the loss of maximum power (again). What would we rather have, quality or power?

All I have to hand is some 0.27 ohm 3W resistors. One either end of each secondary will lose 1.2 volts AC in total, and might translate to a 2W loss in output.

But, as we now know, this is part of the lever which works; it will have to be tried.
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