Allen & Heath- Can't identify the model

[Deepdark]

Hi there

Is anybody has ever seen this Allen & Heath mixer? There is no model number or whatever on it. The quality control sticker Inside the unit is dating from 1977.

 

[bananamoon]

I've just seen it too! .  It looks like a Quasi, but black.

 

[analogguru]

That is an Allen & Heath so called "Pop-Mixer" 16-2:

http://www.allen-heath.com/ahproducts/pop-mixer/#tab4

 

[PRR]

"....a price which manages to limit capital expenditure."

 

[Deepdark]

That is an Allen & Heath so called "Pop-Mixer" 16-2:

http://www.allen-heath.com/ahproducts/pop-mixer/#tab4

Thanks a lot. Spot on

 

[musika]

Looks like a great little mixer. ...a mini MOD2 without the Sowter transformers?  I don't see any input transformers in the schematics.  Maybe that's why they could advertise "limited expenditure'.  Some of the power amp circuits look very  similar to  the MOD2.  Is it working? 

 

[Deepdark]

actually, it's a friend of mine who got it for like less than 50$  I don't know yet if it work. I'll let you know

 

[Deepdark]

Quick one. Is it safe to assume a power consumption of about 400ma, or so? I need to made a new psu but can't find suitable information about it. Fuse is rated 1A fast blow in the review,  22vdc. On the schematic it recalls for 21vdc, so it's pretty close. I plan using 50VA power transformer. I should be ok if my previous assumption is right

I could get a 24v  / 48v dual coil rated 100VA for like 10$ more, but why overrating the power transformer if I can go safely with a 50VA.

For what we know, all channel strips are made of 6 BC168 and 169 (16x) and the stereo output is made of a total of 8 BC168 and 169 transistors. No opamp, no dual voltages rail. Simply 21VDC, recalling to the schematic.

Thanks guys

 

[Deepdark]

ok, little update. We test the mixer with the actual power supply (actually, the previous owner did a terrible job. He just put a wall wart, 12V/300ma  , hard wired into the mixer, with no fuse). Weird, choice, because the schematic recall for 21V. Anyway, to have an idea of the power consumption, we just tried the mixer with the little wallwart. We put microphone on 8 channel and test simultanously with some sounds, snare, etc. The wallwart didn't fried, came hot, or whatever. it sounded good. So, with 21v, I guess I should expect more gain, and my 50VA power transformer should be allright, since we didn't had any trouble at 300ma.

 

[Deepdark]

here is the power supply I'll make to replace the wallwart

 

[PRR]

> with 21v, I guess I should expect more gain

Why?

Headroom, but not gain.

 

[PRR]

I don't think the block diagram or board-series is complete. I don't see the main mix stages on the block; stereo is implied but not drawn out. Nevertheless we know most all of it. The input channels account for most of it just because there are so many. The headphone driver is the next heavy load, but there's just one (pair) of that. The line outs are thriftier. I've assumed some 5mA stages as well to cover missing bits.

13mA * 16 = 210mA
5mA  * 6 = 30mA
(2.5mA+3.5mA) * 4 = 25mA
(2.5mA+17mA) * 2 = 40mA
=======================
305mA DC
21V DC
6.4 Watts DC

This is very similar to a 3-Watt 8 Ohm audio amplifier.

All stages of the PopMix have significant B+ filtering. The main supply does NOT have to be very clean.

Double DC Watts for PT AC VA: 13VA

Get 12V 1 Amp PT, bridge rectifier, 2,200uFd 25V cap. This will come to 18V-20V DC. Not the noted 21-22V, but in any case this board will NOT output "Pro Audio Level" (10Vrms), about half that, and a couple volts either way on the supply won't change it.

The power supply is so simple, and the mixer so rudimentary, that IMHO no "board" is warranted. Solder FWB ~~ legs to PT 12VAC, solder cap to FWB +/- pins, apply shoe-goo to a box and slap it in. If the PopMix turns out to be a life-time keeper, you can do-up a "nice" supply when you get a roundtuit. If not, you have a 20V DC supply for general mayhem.

