Got a Mopho Keyboard and want it to sound better?

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Synthbuilder
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Got a Mopho Keyboard and want it to sound better?

Post by Synthbuilder » Mon Sep 16, 2013 5:55 am

I posted this on Gearslutz but it may be useful to some of you wigglers:

I've got a Mopho keyboard and it's a great little thing. But one thing puzzles me; when listening to any patch using a pulse wave, I notice the overall sound somewhat dirty sounding. It's not very nice really.

You can hear what mine sounds like here:

http://www.oakleysound.com/Mopho&Pulse.wav

I popped the hood and found that it is caused by the internal CVs to the voice chip that control the pulse wdith having some sort of randomised voltage added to them.

However, it is not only the pulse width control voltages that have this unwanted randomisation. All of the CVs do, but it is most noticeable on the pulse widths. What got me onto the right track was the fact that all the CVs had worse noise when they were set to high values. This points the finger towards a noisy reference voltage to the system's DAC.

The DAC used on the Mopho Keyboard appears to use a 5V reference derived from the +12V supply. Now, the 12V supply gets tainted by noise from the other parts of the electronics of the Mopho - but in particular from the -5V charge pump power supply which generates horrible 80kHz perturbations.

However, the real problem is that the Mopho creates the 5V reference by simply dividing the +12V rail with a pair of resistors (R67 and R71). This divided down signal, complete with a portion of the noise from the +12V rail, gets sent to a voltage follower (U18b) and then sent to the DAC. Oh dear... because this noise now ends up on the CVs. The higher the CV the more noise gets through.

Solution: Filter the +5V reference voltage before it goes to the voltage follower. I did it with a 10uF tantalum cap soldered directly across R71. The reference voltage is now nice and quiet. The Mopho behaves better, not only with respect to the pulse wave, but also seems smoother in general tone too.

The Mopho Keyboard is primarily built from surface mount components using lead free solder. This makes it slightly more awkward to work on than older through hole designs. However, it is quite possible to do a nice job if you take care.

You'll need one 10uF, 16V tantalum capacitor. You can use a decent quality electrolytic if you wish, and in that case, a 10uF, 25V will suffice.

Solder the cap across the resistor R71. With the keyboard to the front of the unit, the positive terminal of the cap should be facing left. That is the negative end should go to the ground plane which is attached to the end of R71 that is nearest to the number 1 of its name.

Image

And a closer view:

Image

Don't pull too hard on the cap once it is soldered in place because you might pull off the solder pads of R71.

For the Mopho Desktop:

This seems to be built in much the same way with the DAC reference being taken off the +12V line. I am guessing it too will have the same unwanted noise on the DAC reference. However, the board is different in several other respects and the parts numbering is different.

Now I haven't tried this but from online photographs I can assume that you will need to solder the 10uF cap across R52. This is situated, again above the DAC, near U12. The negative pin of the cap should be soldered towards the front of the unit, ie. the positive pin will be mounted nearest the R52 legending.

Tony

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Post by Tombola » Mon Sep 16, 2013 6:08 am

I don't have a Mopho, but this is a great post.

Do you have an 'after' sound sample?

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Post by Synthbuilder » Mon Sep 16, 2013 6:42 am

Tombola wrote:Do you have an 'after' sound sample?
Not yet. But in some ways there's not a lot to hear. The Mopho now behaves as it should - with the pulse sounding clear even at the extremes and not grainy like you hear in the above wav file.

Tony

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flo
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Post by flo » Mon Sep 16, 2013 6:48 am

Wow, thanks for that! Any idea about their other stuff, Tetra etc.?

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Post by mattysal » Mon Sep 16, 2013 3:03 pm

This is crazy! I always noticed this on my mopho keyboard, gonna get my tech to do this upgraded as i am super not cable of doing this myself!

I wonder if the dsi tempest has a similar "issue"?

thanks for the great work! I'm excited to see the response of DSI support team! I've heard great things about them so I wonder what they will say!
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Post by Skaput » Mon Sep 16, 2013 6:19 pm

Interesting, going to listen to mine now.

By the way what type of solder did you use for the modification, lead free or the good stuff. Just wondering if it would make much difference.

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Post by lvoemachine » Mon Sep 16, 2013 6:50 pm

This is cool. I forwarded this on to Carson Day at DSI. They might find this kind of interesting.

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Post by Paraphonique » Mon Sep 16, 2013 10:12 pm

Another vote for an "after" sound test. I'd really want do this mod on my Tetra.
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Post by Synthbuilder » Tue Sep 17, 2013 2:42 am

flo wrote:Any idea about their other stuff, Tetra etc.?
I've not seen the insides of any of the other stuff or had one to play with long enough to investigate further. I would hope they haven't repeated the same thing.

But just listen to the pulse wave at 99. If it sounds nasty and garbled like the wav file above then it could well have the same noisy DAC reference.

Tony

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Post by Synthbuilder » Tue Sep 17, 2013 2:44 am

lvoemachine wrote:This is cool. I forwarded this on to Carson Day at DSI. They might find this kind of interesting.
They already know about it. I've been in contact with them all last week. The support lady was very kind and helpful and has forwarded my e-mails onto the technical department. We'll see what they say but I haven't heard personally back from anyone yet.

