Upgrading Rolls mini mixer MX-28 to get an Hi-Fi sound

[rantony]

Hello everybody,

I have a mini Rolls mixer, model MX-28. It's a very cute and useful product. It's very small, so I can put it on the top of a keyboard to monitor my sound. I like it a lot, but it has a flaw. When I feed it with a bass sound, it won't reproduce every details and some warmness is lost. I would like to mod it to get a better frequency response and thus a more accurate reproduction.

Here is a link to the schematic (it's public):

http://www.rolls.com/pdf/M_MX28.pdf

As you can see, it is using five BA4560 opamps. The 120pF (x8) and the 50nF (x1) capacitors are ceramic. I thought about changing the opamps for better ones, and changing the ceramic capacitors for mica's. What do you think about this ? Any suggestions for the opamps ?

Another question I have is about the power source: The unit uses 12V. Isn't it a limitation ? Aren't opamps giving better results when they're powered with 18V or 24V ?

Thanks for your help !

[WSJ]

Did you upgrade your MX-28? If so, what imporvements did you make?

The BA4560 has been eclipsed by the NJM2068, with lower noise and 0.001% THD.

NwAvGuy: Op Amp Measurements

Mouser has them in stock for 39 cents.

NJM2068D NJR | Mouser

[indianajo]

I'd change the power supply to at least +-8v on a supply not grounded on the minus rail. That would take at least a 18vdc wall transformer. Then I would ditch all the 1 uf electrolytic caps on the inputs. How old is this unit? Electrolytic caps go to **** in twenty years. I might stabilize the supply with two 1n5344 5 w zeners and I used 22 ohm resistors to protect them from the wall transformer.
RC4560's aren't half bad, I've got some in my Peavey mixer. I don't understand the 120 pf caps, I didn't have to install nearly that much feedback resistor bypass in my RA-88a op amp mixer to get it to stop oscillating when I went from 4558's to 33078's. I did have to put .1 uf on the power supplies at each socket.
Is rantony taking the sound out of the op amps, or out of the MPSA76/2n2907 transistor driven jack? There seems to be no crossover bias network on the transistors to smooth the crossover distortion.
NJM2068's seem to be about the same slew rate and noise performance as ST33078. Six of one, half dozen of the other.

[wahab]

Originally Posted by indianajo

Is rantony taking the sound out of the op amps, or out of the MPSA76/2n2907 transistor driven jack? There seems to be no crossover bias network on the transistors to smooth the crossover distortion.

The op amp provide the drive through the 30R for the first 20mA
or so output current above wich the transistors will supply the reminder.

True that the method is somewhat crude....

[rantony]

Did you upgrade your MX-28? If so, what imporvements did you make?

Unfortunately, no I didn't. I'm working on too many projects at the same time...

If you want suggestions to upgrade the MX-28, you can get some informations here:

Upgrading Rolls mini mixer MX-28 to get an Hi-Fi sound

Same thread title, on another forum. You could refer to this and continue the discussion here...