Hi all,
I've recently gone through the process of tweaking some of my RC971 pre-amp's innards around the op-amp to great effect, including:
- Power supply caps changed from electrolytic to OS-CON
- Decoupling caps changed from electrolytic to MKC (polycarbonate)
- Filtering caps to polystyrene
Howeve, while I carried out the mods myself, the guidance was provided by another person who I can't refer to any longer.
Now I've turned my attention to my RCD991 and despite now understand the principles he introduced to me I'm struggling to pick out the equivalent positions in the CD player that I changed in the pre-amp.
I have new opamps and DAC chips to install, which I'm entirely comfortable with, but in terms of the power supply, decoupling and filtering components surrounding the ICs, I don't feel as confident diving in (especially when in this unit they're pretty much Blackgates, WIMAs or nothing else and likely carry out a variety of functions).
Here's a link to the service manual if anybody wants to throw some parts numbers at me...
https://drive.google.com/open?id=0B0F0XG7w8C0TYV9GMUFyS1VUWUU
Thanks!
I've recently gone through the process of tweaking some of my RC971 pre-amp's innards around the op-amp to great effect, including:
- Power supply caps changed from electrolytic to OS-CON
- Decoupling caps changed from electrolytic to MKC (polycarbonate)
- Filtering caps to polystyrene
Howeve, while I carried out the mods myself, the guidance was provided by another person who I can't refer to any longer.
Now I've turned my attention to my RCD991 and despite now understand the principles he introduced to me I'm struggling to pick out the equivalent positions in the CD player that I changed in the pre-amp.
I have new opamps and DAC chips to install, which I'm entirely comfortable with, but in terms of the power supply, decoupling and filtering components surrounding the ICs, I don't feel as confident diving in (especially when in this unit they're pretty much Blackgates, WIMAs or nothing else and likely carry out a variety of functions).
Here's a link to the service manual if anybody wants to throw some parts numbers at me...
https://drive.google.com/open?id=0B0F0XG7w8C0TYV9GMUFyS1VUWUU
Thanks!
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1st, the digital filter chip (PMD100) is excellent, among the best ever produced.
2nd, the DAC chips (20-bit PCM63) are excellent, also among the best ever produced.
So, no need to change the digital or DAC chips.
That leaves the analog circuitry. However, I won't hazard to suggest any specific component changes, sorry.
2nd, the DAC chips (20-bit PCM63) are excellent, also among the best ever produced.
So, no need to change the digital or DAC chips.
That leaves the analog circuitry. However, I won't hazard to suggest any specific component changes, sorry.
Changing decoupling caps from electrolytic to plastic film could make things worse, unless you are sure the original designer was not relying on the ESR to damp local resonances in other caps or PCB traces.
As I always say, to genuinely improve a circuit you first need to understand it even better than the original designer. Then most improvements will consist of minor circuit changes, not component changes.
As I always say, to genuinely improve a circuit you first need to understand it even better than the original designer. Then most improvements will consist of minor circuit changes, not component changes.
If they used blackgates and Wimas from the factory, the player was obviously already audiophile targeted....
You will have audiophiles turning in their graves if you replace the black gates! 😀
Tony.
You will have audiophiles turning in their graves if you replace the black gates! 😀
Tony.
I know that Rotel 991 very well. The original sound was quite cold and non-motivating.
You'll need good 16.9 MHz oscillator with clean supply. It is as important to supply the 74HCU04 with clean power.
AD8065 will work very well as I/V OP. R307 and 308 are I/V resistors - correct replacement will help fine tune the end result. TX2575 series are outstanding resistors for that job. If you don't need XLR output (it's "fake" anyway...), remove R321 and 322 and R333 and 334. Bypassing electrolytes with film caps in signal path is questionable...
The real improvement would be to use 811 OP that will provide close to zero impedance to those PCM63's, but this requires a bit more work to reduce rails to around 10 to 11V for analog stage, and to implement that 811 correctly to avoid oscillations... but this would make that CD player sign.
You'll need good 16.9 MHz oscillator with clean supply. It is as important to supply the 74HCU04 with clean power.
AD8065 will work very well as I/V OP. R307 and 308 are I/V resistors - correct replacement will help fine tune the end result. TX2575 series are outstanding resistors for that job. If you don't need XLR output (it's "fake" anyway...), remove R321 and 322 and R333 and 334. Bypassing electrolytes with film caps in signal path is questionable...
The real improvement would be to use 811 OP that will provide close to zero impedance to those PCM63's, but this requires a bit more work to reduce rails to around 10 to 11V for analog stage, and to implement that 811 correctly to avoid oscillations... but this would make that CD player sign.
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