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Old 1st September 2012, 09:33 AM   #941
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Quote:
Originally Posted by magnoman View Post
A resistor paralleled by a diode sort of fits that. Just saying the resistor and 2nd stage capacitor forms a filter, but when higher current is demanded (low frequency) the diode conducts and the capacitors appear in parallel. I just mentioned this suggestion to add to the previous posts. I haven't actually done this and would have to think about all the implications before I really used or recommended it.
C,R//D,C? Catch diodes for the CRC? Sure. I've done it a lot.
With the forward voltage drop of the diode (none are zero), the filter effect never disappears.

Parts:
The diodes usually have to be heatsinked (especially 2 silicon diodes series for ~1.35v or similar scheme can incur the need of heatsinking). The thermal runaway needs bolted down, else your voltage drop shrinks a lot. However, very low power amplifiers (like a T amp) don't require as much voltage drop, 1 diode with 1R can do, and that doesn't need heatsinked.
Probably, you won't need high wattage resistor with such little voltage drops.

Tuning:
You need to to arrange the resistor current versus diode drops so that the harmonic differentiation from the diodes switching to the bass beat happens corresponding to the amplifier run at near maximum, in which case you've cancelled some blare in trade for a more open sound. That is the practical use of this filter. I've found this particular power filter, C,R//D,C quite helpful during MP3 playback.

Caveat:
You don't want the diodes to switch on too early (proportionate the amplifier output). Insufficient catch diode drop at the "R" of the CRC for a high power amplifier could be a problem necessitating increased resistor current for less effective CRC filter. . . exactly the same caveat as an ordinary CRC. A partial solution is to increase the diode drop and that may incur the need of heatsinking the diodes.

Case:
We could say that power supply reservoir size of a CRC power supply could be decreased by either shorting/bypassing the resistor to remove both the loss and the filter, or adding catch diodes to constrain the loss. This infers that the loss of a CRC was present to begin with. Decreasing loss works like increasing transformer VA.

Thanks:
Thank you for reminding me that if I'm going to install any sort of CRC filter (including C,R//D,C), I'll need a bigger transformer to make up for the losses of the filter. Generally, I like the catch diodes for decreasing this problem, but its no substitute for buying a big enough transformer. It seems that only a plenty big transformer avoids the need of epic capacitance.

P.S.
Series filters didn't decrease needed power supply reservoir size. Howabout parallel filters? Will they do it?
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Last edited by danielwritesbac; 1st September 2012 at 09:36 AM.
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Old 1st September 2012, 10:18 AM   #942
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new science? or just waffle?
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Old 1st September 2012, 10:48 AM   #943
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Quote:
Originally Posted by DF96 View Post
No, a diode won't do it - it is not frequency sensitive. All a diode does is turn what you think is a smoothing cap into a second reservoir cap. A low value resistor would be better than a diode.
I have seen a lot of old time designers (me included) place a diode and small resistor in series followed by some caps. My thoughts were that the diode will block current being "sucked back". The series resistor would form part of an LP filter also.

Any comment on using both as explained?
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Old 1st September 2012, 10:51 AM   #944
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or you can use the diode alone.

It's Daniel's diode parallel to the Resistor that is weird.
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Old 1st September 2012, 11:04 AM   #945
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I've not tried that one, Daniel - easy enough to do, waffle or not!

What I did try a bit differently but also a bit weird was to add an extra diode between the bridge and the first cap and added a // R in a similar power sharing scheme - I first saw this in one of John Brown's 1541A dac power supplies and tried it on the F3 power amp - a definitely change but not sure if it was an actual improvement - added a further RC snubber across that and seemed a bit "quioeter" - very dependent on the diodes but no discernable effect with different size caps from 4700uF, 6800, 10mF and 15mF, apart from different types (ie Nichi FG, Elna for A, Siemens, BHC, etc)

Tried much of these variations on the supply of the F3 amp that has a Cmultiplier after the supply and quite surprised that nearly all of the variations were quite so clearly audible, altho using the headphones better test.
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Old 1st September 2012, 11:06 AM   #946
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If you want smoothing you want current to be 'sucked back' when appropriate. A diode ensures that sharp upward movements in the first cap get transfered to the second, so no smoothing. I have seen so-called 'diode isolation' in circuits, but I have never been convinced that it works. I suspect it is a popular myth.

There is one place where a diode might do some good: isolating a separate supply rail for the earlier part of the circuit if you want to guarantee that it doesn't droop as much as the main supply rail during LF peaks. The aim is not smoothing.
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Old 1st September 2012, 11:40 AM   #947
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Quote:
Originally Posted by DF96 View Post
If you want smoothing you want current to be 'sucked back' when appropriate. A diode ensures that sharp upward movements in the first cap get transferred to the second, so no smoothing.
The resistor paralleled to it stops that problem--the diode is off except when the amplifier is near full blast.
Edit: For a larger amplifier this can take more than one diode in series to get appropriate voltage drop.
Quote:
Originally Posted by DF96 View Post
I have seen so-called 'diode isolation' in circuits, but I have never been convinced that it works. I suspect it is a popular myth. There is one place where a diode might do some good: isolating a separate supply rail for the earlier part of the circuit if you want to guarantee that it doesn't droop as much as the main supply rail during LF peaks. The aim is not smoothing.
Tha's a different topic entirely. Left, right channels schottky outputs could be added to the power board for broadening a stereo amplifier. Right, The aim is not smoothing. Apples versus oranges. That's a different circuit.

Perhaps it is useful that the amplifier board bypass caps don't need to fight with all of the other power caps?

P.S.
If I did All of that to a dual secondaries supply, it would have 16 diodes (8 for dual rectifiers, 2 for 1.4v V+ catch diode, 2 for 1.4v V- catch diode, and 4 schottky for outputs). How many audiophile sound effects does one power supply need and who pays that much attention to power supplies anyway? I think a lot of that enhancement is possibly noise, up until trying to replay an mp3, whereupon the enhancements could be of service.
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Last edited by danielwritesbac; 1st September 2012 at 12:07 PM.
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Old 1st September 2012, 01:19 PM   #948
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I think this parallel R and D is a bit of a tangent to the thread, but I must be missing something.
This approach simply allows one to use a larger resistor value for filtering and not incur the IR voltage drop during peak currents.
At lower power there is just an RC filter, for the higher peak currents the 2 capacitors act as one reservoir, the resistor also acts to dampen potential resonances between the two capacitors.
Dont know of the overall benefit, as stated it adds some R to the filter during low power times.

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Old 1st September 2012, 01:36 PM   #949
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Quote:
Originally Posted by magnoman View Post
I think this parallel R and D is a bit of a tangent to the thread, but I must be missing something. This approach simply allows one to use a larger resistor value for filtering and not incur the IR voltage drop during peak currents. At lower power there is just an RC filter, for the higher peak currents the 2 capacitors act as one reservoir, the resistor also acts to dampen potential resonances between the two capacitors. Don't know of the overall benefit, as stated it adds some R to the filter during low power times.
That's about it.
The resistor can be set same as for an ordinary CRC. In this case the catch diodes (usually a series pair unless the amp is tiny) just make the CRC's voltage drop more predictable despite the reactive speaker load. Reducing loss reduces needed power supply reservoir size.
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Last edited by danielwritesbac; 1st September 2012 at 01:51 PM.
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Old 1st September 2012, 01:40 PM   #950
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R//D is a non-linear CRC filter. Electrically its a nice tradeoff - extra LF ripple rejection (higher R) at low currents, lower impedance (less LF ripple rejection) at high currents.
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