Rotel RB850 PS Capacitor query - diyAudio
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Old 23rd February 2012, 09:40 PM   #1
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Wink Rotel RB850 PS Capacitor query

Hi there - sorry to start a new thread on this - however the older threads on this amp didnt address this point - i have 2 x RB850 ampls - i want to upgrade 1 and do an A B comparison to see if things have improved. The slit foil PS caps are over 25 years old and i want to replace these with standard Panasonic PS caps. These have improved the sound of my pioneer c21 preamp. The issue is - the Rotel slit foils have 3 terminals - how does this translate to repalcement w/ standard 2-terminal PS cpas? Many thanks!
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Old 25th February 2012, 09:19 PM   #2
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OK I am a doofus - the existing caps have 3 tags however only 2 are connected to the PCB underneath - so unless anyone has objections i will repalce these with ordinary 2-terminal caps. cheers!
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Old 25th February 2012, 10:56 PM   #3
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Have you measured ripple on the supplies?

The big PS caps usually hold up quite well, assuming they were of good quality to begin with and not operated too close to their maximum voltage rating. It's not like they'd be seeing ginormous ripple currents in hi-fi amps (PA may be a different story). Some still measured fine in terms of ESR after 40 years.

Besides, power amps do tend to have a good bit of PSRR, and looking at the schematic, this particular model should be no exception.

What you can try as sort of a litmus test is deliberately degrading performance by installing smaller caps. Can you hear the effect of only using, say, 1000 F? If so, at what kind of volume?

If you do want to swap some electrolytics, replace C607/608 (100/6.3V) with bipolar types, plus C613/614 (1/50V) with whatever you have that you think won't dry out too quickly. I wouldn't expect any audible improvements though.

What you may want to check is whether 5 pF capacitors C615/616 are installed. That should only be the case for the UK model, but you never know. Not sure why they'd want to limit GBW in that one only.
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Old 26th February 2012, 01:01 AM   #4
tvrgeek is offline tvrgeek  United States
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M experience is somehow Rotel makes these things sound so much better than the schematic would suggest, just keep your hands out of them. I am listening to my RA 850 now and have two 951s. I am afraid to "improve" them for fear of messing up whatever Rotel did so right.
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Old 26th February 2012, 08:48 AM   #5
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Replacing the power supply caps gives the biggest improvement, in my experience. The sound appears to get "tighter".
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Old 26th February 2012, 08:00 PM   #6
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Thanks for the input guys - i agree the amp sounds pretty good as is considering its age however i intend keeping mods to a minimum - you cant really go wrong with PS caps and coupling cap upgrades - fully reversible. There is also a bit of info on the net about upgrading these showing gains can be made. I'm also thinking about doing the 4 x Schottky diode upgrade to replace the standard bridges, however cant find instructions on how to assemble the diodes on the net....cheers!
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Old 27th February 2012, 03:54 AM   #7
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OK another couple of questions re messing around with my RB850 power supplies:

1. There are currently 4 x 10,000uF PS caps (which i will upgrade to panasonics). I am thinking of upgrading the bridge to 4 x Schottky diodes. Would 4 x MUR420 be sufficient - would these fare OK with the start-sup "surge" of the amplifier? The MUR420 is rated at 200V.

2. The existing bridges on the board are snubbed by 4 existing ceramic caps - also mounted on the board. I have read that Schottky diodes do not require snubbing - and that snubbing can degrade your sound anyway - can i remove these snubber caps once the Schottky diode upgrade is installed?

thanks!
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