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Lowering audio voltages

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JC199

New Member
Hey all, I've got a (probably unusual) situation that I've no idea of how to deal with it. :)

I have a project that requires audio to be feed into a VCR (along with a separate video feed) that would then be transmitted to some tv's via a coax cable. (like using the VCR as a transmitter), I've done this before, but in this case, the only supply audio line has a voltage of 100V being pumped through it (from a huge amp, they use to run about 20 large speakers). Anyways, I need a way of getting this signal voltage down to where it would be able to go into the transmitter (VCR) without destroying the device, or at the least distorting the video.

Does anybody know of hardware or way to do this? I assume a transformer will work, but I don't know how that will effect the audio?


PS. I hope this is the appropriate place to ask this, if it's not, my apologies.

thanks for any help or suggestions... :eek:
 

Sceadwian

Banned
Do a google search for audio resistor divider.

Just a basic example. If you have a 100k resistor, and a 100 ohm resistor and you tap the middle, like so.
incoming signal---100K---TAP---100---GND

Take careful notice of the ratio of the first and second resistors. It's like an electrical reduction gear. Mind you because that 100K resistor is in line you can't draw a lot of power from it, so it can only be used to feed the pre-amp / input of something like a VCR, not typically use to feed something like another power stage, which is not what you need in this case. The output impednace at 100mw or 1ma of current is about 10k, which should be fine for typical use.

If you feed 100V in as the incoming signal, at the tap (where you feed the signal into your VCR) you'd get approximately a 100mv signal (typical audio line voltage)
 
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JC199

New Member
Thanks for the reply that makes sense, but, if I use that configuration, what kind of resistors should I use? (the standard 1/4 watt color-coded resistors or the ceramic kind, etc.?)

Also, would it be possible to use a variable resistor in the stead of the 100k resistor (for adjustments), or would that not work because of the ratio of the first and second resistors?

Thanks!
 

Sceadwian

Banned
In my given example a 100k and 100Ω mresistor provide 100mv from 100v input. If you want varible volume, you could do something like a 50k and 100ohm static resistor to give you 200mv maximum and use another 50k variable pot after the 50k one to vary between 100mv and 200mv.

So
input---50k static---50k variable---output tap---100Ω--gnd

In the given total impedance of 100k to gnd, you'll draw 1ma at 100v's or about 100mw's of power so 1/4 watt resistors should work fine.
With 50k static and the 50k variable set to 0 (200mv output) you'll draw about 200mw's of power, that's getting close to wanting to use a 1/2 watt resistor to want to avoid heating effects, a little more power handling if you can afford it is never bad, but I'd be perfectly confident in a 1/2 watt, thin film would be good I think, if I'm not mistaken carbon resistors under load condition add a decent amount of thermal noise so if you have a good pre-amp it'll pick it up. Someone else here might suggest a better resistor type for top quality.
 
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JC199

New Member
Ok, I tried it out an it works great! Only thing to complain is a very small amount of noise, but that might be from the audio system, not the resistors. Either way you can't really notice it unless the audio is quite.


Thanks for your help!
 

Sceadwian

Banned
You could try to go larger, bigger resistors will allow for lower current which will decrease thermal noise picked up, but the output impedance will be much higher so the volume control will be highly exponential over part of it's range depending on the VCR's input impedance. Try doubling or multiplying by 10 all the resistor values.
 
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JC199

New Member
Ok, I'm going to leave it as it is now, since it isn't too bad (and in this situation we don't need excellent quality). But that might come in useful later on. Thanks!
 
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