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Old 4th May 2012, 12:23 PM   #901
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
Elvee, I have a simpler question: How do we adapt the inputs of Circlophone for supporting single rail operation?
Here is a way to do it.


I fact, I have already done it myself, because I wanted to upgrade an old amplifier with Circlophone boards.
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Old 4th May 2012, 01:23 PM   #902
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I am tempted to ask for the inclusion of the output cap in the feedback loop.
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Old 5th May 2012, 01:23 AM   #903
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I wanted to try for auto-center at the output because a speaker doesn't need any sort of DC reference, and I have two single transformers not quite match for either voltage or current, yet wired as a single center tap and thus a trimmer doesn't have any right setting; and I have both a grounded amp and a grounded source with the grounds not quite match either.

So, does this (attached) scheme work with that scenario?

(the purpose of the attached schematic is to ask a question)
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File Type: gif Circlophone-Auto-Center.gif (30.9 KB, 368 views)
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Old 5th May 2012, 06:52 AM   #904
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Quote:
Originally Posted by Elvee View Post
Here is a way to do it.
I fact, I have already done it myself, because I wanted to upgrade an old amplifier with Circlophone boards.
Exactly what quite a few of us would like to do! Thanks, Elvee!
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Old 5th May 2012, 10:29 AM   #905
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
I wanted to try for auto-center at the output because a speaker doesn't need any sort of DC reference, and I have two single transformers not quite match for either voltage or current, yet wired as a single center tap and thus a trimmer doesn't have any right setting; and I have both a grounded amp and a grounded source with the grounds not quite match either.

So, does this (attached) scheme work with that scenario?

(the purpose of the attached schematic is to ask a question)
I see a number of problems there:

-Input and output are referenced to two different capacitive dividers; in an ideal world, it might work, but in practice, any mismatch between the capacitors will introduce ripple, and Elytics are not famed for accuracy.

-The input reference is created by 10K resistors dividers, and there is no blocking cap in series with R16, which means the DC gain will be high, and will amplify any offset disproportionately. Even with an AC coupled output this might cause troubles.


I think a better solution is to jettison completely the (pseudo) center tap, and act as if it was a true single supply. The scheme given in #901 then becomes applicable and suffers from none of these problems
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Old 6th May 2012, 12:27 AM   #906
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Default Meanwhile, back to Split rail. . .

Thank you.

I was also wondering what sort of Split rail power supply you'd like to see used with Circlophone? For the builder's thread, it would be nice to have such an example.

I guess you would like a split rail CRC whereby R could be cable so as to get the "clean pair" very close to the amplifier boards. The boards have very little capacitance and so the speaker return forces needs someplace (else and nearby) to go.

I'd favor a polyester dip cap (or equivalent RC) as a load upon transformer secondaries, but I don't know what approximate values to suggest.

A crazy-simple start on a power circuit could be a pair of KBPC3502 (one per each secondary) with a large cap stuck right to each rectifier (conveniently cooled and mounted by bolting the rectifier to a surface). After that some resistance or cable to get the power near the amp boards and then. . . more cap(s) whereupon that is the location of power star and speaker return. That is applicable to both compact builds (might want a real "R"?) and separate enclosure for amp and power (interlink cable is "R"). I can't guess what values you'd like or even if this is a structure you'd like.

Perhaps you favor a fun schottky rectifier and if so, I'd like to hear about it--variable voltage drop isn't a bad thing since it can help sound be much more live if it doesn't also make noise. Noise? Well, I'm going to need some help for low noise.

Thanks again!
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Old 6th May 2012, 10:16 AM   #907
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
Thank you.

