From: mark_zacharias@sbclobal.net
"GregS" wrote in message
news:hpvniq$nlo$1@usenet01.srv.cis.pitt.edu...
> In article <0001HW.C7E8AAB70005E1C4F01846D8@news.giganews.com>,
> audio_empire@comcast.net wrote:
>>On Mon, 12 Apr 2010 05:47:14 -0700, Arny Krueger wrote
>>(in article ):
>>
>>> "cjt" wrote in message
>>> news:4BC289A0.3070103@invalid.invalid
>>>> Spica wrote:
>>>
>>>>> I'd like to increase the filter capacitance of a 21 year
>>>>> old subwoofer amp from the current two 10,000uf 75v caps
>>>>> to a pair of 20,000uf 75v caps. The current rectifier
>>>>> is a Fagor FB2502. Do you think this rectifier can
>>>>> handle the increase or should I upgrade it? Also, for
>>>>> reference, the amp uses a single power chip per side as opposed to
>>>>> discrete xistors if that matters. Thanks.
>>>
>>>> Why?
>>>
>>> Good question.
>>>
>>> Just about the only reasonable justification I can think of is that the
>>> existing caps have lost significant capacitance. This may or may not
>>> happen.
>>> I've seen electrolytics that were still well over 100% of nameplate
>>> capacitance after over 40 years, and others that were under 10% after 3
>>> years.
>>>
>>> YMMV!
>>>
>>> 10,000 uF is actually quite a bit of overkill - a minimum number might
>>> be
>>> 2,500 uF - 3,300 uF.
>>>
>>> The real question is "what sounds bad"? I suspect that most "capacitor
>>> upgrades" satisfy psychological and social needs, not technical needs.
>>>
>>>
>>
>>My experience is that MOST of the time, second-guessing a piece of
>>equipment's designer(s) results in retrograde performance. SOMETIMES in
>>some
>>(admittedly rare) instances, so-called upgrades and tweaks are worthwhile,
>>but not usually. I replaced some crossover caps in a speaker system with
>>'Wondercaps' of the same value once and speakers immediately sounded
>>cleaner,
>>more transparent. I replaced the LM301 op-amp in an old Crown IC150 preamp
>>with a higher-slew rate pin-for-pin compatible J-Fet op-amp once and got
>>measurably lower distortion (using a borrowed high-end distortion
>>analyzer),
>>but I couldn't actually HEAR any difference. It's a crap shoot. And
>>honestly,
>>the designer generally knows what he's doing and has chosen the values for
>>his filter caps for a reason. It might be something as simple as the
>>determination that a larger value adds only expense to the final product,
>>without adding any measurable or discernible improvement in performance,
>>or
>>something more critical, like the values chosen matched the rectifier
>>diodes
>>and a significantly larger value will over stress them. It's best to leave
>>things alone. If you have determined that the current caps are leaking, or
>>have failed in some other way (resulting in hum), I suggest that you
>>replace
>>them with caps of the same value. Most likely "more" will not equate to
>>"better".
>
>
> I had also considered replacing the Crown op-amp but I didn't think it
> would
> be worth it. Seems like there are many wanting to fool with the filter
> caps, and I
> have never arbitrarily replaced any after working on dozens and dozens of
> amps
> and receivers.
>
> As for amps, a cheap transformer and big caps owe more to peak music
> power,
> and good transformers and moderate caps tend to work out best. Upgrade the
> caps
> if you like. The peak current of the diodes is going to be the peak of the
> transformer,
> not so much the actual capacitance. Of course I make amplifiers with
> a soft start, with a relay and resistance.
>
> greg
>
> the diodes
A friend of mine has a Dynaco Stereo 410 which had the popular filter cap
mod whereas the filter capacitance was doubled.
That thing went through power switches every couple of years until I added a
soft start circuit. That switch is over 10 years old and still working...
Mark Z.
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