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Old 15th June 2013, 02:43 AM   #3561
Bonsai is offline Bonsai  Taiwan
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In a class B or AB amplifier, the rail currents are half wave rectified replicas of the output load current, with attendant harmonics. At any instant, only one of the rails is supplying current into the load. Therefore, twisting the supply cables together offers no benefit in terms of reducing the radiating area. In fact, you are likely to couple noise into the non conducting rail and any sensitive circuitry attached to it - filtering ( and even better, cap multiplier for conducted noise) and careful layout is the solution here. You can also use a sound card while dressing the cabling to find the null point (you will have to drive the amp hard while you do this). I have concluded that the best engineering solution is to co-locate the filter caps on the same PCB as the amplifier. This way you can keep the tracks short and the radiating area as small as practicable.
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Old 15th June 2013, 02:45 AM   #3562
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Originally Posted by kgrlee View Post
[*]local rail to 'dirty earth' electrolytics help both THD20k & stability
Chassis earth? Socket earth? Power ground? Is this unique to a specific grounding scheme, or will it apply to the usual scheme?
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Old 15th June 2013, 04:50 AM   #3563
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Default PS induced distortion & earths

Quote:
Originally Posted by Bob Cordell View Post
I seem to remember him being one who suggested not including the ground in the twist, but I don't recall if he made a scientific explanation for it or came to that conclusion experimentally. I do not know the technical argument for one way vs the other. I'll have to go back and look in Self'd book to see what his reasoning was and how big a difference it made whether the ground was included or not.
The relevant pic is Fig 6.9 'Countermeasures against the induction of distortion from the supply rails. 6.7b is usually more effective' in Self 4th ed.

His error is that he doesn't show the speaker return lead which is what completes the evil current loop. In fact he entwines his 'clean earth' with the PSU leads in Fig 6.9a which almost ensures it is within the evil speaker current loop. No wonder '6.7b is usually more effective' as this gets his 'clean earth' further away from these evil currents.

At the risk of repeating myself to this august audience ..
Quote:
'the +/- rails, the o/p devices, other stuff hung on the o/p' as ONE of a pair of leads to the speaker from the big PS caps. The other lead is the speaker return to the same caps.

To minimize mutual coupling, radiation etc. it's THIS pair of leads that need to be twisted, area within minimized, bla bla.

Ground planes etc only obfuscate the issue.
Ideally, the speaker return should follow the 'live' speaker lead back to the output devices and then the PS leads back to the PS caps .. all twisted of course
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Chassis earth? Socket earth? Power ground? Is this unique to a specific grounding scheme, or will it apply to the usual scheme?
I always define 3 earths; Clean, Dirty & Chassis. They are ONLY connected at one point which will be the star point on a power amplifier.
  • Clean is used ONLY for signal and feedback
  • Dirty is used for decoupling and other yucky stuff. At times this might have to run in close proximity or parallel to 'Clean'
  • Chassis is chassis
  • Speaker returns are a special case and returned separately to the star point. In a complex device like a mixer, you might have to do this with several stages with large 'signal' current ... or adopt some form of 'heirarchical' earthing in addition to this star point stuff.
The important point is to ALWAYS consider what happens if your 'earth' has finite resistance cos it HAS! I pontificate at great length on this for LN amps in the Yahoo MicBuilders Group. You have to join.

Also worth reading is John Roberts on the groupDIY and proaudiodesign forums. Us ex-mixing desk people are paranoid about such stuff.

Self doesn't have clear ideas about earths. Bob Cordell is better. see his Chapter 16.9

Last edited by kgrlee; 15th June 2013 at 05:16 AM.
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Old 15th June 2013, 04:56 AM   #3564
Bonsai is offline Bonsai  Taiwan
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I am in broad with you here kgrlee. I went to a lot of effort on my e-Amp to address these issues. My next amp will take some of the learning s from that effort and refine them further, hence my comment about the PSU bring co- located on the same board as the amplifier where you can really control these aspects
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Old 15th June 2013, 08:50 AM   #3565
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If you're going to put the caps right next to the OPS, why not make the heatsink a ground plane? Cover it with heat-resistant dielectric tape and cut out the areas for the outputs. Or, even undersize the cut-outs for the outputs and just fill the gap with a kind of thermal putty that doesn't run. I read about someone doing this with transparent laser printer paper; something thinner would be better. You have to choose the right thermal paste, watching the breakdown voltage.
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Old 15th June 2013, 10:23 AM   #3566
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Originally Posted by keantoken View Post
If you're going to put the caps right next to the OPS, why not make the heatsink a ground plane?
For power amps (and loadsa other audio), ground planes don't actually help the segregation of clean & dirty currents. In fact they usually encourage poor earthing practice. Clean audio is very different from evil digital in this.
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Old 15th June 2013, 10:54 AM   #3567
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Default CCIR IM & HD

More on chapter 22 which I really like.

