Finally rec'd basic components for this project over the weekend and did brief initial run this morning. Everything worked but had a couple of surprises (nothing bad). More on that later.
Did not attempt to wire anything permanently or mount in a box. All connections made with wire nuts and patch cords with insulated alligator clips. Cables carefully laid out to avoid any accidental shorts. Although I intend to implement the solution using the additional 10 ohm resistor, today's test was of the original configuration with 4 ohm resistor connected between the amplifier's positive (red) output and the 8 ohm connection of the 70V transformer. The negative amp output (black) was connected to "C" (assume "Common") input of the 70V transformer.
For the amp, I purchased a FOSI BL20A commercial box. Although rather pricey ($56) off ebay (US), it is a nicely featured multi-input box, that comes with tone controls, an IR remote and what appears to be a high quality, heavy duty 24V power supply. Amp power rating is 100W x 2, supports bluetooth, line input and USB drive. Also, has a pre-out to drive a powered subwoofer though doubt that will ever be used. Nicely finished box with tone controls. Key features for me were the built-in bluetooth and the remote which eliminates the need to touch the amp during a session. In addition, I wanted it to be discrete, able to use the amp for its normal purpose (audio) without attracting any attention
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For today's test run, I connected to the amp via bluetooth from my laptop. Because I only had a limited amount of time, primary goal was confirm that it would output a signal thru each channel. Using dual conductive rubber (CR) loops and a random stim file (more on that), I did feel current thru each channel. Did not hook up a second set of loops/electrodes so only tested one channel at a time.
One of the surprises was how much volume I had to apply before feeling any sensation. I would estimate that the volume was slightly over 70% (according to the Windows volume graphic) before the signal became "interesting". Could be a function of the stim file I was playing.
One disappointment was that I was unable to use a universal 19V/4.5A laptop charger ("iGo"). I wanted to use this specifically because it is double insulated, has its own power switch (momentary) and compact. Although it would power up the amp, it would go into standby mode after about 15 seconds or so, cutting off power to the amp. Tried several times and always same result. Cautiously, I hooked up the FOSI 24V power supply. It is not marked as being double insulated though it is powered by standard two prong plug (no ground lug). Although there were no issues, glad that I was able to use the remote to control the amp. Not sure if I need to invest in another power supply. Would welcome advice.
Regarding the experience itself, was very brief, perhaps 5 minutes with each channel. The signal felt a little on the "spiky" side, not necessarily unpleasant but a little bit sharp at times. Because I didn't try any other MP3's, not sure if what I felt was representative of other files. Also, from past experience with TENS unit, the initial play can be a little harsh until you adjust or perhaps de-sensitize a bit and then begin cranking up the power! Hoping that is the case here. Next test will involve using the 10 ohm resistor configuration.
Because the play time was so brief, did not notice anything heating up. Will provide update after I have opportunity for an extended session.
One last thing, I would appreciate any suggestions / recommendations on stim files (author/title/link?). There are literally thousands out there and don't even know where to start. Initially, will be doing either mono or stereo so that is the priority for now. Not sure how to safely configure for tri-phase so will try that later. PM's are fine.
Thanks to everyone on the forum!