Help with Vintage discrete console direct out mod and grounding

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Vyvyan

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Nov 25, 2020
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recently scored an allen & heath quasi (quasi-modular) 10 channel mixer. I have removed the channels and retro fitted them into an empty (psu is still there) Yamaha m512 im using its recapped psu to power the channels as both mixers use 24v, leaving the -24 floating. ive recapped the modules, raised input coupling from .22 to .47, i have some wima 1uf and 3.3uf caps i may try to. im looking for help with coupling caps values like the 10uf out of the eq, its going to 33k ohm load, if i change it to 3.3 wima
that would give me 1.46hz -3db point, correct? any other suggestions of the coupling values?

Looking for help with direct outs, im taping the post fader buffer just before the pan pot, i have plenty of 600:600 transformers, but im not sure if what the post fade buffers output impedance is and if can drive a transformer. direct out of the post fade buffer with a 330uf output cap sounds fine but i would love to get some use the transformers i have im also not using the echo send or summing if that matters, i may use the pre fade foldback as an echo send

for grounding im a bit confused with the pcb boards. i plan on having all chassis sheild and signal grounds meeting at the psu, but the original quasi E buss is connected to the casing of the foldback output pot would that give me noise if the pot connects to the chassis from multiple channel pcb's or will the chassis just act as the ground buss back to the psu?

all help appreciated

here are the schematics, sowter 200:7k2 mic in transformer has been left out of the schematic but its there

https://www.allen-heath.com/media/QUASI-SCHEMATICS.pdfhttps://www.allen-heath.com/media/QUASI-USER-GUIDE.pdf
heres schems for their minimix which has similar preamp design and may be easier to read (mod1 mod2 and mod3 are also similar)

https://www.allen-heath.com/media/MINIMIX-MANUAL.pdf
 
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for grounding im a bit confused with the pcb boards. i plan on having all chassis sheild and signal grounds meeting at the psu, but the original quasi E buss is connected to the casing of the foldback output pot would that give me noise
The casing of a pot is just more shield and it's generally ok to connect shielded surfaces and shields of wires together in whichever way in convenient. The exception would be any shield coming from outside the enclosure which should be connected to the chassis as close to the entry point as possible. And phantom power returns through the chassis (usually) in which case you want to make sure that path is solid and neat to the power supply ground. Signal grounds (which is frequently referred to here as "0V") should not be connected at more than one point. In an ideal scenario each channels 0V would be connected to each other only through one point at the power supply. But as a practical matter there is usually a ground bus giving you more of a branches-of-a-tree arragement as opposed to a star-ground system. What is critical is that you don't want loops because EMI can induce currents in that loop which will become part of signal since the signal is just it's relative to 0V (in unbalanced circuits anyways).
 
If I may ask, is there a part number with the input Sowters? I would be interested in that.
Re the output transformer drive capability, is there a schematic of the after fader buffer?
EDIT: found it from your other thread
There´s nothing wrong with trying out, if you plan to drive a really a 600 Ohm input it could be a problem with high distortion, but a 10k interface input it should work (a little distortion is the point of using this mixer?).
With the pan pot turned fully one way and fb and echo turned up, the buffer is already driving at least 1kOhm (4x 47k, both 4.7k and one 1.8k in parallel), another 10k in parallel would not make much difference here...
QUASI-SCHEMATICS (verschoben) 1.jpg
 
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If I may ask, is there a part number with the input Sowters? I would be interested in that.
Re the output transformer drive capability, is there a schematic of the after fader buffer?
EDIT: found it from your other thread
There´s nothing wrong with trying out, if you plan to drive a really a 600 Ohm input it could be a problem with high distortion, but a 10k interface input it should work (a little distortion is the point of using this mixer?).
With the pan pot turned fully one way and fb and echo turned up, the buffer is already driving at least 1kOhm (4x 47k, both 4.7k and one 1.8k in parallel), another 10k in parallel would not make much difference here...
View attachment 86923
yes definitely using for color/saturation. im actually tapping off the 47uf (ive changed to 330uf) before the panpot and echo send, im not doing any summing just 10 in 10 out, so ive removed the pan pots and everything after the final coupling cap. also the foldback is pre-fade/buffer,its signal is just coming in from the previous stage on the schematic
 
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That little BC168 emitter follower is going to struggle driving a 600:600 transformer and the signal level at that point in the mixer is likely to be in the region of -8dBu or so. The worst case load on that poor little transistor is just over 900 ohms (with both AUXes fully up and pan set hard left or right). With a 24V supply and the emitter at 12V the quiescent current would be 12mA. This represents the peak current that can be put out into the load. 12mA peak is 8.4mA rms which into a 900 ohm load which is 7.56V or nearly +20dBu. This ignores a whole bunch of other factors like distortion but this represents the absolute best it can be. If you load this with another 600 ohms the load drops to 360 ohms and the maximum output drops to about +12dBu.

So, as others have said, you really do need to be thinking about the direct out driving a 10K load. Do you really need a transformer. Why not just make an impedance balanced output. It really need be little more than a couple of 100 ohm resistors.

Cheers

Ian
 
the unbalanced direct out from the 47uf with everything after it removed does sound fine, but i have around 9 or 10 quality 600:600 transformers, that im trying to get some use out of. i have a A&H schem for direct outs of its big brother the the 1975 mod2. maybe i can just scrap a few channels and use the parts to replicate that, im squeezing and Yamaha pm-2000 channel and 3 pm-1000 channels in the chassis as well and possibly retrofitting a stereo 500 series mix buss comp & eq so i dont need all 10 of the A&h channels, there are extra unused holes and traces for components after the pan pot possibly intended for that option. looks like it has two outs for some reason tho which is confusing me. i have run one channel through one of the transformers briefly and didn't notice much difference compared to the unbalanced out, so im not sure how much of a difference it will really make.

im also thinking of just making a passive transformer box with all my spare transformers and just leaving the mixer unbalanced, that way i would have the option to patch channels thru the transformers or go unbalanced and ill be able to use the transformers with other outboard gear
 
i also have thought about using the line amps i pulled from the yamaha m512 and an m916 and coupling to those post fader as an output amp, they are discrete and made to drive a 600 ohm transformer
 
i also have thought about using the line amps i pulled from the yamaha m512 and an m916 and coupling to those post fader as an output amp, they are discrete and made to drive a 600 ohm transformer
They would be more likely to work. A lot depends on the actual spec of the transformers you have. Which ones are they?

Cheers

Ian
 
i have 8 from the m512 and 916 tamura ga81720 and the two collins freed in this picture
 

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heres the yamaha line amp
 

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The line amp looks fine for driving a 600:600 transformer. I cannot find any spes for Tamura audio transformers like the ga81720, even on the Tamura web site so I cannot say if the little emitter follower would be able to drive it. Line amp looks like the best bet.

Cheers

Ian
 
gonna try it out tonight using a 3.3u wima coupling off the fader. will i run into any problems powering the allen heath pres with +25 from the yamaha psu using ground as 0v and running the yamaha line amp with a separate +25 rail and -25 from the same psu?
 
gonna try it out tonight using a 3.3u wima coupling off the fader. will i run into any problems powering the allen heath pres with +25 from the yamaha psu using ground as 0v and running the yamaha line amp with a separate +25 rail and -25 from the same psu?
Off the top of my head I don't know. They would both have the same ground reference so my initial thought is is should be OK.

Cheers

Ian
 
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