Modding Motu 24 i/o - no regulated PSU?!

Hi.

I recently bought a used Motu 24i/o and after some test to make sure the unit was in perfect condiction i opened it up to see what's happening.
I must say i was impressed with some building details such as separate tranformers for analog and digital circuitry - both linear psu's-, all schotky diodes with huge capacitances in smoothing and a low dropout burr brown regulator for the digital circuit.

Now the strange stuff. The psu for the analogue circuit is not regulated, just the schotkys and 9400uf of capacitance for each rail (-10v and +10v) ???;
Also the unit doesn't have any input or output coupling capacitors... Dc coupled? those NJM2115's and JRC4580'sdon't look like precision opamps to me... ADC/DAC chip have built in hi pass filter, so no DC on the ADC/DAC.

1 - Should i work a way fit some 7806's on the positive and negative rails?
2 - Leave as it is, makes no difference?

Already swapped two NJM2115 for LME49720 on one channel. Not yet tested the sound but the modded channel gave slightly better results in Rightmark Audio Analyzer.

Any opinions are welcome.
thanks!

I wouldn't implement 7806/7906 regulators, beacouse you'll get smaller voltage (read smaller headroom).

I've made similar mod on MOTU 24 I/O, but probably on older unit that you have. it had 5532's on analogue I/O, so i put some Burr Brown instead (I don't recolect witch one exactly)

Sonic improvement was huge. the goal was to make converter to work in manor that there is no audiouble degradation of recorded signal (before mod there was distinct difference between sound on monitors before and after AD/DA conversion).

If my memory is correct, it had +/- 5V rails on analogue portion of circuit. if you have noise floor in parameters described in technical manual, I'd leave it be with out regulation, or if you really make serious mods, IMO would be better to make +/- 15 V supply for analogue part (but you should check in schematics if they are available to you). maybe even +/- 17V for LME49720.

Did that motu had psu regulators?

yes, it had

Well, it measures -98.7 db noise floor on RMAA. Motu says it has a 108db dynamic range (Same as converter chip datasheet).

on RMAA you see dBFS scale if i'm correct. in that case, to obtain dinamic range of 108 dB, you should have noise floor on -108 dBFS, so you are short.
If you measure -98 dBu, then yo are in the ballpark (-98 dBu + 4dBu = 102 dBu, signal is calibrated to +4 dBu, plus, few dB of headroom)

rmaa shows in dbA  ???

Are the opamps smd or dip8 types? If dip8 .. sockets?

smd's

dBA is A-weighted dB curve.... find more info on the net (wikipedia or something)

let's say this way:

dynamic range of device (in this case, converter) is difference between noise floor and maximum allowed signal. i don't have MOTU 24 i/o manual at hand, so I really don't know what is maximum level for your converter, but you can find that info in your's manual.

soooo... it's supposed to be less than the dynamic range of the device?

Anyway, the motu measures 1 to 2 db worse than my previous terratecs ews88 that were fed by the computer's "noisy"switching psu...

maximum allowed input minus dynamic range = specified noise floor

if noise floor is in the neighbourhood of -100 dBFS, it should be OK

hmmm... ok. but i wonder if i can clean the noise up by adding a basic regulator like lm7808 or lm317.
Or do you think the differences will be too small?

if you measured -98 dB noise floor it should me noisless. 10V input is too small for proper operation of 7808 (according to datasheet, in/out difference should be 3V minimum). with that level of noise floor diference would be negliable, plus, you'll decrease headroom on opamps, and that could make your converter to sound "thinner"

:-*

Hi.

Did an harcore mod.
I removed both tranformers out of the case to an external box with a switch and fuse holder connected with a 6 core X 1mm cable.

Thd before:



Thd after:



Noise before:



Noise after:


Now i'm thinking of swapping the transformer for amother that works at +15/-15 and add a lm317 regulator for the analogue side.

How can i clean that 100hz harmonic?  ;D

great results... congratulations.


100 Hz.... well, fist thing are quality input caps....
second... i like to make PI filter with inductor
http://www.tpub.com/content/neets/14179/img/14179_229_3.jpg
I like to use for that toroid high permeability powder core (Al=2000 uH, or more), and make few wounds of laminated 1mm copper wire.

make note that you are already have GREAT results and further improvements are questionable (as result and cost), alldough, that PSU mod (+/- 15V) will make great difference in audiuble performance.

in my experience, if OPAMP work with voltage max recommended, gives "more juice"... for example... devices with NE5532 that nominaly work on +/- 12 V sound rather "unnatural cold and thin", but, when You rise suply to +/- 15V, device "sings", due to improved headroom

regards, Ivica

Bump for this thread !

Great reading. I once started modding my 24 I/O, but at a certain point I decided to call it a day. I knew the psu was a problem, but I didn't want to search for a suitable transformer (spec and sizewise), and to build an external PSU for it.... well, I wanted to keep the PSU inside the original unit.

However, your results are simply great, and I'm going to follow your lead !

Unfortunately I'm not able to open up my 24 I/O right now,  because I have a mix to do these days, but when that's done I'll take out the transformer as well. And probably replace the transformers as well, maybe build a decent chunky superregulated PSU for it.

About my previous attempts of mods :

I have switched a few opamps on the outputs, about a year and a half back. I did some thorough listening tests, but I couldn't tell the difference. This wasn't an upgrade. I remember that I've placed 2 BB and 2 AD's, but types... I can't recall , I'd have to open it up again.
I googled a lot for possible opamp mods for it, but most conclusions were that it's not worth all the effort. I also looked for doing some additional decoupling for the opamps, but... (maybe I'm wrong), the opamps were driven by a single voltagerail. But as said, maybe I didn't look close enough, or maybe I just don't remember correctly.

So, I came to the conclusion that the PSU was the main problem, and I wasn't ready for messing with that, because of this simple reason : No audio interface = No studio.  However, if it's possible to change the psu the difference between opamps will become clearer. (and also what Elektroivo says : the opamps will perform better).

When the mentioned mix is done, and I'll take out the PSU, I'll have a look again at the PCB's, because I'm very interested in swapping the complete PSU for a steady dual voltage PSU (if possible) . Also because the original transformers generate hum (maybe electronically, but also in a physical way).

Boy, I'm getting excited again about modding the 24 I/O.

If you swap the transformers into an external box put also the rectifiers and some big caps inside. Maybe the 50/100 Hz problem will be gone afterwards.

This is crazy! I didn't think PSU mods would be so noticeable on the RMAA!
I been swapping opamps on my motu and echo units, improvements were audible, but barely detectable in RMAA, next I was thinking to try transformers/tubes to see if I can get more smoothness. But after seeing your results, PSU mod goes up on the list...
A 1RU soundcard with 2RU PSU - wouldn't that look funny...