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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: Yesterday at 10:18am
A rather big blunder...

Dinsdale did it; Reg Williamson did it; scores of others did it; and this time I did it!

"an inevitable consequence (of rail decouplers) is that rail-voltage variations cause current to flow into the ground connection" (Self. D, annotated by me)

See images:



Above image subject to copyright: Doug Self, Audio Power Amplifier Design Handbook, 5th edition
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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 11:57am
But is there still room for the stereo loop resistor? Does the ground distortion introduced by the decoupler, having been solved, remove stereo loop distortion?

Theoretically it must still exist simply because the smoothers have ESR (and ESL), and ground here is one of the HT rails (where it isn't in a DC coupled amp).

There is also the problem that ground being -HT it cannot be decoupled to itself. Therefore, it could be thought that we've only gone half way, and that is probably true.

If this is the case, then the ground will still want to shift slightly in sympathy with the current being drawn by the load, and taking the decoupler away from the star is simply kicking the can down the road - but audibly, it helps.

So, this current is bound to flow contra from one channel to the other (via the input ground connection).

This current has been reduced by removing the positive rail distortion (being unregulated) further from the star ground, but there is still the pull due to the load (the speaker) which is different to the other channel.

An early RCA design, and the Boosey & Hawk variant used -HT resistance mixed with output stage voltage at AC. If you think about it, all rails are the same at AC. But their's was probably a grounding blunder like above, as it didn't feature in later designs.

The stereo loop resistor can have unwanted consequences including a phasing effect which can induce tinitus. The larger the resistor, the worse this gets.

There is also the stability issue if not decoupled at very high frequencies. In this single rail design the decoupling is back to the star, where in the RCA it was to ground (dual rail supply remember).

10 ohms produced the worst phasing; 1 ohm did hardly anything, but sounded better than nothing; 6.8 ohms sounded least distorted but some phasing evident; 3.3 ohms was just about right.

With the rail distortion "half improved" perhaps 3.3 ohms will do the rest?

But it would be good to do this scientifically. If only we knew how.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 1 hour 6 minutes ago at 7:26pm
Commercially recorded vocals can contain occasional sibilance due to microphone proximity being too close. Essing as it is called can cause clipping of the microphone signal making the sibilance worse. In replay the amplifier will attempt to reproduce the signal which may be spread between left and right channels but might not be same in both channels, and might have been mixed over to one side using stereo panning.

A signal rich in such sibilance to which is added regular high hat and cymbal strokes, will have the output stage fluctuating wildly the current being drawn from the power supply, which will be in sync and out of sync with the current being drawn by the other output stage.

These signals ride on the low, more regular rhythmic sound frequencies. The power supply is therefore not optimal for both channels together, and the HT can fall such that its voltage is in the dips of ripple. The current flowing in the return - the 0V power supply connection - will be different in one channel with respect to the other, and rapidly changing.

The unavoidable loop formed at the input ground (the common connection shared by both left and right signals) will have current being pulled for one channel through the opposite channel's input ground, and this is an asymmetrical modulation voltage. Not only does the crosstalk suffer, the signal is badly distorted.

Breaking the connection by 10 ohm grounding resistors simply moves the ground common to the source equipment, and often the connection is made at the source's input. If that is a phono cartridge input, the signal will suffer a greater degree of distortion than a line level signal.

Should the 10 ohm resistors go open circuit due to mains breaking through because of a burning-out transformer, there is no earth protection, and the source equipment not being earthed can become live.

10 ohm input grounding resistors, although often used, should never be used.

Because of the resulting direct common (using the safe method) the loop cannot absorb the "current jolts" because of the music being played. The input ground wiring being the weakest point, will distort the input signal.

Shifting the decoupler ground helps reduce this effect, but cannot resolve it completely, as the outputs to the loads depletes the HT unequally. The complete cure is to use monoblock power amplifiers, but in designing a stereo amplifier there has to be some clever solution to achieve the same.