Tony

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Post by Synthbuilder » Tue Sep 17, 2013 2:49 am

Skaput wrote:By the way what type of solder did you use for the modification, lead free or the good stuff. Just wondering if it would make much difference.
With my own gear I always use Pb/Sn solder - it's just easier to use especially if I'm moving things on and off several times which I need to do for comparison* purposes. For a customer I'd stick with whatever was used by the manufacturer.

* No wave files for the 'after' comparison from me for a while. I've not got time to do it this week. But dabbing that cap on and off while listening to the output made it perfectly clear it is a worthwhile mod to do.

Tony

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Post by flo » Tue Sep 17, 2013 4:49 am

Synthbuilder wrote:
flo wrote:Any idea about their other stuff, Tetra etc.?
I've not seen the insides of any of the other stuff or had one to play with long enough to investigate further. I would hope they haven't repeated the same thing.

But just listen to the pulse wave at 99. If it sounds nasty and garbled like the wav file above then it could well have the same noisy DAC reference.

Tony
Will check this on my Tetra and report back...

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Post by widdly » Thu Sep 19, 2013 8:47 am

Just did the mod on my mopho and it sounds good. Before with the pwm at 99 it sounded like the sample Tony posted. Now with PWM set at 99 it sounds about the same as when the PWM is set at 1. I used an electrolytic because I didn't have a tantalum.

:tu:

JohnLRice

Post by JohnLRice » Mon Sep 23, 2013 3:55 pm

That's awesome, Tony! :yay: :hail:

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Post by pre55ure » Mon Sep 23, 2013 5:55 pm

Wow

I'd love to know if this type of thing is a problem on any of their other boards as well. I have an Evolver that I've always noticed has sort of a weird "extra modulation" happening on the analog waves. I wonder if it might be related to something like this. :hmm:

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Post by computer controlled » Tue Sep 24, 2013 1:21 am

Huh, i never noticed this before. I don't have a Mopho anymore, but if i end up with one again, i'll most certainly do this. If anyone does this with the desktop, post a pic!
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Post by wavehead » Tue Sep 24, 2013 4:10 am

damn that looks like some serious corner cutting, but i guess anybody mass producing this stuff at "competitive prices" is going to have a hard time not cutting corners.

makes me wonder what else is going on in some of the DSI stuff that might be partly the reason some of their gear gets a bad rep.

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Post by dafus » Mon May 25, 2015 5:22 am

Thank you Tony for this post. I just did it and it definitely sounds much better than before.
At least for the high Pulse range. Not sure it affects other OSC shapes too but it certainly "feels" better. :hihi:
60€-cent for the Tantalum and 15 minutes unscrewing, soldering and screwing it back on - totally worth it!

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Post by brokensolderingiron » Mon May 25, 2015 2:48 pm

Synthbuilder wrote:
lvoemachine wrote:This is cool. I forwarded this on to Carson Day at DSI. They might find this kind of interesting.
They already know about it. I've been in contact with them all last week. The support lady was very kind and helpful and has forwarded my e-mails onto the technical department. We'll see what they say but I haven't heard personally back from anyone yet.
Tony
Great find and solution!

Wat's puzzling is despite DSI designed in RC smoothing filters on most CVs and the DAC is 16bit yet PW was rattling that bad and clearly no more within specification of the 16bit DAC rendering to 9 10 bits was fixed to a lot healthier state with a simple cap.
Did DSI fix the Mopho desktop "dead oscillator cycles" that was found on SDIY list 6 years ago?(Most noticeable with bass sounds)

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Post by Karg » Thu Jan 28, 2016 5:14 am

Hi,

inspired by Tony's mod, I improved my Mopho Desktop. Sound is much cleaner now! Thanks for sharing your insights!

I also added a USB port, indicator LEDs for clock and notes played, and as a bonus some wooden sides.

Image

Detailed instructions are published on my blog: http://karg-music.blogspot.de/2016/01/m ... hosis.html

, Karg

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Post by Ahinga » Thu Jan 28, 2016 10:26 am

did anyone do this mod with an mopho x4?
i have not opened it yet, but discovered at least 2 dacs on pictures...

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Post by catzrcool » Fri Jan 29, 2016 3:03 pm

Great tip on the DAC-filtering, I'll have to get around to doing it on my Mopho. But is there any technical reason to use a tantalum cap?

The thing is, there are some pretty strong ethical reasons to forego tantalum whenever possible, it's mined under horrible conditions and the ecological impact is bad.

One poster mentioned that he used a regular cap, if that works the same, that's what I'll do, and that's what should be recommended imo.

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Post by Synthbuilder » Fri Jan 29, 2016 3:32 pm

catzrcool wrote:But is there any technical reason to use a tantalum cap?
None at all. You can use any decent quality electrolytic too. I just used tantalum because I had a bag full at the time. One good thing about them is they do have a resin case which makes it less likely to melt when soldering them - which is quite easy to do when soldering the device from the topside of the board.

Tony

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Post by catzrcool » Fri Jan 29, 2016 4:24 pm

Allright, thanks a lot for the help!

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Post by Karg » Mon Jun 13, 2016 4:02 pm

While I do not have a Mopho 4x, in the Tetr4 it is also two DACs. Each of them covering two voices. Here the mod works equally well.

Locations are the following:
Image

Text description is posted elsewhere: http://karg-music.blogspot.de/2016/06/t ... hosis.html

, Karg

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