Bob Cordell Interview: Power Supplies - Page 4 - diyAudio
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Old 21st November 2006, 09:14 PM   #31
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Quote:
Originally posted by janneman
Hugh,

I have a problem with that. The speaker current ultimately comes from the first cap, the one closest to the rectifier. It therefore also needs to return to that. The ripple current may divide in some way between the caps but you can't say that one current goes to one cap and another to the other cap.

Jan Didden
The speaker current comes from the transformer, it has nothing to do with the Filter Caps, contrary to popular notions, it's return loop terminates at the transformer. It's all about the loop, this much is true.

Regards, Mike.
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Old 21st November 2006, 09:30 PM   #32
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Originally posted by MikeB


Bob, can you tell any number, what level of PSRR is recommendable ? Is -90dB a good number, or should it be even more ? (getting difficult)

Mike

The simple answer is as much as you can get without going to heroics that throw you off course.

There are many topological decisions you can make that affect PSRR. Always think carefully about where the current flows when the supply line changes voltage. Think about where you are dumping that current - are you unintentionally dumping it into a ground that wants to be quiet?

Of course, use current sources with a nice high impedance output and a supply-independent current-setting mechanism. Cascoding can be helpful.

Also be mindful of how you do feedback compensation. Common Miller compensation has rather inferior power supply rejection.

Differential circuits tend to have better PSRR than single-ended circuits.

My use of MOSFETs "forces" me to do something good: I always use a boosted B++ and B-- for the input and driver circuits so that I don't waste precious high-current main-supply voltage on gate drive and VAS headroom needs. This means that I cannot take the usual shortcut of just taking the main supply rails and R-C filtering them to get the rails for the input and driver circuits.

I always derive the boosted supplies with extra small a.c. windings connected at the ends of the main transformer windings and then feeding the resulting larger a.c. to a separate bridge rectifier to supply the raw boosted supply voltages. Those extra windings can be provided by a small extra transformer with two isolated secondaries on the order of 10-20V each, or they can be extra windings placed on the main torroidal power transformer.

Bob
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Old 21st November 2006, 09:45 PM   #33
gerhard is offline gerhard  Germany
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Quote:
Originally posted by Bob Cordell

Those extra windings can be provided by a small extra transformer with two isolated secondaries on the order of 10-20V each, or they can be extra windings placed on the main torroidal power transformer.
I'd prefer the small extra transformer for the "small signal" stages, and a 2-chamber non-toroidal type, because it has much less capacitance to the bad outside world.

Gerhard
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Old 21st November 2006, 09:57 PM   #34
gerhard is offline gerhard  Germany
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Quote:
Originally posted by MikeBettinger
The speaker current comes from the transformer, it has nothing to do with the Filter Caps, contrary to popular notions, it's return loop terminates at the transformer. It's all about the loop, this much is true.
If you see it this way, you must continue the loop at least to the generator in the next power plant.
The conduction cycle in the transformer can be surprisingly short, especially if you have heroic capacitor banks. The transformer will deliver current only when its secondary voltage is larger than the instantaneous capacitor voltage + rectifier drop. But when it delivers, it will be like feeding a short circuit. Add ultrafast rectifiers and you have a serious EMC problem.

regards, Gerhard
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Old 22nd November 2006, 12:08 AM   #35
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Hmmm... I feel like I baddly need some small devices to replace huge chokes for class A amps that consume 1300W...
May be, some switched mode devices...?
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Old 22nd November 2006, 01:58 AM   #36
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Quote:
Originally posted by gerhard


If you see it this way, you must continue the loop at least to the generator in the next power plant.
The conduction cycle in the transformer can be surprisingly short, especially if you have heroic capacitor banks. The transformer will deliver current only when its secondary voltage is larger than the instantaneous capacitor voltage + rectifier drop. But when it delivers, it will be like feeding a short circuit. Add ultrafast rectifiers and you have a serious EMC problem.

regards, Gerhard
Correct.

Bob
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Old 22nd November 2006, 02:39 AM   #37
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Quote:
Originally posted by gerhard


If you see it this way, you must continue the loop at least to the generator in the next power plant.
The conduction cycle in the transformer can be surprisingly short, especially if you have heroic capacitor banks. The transformer will deliver current only when its secondary voltage is larger than the instantaneous capacitor voltage + rectifier drop. But when it delivers, it will be like feeding a short circuit. Add ultrafast rectifiers and you have a serious EMC problem.

regards, Gerhard
Nope, I see the loop as starting and ending with the secondary winding. But then I see the power as being drawn from the secondaries with the filters being there only to prop up under heavy load. Charging and discharging the caps is is a slow process relative to the demands of the output transistors. The conduction cycle is a bit more complex than it seems and very much affected by the layout.

Why should one place the filter charging currents in series with the supply and the returns for the gain stages? Seriously.

The differences in approaches are measurable, audible and easy to verify.

EMC is minimized through controlling the loop (again) and is also easily measured (relatively).


Regards, Mike
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Old 22nd November 2006, 07:13 AM   #38
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Quote:
Originally posted by MikeBettinger
[snip]But then I see the power as being drawn from the secondaries with the filters being there only to prop up under heavy load. [snip]

But, Mike, the secondaries only deliver power for 20% of the time (less with higher cap values), so it is really the caps that deliver the power on an ongoing basis and it is the secondaries that prop up the caps now and then.

Jan Didden
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Old 22nd November 2006, 08:39 AM   #39
PMA is offline PMA  Europe
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Quote:
Originally posted by gerhard


I'd prefer the small extra transformer for the "small signal" stages, and a 2-chamber non-toroidal type, because it has much less capacitance to the bad outside world.

Gerhard
Good point. Toroidals are a bit of audio myth.
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Old 22nd November 2006, 01:31 PM   #40
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Quote:
Originally posted by janneman



But, Mike, the secondaries only deliver power for 20% of the time (less with higher cap values), so it is really the caps that deliver the power on an ongoing basis and it is the secondaries that prop up the caps now and then.

Jan Didden

I agree with you, Jan. However, I'm having a little bit of trouble visualizing Mike's approach.

I prefer to have some large capacitance (e.g., 1000 uF + 1 uF film + 0.1 uF ceramic) very close to the output transistors to force a tight loop for circulation of the output transistor currents for this very reason. Those currents are high, have fast edges, and are highly non-linear. As mentioned earlier, an X-capacitor combination from rail to rail can also help in this regard, since you really want to try to sum the positive and negative half-cycle Class-AB currents back to a linear current before they travel very far. And a little bit of impedance in the +/- lines back to the main reservoir capacitors can actually be a good thing. Then, the rectifiers replenish the reservoir capacitors, and the reservoir capacitors replenish the local output stage storage capacitors.

Bob
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