Help with Allen & heath quasi (70s discrete mixer) recap and mods

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Vyvyan

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Nov 25, 2020
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I recently got this little mixer I have been recapping it but want direct outs just not sure where to tap it. I have some 600:600 and 250:600 transformers and wants a few channels to transform balanced outs and the rest can be unbalanced.

Also if you have any mod ideas or suggestions for different cap values I'm open to anything. Already changed the 250uf and 125uf in the mic amp to 220uf, I also have some 330 uf and 470 if laying around that I might fit in those spots for a few channels.

Attached a link where u can find the schematics and block diagram.

Thanks Quasi | Allen & Heath
 
Last edited:

Vyvyan

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Joined
Nov 25, 2020
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5
bump!

The input cap after the transformers are .22 tantalums, are these replaceable with film?
 

Vyvyan

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Nov 25, 2020
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Thanks I replaced a few on the inputs with .22 Mallorys and have everything else recapped now just looking for advice for direct outs. I'm thinking of throwing a switch on each channel and taping off of the 47uf just before the echo send, would this work? I have plenty of 600:600 transformers would the out put of the 47uf before the echo send pot be able to drive a transformer? I'm very new to this have only recapped equipment and done simple mods. Thanks in advance. I've attached some photos if anyone's interested in this cute little rare console (currently has all knobs pulled except for channel 1PXL_20210716_003328986.jpgPXL_20210716_003551032.jpgPXL_20210716_003344974.jpgPXL_20210716_003350137.jpgPXL_20210716_003559621.jpgPXL_20210716_003606332.jpg
 

L´Andratté

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Jan 17, 2013
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Hamburg Germany
Hi Vyvyan, very nice mixer, a little problem for me is that block diagrams and schematics are not consistent in nomenclature, but your guess is worth a try if you can spare the echo send, but you should take the signal from after the 47u! Where physicaly do you want to put the direct out jacks? And where to put the output transformers? Do you really need transformer balanced out?
 

Whoops

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May 9, 2008
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Portugal
Hi,
that mixer looks really similar to the Alice 828,
Does it also has a Limiter in the Master?

here are pictures of the 828

1626488468915.png

1626488508125.png
 

musika

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Feb 11, 2009
Messages
284
Location
Ohio USA
I own a couple of these mixers. No limiter circuit like the Alice 828. But very nice basic circuit. One is 22x2 a and the other is 10x2. No phantom power or direct outs. I have thought about those features as well as channel mute, phase reverse, or line inputs but there is not much physical room to implement these. A buffer amp may be needed for the direct outputs. I have the Allen & Heath buffer card schematic from the MOD2 console if you would be interested. It takes a direct feed pre fader into the buffer amp then to the patchbay.
 

Sounder01

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Feb 9, 2021
Messages
3
I own a couple of these mixers. No limiter circuit like the Alice 828. But very nice basic circuit. One is 22x2 a and the other is 10x2. No phantom power or direct outs. I have thought about those features as well as channel mute, phase reverse, or line inputs but there is not much physical room to implement these. A buffer amp may be needed for the direct outputs. I have the Allen & Heath buffer card schematic from the MOD2 console if you would be interested. It takes a direct feed pre fader into the buffer amp then to the patchbay.
Hi Musica,
I interested in the Allen & Heath buffer card schematic from the MOD2 console .
Could you please share?
Best Regards,
Yongyut
 

Vyvyan

Member
Joined
Nov 25, 2020
Messages
5
I own a couple of these mixers. No limiter circuit like the Alice 828. But very nice basic circuit. One is 22x2 a and the other is 10x2. No phantom power or direct outs. I have thought about those features as well as channel mute, phase reverse, or line inputs but there is not much physical room to implement these. A buffer amp may be needed for the direct outputs. I have the Allen & Heath buffer card schematic from the MOD2 console if you would be interested. It takes a direct feed pre fader into the buffer amp then to the patchbay.
Definitely interested in the schematic thanks!
 

musika

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Joined
Feb 11, 2009
Messages
284
Location
Ohio USA
Here's the documentation that I have on the Line Buffer Amp that Allen & Heath implemented on the 1975 MOD2 console. This is about the same vintage as the Quasi mixer. They called it AH75 and there is a pencil drawing of the circuit because it has never been published by Allen & Heath anywhere that I could find. I also included a photo of the tiny AH75 card attached to an input module. The pcb was designed for stereo, but obviously used as mono for the direct outputs. It may be useful in the Quasi for your direct output modifications.
 

Attachments

  • Allen Heath Mod2 Direct Outs.jpeg
    Allen Heath Mod2 Direct Outs.jpeg
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  • Allen Heath Mod2 Line Buffer AH75.jpeg
    Allen Heath Mod2 Line Buffer AH75.jpeg
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  • Allen Heath Mod2 Line Buffer Schematic .jpeg
    Allen Heath Mod2 Line Buffer Schematic .jpeg
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musika

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Feb 11, 2009
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Ohio USA
I mistakenly mentioned stereo, but meant to say there are two input locations and two output locations on that line buffer pcb.
 

Vyvyan

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Nov 25, 2020
Messages
5
Thank you! Much appreciated! I just scored a great deal on a ferrofish pulse 16 mx so having 10 outs will definitely help feed my extra I/O
 

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