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Building a Power Supply

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Building a Power Supply

Postby Andrew_Richards » Mon Apr 08, 2013 8:45 am

Hello all,

Unfortunately I have managed to lose my power supply for my headphone amp. Normally this wouldn't be too much of a problem and i'd buy a replacement but the amp in question has a very strange pinout and I've been quoted £150 for a replacement!

So, is there any chance I could build my own?

The specs given by Symetrix are:

There are two separate AC supplies coming out of the PS-3: 8.5 VAC @ 1A and 34.5 VAC CT @ 250mA. Pinout is as follows: 8.5 VAC on pins 6 and 1, 37 VAC on pins 7 and 3, center tap @ pins 4 and 5 are connected to earth ground. This is when viewed at the end of the PS-3 7-pin DIN plug.

Any idea where to start? I'm assuming I would have to buy two wall warts and wire them up to one 7 pin din?

Many thanks for any help!

Rich
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Re: Building a Power Supply

Postby Hugh Robjohns » Mon Apr 08, 2013 9:21 am

This can be done with a single transformer with several secondary windings to provide the different voltages... but the confusion is over which winding is centre-tapped! I'd assume it was the 34V winding, but it would be messy if you got that wrong!

H
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Re: Building a Power Supply

Postby Andrew_Richards » Mon Apr 08, 2013 11:16 am

Hi Hugh,

Thanks for that - unfortunately my power supply knowledge is fairly limited. I've had a google of 'centre-tapped power supplies' and have read the wikipedia entry but am not sure which transformer I would need to buy in order to make one?

A friend has offered this advice:

"Depending on how it goes into the unit, I think you will need to have a 34.5V signal (or slightly less). Important thing about it though, a 15VAC supply will only do what you want if you only need the positive rectified signal (or negative) going into the box. Then you can just us a simple rectifier to get the voltage. However, I don't think that this is likely to be what you need, as if it were, they probably would just have used a DC supply."

Would you agree? The item is a Symetrix 304 4-way headphone amp...

Thanks,

Rich
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Re: Building a Power Supply

Postby Hugh Robjohns » Mon Apr 08, 2013 11:46 am

Sorry -- I misread your original post -- all the info is there in the spec, and it is the 34V winding that is centre tapped. The centre-tap just means that the centre of the winding is grounded, so that it can be used to generate matched positive and negative supplies.

There should be plenty of mains transformers around that will meet this spec -- I'll have a look later when I get a chance... but it will some some basic DIY electronics skills to safely house the transformer in a box and wire it up. We are talking about 250V mains wiring here, and so it is a safety critical area. Is that something you feel capable of doing, or do you know someone with the appropriate skills that could do it for you?

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Re: Building a Power Supply

Postby Paul Ochsner » Mon May 27, 2013 6:53 am

Hi Hugh, Hi Andrew!,
This is my first post and I can see that I am going to enjoy myself in here, as long time fan of Sound On Sound magazine.
Regarding Andrew's question I too am in the same situation having prehaps slightly ignorantly bought a used Symetrix 302 two channel pre-amplifier with out power supply at a bargin price off Ebay thinking I could easily get the supply for $20 or so, only to also find that the particular rated supply was both very expensive and very hard to obtain.
Being in Australia I also need the 240/250 VAC model and was wondering if you did happen to find any transformers that would suit for a bit of a D.I.Y. solution Hugh?.
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Re: Building a Power Supply

Postby skintwang » Sat Mar 10, 2018 8:29 am

Sorry to jump into a five-year old thread but has anybody here had any luck building a power supply for the Symetrix 300 series? I have a perfectly good 302 dual microphone preamp but the PSU got fried in a voltage surge. I am also after a 240v version since I am in England.

Symetrix are selling the PS3E for £255 including VAT via a Scottish agent. This naturally makes me lean towards a DIY solution. Any advice would be appreciated. Thanks.
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Re: Building a Power Supply

Postby ef37a » Mon Mar 12, 2018 12:08 am

skintwang wrote:Sorry to jump into a five-year old thread but has anybody here had any luck building a power supply for the Symetrix 300 series? I have a perfectly good 302 dual microphone preamp but the PSU got fried in a voltage surge. I am also after a 240v version since I am in England.

Symetrix are selling the PS3E for £255 including VAT via a Scottish agent. This naturally makes me lean towards a DIY solution. Any advice would be appreciated. Thanks.

The 34.5V centre tapped is near as 17-0-17 volts and indicates a 17-0-17 DC regulated supply, such a transformer will be hard to find. Much more common are 15-0-15 or 18-0-18 volts, both have snags.
The lower voltage might be fine if mains is on the usual 240V and you rely on a bigger than specified transformer, 1 Amp say, to have less 'droop' on load i.e. better regulation.
18V would be safe from the regulated output point of view but might put extra dissipation on the regulator ICs if they are not on adequate heat sinks.

