can't test the voltage on DC appliance

'Most sound desks' use op-amps as their active components which run most successfully on a dual supply rail of 15V ...
Indeed so.
... the easiest way to derive this from the mains is a 2 phase ac secondary and full wave rectification....
Again, indeed. Having a centre-tapped transformer secondary is probably the simplest/easiest way to get +15V and -15V rails.

However, what I was talking/asking about was this concept (which I think is a 'new one' on me!) of having a power supply which is 'half in and half out' of the equipment it supplies - i.e. having the transformer external to the equipment but everything else internal.

What I would normally expect is EITHER to have the equipment supplied with mains voltage, with the PSU entirely within the equipment OR (if one wanted everything in the equipment to be ELV and/or their were space issues) to have the power supply entirely external to the equipment, supplying +15V and -15V DC to the equipment (still only '3 wires').
Using the voltage doubler/tripler for the 48V phantom is also easy.
The need for a 48V rail might possibly be one of the reasons, but it seems to me to be a pretty lame 'excuse'/explanation, in either of those two ('common') scenarios I've just mentioned. If the equipment is supplied with mains, then there is obviously no issue, and if the PSU is entirely external, then generation of the 48V is again exactly the same, the only difference beeing the need for a 'fourth wire' between PSU and equipment.

Kind Regards, John
 
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So I checked my computer to see if I have a circuit diagram for a soundcraft and sadly I don't, However I do for a Behrenger 1202 which is simply 2 more microphone inputs than OP's:
View attachment 176655
X1 pins 1&3 are the 2phase 15v supply, pin 2 is neutral
I finally found the schematic diagram for my mixer
 

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but I thought you implied that the battery was not really suitable?
I think I will have to test it, battery rated at 60amps/hour, and if I use a 50W speaker dose that mean it will last just over an hour then?
 
(For the curious) phantom power is needed to make condenser mikes work. Most domestic types have batteries in them, most professional ones don't.
intresting indeed!
But I use a dynamic mic and ont a condensor
 

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I finally found the schematic diagram for my mixer
Thanks. Unfortunately, that only shows the electronic 'works' of the mixer (i.e. the audio circuitry), and not anything to do with the power supply arrangements. Are there any other schematics there?

Kind Regards, John
 
I think I will have to test it, battery rated at 60amps/hour, and if I use a 50W speaker dose that mean it will last just over an hour then?
Just for the record, its actually amp.hours ("Ah", amps times hours), not amps/hour (amps divided by hours).

Theoretically, if it were in 'new condition', if you were lucky you might get something like 50A out of it for an hour.

As for the speaker, 50W equates to only about 4.17A (NOT 50A) at 12V (50/12). Furthermore, I doubt very much whether a 50W speaker (or, rather, the amplifier driving it) would normally consume an average of anything like 50W, so the average current would probably be well under 4A. The bottom line of that is that I would expect a 60 Ah battery in decent condition to be able to power a 50W amplifier/speaker 'for many hours'.

Kind Regards, John
 
Unfortunitly not!
As I feared.

I don't know to what extent you have been able to follow the technical discussion above but I think that one conclusion of it is that it's very probable that the 48V 'phantom power' (assuming you have, and need, that - but what you say in post#49 maybe implies that you don't need the 48V?) is probably derived in a manner that would not work if you tried to power the mixer with DC (i.e. the original thought of "2 x 21V batteries"). On the other hand, if you use a battery+inverter (or even a generator!) to produce AC mains voltage to feed the existing transformer, then the situation would obviously be the same (including as regards the 48V) as when running the transformer directly 'off the mains'.

Kind Regards, John
 
Indeed so.
Again, indeed. Having a centre-tapped transformer secondary is probably the simplest/easiest way to get +15V and -15V rails.

However, what I was talking/asking about was this concept (which I think is a 'new one' on me!) of having a power supply which is 'half in and half out' of the equipment it supplies - i.e. having the transformer external to the equipment but everything else internal.

