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 so 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 120pF 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 !

[amptramp]

Operating from the 12 VDC supply they recommend, the op amps are biased at 5.67 volts.  This is set by R43 and R44 which gives the base of the power supply transistor 0.55 times the input voltage after the drop at the input protection diode and this is followed by the Vbe drop of the power supply transistor, which is not a regulator - it is a capacitance multiplier that increases C26 by its h_fe.  The 0.55 multiplier is set by R43 and R44.  This should be OK.

The 120 pF caps in conjunction with the 47K feedback resistors give a rolloff at 28.2 KHz, which is OK for value, but ceramics do not belong in audio, especially if they are used in any loud ambient where they may become microphonic.  The ganged level pots at the input have a large effect on frequency response.  When set for maximum gain, they cutoff the low frequencies at 33.86 Hz at the -3db point for the input stage.  The C1 and C2 electrolytic capacitors and the corresponding ones in other channels should be raised in value.

The output stage for J8 and J9 use transistors to boost the op amp output when the op amp output current reaches 23.3 mA.  This is set by the 30 ohm R38 and R41.  At the 5.67 volt bias level, this may not be reached.  Raising this resistor will allow the power output to be handed off to the transistors at a lower level.  If you were to replace the op amps with some of the popular ones like the TL072, which has 192 ohms in series with the output internally, you may never get these transistors to turn on.

The worst problem at this stage is C22 and C24 which set the output bass response.  Going into a zero ohm load, the bass cutoff is 341.4 Hz with this number going down as the load impedance rises.  C19 and C20 are also bad with a cutoff at 159 Hz at low impedances.  Raise the capacitance there for better bass.

The 50 nF ceramic should have a minor effect - it is just there to keep the ground at chassis potential for AC signals, but if it turns microphonic, this will not be good.

This unit is a good starting point for a better mixer (and there are many worse designs out there).  Just do the calculations and set your values where you want them.  I am not familiar with these op amps, but you should determine whether they have the input and output range to handle the signal levels.

[rantony]

Thanks a lot amptramp for your analysis and all the details. It seems that I will have to go through the 'trials and errors' process. I'll try Mica for the 120pF caps. And I'll try a higher value for the 1uF's. But what kind of value do you have in mind for these ? The higher the best ?

The output stage for J8 and J9 use transistors to boost the op amp output when the op amp output current reaches 23.3 mA. This is set by the 30 ohm R38 and R41. At the 5.67 volt bias level, this may not be reached. Raising this resistor will allow the power output to be handed off to the transistors at a lower level. If you were to replace the op amps with some of the popular ones like the TL072, which has 192 ohms in series with the output internally, you may never get these transistors to turn on.

Ok, what you are saying is that it's best to avoid that the transistors turn on ?

The worst problem at this stage is C22 and C24 which set the output bass response. Going into a zero ohm load, the bass cutoff is 341.4 Hz with this number going down as the load impedance rises. C19 and C20 are also bad with a cutoff at 159 Hz at low impedances. Raise the capacitance there for better bass.

Again, which values would you try first ?

Regarding the opamp, I think I'll try to change only the one that handles the signal to the J8 and J9 jacks, as I'm only monitoring my sound with headphones. I might try an OPA2134. Any opinion on this ?

Oh, and by the way, I add the same frequency response issue with a Tapco Mix50 with headphones ...

[amptramp]

If you want hi-fi sound, I would advise going up by a factor of 10 on all electrolytic coupling capacitors.  Remember, the frequencies I have calculated are -3db (half power) points.  This is a substantial drop.  You really want something with no more than 0.5 db drop and the only way to do this is to put the -3db point around a tenth of the lowest frequency you are expecting.  Note that the resistance of the load also affects the frequency response so that higher resistance would reduce the low-frequency turnover (which is what you want).  But if you want a circuit that is compatible with any load, higher capacitance helps.  Note also that most electrolytics have a wide variation in capacitance and going to much larger values assures that there will be less variation from channel to channel both at room temperature and any variations in temperature (and typical electrolytics have a pronounced loss of capacitance both above and below room temperature).

You are not trying to avoid turning transistors Q1 - Q4 on.  These transistors are used as current boosters for the op amps, but you have to be able to drive the base at least one Vbe drop beyond the output in order to get them to supply output current.  What my calculations show is that certain op amps will current limit before they reach an output that would turn the transistors on.  The OPA2134 that you are proposing can source and sink at least 30 mA, so it should work with the existing 30 ohm values for R38 and R41.  BTW, this is a nice amplifier with 8 nV/(Hz)^0.5 noise voltage and wide bandwidth.

The OPA2134 bandwidth may allow you to reduce the value of the 120 pF capacitors on the input stages to permit better high frequency response.  The whole idea of these capacitors is to provide a rolloff before you get to the inherent rolloff of the amplifier and since the OPA2134 is a wideband amp, this could be reduced to a quarter of the 120 pF value (and 30 pF does happen to be a standard value).  Without these capacitors, the input capacitance to ground from the inverting input may be enough to cause a rise in response at high frequncies.  When this response crosses the falling response of the op amp, you get a large phase shift in the feedback which, even if it is not enough to cause oscillation, can cause poor transient response and the possibility of ringing transients that force the amplifier to run out of headroom even at low signal levels.

I would use a socket for the op amps if you intend to experiment with various types as this is a lot better than resoldering a printed circuit board a number of times.

[rantony]

Again, thanks for your nice explanations and advices. I will try to modify only one channel to see if it brings improvements. I will order the required parts and I will do the follow-up once the modifications are done.

Talk to you later !