I was also wondering what sort of Split rail power supply you'd like to see used with Circlophone? For the builder's thread, it would be nice to have such an example.
Keep it as simple as possible: transformer, rectifier bridge and 2 x 4700µ per ~60W channel is OK.
No need for something unusual or fancy
Quote:
I guess you would like a split rail CRC whereby R could be cable so as to get the "clean pair" very close to the amplifier boards. The boards have very little capacitance and so the speaker return forces needs someplace (else and nearby) to go.
Anything that increases the supply impedance should be avoided, and this includes the R in the CRC.
The on-board capacitors are sufficient for the HF return, and if there is a good wiring to the main filter caps, the supply will be nice and stiff at all frequencies.
With electrolytics, there is no need to damp parasitic inductance, which means there is no need to introduce additional resistance in the circuit
Quote:
I'd favor a polyester dip cap (or equivalent RC) as a load upon transformer secondaries, but I don't know what approximate values to suggest.
Such a capacitor is not needed, and if used indiscriminately could do more harm than good.
A snubber (10Ω + 220nF) could be useful if RF bypass capacitors are placed across the bridge's diodes.
Small ceramic capacitors (4.7nF) mounted directly on the diodes can be useful to suppress the HF noise caused by switching, and the PIN-diode-modulator effect in the presence of strong electromagnetic interference.
But without damping, they will cause ringing in the transformer, and this can be eliminated by using an appropriate snubber.

Quote:
A crazy-simple start on a power circuit could be a pair of KBPC3502 (one per each secondary) with a large cap stuck right to each rectifier (conveniently cooled and mounted by bolting the rectifier to a surface). After that some resistance or cable to get the power near the amp boards and then. . . more cap(s) whereupon that is the location of power star and speaker return. That is applicable to both compact builds (might want a real "R"?) and separate enclosure for amp and power (interlink cable is "R"). I can't guess what values you'd like or even if this is a structure you'd like.
See above

Quote:
Perhaps you favor a fun schottky rectifier and if so, I'd like to hear about it--variable voltage drop isn't a bad thing since it can help sound be much more live if it doesn't also make noise. Noise? Well, I'm going to need some help for low noise.
The only benefit of schottky's in a well designed supply is the lower voltage drop.
Any reverse-recovery effect of standard diodes can be addressed by suitable bypass capacitors.
In summary, the main design rule of a Circlophone supply is KISS and no-nonsense.
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Old 6th May 2012, 06:36 PM   #908
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Quote:
Originally Posted by Elvee View Post
... In summary, the main design rule of a Circlophone supply is KISS and no-nonsense.
I couldn't agree more with Elvee's reminder about the KISS formula, and experience tells us it often is the best approach.
Still let me share with you the approach of one of my junior friends vis-a-vis a 'belt and braces' power supply for his 'Gainclone' (LM3886) amp, out of which he wanted to squeeze the best possible performance. Though many agree that the chip amp performs well with just a couple of thousand microfarads of filtering, this PSU with its large capacitor bank audibly improved the low end and the transient performance, while at the same time it "took away" something from the amp. But when the cap bank was isolated with a couple of diodes, the performance had both the goodness of the 'light filter' as well as the 'ease' provided by the heavy cap bank. I dont know what to make of it.

Perhaps Elvee would be kind enough to examine the question in detail and give us his recommendations.

Though that shouldn't bar us from experimenting with all sorts of combinations, looking for an audible improvement...
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Old 6th May 2012, 09:09 PM   #909
jerryo is offline jerryo  Isle of Man
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Is there any possibility of a pcb group buy being set up by someone for this amp with a finalised BOM and build manual?
It seems to me that it would be a good addition to the amps offered as group buys normally.
Just a thought. I would be interested if there was a group buy.
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Old 7th May 2012, 11:39 AM   #910
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Quote:
Originally Posted by The Prof View Post
. . . But when the cap bank was isolated with a couple of diodes, the performance had both the goodness of the 'light filter' as well as the 'ease' provided by the heavy cap bank. I dont know what to make of it.
If I squint at that, it is a CRC with "D" instead of "R" and so there is some voltage drop but enforced to a regular amount.
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