Preamplifier recap: voltage values

KanedaK

Member
2011-02-07 8:32 pm
I own a rare Korn & Macway SP100 preamplifier
I would like to recap it since it's 30 years old and runs a bit hot, but it's impossible to find any schematics or service manual.
Main PSU caps are rated at 63v; then theres quite a few 100uF rated at 50v.
I have a bunch of Elna 100uF but they are rated at 25v

what's the average voltage in a solid state, discrete preamplifier? Is there an "average" or it's very design dependant? I could just stay with the same values to be safe of course, it's just that i have these "expensive" Elnas at hand...
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
100% design dependant.

You could always measure and note the measured DC voltage across each as guide to choosing a replacement, but if you are at all unsure what voltage a cap is going to see in actual use then you must stick to the original (or higher) rating.
 
50V and 63V capacitor in the PSU and decoupling probably indicate that the pre-amplifier is operating with a single polarity supply.

Most dual polarity pre-amps will operate with ±12Vdc upto ±24Vdc and use 16V to 35V capacitors.

You either play safe as Mooly said, or get someone knowledgible to investigate inside for you.
 
Hmmm... I'm seeing 4700 uF/ 50 V axial type capacitors on some photos that appeared by typing "Korn & Macway SP100 preamplifier"
Already re-capped, perhaps ?
The issue of running hot : it's because the case doesn't allow for air flow
If you just dill some (many ) holes where the heatsink (supporting the 4 transistors ) is located.... or 3-4 slots ( 1mm X 100 mm )
 

KanedaK

Member
2011-02-07 8:32 pm
Hmmm... I'm seeing 4700 uF/ 50 V axial type capacitors on some photos that appeared by typing "Korn & Macway SP100 preamplifier"
Already re-capped, perhaps ?
The issue of running hot : it's because the case doesn't allow for air flow
If you just dill some (many ) holes where the heatsink (supporting the 4 transistors ) is located.... or 3-4 slots ( 1mm X 100 mm )

My mistake, the original 4700uF are indeed 50v, it's the ones I bought to replace them that are 63v.
 

KanedaK

Member
2011-02-07 8:32 pm
I've replaced the two main PSU capacitors.
The original were Frako 4700uF 50v, the new ones are Roederstein gold (new old stock) 4700uF 63v 105°c.
I'm shocked at the difference I hear. Width and depth of soundstage, bass depth, overall ease and clarity, everything made a massive jump in SQ, just with two new caps.
I guess the old Frako caps were in urgent need of replacement.
I will most likely replace all caps, but I suspect I have to be very cautious with what I will choose.
 

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If you feel like experimenting you could find a pair of 50V 10mF capacitors that fit the space.
This doubling of PSU capacitance may give a further improvement in sound quality.
But do check that your rectifier can cope with charging the bigger capacitance.
10mF is not a big increase. Going to 15mF, or 22mF, might eventually break the rectifier.
 

KanedaK

Member
2011-02-07 8:32 pm
If you feel like experimenting you could find a pair of 50V 10mF capacitors that fit the space.
This doubling of PSU capacitance may give a further improvement in sound quality.
But do check that your rectifier can cope with charging the bigger capacitance.
10mF is not a big increase. Going to 15mF, or 22mF, might eventually break the rectifier.

I'm wary of drastic capacity increase. I have no explanation why but I know sometimes it doesn't bring the expected results on the table; slowing down the music and killing dynamics (quite the oposite of the expected result).
This preamp sounded very good with the old caps, and 100 times better now; I wouldn't touch any values here, such is the quality of the device. what I will probably do, tho, is changing the transformer: this one is 220v, and i'd rather install a 230v version. I'm in touch with an ex-employee of the firm, who now went back to his country in eastern europe, and he advised me the correct replacement.
 
You may want to look at and assess the quality and implementation of the supply rail decoupling.
It is possible that if the smoothing had degraded significantly, that the decoupling has also degraded to some extent.

I think that when the PSU smoothing capacitance makes a noticeable difference to medium and high frequency reproduction, that it's a shortcoming in decoupling that has led to the poor performance. Effective supply rail decoupling is crucial to minimising the voltage glitches/spikes on the supply rails and their effect on the amplifying performance.
 

KanedaK

Member
2011-02-07 8:32 pm
You may want to look at and assess the quality and implementation of the supply rail decoupling.
It is possible that if the smoothing had degraded significantly, that the decoupling has also degraded to some extent.

I think that when the PSU smoothing capacitance makes a noticeable difference to medium and high frequency reproduction, that it's a shortcoming in decoupling that has led to the poor performance. Effective supply rail decoupling is crucial to minimising the voltage glitches/spikes on the supply rails and their effect on the amplifying performance.

I'm pretty sure this is the case here indeed. Please see my new post about recapping this preamplifier. Any advice is welcome as I'm new to this, even tho I learned quite a bit by recapping my Revox B226 CD player.