 

[Deepdark]

I don't think the block diagram or board-series is complete. I don't see the main mix stages on the block; stereo is implied but not drawn out. Nevertheless we know most all of it. The input channels account for most of it just because there are so many. The headphone driver is the next heavy load, but there's just one (pair) of that. The line outs are thriftier. I've assumed some 5mA stages as well to cover missing bits.

13mA * 16 = 210mA
5mA  * 6 = 30mA
(2.5mA+3.5mA) * 4 = 25mA
(2.5mA+17mA) * 2 = 40mA
=======================
305mA DC
21V DC
6.4 Watts DC

This is very similar to a 3-Watt 8 Ohm audio amplifier.

All stages of the PopMix have significant B+ filtering. The main supply does NOT have to be very clean.

Double DC Watts for PT AC VA: 13VA

Get 12V 1 Amp PT, bridge rectifier, 2,200uFd 25V cap. This will come to 18V-20V DC. Not the noted 21-22V, but in any case this board will NOT output "Pro Audio Level" (10Vrms), about half that, and a couple volts either way on the supply won't change it.

The power supply is so simple, and the mixer so rudimentary, that IMHO no "board" is warranted. Solder FWB ~~ legs to PT 12VAC, solder cap to FWB +/- pins, apply shoe-goo to a box and slap it in. If the PopMix turns out to be a life-time keeper, you can do-up a "nice" supply when you get a roundtuit. If not, you have a 20V DC supply for general mayhem.

Thanks PRR. So my first though were near, I'll have plenty of juice with my 50VA PT. Actually, we tried the mixer with the cheap as hell 12V/300ma wallwart and it sounded really good actually. So we decided to go on with a Chuck Norris approved PSU  8)

 

[musika]

FWIW.  When Allen & Heath was building these mixers in the mid seventies, they supplied them with 24V supplies.  I have a MOD2 and a Quasi with the original 24V supplies.  They also both have a (100R) resistor network that was usually hidden from view under the panel  that I think was designed to cut the supply down to the 21V shown in the specs. 

 

[analogguru]

So I had a look inside a spare, original Allen & Heath "MPS 2" power supply for the POP-Mixer laying around.

It consists of a transformer 30V/25VA, a rectifier bridge WO-04, cap 2200µ/40V, series resistor 10E/2W, another cap 2200/40V, fuse T2A and then a stabilisator circuit with a TIP31B, a zener diode 24V and 2 resistors, 1 cap . The unit has 4 output jacks.

 

[Deepdark]

Thanks Guys, plenty of usefull information here. Looks like I could have go with a cheaper way to build the power supply hahaha. Anyway, I have plenty of juice to make a phantom power circuit within the same transformer.

 

[Deepdark]

I tried the mixer yesterday with the 20V psu I made. Unfortunatly, it didn't work, for an unknown reason. The psu work, the led lit upt, I got 20Vdc, There is 20V all along the b+ rail, so my voltage enter the board for sure, but there is no sound coming out when trying it. With 12V wallwart, it works like a charm but not with 20V.

Maybe the mixer has been modify to accomodate 12V instead of 20V. I don't know. It's weird. Any idea, just out of curiosity?

I have a 14V transformer here, so I should be ok with the LM317 and the correct resistor value for the voltage divider to get my 12v easily, rigth?

 

[Deepdark]

Other than that, I noticed the 100R resistor on the pin 5 rail (b+) are still there on the strips. It's really strange that I got 21V on my pin 5, but no sound other than a faint noise comming out of it. Is it possible the transistors got overloaded somewhere in the chain and causes that?

 

[musika]

It seems to me that somewhere inside the Pop Mixer,a previous owner must have implemented a voltage converter.... like the one I found attached.  Adjustment of R5 to 21V (from 24V ) for the mains mixer power.  Did you look for something like this inside the power module?  Glad to hear the 12V supply works...

 

[Deepdark]

It seems to me that somewhere inside the Pop Mixer,a previous owner must have implemented a voltage converter.... like the one I found attached.  Adjustment of R5 to 21V (from 24V ) for the mains mixer power.  Did you look for something like this inside the power module?  Glad to hear the 12V supply works...

I didn't look for something like this, but it's possible lol. Since we have no informations about it's history, we don't have any cue about why did he choose a 12v wall wart.