Bob, in 22.4, you touch on the equivalence of the products of CCIR IM test with the order of the non-linearity.

In Jurassic times, I worked out the exact equivalent numbers for these vs 2nd, 3rd .. up to 7th Harmonic Distortion products and scribbled all this in our B&K 1902 manual which was used to measure CCIR IM with our B&K 2010 (This really dates me!) Scaling was as adjusted to exactly how this gear did the measurement.

My few remaining brain cells won't allow me to repeat the exercise But you might be able to persuade a tame Mathematician to do so for inclusion in your 2nd ed.

The exact relation exists for wide bandwidth devices .. ie if Closed Loop response is flat to 200kHz, there is a simple factor to convert each 20kHz Harmonic Distortion up to 10th order .. to the corresponding CCIR IM product(s).
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A useful point on doing CCIR (& other) IM is that you don't really need a supa dupa mixer. Just mix passively directly into the PA input. Two 10k metal film resistors have rather less distortion than even the best uber OPAs at the levels required to drive any PA to clipping.
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Old 15th June 2013, 10:56 AM   #3568
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bob Cordell View Post
Hi Andrew,

Ever since I read Cherry's paper long ago I have been concerned about the "half-wave rectified" high currents flowing in the output stage. The two main issues are to keep the loop area small and to electrically or magnetically sum these two currents/fields to a linear field representing the audio signal as close to their physical origin as possible.

One thing this means is that in output stages with multiple output pairs, the NPN and PNP devices should be interleaved, rather than grouped together by sex.

A second thing is that good bypassing sould be provided right at the rails on the board to provide a local low-resistance, low-inductance path to allow local summing of the currents to a linear one. This could even involve a local rail-rail X capacitor.

Note that this implies that some small resistance in the rail lines back to the supply is not necessarily a bad thing, and that the use of very heavy lines, as is often done, may be a well-intensioned but misguided approach.

A third thing, as we all know, is to twist the pos and neg rail wires together on their route back to the power supply, so that the magnetic fields of the two rails sum to a linear field.

There has been some disagreement as to whether the ground return back to the supply should be twisted together with the rail lines, or whether it makes a difference.

Here is an interesting idea, if not controversial. Since some resistance in the rail lines back to the supply is OK, and perhaps desirable, consider using a shielded twisted pair to take both rail lines and the ground back to the supply. Even 20 AWG microphone cable, at 33 mohm/ft, would probably be OK for this short route, even for transients of 10's of amps. The smaller wire allows for a much tighter twist, and shielding that line would seem not to do any harm and maybe some good. Note that a good copper-braided shield has a much higher effective AWG..........
I agree with everything in your reply except
Quote:
There has been some disagreement as to whether the ground return back to the supply should be twisted together with the rail lines
In my mind there is no disagreement. The PSU supply must be a twisted triplet. Your suggestion of using the shield as the return may be even better, I don't have any means to confirm that.
I have stated on this Forum every point that you have confirmed as worth consideration, even your "here is an interesting idea.
You and I are certainly not in disagreement.
I am currently reading H.Ott. He says much the same message, although I am learning that at VHF the practicalities are slightly more complicated.
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Old 15th June 2013, 11:44 AM   #3569
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bonsai View Post
In a class B or AB amplifier, the rail currents are half wave rectified replicas..............At any instant, only one of the rails is supplying current into the load...............
Quote:
Originally Posted by kgrlee View Post
...........error is that he doesn't show the speaker return lead which is what completes the evil current loop...............Ideally, the speaker return should follow the 'live' speaker lead back to the output devices and then the PS leads back to the PS caps .. all twisted of course...........
Exactly.
It's the Flow and Return currents that are balanced and it's these that must have minimum loop area.
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Old 15th June 2013, 01:14 PM   #3570
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Originally Posted by kgrlee View Post
For power amps (and loadsa other audio), ground planes don't actually help the segregation of clean & dirty currents. In fact they usually encourage poor earthing practice. Clean audio is very different from evil digital in this.
I definitely agree. I don't propose a ground plane as a way of cutting corners. The reason I'm interested in ground planes is that they shield the sensitive nodes and control parasitics. They make the parasitics more predictable and controllable. The relaxed restrictions result in greater performance in capable hands. High-performance circuits need shielding at their sensitive nodes.

Andrew, what are you reading? Any links? I would at least like to have some idea of how VHF really behaves.
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