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Termyn8or -> RE: Guitar Amp? (7/13/2007 2:08:29 PM)
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tag we need to have a little talk. I did a jumpthrough, sorry but we need to clarify things.
The frequency an amp puts out is determined by the input frequency. The amplitude of the output is dependent on the input level.
In the case of a hifi amp you put in a voltage and it attempts to reproduce it as accurately as possible in voltage. Current drain on the output is determined by the impedance of the speaker system, which can vary drastically with frequency. The hifi amp puts out a constant voltage GAIN, based on the settings.
A guitar amp is designed not to do that. It could be said that they do not desire a high damping factor, which means it will have a higher effective output impedance. In a hifi amp the low impedance is capitalized on to make enough low bass out of a woofer that is much smaller than the waves it must reproduce. The effect is caused by the inductance of the speaker. Better woofers are more turns of heavier wires vs less turns of thinner wire, which may result in the same nominal impedance, but a drastic difference in the impedance curve vs frequency.
A guitar amp is not designed that way. Even solid state ones make an attempt at soft clipping, and tubes are even better. They want the speaker to interact with the output stage when clipping. This is what gives you the sweet Fender sound or whatever. While many of them do have overdrive stages at the input, the best squeal and grind is obtained by clipping the output stage.
Now there are certain caveats if you want to stay alive. First of all if you have a tube amp, don't even think about it, what's more on a solid state amp you are going to have to float it, that is interrupt the ground wire. The ground wire can kill in this situation.
Even if you use a stereo amp which is alot better choice, do not connect it to any other equipment that may be grounded. Also you might want to take it to a local TV shop and have a hot leakage current check done before you start. I know how to do such a test, the right way as well as the improvised way, at any rate it is tested at real line voltage, not with an ohmmeter.
There are other options, and really a battery powered boombox will never give you a lethal ground fault as long as it is running off batteries. There are many models with seperable speakers which means you have access to the speaker wires. That should be all the power you need. It does depend on how you wish to place the electrodes though. If you are going to use a urethral electrode you'll have plenty of voltage as long as you have a good other side (like a ground) on your body. Remember nothing above the waist, at least for now. You need a bipolar system for that, and this is not. Even the two channels share a common ground, even if the output is a BTL configuration, the only thing that can make it bipolar is transformers.
You can actually make electrodes with paper towels and duct tape. Get the towels stuck, put the wire under them first and then pour on some saltwater. Just enough, but make sure it is enough. Then you can use medical tape, or more duct tape to adhere it to the skin.
Depending on your choice of electrodes you might need more voltage. If you want to persue this I think the best solution is line drop transformers used in PA systems. Each transformer should be the kind that isolates, not all of them do, but when you get it, there will be 35.3 V and 70.7 V taps on the primary and hopefully 4, 8 and 16 ohm taps on the secondary. You can get many voltage step up ratios, but I am not saying to use the 70.7 V tap. All you might want to do is feed the 4 ohm and take off the 16 ohm. That will double the voltage. For a little more kick you can feed the 16 ohm and tap off the 35.3 V winding.
If you are ever going to use more than one channel you must be sure that the transformer effectively isolates. You might just have to learn some more electronics, but there's nothing wrong with that.
I also suggest that you go to a garage sale and pick up a used keyboard. Going through all the notes etc will give you an idea of what frequencies are good and which are bad. The human body also has an impedance curve and to me it seems inductrive, although that has no basis in physics. I only know this because I have a Wavetech waveform generator I use sometimes. Has a big vernier for the frequency and I notice that at certain low range no stim is best. It can be painful. Getting to the higher frequencies, especially in sinewave mode, the voltage needs to be turned up to feel it, and then it starts feeling good.
So I can't explain why it seems inductive, it is completely subjective, but that seems to be the way it is.
At this point I would suggest you visit PES, I think it is PES.com but not sure. Personal Electronic or Electrical Stimulation is what it stands for. They have these gnarly looking electrodes for both male and female that are made out of this hypoallergenic conductive silicone based shit. No matter what you feed them with, good electrodes are a must. If you don't have a contiguous contact area it can burn like hell. I've tried a bunch of different things, and but a few were even worth a mention.
But I still don't think I would use a guitar amp. I think even a moderately powered stereo reciever would be better. Anything battery powered would be even better. If you get a boombox that runs off of 8 D batteries, you should have plenty of juice.
T
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