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Old 28th June 2005, 03:51 PM   #1
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Default Re: Recommendation on capacitor value needed

I am changing the e-cap on my pre-amp at the differential input stage. The rail is a + - 15V regulated and the capacitor is 330uF 25V. I wish to change the pair into Black Gate FK series but it also has 470uF 16V or 220uF 25V. Can anyone tell me which value would be better for a tighter bass?

By the way, I am also considering to change the coupling caps on the pair of differential opamps using OPA627BP. The current value is 0.047uF WIMA. Is there any suggestions on which brand/value will give a tighter bass? Please bear in mind the mid and high are just enough and I don't want to loss them.

Thanks
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Old 29th June 2005, 12:37 AM   #2
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Most differential input amps have a high enough input impedence that a coupling capacitor at the input need only be 1f - 4.7f, and the vast majority of amp use a value in that range (although I saw an amp here a few days ago using a 1000f input coupling cap ).

There may be a point where too large a value results in inferior sound...then again, if the input impedence of your amp is really really bad, you might need a input cap larger than normal, but even then anything over 47f for an input cap seems huge and would indicate...if that value was necessary to sustain bass...faulty design.

Perhaps you'd like to provide some info on the amp...
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Old 29th June 2005, 12:43 AM   #3
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Best input cap is no input cap!


Better design

Greg
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Old 29th June 2005, 12:53 AM   #4
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Tough to do with a bipolar input stage. FET maybe...
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Old 29th June 2005, 02:47 AM   #5
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Hi Echo Wars,

"Tough to do with a bipolar input stage. FET maybe..."


Not so. A simple complementary bipolar stage with matched Hfe,
OR bias cancellation as used by Linear Tech. in their LT1028 low noise bipolar input op amp -which I used as a MC/MM RIAA preamp direct coupled to the cartridge with absolute success.

The other big advantage of no input C at preamp input levels is less hum and noise stray pickup as the input node is then at source impedance not raised by Zc.

Cheers,
Greg
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Old 30th June 2005, 08:13 AM   #6
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I don't mean for a moment to suppose that a cap-coupled stage is preferable to a DC stage (maybe for some so they may sleep better knowing DC is blocked), but Sunsun22 (who seems to have abandoned the thread) appears to be battling an existing design. In that case, I was simply trying to steer him in the right direction...

...I mean, ya gotta admit, the size of input caps he's discussing are a tad unusual...
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Old 30th June 2005, 03:53 PM   #7
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I was traveling in the past 2 days therefore didn't have time to catch up this thread. Thank you for all the inputs but I'm sorry that I'm using the wrong terminology. The 0.047uF are not coupling caps but instead, being soldered to the +ve to ground and -ve to ground of an opamp in the differential input stage. You can go to my thread in Chip Amp telling my experience on how a passive pre-amp out-performed my Mark Levinson 38S preamp. The attempt of this thread is to look for a swap in caps to tighten the bass of my preamp. The current value is 0.047uF and the caps in use are Vishay (blue in color). I'm figuring to use either Auricap or, at least, Solen to replace them hoping to get a desired bass output. Would appreciate your inputs in this respect.

Sunny
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Old 1st July 2005, 12:38 AM   #8
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Hi Sunsun22,

Do you have a schematic of the preamp - without that we're guessing. The product may have been poorly designed in the first place by a staff engineer with little audio knowledge, loosely based on prior competent designs by a consultant. My experience.



Cheers,
Greg
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Old 1st July 2005, 01:22 AM   #9
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The preamp that I'm talking about is a Mark Levinson 38S. I've did a number of modification on it by following Peter Daniel's project. I WAS pretty happy with the improvement UNTIL I met the passive pre-amp from Django (please already refer to my other thread in Chip Amp).

Since then, I've changed 8 caps to BlackGate FK series and it sings after a few days of buring in. The mid and high and more open now but the bass still cannot produce the tight and deep effect of the passive preamp. That's what I'm aftering.

Thanks
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Old 1st July 2005, 01:26 AM   #10
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Is it phono RIAA you are listening to? If so it could be too much rolloff at the LF pole (20Hz) which is improved by setting this lower, say 10Hz!

Cheers,
Greg
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