Rolls MX51s Channel 1 Delete

[Michael1986]

Hello,

I am having issues with my Rolls MX51s mixer and static on channel 1 (unused channel). Essentially, when the unit is powered, there is static on channel 1, if I turn the potentiometer the sound increases. Since this channel is not currently being used, I would like to “delete it” from the circuit and just use inputs 2, 3, and 4.

Rolls provides their circuit diagrams free of charge in the user manuals (free online download), however, I find it hard to follow the one for the MX51s so I’ve opened it up and traced the tracks and created my own diagram (attached). I have also attached the public Rolls MX51s schematic. I feel like theirs has some mistakes, however, I could also have made the mistake on a few resistor values during my visual inspection.

I am hoping to get some information on how these two op amps are interacting with one another and if possible, a way to “delete” channel 1 from the output signal while leaving the rest of the 3 channels intact. If needed, I can only use 2 channels rather than 3, if for some reason this is required.

I have put a cap on the XLR input, as well as removed the XLR input incase that was causing static, however, that did not work. I tried a few aftermarket choke and capacitor noise filters on the input power line with no change.

Word of Caution: I searched BA4560 and found the pin layout as shown in my diagram. I do not think there are other layouts and it seems to make sense as shown in my diagram. Please correct me if you think I made an error and I can re check the pins with my multimeter.
 
Summary of Questions:

1)   Is it possible to “delete” only the channel 1 (XLR input) circuit from the mixer signal while leaving channels 2, 3, and 4 alone?
       a.   If I removed Pin 1 from Ch1 Potentiometer, R24, R17, Pin 8 from BA4560 (2), and Pin C from the transistor, and Pin 1 and 2 from BA4560 (2), would that isolate the circuit                  and allow BA4560 (1) to mix the first 3 channels? I am concerned with PIN 3 and 5 on BA4560 (1) with this approach. What would this do?

2)   What is the purpose of having the polarized capacitors C8 and C11 (and C10 and C13) positive to positive? I didn’t think that was possible?

3)   Similar to #2, why would C1 and C6 be neg to Pos in the flow? What does it do?

4)   Any thoughts on why I am hearing this static? Anything I can do to get rid of the static?
      a.   Could it be the DC input to the unit?

5)   If someone could explain how the two op amps / this circuit is operating it would be greatly appreciated.

Thank you!

Michael

[amz-fx]

BA4560(2) is the XLR microphone amplifier. If you take it out, there will be no Input1 signal going to the mixing amps in BA4560(1).

OR... The amplified microphone signal from Input1 is going from pin1 of BA4560(2) and into the output mixing stages through R3 and R14. You could take out these 2 resistors instead of the op amp, and there will be no IN1 signal going to the output. This is probably better. You can always put them back if it doesn't fix the noise problem.

Pins 3 and 5 of BA4560(1) are connected to a bias voltage and will not be impacted by the change.

Electrolytic capacitors that are placed plus-to-plus (or minus-to-minus) are a way of creating a non-polarized capacitor. This technique is widely used in mixer circuits since it is probably cheaper than buying a non-polar cap of the required value.

regards, Jack

[PRR]

Welcome!

Very nice trace-out.

Rolls is good value for money; but also you get what you pay for.

There is a slip-up: the user-sheet for the MX51s has the schematic for the M51s (no X). The no-X has a transformer based mike amp; the X has an opamp mike amp. That's just wrong, product got ahead of the documentation.

Also the inputs are numbered differently??

The mike amp is basically top-left of this:
http://archive.siliconchip.com.au/static/images/articles/i1021/102159_4lo.jpg

Rolls runs that to a volume control and then to a buffer opamp.

The mixer is basically a Virtual Earth, standard fare (with extra resistors, and back-facing level pots).

Your observations have not proved the static comes from the mike-amp, since it continues with that pot full down?

Basic trouble-finding. Bad Solder joints!! Get the board out in bright light with a magnifier.

After that: bad caps? Start replacing electrolytics. I'm not even sure where to start because the whole thing is interactive.

