Kenwood Basic C2 Op Amps

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.

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.

Too fuzzy to read the scats, but in general, if you see a larger value of cap (.1µf 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 makes a wide range of these).

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.

kennedy6949 said:

Don't see the capacitors your talking about

Click to expand...

Please note the all-important word:

EchoWars said:

but in general, if you see a larger value of cap

Click to expand...

There 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.

I wasn't applying that you said they were there, I apologize for not making myself clear.

Thanks for your expertise.

Rgr that.

Has anyone here had any experience with the OPA1656 or OPA1642. Both seem to be getting very good reviews and are pin compatible with the stock opamps except for being SOIC configuration (not a big deal).

I have read that the 1656 is CMOS based which goes against my normal acceptance whereas the 1642 is JFet and has been described as having silky smooth highs?

On other thought: Since the voltage to the opamps is being regulated by a zener diode, would it make since to follow the diode with a Schottke Ultrafast Stealth diode to further smooth the current and lower the incoming noise? Pricing is negligible.

Any ideas or thoughts? Thanks

Thx1326 said:

Since the voltage to the opamps is being regulated by a zener diode, would it make since to follow the diode with a Schottke Ultrafast Stealth diode to further smooth the current and lower the incoming noise?

Click to expand...

Well, I can say for sure that it doesn't make sense to me, as I can't imagine what you mean by "follow" in this context. Indeed, I can't imagine any circumstance in which just adding an active component to any kind of circuit would somehow reduce noise. My imagination is limited, of course.

If you wanted to replace the Zeners with a series string of LEDs chosen to add up to close to the original Zener voltage, that could reduce noise. And look cool, to boot. You'd want to investigate (and possibly modify) how much current the original Zeners are being supplied with, to make sure it's compatible with the LEDs.

I have a C1 to upgrade and bought the factory manual. I'll be very interested in your choices and out come. Please keep us informed

I got my stuff from DRM you can look at their website if it helps any.