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A question for the electronics experts...


daflewis
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Hi all,
I have a raven labs MBD-1 (basically a mini mixer) which runs on two 9v batteries. I had the idea that I could run it using a one spot daisy chain with battery clip adapters, but it won't work... It will work with one and a battery, but not 2 from the one spot. I guess this has something to do with the 2 feeds coming from the same power supply, but I wondered if someone could explain it to me?
Also, am I ok powering it 1 and a battery? I don't know what effect the different voltages would have as the battery dies.
Thanks in advance,
Daf

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[quote name='LawrenceH' timestamp='1339259749' post='1686051']
The mixer is probably running at 18V, ie using the two batteries in series. Two parallel connections from a 9V supply won't substitute for this
[/quote]

Yes, and possibly even as +9v : 0v : -9v

It should be possible to power the mixer from two separate 9V power supplies, but only if the two supplies have floating grounds - which actually might be unlikely for a PSU designed to power multiple 9v devices.

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Thanks for that. I do remember seeing something about the raven labs power supply (now unabailable) being +9 0v -9v - not that that means anything to me.... Just like floating grounds - there should be an emoticon for "this is going straight over my head!" ;) butI'm glad I asked before going and buying another power supply.... Still, if I run it with 1 battery in I suppose it'll at least cut my battery costs in half... :)

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It's a bit difficult to describe without drawings, but imagine an audio signal as being a sine wave that moves up and down (i.e. the voltage increases and decreases). The louder the signal the higher (and lower) the sine wave will reach (its amplitude). As the audio signal gets quieter and quieter the sine wave amplitude becomes less and less until, when there is no audio at all the "sine wave" is just a straight line.

This straight line corresponds to the ground level, sometimes called 0volts. When the sine wave is above this ground level it is a positive voltage and when below it is a negative voltage. An amplifier operating with a -9 : 0v : +9 volt power supply will be capable of outputting an audio waveform that extends up to (just under) 9 volts at the 'top' and to -9v at the 'bottom'.

But batteries don't have 'ground' terminals, only + and -. They are 'floating' power sources.

So, to connect two 9v batteries to give -9:0:+9 connect the + of one battery to the - of the other and connect this point to the ground wire of the amp/bass/whatever. Now, the spare + terminal will be +9 volts above ground and the - terminal will be -9 volts below ground.

However, trying to do this with a non-floating power supply is not possible because the '-' terminal is already tied to ground, thus it is not possible to connect the '-' of one output to the '+' of another output - or rather it is but it will short-circuit the power supply outputs, which is not a good thing.

Hmm. Re-reading this, I'm not sure I've explained it very well. I guess the key point is that batteries can be connected in a 9-0-9 configuration because they are 'floating' devices and any replacements for them must be similarly floating (or a purpose built 9-0-9 power supply).

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Hi Flyfisher,

Well, that's the best description of voltage i've ever had! thank you. :) and i get the idea about batteries being "floating devices" - i even got the bit that if you try to use a grounded power supply (or if batteries weren't "floating" for example) they would short, as one would end up connected to ground at both the + and - .... i think .... but this leads me to think that i was lucky not to damage the unit when i tried to power it with two feeds from the power supply - would that be right? :unsure:

Edited by daflewis
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yes, i use rechargeables at the moment, but whereas alkalines will last a couple of weeks (i do 8 shows a week) the rechargeables only last maybe three days, and when they do go there's no warning which is a bit embarrassing in front of two and a half thousand people... add to that the fact that i have two of these units, and the fact that they aren't particularly accessible and you can see why i was looking into power supplies. the other problem is, as far as buying a power supply with a normal plug - the earlier raven labs (the ones that i have) don't have that option (they added a 2.5mm input on later models)
Actually, i have a later raven labs pickup blender unit with a power input and i have tried to find a supply for that without success (though only maplins and the usual other suspects) so if you know where i could get one that would be useful anyway! :)
cheers,
Daf

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[quote name='daflewis' timestamp='1339288458' post='1686551']
Hi Flyfisher,

Well, that's the best description of voltage i've ever had! thank you. :) and i get the idea about batteries being "floating devices" - i even got the bit that if you try to use a grounded power supply (or if batteries weren't "floating" for example) they would short, as one would end up connected to ground at both the + and - .... i think .... but this leads me to think that i was lucky not to damage the unit when i tried to power it with two feeds from the power supply - would that be right? :unsure:
[/quote]

Probably not lucky. You wouldn't damage the mixer because with the PSU shorted there would be no voltage going to the mixer anyway and any decent PSU wouldn't be damaged by being shorted because the designers should have realised that such a thing was almost inevitable at some stage and would have built in some form of short-circuit protection. A simply fuse would do the job, though these days some form of electronic and reversible protection is generally used.

