Allen & Heath System 8 power supply...

[JohnnyDMonic]

Hi,

A couple of years ago I bought a System 8 16-4-2 desk but it came with a non-standard power supply which works fine but doesn't have phantom power. I was wondering if there is a way I could adapt it to provide phantom power as well.

The current power supply transformer has a +15v and -15v AC secondary winding which is centre-tapped for the audio ground. Each AC supply is seperately rectified, smoothed and regulated to provide +15v and -15v DC to the mixing desk.

What I was thinking was that I could somehow rectify the +15 and -15v AC supply to around +30v DC which, when smoothed, would give around +42v DC. But I'm not sure if this is possible without shorting out the transformer.

I have taken the power supply apart and drawn up a schematic of it:

There were a couple more caps and diodes after the regulators but I didn't have time to add them to the schematic because i needed to use my desk in a hurry.

So, is there any way of having phantom power as well using the existing transformer, or will I need a second transformer?

Thanks a lot,

John.

[Tom Phillips]

John,

I don’t know of a practical way to generate the new 42V supply AND keep the existing +15 and –15V without adding a second transformer.

By “practical” I mean without a complete re-design to a fancy switching type supply. That wouldn’t be worth the trouble just to use the existing transformer.

Regards,
Tom

[JohnnyDMonic]

Thanks Tom, you've confirmed what I thought. I actually need 48v so what I was thinking was to get a small 36v AC transformer which should give me just over 50v DC after rectification and smoothing so it should be enough to get a steady 48v DC out of the regulator. If not, I can use a 44v transformer but I'm not sure...

I was going to use this schematic as a guide - http://members.aol.com/ecc81/ps48.html

Should I use a toroidal transformer for this? I understand they are better at not introducing noise into the supply or something along those lines.

Thanks,

John.

[Tom Phillips]

John,

The circuit in your link looks like it will work very well. Using the linear regulator IC will give you very clean phantom power. The TL783C is also adjustable and short protected. A big plus. If you can get the completed board for $35 I thinks it’s worth it rather than building from scratch. It’s interesting that the direct from Triode Electronics price is more ($54). Let me know if you find out anything about that.

You will not get cleaner power with a toroidal transformer. They do, however, radiate a much lower magnetic field than a transformer built with a conventional core. That’s what you are thinking of. Unless you are going to mount the supply close to some very sensitive circuits that would be affected by a transformer’s field, I don’t think a toroid is worth the extra expense.

Regards,
Tom

[Top of]

Circuit Diagram A&H Sys 8

Hi, If it is any help to you. The original Trans has 2 x secondary outs, 15-0-15 and 1-40. If it is any help I have an original circuit for this desk and I can scan it for you. I only found this by accident and joined just to let you know. So you tell me how to deliver it to you and I will.
John

[Top of]

Addendum

Sorry, Silly me I stated one of the secondary's was 1-40 it should read 0-40 and the circuit is for the Power Supply and the PP board. I also have copies of all circuits for the other boards. All reprints from factory drawings not the one from the manual, which I also have. I also have a spare parts list and nº's. All are relating to a MkII board.

[yellodoggmusic]

[QUOTE=JohnnyDMonic;15421]Thanks Tom, you've confirmed what I thought. I actually need 48v so what I was thinking was to get a small 36v AC transformer which should give me just over 50v DC after rectification and smoothing so it should be enough to get a steady 48v DC out of the regulator. If not, I can use a 44v transformer but I'm not sure...

I was going to use this schematic as a guide - http://members.aol.com/ecc81/ps48.html

QUOTE]

hi john, i'm new to this forum, i just picked up a system 8 mk III 16-8-2 today, sounds great, .... and like you i have a non-original power supply. i need to get the phantom power going so did this transformer work alright for you? Please let me know, e-mail me, i'm very poor at everything besides wiring electric guitars.... so any help from anyone would be appreciated... can i get some tech shop in the tri-state area to rig my power supply or whatever? thanks
\

please e-mail me at yellodoggmusic@yahoo.com.