Motu M4 - Tear down, bit of internals analysis and few in-house measurements

[AnalogSteph]

So, I should accept the current behaviour as normal and that there's not much they can do 'practically' to make the knob behave more like a normal smooth volume dial, correct?

Pretty much.

I wouldn't expect too much in terms of "easy fixes" in this price class with years worth of design experience on behalf of the manufacturer (or rather the engineering firm they're contracting out to, as seems to be the case for MOTU). You'd stand a better chance in the super budget class, which seems to be the Wild West of audio interfaces.

Poking around inside a Swissonic UA-2x2

Can a 2x2, 24/96 capable (UAC1) audio interface for a mere 65€ from Thomann's house brand be any good? Well, I just happened to need something to take care of an XLR condenser mic so I could finally liberate the Audient EVO 4 from my office PC and use it for portable measurements again, and I...

www.audiosciencereview.com

That's hardly the most outrageous phenomena I've seen either - a review of the ESI Neva Uno/Duo series had a vocal sample with a dynamic mic that sounded like genuine telephone quality (no bass, no highs and noisy), and while a Swissonic Audio 1 had decent noise level on the dynamic it was kind of distorted (not to mention that both Audio 1 and Audio 2 seem to feature a bass boost of sorts).

 

[richlooker]

So, I should accept the current behaviour as normal and that there's not much they can do 'practically' to make the knob behave more like a normal smooth volume dial, correct? This stuff is a bit beyond me so I was thinking that maybe it was a simple tweak/fix on their end, and that they just didn't really consider it since they weren't making a mixer but instead an interface (where you adjust your gain once or twice and move on to the DAW). I'm always finding different ways to use my gear and always reveal these types of problems as I go. Maybe I'm the one needing tweaking/fixing, haha! Thanks for your input!

I think it's both normal behaviour and what should be expected. The pots in question are labelled "Gain", not "Volume", indicating they are not meant to be used as volume controls. They should be set such that the signal from the connected source will not lead to clipping. As you have not said what you are using it for, I can't make any assumptions about your setup and workflow, but I set and forget the gain, and use volume controls in my DAW and mixer.

/Richard

 

[richlooker]

This took some time; I have too many projects ongoing at once...

Voila, two 7-step precision attenuators to replace the gain pots. They should give -17, -10, 0, +10, +20, +30 and +40dB. Not exactly beautiful SMD work, but they'll do the job.

I have butchered a couple of cheap pots to make mounting flanges; they need a bit of drilling and machining, and will then be glued on top of the switches.

Nicely and securely mounted, now I need to shave off the flange that protrudes over the hex nuts, and make 4.2mm-6mm adapters so the knobs will fit.

It fits in perfectly in place of the pot.

 

[MCH]

This took some time; I have too many projects ongoing at once...

Voila, two 7-step precision attenuators to replace the gain pots. They should give -17, -10, 0, +10, +20, +30 and +40dB. Not exactly beautiful SMD work, but they'll do the job.

View attachment 362119

I have butchered a couple of cheap pots to make mounting flanges; they need a bit of drilling and machining, and will then be glued on top of the switches.
View attachment 362120

Nicely and securely mounted, now I need to shave off the flange that protrudes over the hex nuts, and make 4.2mm-6mm adapters so the knobs will fit.
View attachment 362121

It fits in perfectly in place of the pot.
View attachment 362122

Wow that took some work!
Probably too late now, but for these kind of small mods, I would recommend you jlcpcb. You can get a bunch of those PCBs printed any shape you want and shipped to you, probably for ca. 5 euros or less, being so small.

 

[richlooker]

Mission completed. Or, at least functionally. Aesthetically not quite there; I have to make the original knobs fit the tiny shafts of the rotary switches. I'll butcher the potentiometers they replaced, drill 4.2mm holes into the 6mm shafts from the back end, and fit set screws.

Output from the signal generator left channel (both channels read the same):

And reading in Audiotester:

Which is spot on.

Now I know I can just hook things up, fire up Audiotester and the M4, and AudioTester will display the absolute value of the input signal. With the original pots for gain control, I would have to calibrate Audiotester every time the pots had been touched.

@AnalogSteph you were right about the resistor values, even though I was lucky and hit exactly 0dB. The attenuation did not turn out exactly as planned; the next lower gain step was supposed to be -10dB, but it turned out to be -11.05dB. No big deal, I'll use the 0dB setting 99% of the time anyway.

@AnalogSteph you were right about the resistor values, even though I was lucky and hit exactly 0dB.

If it's really a pot rather than a mechanical encoder, I would assume it's being used to tap off a varying percentage of a DC voltage which is then filtered and fed into an ADC (8 bits as commonly available on micros would do and provide 256 discrete values, in which case I'd guess it would be a linear pot). Prototyping may involve fiddling with a bunch of pots to get the set points just right, followed by playing with resistor ladder values in simulation.

Gain control itself is probably a hybrid of THAT6263 control in 3 dB increments + digital gain adjustment for the intermediate steps plus extension on top (the chip itself only has a 42 dB gain range, -8 to +34 dB).

But why use THAT6263 in 3dB increments? According to the data sheet, it has a mode with 1dB increments.

Richard

 

[AnalogSteph]

But why use THAT6263 in 3dB increments? According to the data sheet, it has a mode with 1dB increments.

From the horse's mouth:

So what makes a 6263 a 6263 is precisely its 3 dB step size, you don't get to choose anything else.