Bad chip is way-way down on the likely-list, but if Rolls was low-balling to control costs they could have been bit by grey-market chips.

[Michael1986]

Hello again! Thanks for the reply's so far!

FYI, I've also posted this here:

https://www.diyaudio.com/forums/headphone-systems/328569-rolls-mx51s-mixer-channel-1-delete.html#post5570586

Hey Jack, in regards to your answer: From looking at it, I assume that R3 is the path that the output from CH1 would take for the L ch and R14 for the R ch. On that note, I did lift Pin 1/2 of BA4560 (2) to remove that from the circuit but that resulted in no output sound. Any idea why this would be different from removing R3 and R14?


PPR: Thanks for the welcome! And it was fun doing the trace-out, I like to learn as much as I can and it has been helpful to understand the circuit. I'm still talking to Rolls about their schematic and will see what they say. I was confused as to the transformer sign when there was clearly no transformer and you are right, the XLR is Ch4 in that diagram. I'll have to research more into Virtual Earth and the back-facing level pots as I'm don't understand those at the moment.

Yes the noise continues with the pot full down, but increases with that pot going up so I figured that it was coming from that circuit somewhere. I took a look at the solder joints under my microscope and I don't see any glaringly obvious issues but the solder definitely doesn't fill the barrel (almost at all) on some of the leads. I was trying to look for cold solders but none stood out. I'll reflow some of them when I get a chance. If I can "delete" Ch1 from the mix first (going to try Jack's suggestion), then I could at least isolate where the issue is coming from and then I would start my individual component replacements / reflow efforts from there.

I really appreciate both your responses. They are really helpful.

Thanks,

Michael

[amz-fx]

Yes the noise continues with the pot full down, but increases with that pot going up so I figured that it was coming from that circuit somewhere.

Interesting. You have to consider that the pot is "grounded" to the bias supply that is derived from R18 and R19. The C12 capacitor should keep the bias supply relatively noise-free but if it has a poor solder joint or is in some way defective, then the bias supply could become a noise source since it is feeding inputs on all 4 of the op amps.

You should check the solder joints on R18, R19 and C12. As a last resort, you could temporarily add another capacitor in parallel with R18 (or replace C12) and see how that impacts the noise.

regards, Jack

[Fender3D]

... if it has a poor solder joint or is in some way defective...

... Or if BA4560 (2) is dead or a dead wannabe...
Measure voltages @ IC's pins, they might give some clues
or, if you have a desoldering pump at hand, try removing it.

[Michael1986]

Update!

So I had some time last night and I did a few things.

1)   Shorted Pins 2 and 3 together on XLR – no change

2)   Removed U2 chip – everything else attached as per schematic. This removed channel 1 input as expected, however, with the noise gone from channel 1 I was able to hear it more pronounced on channel 2, 3 and 4. Channel 1 was just much louder than the other 3. So there is still an issue.

3)   Reinstalled U2 and just to see, removed R3 and R14 and had the same effect as 1. (then reinstalled them)

4)   Checked solder joints on R18, R19 and C12 – all looked good but I reflowed them all the same. No change in sound

5)   Reinstalled everything and hooked the unit up to my car battery, no hissing anymore. With the engine on, hissing.

So in conclusion I am getting the hiss when connected to wall outlets in my house with the rolls adapter as well as when I hook the unit up to my running car (the unit is rated for 9-15 volts input). The only time I get no hiss is when it’s connected to a clean 12v DV source (my car battery, engine off).

The only suggestions I have not yet tried are replacing (or adding a cap in parallel with) C12 to test since I have not had a chance to look through and see if I have anything on an old PCB laying around or if I have to go and purchase one.

I assume that since the board is rated for 15V that the Capacitor should be 15v minimum rated? For the purpose of testing would a lower rated one (10V or higher) pop with 12v applied? How many µF should I be looking to go? Is there such thing as too big in this case?
As a separate note, here is my end use and some potential issues I’m thinking of.

