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OPA627: OPA627 Compatibility in Kenwood Basic C2 Preamplifier

David Sherfy
Prodigy 10 points
Part Number: OPA627
Other Parts Discussed in Thread: OPA1656, OPA1642

I am in the process of restoring and upgrading several pieces of the vintage Kenwood Basic C2 preamplifier.  it has 4 op-amps as follows:

1 - NJM4580D mounted in the phono preamp Section (originally NJM4560D)

1 - NJM4580D mounted in the Tone Control Section (originally NJM4560D).

1 - NJM4580D mounted on the Filter Board (originally NJM4560D)

1 NJM2068D mounted in the Flat AMP section. (originally NJM2043D)

Question:  Should the OP627 be a drop in replacement for the above 4 opamps?

Also, is this available in a standard 8 pin (2 rows of 4) dip socket configuration?

Thanks in advance for your reply.

  • 0
    TI__Mastermind 22630 points

    Hey David, 

    Have you looked at the OPA1656? This is one of our modern Burr Brown devices that has ultra low noise and has great THD specifications. It does not come in the dip package however we do have coupon board that converts into pins that can be used as a dip adapter. I have linked to this below as well. 

    https://www.ti.com/product/OPA1656

    https://www.ti.com/tool/DIP-ADAPTER-EVM

    https://www.ti.com/amplifier-circuit/op-amps/audio/overview.html

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Prodigy 10 points

    Chris...  Thanks for the reply...

    I Have been doing quite a bit of research and reading and have basically narrowed it down to either using the OPA1642 or OPA1656.  I have a few reservations that I would like to get your comments.

    General rule of thumb in the audio community is to "avoid" CMOS based opamps due to non-linearity and higher noise levels.  There have been reports that the 1656 does have higher noise levels in the lower frequencies.

    On the other hand, the 1642 is the preferred JFET based, has lower noise in the low frequency range.  Plus, it has been described as having a very linear response and "silky smooth highs".

    How would you compare these two models and please offer your recommendation.

    One more question, in the circuits these are be used in, the operating voltage of the opamps are be regulated/controlled by a standard zener diode limiting the voltage to 18 volts.  What would you recommend as the optimum working voltage for each the 1656 and 1642? 

    Also, do you feel there would be any benefit in inserting a Schottke Ultra Fast Stealth diode between the zener and the opamp.  Some have suggested this to smooth the voltage and lower the residual noise.  Could you offer your comments on this.

    Thanks again!

  • 0 in reply to David Sherfy
    Guru 281000 points

    The OAP1656 actually has better linearity (its THD+N is better, although I doubt the difference is audible). It has lower noise at higher frequencies, but this does not matter for music.

    You have to decide yourself whether to go for noise or for THD+N.

    There is no reason to use supplies less than ±18 V.

    An additional diode would increase noise (probably by an unnoticable amount). "Stealth" diodes make sense only in fast-switching power supplies; this is the opposite of what audio waveforms look like.

  • 0 in reply to David Sherfy
    TI__Mastermind 22630 points

    Hey David, 

    You identified some great points and some tradeoffs! The OA1656 is running at a higher Iq than the OPA1642 and therefore the noise will be lower. Based on the application I don't believe pulling the extra power will be of concern and the OPA1656 with the lower noise floor is a great choice. When it comes down to the THD I don't think it will be audible with either device. The THD for both devices is even below the measurement noise floor of our measurement equipment and we have to use a special test circuit in order to test the THD. I just released an app note discussing this topic (link below).  A listening test would have to be performed between the devices in a well controlled room to have a chance of hearing any difference in my opinion. I think you will be pleasantly surprised with either device. 

    https://www.ti.com/lit/an/sboa580/sboa580.pdf?ts=1708473196956&ref_url=https%253A%252F%252Fwww.google.com%252F

    This is a cool project. Let us know if you have any further questions!

    Best Regards, 

    Chris Featherstone