What I would normally expect is EITHER to have the equipment supplied with mains voltage, with the PSU entirely within the equipment OR (if one wanted everything in the equipment to be ELV and/or their were space issues) to have the power supply entirely external to the equipment, supplying +15V and -15V DC to the equipment (still only '3 wires').
The need for a 48V rail might possibly be one of the reasons, but it seems to me to be a pretty lame 'excuse'/explanation, in either of those two ('common') scenarios I've just mentioned. If the equipment is supplied with mains, then there is obviously no issue, and if the PSU is entirely external, then generation of the 48V is again exactly the same, the only difference beeing the need for a 'fourth wire' between PSU and equipment.

Kind Regards, John
Now we have SMPS out of this discussion, analogue PSU's are simple, reliable and low noise.

Ideally the smoothing and regulation should be as near as possible to the load to reduce losses, increase stability and reduce the potential for picking up noise.
To reduce the potential for the transformers magnetic field to induce noise into the sensitive electronics it should be as far away as possible.

The simple solution is to do as seen in this thread. As I hinted at earlier 50:50 is a well proven concept and is a very common arrangement in pro audio kit. If a remote dc PSU is used, it's fairly likely to have minimal smoothing and further smooting/regulation within the device.

The larger Soundcraft (older) desks typically had a 150VA transformer, brigde rectifier for +/- dc of around 20-24V and say 100μF caps and another winding/bridge rectifier/100μF cap for about 60V and an expensive piece of 7core cable (loads of thin strands of copper to keep a thick cable flexible) and high quality multi-pin plug and socket. In the desk is more smoothing and regulators for +/- 17V & +48V.

I'm not trying to indicate it's the only way it's done and some manufacturers do not do this so some pro audio guys may not even come across it
Thanks. Unfortunately, that only shows the electronic 'works' of the mixer (i.e. the audio circuitry), and not anything to do with the power supply arrangements. Are there any other schematics there?

Kind Regards, John
I have the full circuit for the 1202 (but not the FX version) which is probably identical to the 1002 if that is of use, I've already posted the PSU element of it.

it.
 
Ideally the smoothing and regulation should be as near as possible to the load to reduce losses, increase stability and reduce the potential for picking up noise.
Fair enough. Some 'smoothing' (to deal with any noise) would be required in the equipment, but I would have thought that all the 'primary smoothing' )which might involve some fairly bulky components) could be done in an external PSU. As for regulation, unless we were talking about high power/current equipment (and a mixer clearly isn't that), I would again have thought that regulation within an external PSU would be adequate.
To reduce the potential for the transformers magnetic field to induce noise into the sensitive electronics it should be as far away as possible.
Indeed - but that, in itself, is obviously served by having the entire PSU 'external'.
....I have the full circuit for the 1202 (but not the FX version) which is probably identical to the 1002 if that is of use, I've already posted the PSU element of it.
Indeed, and that's why I wrote:
.... I think that one conclusion of it is that it's very probable that the 48V 'phantom power' (assuming you have, and need, that - but what you say in post#49 maybe implies that you don't need the 48V?) is probably derived in a manner that would not work if you tried to power the mixer with DC (i.e. the original thought of "2 x 21V batteries").

Kind Regards, John
 
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.
Actually, it's not at all uncommon for this sort of application - and prior to the ubiquity of SMPSUs, for non-audio supplies in the past.
You can get the same DC voltages with half-wave rectification from a single AC supply - but then you only have half-wave rectification. By having a split AC, you can have full wave rectification which means less ripple and/or smaller smoothing caps. With a single AC input, you can only have a signle (either positive or negative) DC supply with full wave rectification.
As the saying goes, been there, done that ...
Again, indeed. Having a centre-tapped transformer secondary is probably the simplest/easiest way to get +15V and -15V rails.
Indeed it is - and with full wave rectification as above.
However, what I was talking/asking about was this concept (which I think is a 'new one' on me!) of having a power supply which is 'half in and half out' of the equipment it supplies - i.e. having the transformer external to the equipment but everything else internal.
Again, not at all uncommon at one time - and as already mentioned, in this case means not taking a magnetic field into the equipment.
In this application, to have the full PSU external would need another input pin (total of four: gnd, ± rails, phantom power) and also a custom PSU. A simple X-0-X transformer will be an off the shelf* item, and available in multiple input voltages - thus simplifying catering for different markets.
And lastly, it also means not taking "mains" voltages into the unit - which somewhat simplifies all the design/regulatory/certification issues involved which are now "exported" to the off the shelf external PSU (and hence an SEP**). You may have noticed that few bits of equipment with external power supplies has the equipment maker's name on the PSU - the PSU will have been picked from a catalogue and all the issues (safety, EMC, load waveform (PFC correction), etc can be dismissed with a few lines in a contract and a little bit of due diligence to make sure the manufacturer is delivering what it promised.