The reason I bought such a small mixer is because my primary use will be mixing Bluetooth and audio in my car. I have some adum4160 USB to USB isolators which I have been using to filter my Bluetooth power and power going into my USB dac but now I’m starting to wonder if I need to have some sort of 12v filter on the 12v line going to my amplifier (I’ve been having some background noise on the speakers with nothing being output to them). The rolls mixer will be mixing the signals and then sending to the amp (it accepts line level inputs – Alpine KTP-445u).

Is there a circuit I could make to clean the supply (13.7v-14.7v) coming into the mixer/amplifier? I’m very interested in how to design this sort of dc scrubber (if I can call it that).
Thanks,

Michael

[amz-fx]

The only time I get no hiss is when it’s connected to a clean 12v DV source (my car battery, engine off).

There is your answer. You get no noise from a clean battery power source. This indicates that the AC power supply is somehow contributing noise to the mixer.

We don't know the current draw of the mixer but  the op amp chips are about 6 ma each. The LED is an additional 2 ma. With the phantom power off, that's about all. The draw from the bias supply is negligible.

So...  a quick back-of-the-envelope calculation says that you could replace D1 with a 68 ohm or 75 ohm resistor, and get about the same voltage drop across it as with D1. The resistor is going to make a better low pass filter with C7 that relying on the output resistance of the power supply and diode. This change will be more effective than increasing the value of C7. A 1/4w resistor will be good enough, even if the current draw is 2x  the quick-and-dirty estimate. You can use a 1/2w resistor if you have one. Also, touch up the solder joints on C7.

If it works and you decide to go with it permanently, I would get a 1/2w resistor to install for the extra safety margin (even though the 1/4w is adequate).

You could even replace D1 with a diode/resistor series pair, if you want to retain the polarity protection. Use a 1N5817 Schottky diode followed by the 68 ohm resistor.

regards, Jack

[Michael1986]

Hello Jack, thanks for the response! I'll see if I have the proper values lying around and try it tonight. I might have to wait a bit before I have time to grab parts otherwise.

On another note, a quick update - I have also now tested this in four different ways:

1) On 12v battery (Car off) - No hiss/whine
2) On 12v battery (car on - 13.7-14.7v) - Hiss and whine present
3) On A/C wall adapter - Hiss/whine present
4) On 12v DIY bench top power supply (old ATX power supply) - No hiss/whine present

What I found interesting was that in both cases when the hiss/whine was present, touching the potentiometers caused and increase in hiss/whine, however, when touching them in the  other two cases there was no change - ie: nice clean sound.

In addition to trying what you have proposed above, is there a way to avoid this in the future with other electronic equipment in the 12-15v range? I would like to build some sort of "filter" if possible to make it more versatile.

I am considering building this L3999 circuit (in addition to Jack’s suggestion; for future projects) but want to make sure that this would do what I’m thinking it will do? From the datasheet it recommends using:
- LT3999 (x1)
- Coilcraft PA6384 transformer (x1)
- 10uF 16v (x2), 0.1uF
- 10k, 15.8k, 28k, 49.9k, 255k resistors
- 15.3uH inductor
- assuming 2x Schottky diodes based on page 10 of the datasheet (switching diode selection). Recommendations? I was thinking 1N5817 (x2)

From my understanding, the transformer will remove and noise that was on the power line and what I get out would be clean DC at 12v?

I would connect the SYNC pin to ground based on page 5 of 16. Any thoughts on this?

What I’m unclear of is whether or not I would need to program the chip?

Finally, on Page 12 of 16 it also shows some other schematics of typical applications. One is 10v-15v in and 12v 200mA out (and -12v 200mA as well). My question on this is for the one I am proposing to build, would 10-15v be accepted on the input? The chip itself is good for 2.7-36v input but if I use this for another accessory in a car when the power goes from 10-15v would it harm the circuit as is? Would it run on 13.7-14.7v)

Thanks!

Michael

[Michael1986]

quick question on the 68 Ohm resistor. Won't it burn up with 15 volts applied (from the ac adapter)?

V1/4W= √(68 Ω / 4) =  √(17) = 4V ? or is it because it isn't the only resistor in the circuit? I am definitely not an EE. Thanks!