AC vs DC - an electrical question

[gibsonlp]

Hi guys.

As part of my home studio purchase I bought a couple of M-Audio DMP3 pre-amps and an aphex headphones amplifier.
The Aphex takes 12VAC
The M-Audio preamps take 9VAC
I can't use these power supply units as they accept only 110v and Israel is wired with 220v.

I have 2 questions:
1. Is it possible to supply it with 12VDC and 9VDC instead of 12VAC and 9VAC?
2. Does the polarity matter for an AC complaint device? if it is - how do I know the polarity that is required? (the original PSUs were AC so no polarity diagram exists on either the PSUs or the units themselves).

Thanks in advance.

[leftyguitars]

The easiest way to do this is to buy a 220 volt to 110 volt dropper (transformer) if the frquency is the same (50hz or 60hz), and then use the original power supplies. These are available from online stores. If you can't get one I can get one for you and ship it to you, they don't cost much.

The units probably are DC but you would need to check inside for a bridge rectifier or similar and if there was one you would need a meter to work out the correct input polarity.

[gibsonlp]

Thanks.
The frequency is NOT the same (50Hz in Israel vs 60Hz in the US), that's why I didn't get 220 -> 110 dropper as they don't transform the frequency, only the voltage.

If the units are DC, how come they have 9VAC and 12VAC written on them and on the transformers they come with?

Thanks again!

[headbanger]

Gil,

I would go to an electronics components type store and see if they have an off the shelf 12vac & 9vac plugpacks.
Alternately you should be able to buy 220 - 12vac & 9vac multitap transformers @ wire one or both appliances to them.

Strangely enough, it may well be possible to supply them with dc in as the rectification circuitry is inside the appliance just not the transformers. Although I wouldn't recommend this unless you know what you're doing.

AC is non polarized

you could go the 220 -110 step down transformer route but it might be more expensive that simply finding 220-12 or 9 vac adaptors.

Do you have an RS Components equivalent in Israel?

[headbanger]

something like this

http://australia.rs-online.com/web/sear ... &R=4006535

[gibsonlp]

Thanks.

I managed to find a few local stores that sell 220->9v and 220->12v AC transformers.

Apparently, at least my Aphex headpod cannot use a DC power supply, from the manual:
Caution: 12-16V AC Sources Only. At least 6 Watts capacity is required. A DC input may damage your HeadPod and possibly your headphones.

[gibsonlp]

I think this might be good:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0300375681

And apparently, there is RS in Israel... - http://il.rs-online.com/web/

[bails]

Devices requiring AC power will certainly not work with DC power.
Devices requiring DC power will certainly not work with AC power.

Not even worth trying, because you risk frying the device and your power supply at the same time...

Off the record (though is very much on the record!), I do occasionally mismatch voltages if they are close enough, but never AC with DC.

[gibsonlp]

Yeah, thanks, I read some more and apparently AC needs AC, the sales person at the store that sold me a 9v 1A DC adapter said that AC can work with DC but DC can't work with AC, that sounded a bit weird so I decided to ask before I make any horrible mistakes...
I am glad I didn't do it...

[octagon]

Yeah, thanks, I read some more and apparently AC needs AC, the sales person at the store that sold me a 9v 1A DC adapter said that AC can work with DC but DC can't work with AC, that sounded a bit weird so I decided to ask before I make any horrible mistakes...
I am glad I didn't do it...

Yeah,this could have happened if you did..

[cjj]

Devices requiring AC power will certainly not work with DC power.
Devices requiring DC power will certainly not work with AC power.

Not even worth trying, because you risk frying the device and your power supply at the same time...

Off the record (though is very much on the record!), I do occasionally mismatch voltages if they are close enough, but never AC with DC.

Well, the first statement is not an absolute truth. There are many devices out there that have an AC input which will work just fine on DC. Many of these AC devices, ones that have an AC "wall wart" transformer actually rectify the AC (turn it into DC), then filter and regulate it to the proper voltage. Feeding the right DC voltage will simply pass through the rectifier and get regulated just fine.

But, unless you know what the circuit inside looks like and have a fair understanding of electronics, the third statement should be followed, it's just not worth it.

Now, as far as worrying about the 50Hz vs. 60Hz, that's probably not something to really be concerned about. In just about any sort of audio application, the internal circuits will be running on DC. So, since the AC will be getting turned into DC anyway, the frequency really doesn't matter. The only thing that will happen is that the transformer won't work quite as efficiently, but at these frequencies, that inefficiency is negligible, which is why many devices state 50/60Hz. It's far cheaper to use a generic transformer that's capable of either rather than try to design to the ultimate efficiency point for just one frequency.

Now, for Gil's second question, no, there is no polarity with AC voltages, especially on the output (low voltage) side of a transformer. With 110V AC house current in the U.S. there is a "polarity" in that one side is "hot" and the other is "neutral" because the neutral side is actually tied to ground/earth, but the device generally doesn't care. However some devices actually tie the neutral side to things you can touch, so you could get shocked if it's done wrong and you touch another grounded/earthed object. But, back to the low voltage things, there is almost certainly a transformer there to step the voltage to a lower level. This isolates the output side from the input so the hot/neutral thing doesn't exist any more.

There is one place where AC can have a "polarity". On some devices, they actually use a "center tapped" transformer, which will have say, 12V AC between the center and either side, these will have 3 wires/connections. If you only have 2 wires, you don't have to worry.

It should be fine to use any sort of transformer that has the proper AC output voltages. A 220V to 110V transformer on the input will work fine too.

[gibsonlp]

Thanks for the explanation!

[cjj]

Anytime! I guess I haven't spent the last 33 years in electronics design for nothing.

Seriously, if you have any further questions on power supplies, etc., please feel free to ask!

[gibsonlp]

Ok, I went to a nice electric shop today, this is a nice old-style store with tons of interconnects, capacitors, resistors, etc... a really knowledgeable owner and decent prices.
What he said is that connecting a DC power supply to an AC unit will NOT fry anything, but will most likely not work for various reasons I forgot (something like "too much current will be lost when the transformer inside the AC unit will try to transform the DC current to DC again"), I don't remember, in any case - I bought a 2A 9VAC transformer, he changed the plug with 2.5mm jacks as the manual says and added another plug on the wire so I can connect both DMP3 pre-amps on the same line (they need less than 1A each). I also got a 12VAC 1A transformer that came with the required 2.1mm jack for the headphones amp.
Total - 151NIS ($36) including the 2 extra plugs and the wiring job, not bad...

Everything works like a charm! now all I need is time...

[jps]

Check out that nice, old, vintage Time store just down the street from the nice, old, vintage electronics store you just visited!