View Full Version : Kenwood Basic C2 Op Amps


kennedy6949
06-19-2012, 07:46 PM
I am about to recap my Kenwood Basic C2 preamp and while doing some research became curious about swapping out several of the op amps for better performance.

Since this system is about 90% vinyl, I want to start with the phono amp and then the flat amp. If it goes well then the tone amp would be next. The filter amp also has an op amp, but I don't use them at all.

The eq, or phono, amp uses a NJM4560D op amp, and is powered by +/- 19v. The flat amp uses a NJM2041D and also uses 19V. These are both DIP8 with standard pinouts. Because of the high voltage, I was thinking the OPA2604 would make a good match. After doing some research, I am leaning toward the LME49860, due to the fact that it is bipolar, as is the original 4560. It also appears to be fairly stable.

As for the flat amp, I am not as sure, but it looks as the LME49860 should work also?

I've included the schematic from the service manual, if it helps.

If these work out, would it benefit to change the tone amp and headphone amp op amps also?

Thanks.

kennedy6949
06-21-2012, 01:30 AM
Well from what I can tell it appears the op amps will work. However, I've read that in some preamps the op amps serve in a dc servo application and not it in the signal path, minimizing their sound impact.

Can anyone tell me if the op amps in the phono amp and flat amp work in this manner? Both circuits seem similar, and I believe they are in the signal path, but I can't be sure.

EchoWars
06-21-2012, 02:17 AM
Too fuzzy to read the scats, but in general, if you see a larger value of cap (.1f or larger) between the inverting input and the output, the opamp is being used as a DC servo to null the offset of some other circuit.

Limiting yourself to 8-pin PDIP does limit your choices for parts replacement as well. For instance, I'd be wanting to use something like the Linear Technologies LT1115 for the phono, but it's only a single opamp so some sort of adapter would have to be used (Cimarron Tech (http://cimarrontechnology.com/) makes a wide range of these).

kennedy6949
06-21-2012, 04:45 PM
Man, didn't realize how bad that schematic got. Here is a better look.

Don't see the capacitors your talking about, but just want to make sure it isn't a different design.

EchoWars
06-21-2012, 05:33 PM
Don't see the capacitors your talking aboutPlease note the all-important word:but in general, if you see a larger value of capThere would be no purpose to using a large(ish) value of cap in the feedback loop except to filter AC and integrate the average DC value in order to correct for it.

The opamps in the phono section are doing what opamps do best...amplifying a signal.

kennedy6949
06-21-2012, 07:11 PM
I wasn't applying that you said they were there, I apologize for not making myself clear.

Thanks for your expertise.

EchoWars
06-21-2012, 08:09 PM
Rgr that.