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In Reply to: Re: Rotel RA1060 - upgrade OpAmp posted by jyung@maths.uwa.edu.au on March 07, 2003 at 16:53:52:
Thanks for your comments, which I broadly agree with.In the case of the Rotel, I did not suggest any upgrade of the main power caps because my RA 971 has rather nice BHC Aerovox caps, specifically made for Rotel. I feel they are a strength of the Rotel range, so I'm not tempted to try to "upgrade" in this area :).
I'm familiar enough with the circuit to be confident that removing the input DC block caps from the OPA2604 is ok, because they have a pair of output DC block caps. The caps are BG, but polar (ideally non-polar), so they're still a bit of a limitation.
Changing the feedback caps to Caddocks or Dales are a good idea. If you look carefully at the values chosen by Rotel for the RA 971, the gain is higher than neccessary (amp plays too loud at 11 o'clock), so there's an opporunity to fine tune. Also the values chosen are not optimal for minimization of distortion and DC offset. I might look at this upgrade sometime in the future.
A very worthwhile upgrade is to replace rectifier diodes with soft recovery diodes (of correct rating, of course). This is a simple, well-tested upgrade, and imho certainly improves the subjective quality of bass.
Follow Ups:
mb,
Can you tell me the original 971's
components? A list should do. list
the brand and type as well.
I guess you're asking about the caps, so here they are. Of interest are the 50BG and 25BG -- those are Black Gates! The main power caps are BHC ALC 10000uF. Not bad for 'budget' amp?? The voltage regulator for the preamp section is disappointing -- just using zeners. I thought of using LTxxx regulators, but the layout was just a little difficult to modify.Sound-wise, the major improvement came with low ESR caps (Rubycon ZL) for the power supply of the 2604. Faster -- yes. Loss of bass -- no! Faster and deeper in fact. Yes, I have bypassed the DC blocking caps with Bennic XPP (film/foil polyprops). Added clarity, but not as much as from improving the 2604's power supply caps.
Cheers,
MervinC501,502 041 50BGF10M CAP,ELEC.50V10UF
C503,504 044 LFSC160V470H CAP,STYROL 160V47PF
C505,506 041 50BGF10M CAP,ELEC.50V10UF
C507-512 044 50MSAN104J CAP,FILM 50V0.1UF
C513,514 042 LTP50V472J CAP,MYLAR 50V4700PF
C515,516 042 LTP50V152J CAP,MYLAR 50V1500PF
C517,518 041 50BGF10M CAP,ELEC.50V10UF
C519,520 044 LFSC160V101H CAP,STYROL 160V100PF
C521,522 041 50BGF10M CAP,ELEC.50V10UF
C523,524 041 ECEA1EFS221 CAP,ELEC.25V220UF
C527,528 041 ECEA1EFS221 CAP,ELEC.25V220UF
C601,602 041 50BGF4R7M CAP,ELEC.50V4.7UF
C603,604 044 LFSC160V331H CAP,STYROL 160V330PF
C605,606 044 LFSC160V221H CAP,STYROL 160V220PF
C607,608 041 25BGF100M CAP,ELEC.25V100UF
C609,610 044 LFSC160V331H CAP,STYROL 160V330PF
C611,612 041 50BGF10M CAP,ELEC.50V10UF
C613-616 044 LFSC160V221H CAP,STYROL 160V220PF
C617,618 044 MKT18-100V104K CAP,FILM 100V0.1UF
C901-904 043 LTC500V103 CAP,CERA.500V0.01UF
C905,906 041 ALC20S1044DG CAP,ELEC.50V10000UF
C907,908 041 ECEA1EFS101 CAP,ELEC.25V100UF
C909,910 041 ECEA1EFS471 CAP,ELEC.25V470UF
C911 041 UTSS1H100 CAP,ELEC.50V10UF
ooooooh!!!! they used black gates!?!No--- zeners are not bad. If the opamp
(2604) has high PSRR (power supply rejection
ratio) which all opamps have, the main
thing you need from the power supply,
in my opinion, is not a constant voltage
but low output impedence at high freqency.
3-terminals voltage reg' are not design
to have this. But no gra', your amp
might sound better with reg's.glad to hear improvement after Rubycon
caps. What caps were used before? Do tell me.Could you tell me where to get Bennic
and in fact, any polyprop caps?Mervin, I really appreciate your effort
in listing. If you want my trick of ensuring
the opamp in class A, just send a email
to my private address. You just need to add
1 single component! same supply voltage!there's not much help to me if I know the
components' number positions. If you want me
to help you, I need to know where they are
in the circuit. I guess it's hard to trace
off from the PCB.James
Praise! Professional Audio Equipments
ooh, Rotel has BHC aerovox!?! I didn't know that.
I was pretty ignorant.I forgot to mention why caddock upgrade is important
in the feedback network: In any amp using large amounts
of neg. f.b. the quality of the feedback network is very
vital. Feedback is a very delicate thing. With op-amps
the open-loop gain is almost infinite, therefore large
amounts of NFB required to bring down to a gain say 10.
The more feedback you use around an op-amp, the more
benefits you reap. So if you find your amp too sensitive,
it's good idea to feedback more. I don't suggest changing
the resistence from -ve to gnd; keep it the same. But rather
decrease the resistance from op to -ve.I also forgot to add that I have a way of making sure that
the 2604 is running in class A without lowering the supply
voltage. Send a private email to me if you want to try it out.Have you tried polyprop' caps for coupling between op of 2604
and power-amp? You don't need a large one if your power-amp
has high input impedence. I suggest you to stay with the original
value unless you don't want much bass and want a "faster" sound.Enjoyed this conversation. :-)
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