The thing about rechargeable cells is that they usually provide a lower voltage than their disposable equivalents. Depending on the device being powered, this can result in the rechargeables not lasting as long.

bertbass probably has the right approach - just get a 9-0-9v PSU. ;)

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Yeh, a +9v 0v -9v power supply with two battery clip ends would be great.... Anyone fancy making me one? :) or two actually! ;)

I know there'd be a load of other people after them too.... Bob gollihur (a big on line double bass site/shop in the states) said if I managed to find some way of powering the raven labs gear (both with battery and 2.5mm jack) they'd be very interested... Could be a nice little project for someone... :)

Edited by daflewis
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If you can't find a commercial 9-0-9 PSU then the easiest option would probably be two 9v PSUs. Might not be easy to find such a device with a 9V battery clip on the end, but that could easily be fixed.

Just have to make sure that 9v output is floating, but most of them are because they often have a plastic "earth" pin, so there is no ground reference anyway and they get that through the connected equipment.

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ah, ok - good to know there's an easy way to tell if it's a floating ground... there have been a couple of on line discussions about splicing together two 9v supplies with a 2.5mm on the end - a bit above my soldering skills though! but yes, two individual supplies could work fairly easily for the battery clip ones. :) Ta.

Edited by daflewis
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Probably easiest to use two 9V PSUs with PP3 type connectors on them. Theyre cheap and you wont have any worries about safety.

Last thing you want in a 2500 seat theatre is smoke setting off the fire alarm, or worse....

And then if it turns out to be home made or modified, you wouldnt be very popular !

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Thanks for the concern brancini, but actually one of the guys i was thinking of is head sparks at the theatre, so we should be ok... ;)
(and it would have to be checked over by him and PAT tested anyway)

Edited by daflewis
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[quote name='daflewis' timestamp='1339462805' post='1688919']
Thanks for the concern brancini, but actually one of the guys i was thinking of is head sparks at the theatre, so we should be ok... ;)
(and it would have to be checked over by him and PAT tested anyway)
[/quote]

you sent silent fly a wee message yet? he's the sort of fellow who would take on your wee project and be able to add it to his line up maybe

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Blimey, that Joe Brown is a dark horse - I thought he was an old skiffle player. ;)

[quote name='BRANCINI' timestamp='1339461994' post='1688915']
Probably easiest to use two 9V PSUs with PP3 type connectors on them. Theyre cheap and you wont have any worries about safety.
[/quote]

Frankly, that's the approach I would take. Quick, reliable, safe, probably less than £20 the lot and cheap enough to carry a spare.

I have little doubt that PSU article is sound but I'd bet the components would cost at least £20 and it would take the best part of a day to build (including sourcing and ordering all the components and doing all the box work).

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Thanks flyfisher, i was thinking about using the 2 psu route for the ones with only batteries and having something made up for the one with the power input, but yes, i hadn't really thought about the costs involved in having one made.

Can i check once more, if i want to use 2 PSUs they would BOTH have to have floating grounds? or was it only because i was trying to run both off one unit that it needed to be floating?

Sorry for being dense....

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Using two PSUs with floating outputs is electrically identical to using two batteries (which, by their nature, are 'floating' voltage sources).

Depending on the exact design/construction of the units to be powered, a mix of floating and non-floating PSUs MIGHT work but two floating PSUs WILL work. Besides, keeping the PSUs identical makes carrying a spare one simpler.


I missed the point about the other unit with a power input socket. I've re-read your posts and you mention a 2.5mm jack. I'm assuming you mean a normal audio 'mini-jack' rather than the sometimes used 'barrel jack'. For this to accept a 9-0-9v it would have to have 3 conductors, i.e. a stereo connector. I can't recall seeing a piece of equipment made this way (which doesn't mean such things don't exist) so I would suspect it would be a more standard single voltage input.

Is the equipment marked with the voltage input for this socket or do you have a manual with any details?

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thanks... again!

The jack is apparently a 2.5mm stereo - i've seen a thread (on talkbass i think) about splicing together two 9v PSUs into the one jack so i guess it is also 9-0-9v. (the later ones also have a daisy chain facility to link the units together to power off the one supply - shame i only have one like this! ;) )

i think the easiest, safest and as you say cheapest way is just to get six floating PSUs, six battery adapters and a couple of extra four gang sockets!! :lol:

cheers!

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[quote name='daflewis' timestamp='1339462805' post='1688919']
Thanks for the concern brancini, but actually one of the guys i was thinking of is head sparks at the theatre, so we should be ok... ;)
(and it would have to be checked over by him and PAT tested anyway)
[/quote]

I understand your point, but I think you'll find said sparkie is obliged to only use certified equipment. So dont forget to ask the insurance company and the fire officer if they are happy with you using a non CE certified power supply, built by the local sparkie, and connected to the mains, just so long as he puts a PAT label on it.... :rolleyes: .

For the sake of the cost of two 'Battery Eliminators' it just aint, imho, worth the risk.

Edited by BRANCINI
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I'm sure I can find that talkbass thread again with the technical data on it... I'll take a look and talk to my sparks friend about soldering that jack.
It's kind of gone full circle - I'd been hoping to find a way to power them from one psu (or at least one psu each) but it looks like that's just not feasible... though the floating ground info is important - thanks for that and all the other advice. :)

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