Separate transformers for each channel help greatly because they isolate the 0V's at the power end of each amplifier. In actual fact they don't fully isolate the 0V's because this is AC, and zeros are only relative, so the grounds actually connect at the amplifier mains input. This being the reason for supply polarity affecting, more or less, hum in some phono equipped, double-insulated integrated amplifiers.

Where separate transformers cannot be accommodated in say a compact case design, we could use dual secondary’s, but in effect the secondary’s are joined in parallel at AC without any external connection. This is because of mutual inductance - the intimate contact between secondary’s and the primary - and is why secondary inductance does not obey the parallel inductors rule in a transformer.

The "invisible" connection between "isolated" secondary’s is evidenced when adjusting quiescent current and output voltage centre point to minimise distortion, as adjusting one channel effects the other channel to some degree.

Therefore, in a dual secondary supply there is still power-end 0V commoning, and therefore some stereo-loop distortion might make itself present.

In a dual rail supply amplifier some averaging might take place which assists the power end 0V remaining at zero, but not in a single rail supply.

At worst the stereo-loop distortion could be blamed on other things, and I'm sure it has, and I think the output capacitor has often taken the blame. If you think about it, if stereo-loop distortion is worsened by a single rail supply, and single rail amplifiers have an output capacitor, then it might be seen as prime suspect.

Bringing back the 0V end of the decoupler via a separate trace demonstrates where the "levers" for this distortion are located, and tends to cement the argument for the existence of stereo loop distortion.

If we then add Dinsdale's observations, we can be reasonably confident that we've found the source of the problem.  Inserting a resistor between voltage and current stage 0V's (the VAS 0V to the star) changes the asymmetrical ground current that would flow the wrong way around the loop, into a voltage, which is an error voltage which the global negative feedback corrects for.

The problem in doing this, the mixing of ground currents between left and right, means we have created an accidental stereo-width enhancer.  It is enhanced by the asymmetry, and if too much is used, we will arrive at left-right distortion again.

As Dinsdale commented; "Although this causes a slight increase in overall distortion on mono signals the improvement on stereo signals is impressive". I think this is an indication of the accidental stereo-width enhancer to which I just referred.

The hundreds of millivolts Dinsdale also refers to are in part due to the grounding regime, which was improved to nearer what Doug Self found. This was in a later issue I have not been able to find yet. Dinsdale considered it sufficient to dispense with the resistor. However, when trying this myself I have felt cross-channel distortion is still present.

If the improved grounding regime leads to a smaller distortion voltage of only a few millivolts, then a smaller resistor (than 10 ohms) should suffice, and perhaps the extent of stereo "enhancement" will also be less? In experiments using the 10 ohm resistor I was aware of sufficient mixing because it gave rise to a phasing effect which induced tinnitus.

With this "large" value, compensation by a bypass capacitor was also required to maintain gain margin. The value of capacitor gave a frequency far beyond the audio spectrum, so I don't think that contributed to the phasing effect. Smaller resistor values improved the subjective results, and I am now at 2.2 ohms, having gone as low as 1 ohm, where in my opinion, stereo-loop distortion rose again.

Returning now, to the transformer dual secondary's, and that paralleled windings indicate the same inductance as a single winding. If this was exact then it would indicate zero impedance between the two, thus making the power supply no different to one where both channels are supplied from the same secondary.

However, there are losses in all magnetic circuits, and the inductance isn't 100% the same for parallel versus single - there is a very slight difference in measurement. The inductance meter measures at a frequency which allows it to provide the most accurate result, this not being mains frequency, but around 1kHz.

There might be sufficient difference at other frequencies such that the impedance between secondary’s varies from zero. In that case the loop-resistor value can have very different effects at different frequencies. It was at this point I started to consider bridging the secondary’s which effectively makes it a single, but higher VA, single secondary winding.

So now it's time to try that.
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