modding a behringer autocom mdx1200

[AK79]

yes, changing C4 is a good idea imo. I was concentrating on the input and output so far, beacuse there is is the most potential to improve things easy.

Output balancing: May be we can get away with impendance balancing, this would mean to insert a 47Ohm resistor between X2 ring and ground. Would improve cmrr at the other end(?). but the line driver is a nice idea. The question is how much current the supply can deliver, somehow I´m crap at calculating such things    For one second I had quad input buffers in mind, so all in all there would be much more chips in there.

Will try to replace the BE037 and see what happens. I ordered some fancy Analog Devices chip. Why they came up with a own op-amp ?
I have no idea, maybe they just wanted to get their own chip manufacturing going but keep costs down at the same time.

also will replace R9 and R10 with 10k - i really don´t need 100k Input impendance in my setup. Get rid of some more noise with that.

 

[dagoose]

Just for testing i removed one be037 and tested it with a tl074 which i had and it's working fine with it and i didn't hear any difference between the tl074 and be037 so they are basically the same. I will replace it with the mc opamp when i have them in. Just that you know thzt you can safely dismantle the BE037  ;D

 

[AK79]

thanks for that hint.

I put in a AD467 - which the unit seems to hate. Max Output setting gave gross distortion. I found out that the combination of AD467 and NE5532 does not work in this circuit, maybe
I would have known that before but i was too lazy to read the spec sheets  ;D

Any way now the 5532 was swapped with OPA275 and here we go : 4dB better THD + N compared to the 2nd channel which I left untouched.

Also C4 was changed to 220uF.

I´ve added a picture to show what the bigger capacitors do to the frequency response.

Blue : C24 & C4 increased to 220uf
Orange : C24 & C4 standard 22uF

 

[AK79]

P.S.

When compression is engaged, the modded channel maintains its THD+N while the other channel is getting worse the more compression is introduced.

 

[dagoose]

OK, i think my autocom is finished for now!

Replaced the output opamps with opa2277 and the tl074 and be037 with mc33079 and placed 100nf bypass caps on all of the opamps. What i think is funny is that originaly there is a 10uf elco near the left channel on just the + for decoupling, not in the -, weird.. or not? You can also see it on the schematic.
The output elco's are panasonic 220uf now and, except for the output caps, i bypassed them all with wima 100nf's soldered to the bottom. The 20k resistors on the input are now 10k metalfilm which gives it just a bit more gain and so theoretically a bit less noise.

I didn't do any measurement because i didn't do that before starting off, to me the sound is more important then numbers but i know it will be better.
The lowend is a lot better now and it sounds more open and transparent then unmodded so the mods really made quite a difference. To me it's a really nice sounding compressor now that will do a nice job as a busscompressor i think. I have to use it in practice now but i think i will like it!

 

[Script]

Good to hear that the 220uf really seem to do good to the freq response

The "dynamic enhancer", I'm not so sure about. What's it with this gimmick? I'd probably drop it altogether...

Did you also change that LM339 (IC14) in the sidechain?

 

[AK79]

Hi Dagoose,
nice to hear you are satisfied with the mods. I´m also for now, though I will try some other stuff just to experiment if certain things i´ve read about are worth the effort. Overall I like the sound of my unit, theres less distortion, which was probably the main cause I did not like this thing before. I compared it to a FMR RNC, which is a good one amongst the el-cheapos and must admit my moded MDX sounds a bit more exciting than the RNC now.

For the 10uF - I can´t find it in the schematic....

I also thought to use it as a busscomp but then we´ll have to add a stereolink, any ideas where this has to be ?

@Script:

I lkie the enhancer, for it is after the compression, which gives a interesting effect. Will probably try to make the filter sound less harsh.
No plans to change the LM339 - I think it wouldn´t make much difference .

 

[gtrmann]

I have a few of these in my studio rack.  I really like compressors that can really ugly up a signal,  these cannot.  The attack time is not fast enough.  I have been changing the values of 4 resistors to quicken the attack and release times.  I changed R 145 and R154 from 470 ohms to 47 ohms to modify the attack parameters.  I changed R147 and R156 from 15K to 1.5K to modify the release parameters.  I just breadboarded a gated sine wave generator so I can examine the attack and release times on my scope.  I am still messing with them,  so I might change these values in the future.....  If anyone is interested....

 

[gtrmann]

Ok,  I missed this completely.  After I started testing with the gated generator I found the resistors I mentioned in my earlier post had little to do with the attack times.  The key seems to be C51 and C55.  They are 100nF .  I replaced them with a value I had on hand 1500pF.  This really quickened the attack time.  There is a trade off here with fast attack times and filtering.  I expect to listen to these this weekend and tweak this value !!

 

[BYacey]

I've seen a few mentions here of changing the carbon resistors to metal film. I doubt you'll even find any carbon resistors in the unit; most everybody uses metal film nowadays. I don't even know if you can buy carbon resistors anymore unless they are new old stock sitting around someplace.

 

[JohnRoberts]

I've seen a few mentions here of changing the carbon resistors to metal film. I doubt you'll even find any carbon resistors in the unit; most everybody uses metal film nowadays. I don't even know if you can buy carbon resistors anymore unless they are new old stock sitting around someplace.

Huh?

Carbon film are still widely used AFAIK.

JR