Preamp o/p cap values.....confussed

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Gerry
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#1 Preamp o/p cap values.....confussed

Post by Gerry »

Hi all
Been a while....
A little confused on pre-amp out cap values.
I have a ARC LS7(circuit schematic) which has 2.0uf o/p cap bypassed with 0.01uf. (stock)
The power-amp has an input Independence of 150Kohms (unbalance) and 100Kohms (balanced).
I use it unbalanced.

Reading an old thread on Pinkfish re upgrading the LS7 the suggestion was to increase the o/p cap values to 10uf (better l/f response).
I would try:
1. Increase output cap to 10uF, put something in the level of Mundorf MCap Supreme or similar.
2. Replace the old-fashioned high-voltage regulator with the very best one you can possibly put together.
3. Split high-voltage power rails to L,R, input-stage and output stage, total of 4 regs.
4. Consider reducing the gain by changing FB resistors.
The V-Cap CALCULATOR suggests the opposite and o/p cap value of 0.53uf!!
I have briefly re-read Morgan Jones' Valve Amplifiers who suggests the same, a much lower value. (Jones seems to suggest partly it is a cost reason).
Why?
Surely as wide a frequency response is what one should be looking for so 5hz (2.1uf), better than 20hz (0.53uf)? Though I do note the V-Cap argument about too much capacitor can effect sound.
Any thoughts?
Regards
Gerry
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IslandPink
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#2 Re: Preamp o/p cap values.....confussed

Post by IslandPink »

Depends ... :D
Depends whether this is a phono amp or an actual pre-amp that's in the chain after a phono amp.
In the case of a phono amp like mine I don't have a pre-amp in addition, so I size the cap fairly small ( 0.68uF ) and it just adds a bit in terms of rumble cut-off to the other low frequency corner which is present in the coupling cap after the RIAA bit. The RIAA coupling cap/grid-leak bit is set low-ish , around 5 or 6Hz, so the coupling cap on the end of the phono just adds a bit more but it's still under 10Hz.
If this is a pre-amp then you might not want it to add much LF cut at all, so in that case 2uF or more makes sense.

On the flipside ( that's American lingo ) there is a break-point in cap quality above 1uF in many ranges, where you have to move from film & foil ( eg. Mundorf ZN ) and go to metallised film ( eg. Mundorf silver/oil, Ansar ) & the transparency drops a bit.
For a very high quality bigger cap my best advice is a Black Gate 22uF VK .... if you have a couple in your stash :D
Otherwise I'd probably advise a GE polyprop/oil 'can' cap of 10 or 15uF combined with a Mundorf ZN .
This all assumes the valve in the preamp is a decent current eg. 10ma or more.
If it's a more weedy valve then maybe a small electrolytic might be better if you want vales above 2uF .
"Once you find out ... the Circumstances ; then you can go out"
Gerry
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#3 Re: Preamp o/p cap values.....confussed

Post by Gerry »

IP

Thanks.
It is a preamp.
I swapped out the original for PIO (2.0) with Teflon (0.1)bypass.
The added another 2.0 (PIO) as I had one following comments and the calculator, trying get as much bandwidth as possible.
Regards
Gerry
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Mike H
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#4 Re: Preamp o/p cap values.....confussed

Post by Mike H »

If following amp is 100k input, 1uF is plenty for most practical purposes. My personal preference is ClarityCap polypropylene film, and available with values up to several tens of uF. And don't need extra small value bypass caps IMHO.
 
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