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

[Vyvyan]

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

 

[Vyvyan]

bump!

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

 

[industrialarts]

Yep, in fact probably advisable to do so.

 

[Vyvyan]

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 1

 

[L´Andratté]

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]

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

 

[musika]

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]

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]

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]

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.

 

[musika]

Pin 5 on the drawing refers to the TT patchbay jack for the direct out.

 

[musika]

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

 

[Vyvyan]

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

 

[Vyvyan]

Finally finished recapping and found I can get a pretty strong signal "input module- routing section"(see photo) 47uf cap just before the echo and feedback pots/sends but also get a strong signal from 10k fader on "quasi input amplifier section" there are also under traces and component holes just after the pan pots on every channel see photo

Just curious as to anyones thoughts on Where I should tap for the direct out, the fader or final 47uf in routing section. Would I benefit form the extra post fade stage? And how would I calculate the output impedance? I have plenty of 600;600 transformers to use. Also all input channels are transformer balanced not shown on schematic.

 

[Vyvyan]

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.

So this quasi does already have a post fader buffer matching the above schematic so I took the direct outs from there disconnecting the 47uf's connection to the the foldback pot and running it to a 600:600 transformer and it sounds great!

Now I'm wondering what I should do with ground the outputs should I just ground it to the fader ground bus or to the channels board group marked "earth" on the board? Or possibly add a switch to the already pre drilled holes on each channel for a ground lift?

Fyi I'm not doing any summing with this, just 10 in 10 out. Going to be using the master and echo/foldback amps with some Altec 1588b's as output amps

 

[musika]

I am building some of these Line Buffer cards for my Allen and Heath Quasi mixer to use for direct outputs. What capacitor can I substitute for the 68pf 1KV ceramic disk capacitor (in parallel with the 120K resistor) in the line buffer AH75 schematic pictured below? Would a 100pf 50v polystyrene foil or a tantalum work? Or maybe there is a better alternative. Can anyone help?

 

[Vyvyan]

I am building some of these Line Buffer cards for my Allen and Heath Quasi mixer to use for direct outputs. What capacitor can I substitute for the 68pf 1KV ceramic disk capacitor (in parallel with the 120K resistor) in the line buffer AH75 schematic pictured below? Would a 100pf 50v polystyrene foil or a tantalum work? Or maybe there is a better alternative. Can anyone help?

Honestly I'm pretty sure I used a 50pf polystyrene on a few channels as it was all I had. Works fine

 

[musika]

Also, looking for some modern replacement transistor for C732 GR since I cannot seem to find a supply and I have found that C1815 GR would seem to substitute. What transistor did you use? Any others you can recommend?

 

[Vyvyan]

Also, looking for some modern replacement transistor for C732 GR since I cannot seem to find a supply and I have found that C1815 GR would seem to substitute. What transistor did you use? Any others you can recommend?

My quasi actually had 2sc1000's and bc168's, so I believe the 2sc732s can be replaced with mpsa18's as they are a common replacement for the 2sc1000 and supposedly improve noise performance. Bc550 will probably work. I have some c732 if you would like I can send them cheap. Got see how many I have when home tonight.

 

[musika]

OK. Thanks. Message me sometime. I’ll check into the MPSA18’s datasheet. The line buffer schemo came from my Mod II console direct out cards. The Quasi mixers that I have here are all loaded with BC168B’s.