Battery powered phono pre-amp???

[kpodolski]

I have a Cambridge 640P and had some questions regarding powering it via a battery instead of the wall wart. The pre-amp requires 12v .05a. Before wasting time and money building a battery pack I used my 12v motorcycle battery. When I power the unit up with the battery the power indicator lamp lights but the unit will not turn on...there is a normal several second delay but the unit never trips on. My thinking is that the amperage from the motorcycle battery is way to high. Is there a circuit I can build to get the amperage down to 0.5? Is there something I'm overlooking here? And help, suggestions and /or guidance will be greatly appreciated!

[Alaric]

Is it .05 or .5 amps? I think you're lucky that 10-15 amp battery didn't smoke your pre! Google Watt's Law and Ohm's Law and you'll find the math to allow you to pick the proper resistors to juggle something close , but you may wind up with heat issues. Also , if your battery isn't a gel battery , and has H2SO4 in it , the fumes won't do your equipment any good.
Honestly , I wouldn't try it. 8 D cell batterys , wired in series , would be a better start , but check the amperage!

[Johnloudb]

Your phono preamp shouldn't pull more amps than it needs, so the battery shouldn't hurt anything. But, a 12 volt wall wart will often put out more than 12 volts (like 14-16v) depending on the load. So, it may be that you don't have enough voltage.

You should put a resistor load (240 ohms) across the wall wart and measure the voltage with a multi-meter. That will give you an idea of what voltage you really need.

Other than that I don't know why it wouldn't work.

[Glen B]

You're feeding your phono preamp DC when it appears to require an AC input. Some preamps with outboard supply have the rectifier and filter caps inside the preamp, with the outboard power supply providing only raw AC.

According to the owner's manual for your unit on the Cambridge website, the rear panel power inlet is marked "12V ~ (the symbol for AC) 0.5A, Use Supplied PSU Only !", the latter obviously for good reason.

[joetama]

You're feeding your phono preamp DC when it appears to require an AC input. Some preamps with outboard supply have the rectifier and filter caps inside the preamp, with the outboard power supply providing only raw AC.

According to the owner's manual for your unit on the Cambridge website, the rear panel power inlet is marked "12V ~ (the symbol for AC) 0.5A, Use Supplied PSU Only !", the latter obviously for good reason.

Never mind... I just looked at it and it does use an external transformer so I'm guessing that 12 VDC isn't going to work. Probably wouldn't try it. The words "Use Supplied PSU Only" would rattle me a bit.

[kpodolski]

Is it .05 or .5 amps? I think you're lucky that 10-15 amp battery didn't smoke your pre! Google Watt's Law and Ohm's Law and you'll find the math to allow you to pick the proper resistors to juggle something close , but you may wind up with heat issues. Also , if your battery isn't a gel battery , and has H2SO4 in it , the fumes won't do your equipment any good.
Honestly , I wouldn't try it. 8 D cell batterys , wired in series , would be a better start , but check the amperage!

Sorry, my mistake, it is .5a. I like your idea about the D cell setup. I think I'm going to give it a try.

I'm just curious about this approch because I've heard many postive remarks about battey powered phono pre-amps.

Thanks for all the advise guys!

[joetama]

If you have a multimeter I would test the power supply that came with the unit to see what is coming out of it.

[Glen B]

Even better, the type of output (DC or AC and current in mA) may be marked on the power supply.

[kpodolski]

Even better, the type of output (DC or AC and current in mA) may be marked on the power supply.

My bad...the unit is marked with 12v AC!!! I'm lucky I didn't destroy the pre-amp. Lesson learned!

[joetama]

I was worried for you.

Just out of curiosity do you like that preamp? I'm in the market for one.

[kpodolski]

I was worried for you.

Just out of curiosity do you like that preamp? I'm in the market for one.

I bought this as a holdover until I found a tube stage. For the money, this is not a bad phono stage. I'm having some ground induced hum (very low in level) that I'm trying to nail down. It's not a problem with the unit because it's dead quiet in another system.

Sound wise in my system, it has a nice airy feel. The bass is tight and contolled with no boominess. The bass is a tad on the light side but I'd rather have it that way then bloated and out of control. I'm using an Ortofon OM30 on my Project Perspective. I would highly recommend this unit. I'm happy with it. You could probably spend more and get less sound quality.