* Well almost off the shelf - unless the particular configuration of connector already exists.

** SEP = Someone Else's Problem
 
Actually, it's not at all uncommon for this sort of application ... You can get the same DC voltages with half-wave rectification from a single AC supply - but then you only have half-wave rectification. By having a split AC, you can have full wave rectification which means less ripple and/or smaller smoothing caps. ...
As I've explained (and as you address below), I've never had any issue (or lack of familiarity) with the concept (and advantages) of having a transformer with a centre-taped secondary when one wants positive and negative supply rails. What I was/am unfamiliar with is the concept of having the transformer 'external' and the rest of the PSU 'internal'.
Again, not at all uncommon at one time - and as already mentioned, in this case means not taking a magnetic field into the equipment.
Indeed - but, as I've said, that is satisfied by the (I would have thought more common) practice of having the entire PSU 'external' (not just the transformer).
In this application, to have the full PSU external would need another input pin (total of four: gnd, ± rails, phantom power)
I've acknowledged that - but it is surely trivial. I personally have plenty of PSUs which have a lot more than three (or four) output connections to the supplied equipment.
... and also a custom PSU. A simple X-0-X transformer will be an off the shelf* item, and available in multiple input voltages - thus simplifying catering for different markets.
I suppose so, but there are only really two relevant markets, and transformers which can be switched between the two are readily available.
...And lastly, it also means not taking "mains" voltages into the unit ...
Indeed, and I've acknowledged that as well - but, again, it is not an argument for not having the entire PSU external (which is what we are all used to, even more now than in the past).

Kind Regards, John
 
intresting indeed!
But I use a dynamic mic and ont a condensor
Well that's good news- one less thing to worry about. Still reckon plugging the thing into your 150w inverter will be the best bet, any chance of having the car/van within 50 metres of where you need the mixer (with the engine running)?
 
Well that's good news- one less thing to worry about. Still reckon plugging the thing into your 150w inverter will be the best bet ...
Perhaps, but it still feels a bit OTT for a type of thing that (particularly since he doesn't need the 48V) could probably in theory be powered by a tiny battery.
... any chance of having the car/van within 50 metres of where you need the mixer (with the engine running)?
I suggest that you keep that idea away from your friendly neighbourhood Green Party candidate :)

Kind Regards, John
 
What I was/am unfamiliar with is the concept of having the transformer 'external' and the rest of the PSU 'internal'.
You may be unfamiliar with the idea, but it's not rare, and for an application like this makes perfect sense. As you'll have seen from the diagram previously posted, the internal circuitry is quite simple - apart from the detail of the voltage tripler for the phantom power. It's low power, efficiency is not an issue - but noise is. It would certainly be possible to have a PSU made to do the whole lot in the external PSU - but it would probably be a custom model and a custom unit costs more than an off the shelf one as well as bringing various factors into play in terms of specifying the design and ensuring that the manufacturer meets it. As they've done it, the key bits that control supply quality (especially noise) are inside their own box.
But it comes down to this : whatever any of us think about it, it's how they decided to do it, and that's all that matters !

As for the OP, the unit should work just fine from + & - DC supplies with the exception of the phantom power. A quick check of the specs for a 7815 regulator indicates that the dropout voltage is around 2V, so the + input needs to be around 18V minimum (and the - input around -18V). Maximum limits seem to be higher than I expected at 35V, so damaging the unit is unlikely with (say) ± 24V. It might even be practical to power it (for short periods) with 4off PP9 batteries to give (nominally) ±18V.
 

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