Replacing input coupling caps

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I found audible differences in hiss between carbon comp and metal film resistors in the PAS2 and ST70. Only over 100 kohms.
Carbon film is so much better at hiss than carbon comp I wouldn't bother replacing unless you have 1 meg feedback resistors or something. Thermal noise goes up with the resistance value.
 
Thanks indianjo and Mark.

I regret that I have very little understanding of all but the most basic circuits and certainly not the ones for my amp, but am interested in Mark's comment about feedback resistors.

@MArk: would you mind seeing if you can identify the resistor to look at from the diagrams in my last post? Any channel will do as an example. Taking the "Amp board 1 " diagram L.S channel as an example, there is a resistor R53 (22k), but I can't see where it is in the complete circuit as it bridges the "feedback" (pin 6) connection of the AMP board connector 3 to the "Over Load" connection of connector 2. I find the other side of the feedback connection at pin 6 of connector 29 in the attachment on this post, which comes from the dumper output, as expected, but am very unsure of the other end (Over Load). Or is feedback R40 (470k) + R37 (8.2k)?

Sorry to be dim!
 

Attachments

  • P-Amps.pdf
    387.9 KB · Views: 48
I'm not downloading this. I've gotten viruses from pdf files. I certainly would not worry about hiss or distortion in a 22k resistor.
Feedback goes from output of op amps to negative input. Or if the main signal input goes to op amp minus input, the feedback goes to the plus input.
Or in discrete circuits, from output to the base of some input transistor. Download the datasheet for the NJM4558 op amp IC from datasheetcatalog.com and look at the IC schematic. The minus input is shown on a "long tailed pair" of transistors at the left side. Feedback will go to something that looks like that in discrete circuits. The two transistors below the LTP are constant current sources to reduce sensitivity to power supply variations(hum). Those two ccs transistors may be replaced by resistors in cheaper discrete designs.
 
Last edited:
No problem and thanks for the replies.

It's very unlikely that I'll do anything with any of the resistors anyway. I understand feedback resistors in simple circuits, but can't work it out here, especially as the circuit is split over several boards/slides. Anyway, thanks for the interesting contribution.

BR.
 
Last edited:
Cap Replacement-CA 640R

Hey,
So to be up front, i'll a newbie to the thread forum but am intrigued with this topic of replacing the caps on my Cambridge Audio 640R. Has anyone taken this piece apart to access the backside of the main board? I want to replace the two main power caps but am having issue access them. Any assistance would help.
thanks
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.