can't test the voltage on DC appliance

[JohnW2]

Well it says ac power in on the back.

Indeed it does ....

If that's what the OP is plugging his transformer into, it clearly IS an ac-ac transformer (even in winston's eyes), in which case the explanation is as we have all been saying.

fozzyfester: have you yet tried measuring with your meter on its 20V AC range?

Kind Regards, John

 

[JohnW2]

hear is the transformer

That squiggle after the "V" indicates that what we've all been saying is correct - that the output of that transformer is AC ...

Kind Regards, John

 

[winston1]

It may be true. But it is not helpful.

Of course it is helpful. He was using the wrong range on his meter, DC instead of AC.

I look forward to your apology.

 

[fozzyfester]

Looks like JohnW2 has one him self
OK just tried it with the meter set to the 200AC
and the 3 pins, when I test them next to each other I get 16.9v, but when I test them furthest way I get 34V

 

[JohnW2]

Looks like JohnW2 has one him self

You mean a mixer desk? If so, definitely not

OK just tried it with the meter set to the 200AC. and the 3 pins, when I test them next to each other I get 16.9v, but when I test them furthest way I get 34V

That sounds roughly right - in the latter case you are getting the sum of the two outputs of the transformer.

Kind Regards, John

 

[fozzyfester]

The original plan of all this testing was to build a battery for it and run it from that, but if it is an AC unit then that will never work

 

[JohnW2]

The original plan of all this testing was to build a battery for it and run it from that, but if it is an AC unit then that will never work

Very true.

The AC input into the mixer must get turned into DC within it, to power the electronics, so if one had the knowledge and took the thing to pieces, one could probably adapt it to run off batteries (albeit probably quite large ones). Alternatively, but again requiring knowledge and ability, one could devise an 'inverter' which changed the DC from a (again, perhaps quite large) battery into the AC voltages required by your desk.

I have to say, it's a pretty unusual set-up (to have a transformer supplying fairly low voltage AC to a piece of equipment {which ultimately needs DC}), and can't help but wonder why they've done it like that.

Kind Regards, John

 

[DetlefSchmitz]

If you put dc into a bridge rectifier at a voltage equal to the peak ac voltage (1.414 x rms voltage) then it should work fine. No need to take it apart in that case.

 

[JohnW2]

If you put dc into a bridge rectifier at a voltage equal to the peak ac voltage (1.414 x rms voltage) then it should work fine. No need to take it apart in that case.

Yes, very probably true - but, maybe because I am over-cautious, I personally would be a little nervous about doing that without first taking it apart to see exactly what the AC power input socket was connected to.

For example, the fact that it appears that the two AC inputs are out of phase could mean that the combined voltage of the two was being used in some way (e.g. a bridge rectifier fed by the two out-of-phase inputs). Probably not all that likely (although one has to wonder why there are two AC inputs), but I would at least like to know what was going on 'inside the box'!

If it were simply a single '2-wire' ELV AC input into the box, then I'd probably be much happier to do as you suggest, without looking inside the box.

As I said, maybe I'm just over-cautious!

Kind Regards, John

 

[DetlefSchmitz]

You could be right. It is quite possible that the inputs are turned into a positive and negative rail. So you might need a +21V battery and a -21V battery. All seems a bit over the top for typical audio circuitry, though having positive and negative internal rails for the op-amps is, in itself, beneficial. I probably misunderstood your 'taking it apart', by which I guess now you meant 'opening it up'. I thought you were suggesting re-working the circuit, and was just pointing out that it was likely not to be necessary.
For the benefit of the OP however, batteries may be an option, and it may not require any modification to the unit, but would probably require 2 x 21V batteries.

 

[JohnW2]

You could be right. It is quite possible that the inputs are turned into a positive and negative rail. So you might need a +21V battery and a -21V battery.

Indeed, and unless one were prepared to risk pure trial and error, I don't think that one would/could know whether that were the case (or which way around to connect the batteries) without opening it up (and 'having some knowledge').

All seems a bit over the top for typical audio circuitry, though having positive and negative internal rails for the op-amps is, in itself, beneficial.

I agree - and, even if one wants positive/negative rails, as I said before it is very odd/unusual (at least in my experience) to have two out-of-phase ELV AC inputs into something which is presumably just very low power electronics.

I probably misunderstood your 'taking it apart', by which I guess now you meant 'opening it up'. I thought you were suggesting re-working the circuit, and was just pointing out that it was likely not to be necessary.

Yes, 'opening it up for a look' (with an understanding of what one would looking at) was what I primarily meant. It could be more complicated than we are envisioning. Given that what's going in is AC, it's even not impossible that there are other ('true') transformer(s) inside - e.g. to provide a separate 5V supply as well as, say, +/- 15V rails (something which would obviously fail to work if one had fed DC into the 'AC inputs'). That's another reason that I would want 'a good look inside'!

For the benefit of the OP however, batteries may be an option, and it may not require any modification to the unit, but would probably require 2 x 21V batteries.

Yes, it certainly is a theoretical option(i.e. must be possible, even if modification to unit were required), and may well be possible without modification to the unit. However, even if it were adequate (via the input socket) "2 x 21V batteries" would raise issues. Unless one built the batteries oneself with individual cells, it might well end up as 2 x 24V batteries - and that could raise issues that required some attention.

Kind Regards, John

 

[fozzyfester]

one could devise an 'inverter' which changed the DC from a (again, perhaps quite large) battery into the AC voltages required by your desk.

I do happen to have a 150W inverter I got from halford a while ago and a car battery and a mains charger for it. The problem with the inverter is it cuts off early when there is still plenty of life in the battery and the problem with the battery is its a car battery (60amps/hr) designed for cold cranking and not long drain (like the leisure batteries on camper vans and boats)

I am also going to need to power a small speaker from it too (max 50W) but that could be a 12 or 5 volt DC one that will easily run off of the battery without the inverter. Maximum run time will be 2 hours

 

[Nozzle]

https://www.gear4music.com/G4M/Behringer-PSU6-UK-Power-Supply/KYG

That's just a transformer, the image here has AC outputs.

Switch your DVM to AC, then try again

Nozzle

 

[JohnW2]

That's just a transformer, the image here has AC outputs. ... Switch your DVM to AC, then try again ...

I think you may have missed a page or two of posts

Kind Regards, John

 

[JohnW2]

I do happen to have a 150W inverter I got from halford a while ago and a car battery and a mains charger for it. The problem with the inverter is it cuts off early when there is still plenty of life in the battery and the problem with the battery is its a car battery (60amps/hr) designed for cold cranking and not long drain (like the leisure batteries on camper vans and boats)

That's presumably a mains-voltage output inverter so, even if it were up to the job, you'd still have to use the transformer with it - a car battery plus an inverter and transformer seems a lot to have to cart about to run a small (1.6kg) low-powered electronic device!

If you need to run such a thing off battery power, I wonder if you shouldn't be thinking laterally. These things don't seem all that expensive (mainly £40-£70 on the site you linked to) and at least some seem to be designed to run from 12V DC input. For example, one of the first ones on that site that I clicked on ... (click here) . Although it appears to come with a mains-powered switch mode power supply with an output of 12V 1.5A (max) (see the instructions which can be downloaded from the site), there is a socket on the back of the mixer into which you could just plug a 12V battery. The power is stated as 11W, which implies that it would take just under 1A at 12V. I suspect that many of the other mixers available are similar.

Kind Regards, John