The mains side wiring can be done without death! If you go for wire out toroidal transformers you can connect mains using 5A 'choc-block' screw connectors, either directly to a 3 core mains cable (earth to casing and to output if that is how the original was wired) or, more neatly, to an IEC power connector using 6mm push tags for that. All that can be done without mains present. But! I must be said, as did Hugh, do not undertake this task unless you are sure you can do it safely.

If you also need the 8.5 V at 1A that is just another 9V transformer.

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Re: Building a Power Supply

Postby James Perrett » Mon Mar 12, 2018 1:31 am

The first thing I'd be doing is opening up the PSU to see exactly what was damaged. If it isn't the transformer then there may be a cheap fix. Have you checked the output of the PSU? Are you sure that the problem isn't in the preamp itself?
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Re: Building a Power Supply

Postby ef37a » Mon Mar 12, 2018 8:36 am

James Perrett wrote:The first thing I'd be doing is opening up the PSU to see exactly what was damaged. If it isn't the transformer then there may be a cheap fix. Have you checked the output of the PSU? Are you sure that the problem isn't in the preamp itself?

Very good points and a peek inside the pre amp will tell you how well 'sunk' the regulators are. Very often for low current applications the regulators are just left naked in free air, if that is the case I would avoid an 18V AC input. You could bolt on some extra sinking but things are getting complicated now.

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Re: Building a Power Supply

Postby Edward Mills » Thu Jul 23, 2020 7:04 am

For what it's worth... which may be exactly zero... my Symetrix 304 headphone amp, purchased sans power supply brick for the princely sum of $7 at ye olde Goodwill, is happily percolating along on naught but +15 / com / -15 supplies, apparently without need of the aforementioned 8.5 volt supply ! It sounds nice to me, a little less "warm" than the now-deceased Fosgate tube amp I'm used to (which will shortly be addressed by the addition of a homebrewed 12AU7 preamp) and appears to be functioning quite normally. Here's the saga:

I found the pinouts and voltage requirements that were so kindly provided elsewhere in this thread, and with the reasoning that since everything passes through a bridge rectifier on the board, it should be a simple matter of cobbling the right voltage together, easily accomplished with the right combination of switch-mode PWM adapters, of which I have a huge assortment in a file cabinet drawer.

Got everything twisted together for a test run, and bingo. Nailed it right off. Equal gain off both inputs, cranked all levels full CW, no artifact noise, line hum, nothing. Dead quiet.

Then I noticed that I had accidentally fried the 9-volt wall wart somewhere along the way, probably while fumbling with bare wire ends to determine polarity, and realized that the seemingly successful test had been conducted with only the split 30V supply operational!

Flash forward to the headphone amp board lying naked as a jaybird on my little workbench. It's a very nicely made SMD affair, with three little ferrite-bead filter chokes connected to the DIN socket, along with the chassis ground and absolutely nothing else. Filter choke #1's circuit disappears into a via and reappears as a bottom-side trace straight to signal ground; chokes #2 & 3 go directly to a lovely pair of glass fuseholders, and thence to the usual set of balanced-load filter caps in series-parallel. So those are obviously the +/- supplies. Only two voltage regulators are evident, little surface-mount power tabs in apparent push-pull. After searching until blue in the face, I could find no evidence of any other power supply distribution apart from the split 30V setup. Holding the board up to bright light confirmed that it's only double-sided, no inner layers. Did I mention that it's a really nice glass board that appears to have been made on well-calibrated pick-&-place machines, unlike my Fosgate headphone amp, which in comparison looked rather amateurishly designed despite carrying an original price tag that was audiophile-level obscene?

Next order of business: removed the DIN socket entirely, neatly attached three nice leads to their respective feed-through pads, added a little strain relief to the enclosure in place of said receptacle, and there she sits, working perfectly fine atop my old Yamaha receiver.

I may upgrade the power supply if I can find a matching pair of those nice Hewlett-Packard laser printer bricks - you know, the otherwise totally useless proprietary ones with 32 and 16 volt outputs - so it'll be one volt closer to spec and have a little more current-handling, as the one I used is kinda close to the edge, milliamp-wise.

I wish I'd had the presence of mind to take a picture of the headphone amp board, but for you enterprising gearheads out there, the three connections should be fairly obvious.

Could it be that the 8.5 VAC source is intended for some other device besides my Symetrix 304, but which uses the same power supply transformer unit?

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Re: Building a Power Supply

Postby CS70 » Thu Jul 23, 2020 2:19 pm

Have absolutely no use for the info, but what a nice way to revive a thread! :clap:
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