gssl mixbus compressor

this page is about how i built my individual gssl mixbus compressor, what did work and what did not and also for what reason. thanks to the guys from the prodigy-pro forum for helping out and making all this possible. the shematics are taken from the great fellas from gyraf audio and the addtional boards i got from expat audio.


note: this page is absolutely without any profit orientated purpose and nothing in it should be used as such from anyone. this is supposed to be an educational lecture for its reader. so take a minute and read it if you like, it might help you.


getting started

when i decided to build a gssl mixbus compressor in the first palce, i stumbled upon many different sites which tell very differently about how to build and modify the unit brought to you by gyraf audio and it took a while until i found out which version might be good for me. i, myself decided to go with pre-trimmed that2180bl chips for the dbx202 substitution circuit as well as for the sidechain vcas. furthermore i decided to add the turbo- and the crc-board from gyraf audio and the addtional boards i got from expat audio to my unit, as they appear to give it some very usefull features. i did not have the gerbers for those pcbs so i decided to order them once and scan them in case someone might need them again. so, if you need them you can contact me or just spend a few euros and support those great guys from expat audio. read more about those mods here and here.


when going with that21080bl vcas there are some minor changes you got to take care of concerning some resistors and a few other components. you can find most of its changes at matt's diy page which is where i found them. there still is a questionmark on matt's page concerning wether or not to change the 100k resistor for right unity gain. this thread over here clears that one for you, as it should be replaced by a 127k resistor. to get this value i just hooked up to resistors in series. also note that the 3k9 rsistors in the vca circuit are replaced by 5k1 ones, but the ones in the sidechain path stay the same and should not be changed (this also counts for the ssl turbo pcb).


okay, let's go

first of all i ordered most of the parts from reichelt, which is a pretty good spot to get most of the parts. i put up a list with all the compoments you can easily get from reichelt here. but be careful, i found out that some parts reichelt offers are not very well suited for the circuit, but we'll come to that later. while waiting for the parts to arrive i prepared to get the pcbs done, so i can start right away.


first parts arrival

the pcbs from expat audio

i made those with a technique called "direkt-toner method" about which you can learn here. it's not the most reliable way to make your own pcbs, but ones you figured out how it might work for you it is pretty handy. but always be carful when working with acid and other unhealthy materials - keep the windows open. i used natriumpersulfat for the acid bath which is more clear and less dangerous. but therefor you have to make sure you are working with a constant temperature which sometimes can be very hard.


using a reichelt catalog to transfer the prints

the transfered printing

with earlier pcbs i did for different and smaller circuit, i never stumbled upon any serious problems during the etching procedure.they used to turn out to be pretty well, so i thought i might be on the safe side. but my first and second try (with the ssl pcb) both turned out to be very bad. the acid somehow got its way through the toner and i head to help with an edding a little. the fourth and fifth time around i got it right and sure was lucky i put on my gas mask during that pretty long period of time. be carful when wahsing off the paper, not to rub to hard, cause you might make the toner briddle. hope you got more luck with that.


bad pcb turn-out

but in the end it worked


the mounting

as you can see its with this method it's also possible to make a print for the part placement on top of your pcb which i found very helpful. i checked all the pcb traces so there won't be any shorts or bad connections within the pcb printing and went on. the next step was to solder everything together and to check every value again befor mounting it. lot of work around here.


only a few resistors mounted yet

almost everything in place

but a few hours and a dizzy head later everything seemed to be in place. then i moved on to connect everything and to wire the power connection. always be careful when handling main voltages your friends might thank you. at this point it is a very clever idea to check the voltages on each pin and the main voltag rails while comparing it to the shematics, just to make sure you that did everything right so far. however i didn't do that, because i wanted to get it finished which just brought me some more trouble with the circuit later on. so don't be silly and check it.


finished turbo board

everything connected right


getting it in, making it work

next up was drlling the holes for the 19" rack and try to fit everything in. drilling the holes was a real pain in the ass, as it did not appear to be as easy as thought. but as always, try a few times and you'll get it right. if you want to get sure put on some glasses and breath through a mask, it might take longer than you think and it's not the most healthy thing to inhale.



first fitting tests

cables everywhere

for the peak meter a used an old sifam meter i took out of an old unit i had laying around. so i got lucky, cause theses things can be quite pricy. but so far everything looked fine and i was ready to turn that thing on.


drilling a huge hole sucks

nice vintage meter in there


the trouble

when i first turned the unit on nothing seemed to work. not even the power led did light up. some restless hours later i found a short within the +12V rail which caused the +12V to break down. that was when i decided to take all chips out again and start back from the begining and measure if everything is fine. after i solved the problem with the +12V rail everything seemed fine and all voltages were correct. so i tried to turn it on again and again - nothing. i had no idea what could be wrong and searched and searched. i discovered by trying this procedure that my +15V trace within the ciruit not seemed to be stable. it always stuck at 0,6V when any of those ics was mounted. without the chips everything was fine.


i just couldn't figure out what the problem was. i tried replacing all the fixed voltage regulators by new ones of the same brand, replaced several parts, look for shorts for days. i even started buiding a second mainboard from scratch just to make sure everything was done right, but wasn't able to solve the problem and it occured again. almost by accident i discovered this thread which was about exactly the same problem that i had and explaining why it occured in the first place. it seemed like reichelt offers a bad series of voltage regulators which are very easy to latch-up. the bad one was labeled TS7815 and swapping it with a new one from st-brand solved the problem for me.


there is a mod you can find on the net which solves the problem without exchanging the regulator. all you need is a handfull of parts (2x 2 100nF\50V ceramic, 1 x 1N4007) and it is said to be working. i only could find this thread about it, so check it out, and you might get happy. the picture itself is quite self-explaining, so i don't think you'll need the shematics.


reichelt's bad voltage regulator

modification to get your unit working



after replacing the bad regulator everything worked pretty well. the unit is quiet as hell so the crc-board seems to do its job and the turbo-board's oxford mode is really worth it. i made it switchable so i am able to compare it and it sounds intensely different from the aahrus mode. quite more open to my ears and i like it very much. i painted everything i needed to know right on the front panel because i don't think i got any money left for some nice printings and also don't really care. there are some minor spelling mistakes on the unit due to lack of space but that makes it even more unique. maybe someday i might get a custom made frontpanel just for the sake of it, but so far i am fine and glad that i got it working.


the finished baby

and again!


listen up

here you can listen to an example of what the compressor might sound like. the sample is a rough drum part first of all totally unedited, and then fed through the gssl pretty hard at 10:1, long attack short release.


drum_rough.wav / drum_hard.wav


thanks a lot for reading it, and thanks to all the diy guys out there, that made this possible. the next project won't be far as it seems to be some kind of an addiction..