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Old 24th March 2015, 10:18 AM   #1
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Default Class AB amp with booster capacitor supply

I have good-for-audio filter capacitors and many general quality capacitors. The audio-good will be used as main filters and other will be connected as boosters when the amp demands.

I listen to music at a moderate level which does not need big microfarads.

What happens if I use these as shown in the schematic.

Gajanan Phadte
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Old 24th March 2015, 10:41 AM   #2
AndrewT is offline AndrewT  Scotland
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Your supply voltage droops when demand increases.
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Old 24th March 2015, 10:58 AM   #3
AJT is offline AJT  Philippines
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first time i have seen such a scheme, and why not?
your reserve will be on standby most of the time, or maybe not...
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Old 24th March 2015, 11:46 AM   #4
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Quote:
Originally Posted by AndrewT View Post
Your supply voltage droops when demand increases.
It happens in any case.

Gajanan Phadte
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Old 24th March 2015, 12:01 PM   #5
Mooly is offline Mooly  United Kingdom
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I don't think it would work as you intend tbh. The two supplies are essentially in parallel, the diode gating doesn't really isolate them. As you draw more current you just pull both rails down together.
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Old 24th March 2015, 12:03 PM   #6
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Try simulating the circuit. I am convinced it will give you some interesting voltage and current wave forms.

Depending on the amount of capacitance of each part, there will need to be a enough current demand to drop the voltage by more than the forward voltage of the diodes.
The same with the charging currents to the capacitors, with the main capacitor bank starting to charge first.
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Last edited by Mark Whitney; 24th March 2015 at 12:15 PM.
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Old 24th March 2015, 01:29 PM   #7
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Old 24th March 2015, 05:51 PM   #8
CBS240 is offline CBS240  United States
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Forget the second power bridge. It really is transient current demands that a large capacitor bank works to provide. Just use the main filter capacitors and follow them with small resistors, and then to the rest of the capacitors in parallel. IOW, CRC filter. As long as there is a series resistor to the large capacitor bank driving the circuit, the smaller ones will not surge enough current to overload the bridge/transformer during power up. (hopefully ) Also important to keep AC power current (100/120Hz) and audio signal currents from flowing in the same PCB tracks or wires, thus aiding PSR. It is important that layout to the ground point also separates the return currents for these caps. Don't forget that the supply capacitors are part of the amplifier.
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Last edited by CBS240; 24th March 2015 at 05:57 PM.
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Old 24th March 2015, 08:48 PM   #9
Bigun is offline Bigun  Canada
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To ensure that the good quality caps are the ones that the amplifier experiences for best sound you want them to be where the signal current flows. The signal current will flow through the path of least resistance, as CBS240 says. This means you want to place the audio quality caps close to the amplifier itself, ideally on the same PCB or very close by (but away from hot heatsink) because this minimizes the inductance of wiring between the caps and the amplifier - in other words it minimizes the impedance.

Between the filter caps and the amplifier local caps you can put some resistance like CBS240 suggests or use chokes to isolate the two capacitor banks in order to keep the signal currents out of the main filter caps (except for low frequency). I used a pair of common mode chokes wired to ensure that the dc current through the chokes is balanced: TGM9 - my version of the JLH '69 Class A amplifier

And put the rest of the filter caps further away from the amplifier, near the rectifier and transformer so that the charge pulses are going through short wires that are twisted together to minimize any noise coupling into the amplifier.
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Last edited by Bigun; 24th March 2015 at 08:50 PM.
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Old 25th March 2015, 04:02 AM   #10
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One advantage of my scheme I can envisage is that the short charging currents to the main filter, from the transformer will be reduced in current amplitude due to an help from the booster capacitors.
Definitely there will be some very odd charging waveforms according to the value of the caps and the inductance it sees. This will be differing according to the load currents and will be highly unpredictable with music transients.

Gajanan Phadte
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