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Presonus Firebox CV out success!
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author Presonus Firebox CV out success!
gzifcak
hi all,

i think this may be my first post. i just wanted to let everyone know that i successfully bypassed the DC blocking capacitors in the output paths of a Presonus Firebox audio interface, giving me 6 CV outs. i'm sequencing an SH101 right now from a Reaktor sequencer via outputs 1 and 2. if my multimeter is telling the truth, i'm getting +/-8 volts, and i am clearly getting a very wide range of pitch CV.

just to clarify (in case anyone hasn't heard), you don't need MOTU Volta to do this, just a program that will let you send numbers/MIDI values out of your audio interface. Max, Reaktor, and PD are perfect.

hope this inspires someone else,
greg
criticalmonkey
photos of the hack?
gzifcak
sorry, no photos, it's back together again. i can describe it though:

i basically just bridged the legs of six 10uF capacitors on the upper board. there are eight in total, all along one side, but i left the headphone ones intact, just because it wouldn't offer any additional functionality (built in mult/attenuator i guess? but i can just do that in software if i need to).

if anyone needs more specific information, let me know. if there is enough demand, i maybe convinced to open it and snap a pic.
skrasms
Have you measured the output noise?
gzifcak
what do you mean by output noise?

i measured the DC voltage with no signal present, and it's zero, so it should be fine to still send audio through it, although i don't really use if for that anymore.
os
Good work!

Have you noticed any drift in the DC voltages? E.g. leave it outputting the same voltage for a while and see if it holds steady. You'd soon notice it if e.g. your pitch was drifting.

I seem to recall that was mentioned before as a possible issue when using DC-coupled outputs on interfaces not specifically designed as such.
tragedybysyntax
oh wow.... using reaktor?!?!?! can you post a clip?
os
Sounds like people are using all kinds of software to generate CVs nowadays. Max, Reaktor and pd are the ones that come up a lot, though there was also that guy who was using Reason for CVs.

And on the other side of the fence you have Volta and Silent Way. I guess it's a personal preference thing - some like to roll their own, others like something that works out of the box.
wetterberg
hey, KUDOS for doing this! I don't think I've ever heard of anyone actually DOING the mod!

SlayerBadger!
gzifcak
os wrote:
Good work!

Have you noticed any drift in the DC voltages? E.g. leave it outputting the same voltage for a while and see if it holds steady. You'd soon notice it if e.g. your pitch was drifting.

I seem to recall that was mentioned before as a possible issue when using DC-coupled outputs on interfaces not specifically designed as such.


no droop as measured by my multimeter. i got a steady 8.882 v while sending a "6" out of reaktor. i thought -1 to +1 would equal the max output range, but i guess not.
gzifcak
tragedybysyntax wrote:
oh wow.... using reaktor?!?!?! can you post a clip?


clip of the sequence (new benefit of this paradigm: rendered audio of CV patterns!) or clip of the 101? it just sounds like a standard 16 step sequence i wrote. i'm psyched about sample accurate timing, i never could get MIDI sync super tight coming out of my computer.
rezzn8r
eek!
supercool! I have a firebox...and some upcoming vacation time...hmmm
skrasms
gzifcak wrote:
what do you mean by output noise?

i measured the DC voltage with no signal present, and it's zero, so it should be fine to still send audio through it, although i don't really use if for that anymore.


I mean the AC that rides on the DC voltage. It can be measured as RMS AC voltage. DC measurements can hide problems that exist here.

Think of it this way... all these audio interfaces people are modifying have only been designed to work accurately in the audio band. We are now using them for signals that are below audio frequencies, all the way down to *infinite* octaves lower. No one seems to be doing any measurements on the performance between 0 and 10 Hz, where previously noise performance was not an issue. I asked MOTU about this range in their interfaces, and they gave me an audio THD spec for DC performance. The idea of a THD measurement on a DC voltage was worth a laugh, but it showed how little attention people pay to this stuff.

I have problems with my Audiofire12 because once I removed the DC-blocking caps a lot of noise showed up on those channels. It doesn't show up when measuring DC, but an RMS AC meter shows that it is an order of magnitude greater when I bypass the output caps. It is very audible when controlling oscillator pitch.

It is one thing to know that "DC" comes out of an interface, but that doesn't mean it will make a good CV generator for a synth. The synth community should come up with some measurements differentiating between what is good and what is merely better than nothing.
gzifcak
skrasms wrote:

I mean the AC that rides on the DC voltage. It can be measured as RMS AC voltage. DC measurements can hide problems that exist here.


oh yeah, i did measure the AC and got 0.000. i haven't tried measuring AC while outputting DC, but my oscillator pitch is very clean.
skrasms
gzifcak wrote:
skrasms wrote:

I mean the AC that rides on the DC voltage. It can be measured as RMS AC voltage. DC measurements can hide problems that exist here.


oh yeah, i did measure the AC and got 0.000. i haven't tried measuring AC while outputting DC, but my oscillator pitch is very clean.


Thanks for letting us know.

I did some more testing with my Audiofire and found out that it is actually the ultrasonic noise giving me problems. It is aliasing back into the audible spectrum in my digital modules. It's a good test of anti-aliasing filters.
Sound
Congratulations gzifcak.

Here is a thread where Bird Flu was asking to do it. There are photos. maybe you can tell us wich are the capacitors.

https://www.muffwiggler.com/forum/viewtopic.php?t=7182

Regards.
gzifcak
i saw that thread when i was getting ready to start. it's not any of those capacitors; those are on the lower board, and the correct ones are actually on the upper board. they are the eight 10uF electrolytic caps grouped together on one side of the upper board.

hope that helps,
greg
Analog>Plugins
skrasms wrote:
gzifcak wrote:
skrasms wrote:

I mean the AC that rides on the DC voltage. It can be measured as RMS AC voltage. DC measurements can hide problems that exist here.


oh yeah, i did measure the AC and got 0.000. i haven't tried measuring AC while outputting DC, but my oscillator pitch is very clean.


Thanks for letting us know.

I did some more testing with my Audiofire and found out that it is actually the ultrasonic noise giving me problems. It is aliasing back into the audible spectrum in my digital modules. It's a good test of anti-aliasing filters.


Yeah the Audiofire 12 will light up your mixing desk led's even when you can't hear a sound because of the high frequencies that you cannot hear being so loud.
lys.//
sorry for necroposting figured these cards are so cheap ($10-$50 USD on eBay right now) it's worth boosting.


I completed this hack last night on an old Firebox and took a photo.

Used a multimeter to find out which cap leads back to which output. Desoldered the caps and bridged the connections.

Tested the output from Ableton's CV Tools package and getting -9 to +9v range on all six outs!

BTW the old Lion drivers on Presonus site still work great on Mojave w00t

Cheers to the OP who first attempted this back in 09 thumbs up

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