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Old 9th September 2008, 07:25 PM   #1
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Default Mod ideas for McIntosh MC252

I've taken the plunge and opened up my MC252. It's a 250Wpc SS amp that uses Toshiba 2SC2500 power transistors. I am looking to improve the dimensionality of the sound -- it's a little flat. I'm looking for suggestions on changes that may make the most impact.

So far I've made two modifications:
[list=1][*]Changed out input coupling caps from Rubicon electrolytics to Black Gate Standard electrolytics[*]Bypassed 27,000F/100V PS caps with 47F/160V Black Gate VKs [/list=1] I'm thinking these changes will make a difference (and improvement, hopefully!). I'm considering some other mods as follows but don't know what will make an appreciable impact.
[list=1][*]There are unbypassed Rubicon YXE 10F/100V electrolytics decoupling caps on the rails of the amp boards. I'm thinking of replacing them with Black Gates and bypassing them. Polyester, polypropylene, polystyrene? I don't have loads of room so size is a consideration. [*]There are two other small-value caps on the rails. One is a 100pF/100V NPO ceramic between the gate and the collector of a driver transistor on each rail -- I believe this is to inhibit HF oscillations. The other is a 0.47F/100V metalized polyester film (WIMA RN) that appears to be solely for decoupling. I'm thinking I shouldn't fool around with the ceramic but would doing something with the MPF make any difference.[*]There is global feedback with a 0.0047F/5V MFE WIMA cap in it. Worth some action?[*]There is also a "current limiter" feedback look applied to the driver input of each rail. The gate of each current limiter transistor is coupled to ground with a 47F/35V Rubicon electrolytic. This doesn't seem like a high-impact area but I really don't know.[*]The bridge rectifier might also be an upgrade opportunity but since I don't know how to characterize what I've got already, any change would be a bigger guess than the others.[/list=1]
I think that's it. Any thoughts or suggestions would be most appreciated.

KB
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Old 9th September 2008, 09:41 PM   #2
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Default Schematic?.... picture?.... this use to motivate folks


regards,

Carlos
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Old 11th September 2008, 03:08 AM   #3
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Default Sorry, no schematic

Destroyer, I totally agree that the schematic would make things much more interesting.

Unfortunately, I'm not comfortable posting it.

I was hoping for some general guidance but understand that it may be to difficult.

Cheers.

KB
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Old 12th January 2009, 12:37 PM   #4
clm811 is offline clm811  United States
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Default Non-polar caps for signal coupling

Quote:
So far I've made two modifications...Changed out input coupling caps from Rubicon electrolytics to Black Gate Standard electrolytics
May I recommend that you'll get better sound quality if you use Non-Polar caps for signal coupling. The resulting distortion of polar caps in this position will outweigh the gains in cap upgrade.


Quote:
There are unbypassed Rubicon YXE 10F/100V electrolytics decoupling caps on the rails of the amp boards. I'm thinking of replacing them with Black Gates and bypassing them. Polyester, polypropylene, polystyrene? I don't have loads of room so size is a consideration.
There is some disagreement on bypassing Black Gates.

Martin Colloms wrote that "existing selected quality 0.1uF polypropylene de-couplers on the power amp supply lines had to be removed from the circuit as they were found to audibly degrade the performance advantage even of a 47uF 25V standard polarized BG used with no other supplemental high frequency decoupling"

Personally, if I have to bypass, I use 0.1/50v BG NX-H. If higher voltage is required, try 0.01uf/600v film/foil polystyrene caps(if you can find them).

-Chas
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