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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 2019 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: 10 Dec 2019 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: 10 Dec 2019 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: 12 Dec 2019 at 12:14pm
60 hours in: sounding much better in the highs. Still has bass.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 1:15pm
Originally posted by Graham Slee Graham Slee wrote:

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.

Is this similar to "the tree trunk symmetrical ground"?
Bruce
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 12 Dec 2019 at 3:47pm
Originally posted by BAK BAK wrote:

Is this similar to "the tree trunk symmetrical ground"?


The "tree-trunk" ground doesn't result in a stereo loop, but can give rise to hum and/or increased distortion due to the power earth modulating earlier stages. It all depends on the board layout and proximity of the smoothers to the board.

As Dinsdale explained after being criticised, most early transistor amp designers previously designed valve amplifiers, where smaller currents didn't exacerbate the problem like it does with the higher current transistor amps.

There seems to be a gap between the 60's and late 70's when "star-earth" mania suddenly burst on the scene (or seemed to). I remember asking WW around that time if they'd do an in-depth article on the subject. The editor just laughed.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Dec 2019 at 7:20pm
At 100 hours the SQ has lightened slightly, but hasn't gone in the usual direction of distorted highs, and bass isn't being masked by it.

I had to use the snap-in smoothers due to the single supply being hard wired, plus lack of space, and I think they are responsible for the lightened SQ.

In my opinion they're simply not musically captivating. OK, they'll do the job objectively, as measurements prove, but what about the enjoyment?

When employed in separate L and R supplies the snap-ins did a much worse job than the radial wire types in highlighting the stereo loop distortion. The radial wire types disguised it to some degree.
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