DIY Stereo Stim with amp,transformers and maybe resistors

[Electro]

From the amplifier each channel positive goes to a 5.6 ohm 10W resister which then feeds to the 8 ohm lead of a 70V transformer. Each channel negative goes directly to the common on the same side of the transformer. I use the 1W lead for the electrode positive lead and the common lead for negative. Nothing real crazy. Four components inside the box. two 5.6 ohm resisters and two 70V transformers.

Can't remember why I went with 5.6 ohm resistors or why I went with the 1W lead on the transformer. I also had two 10 ohm resisters somewhere in the mix but took them out. I'm sure I read it somewhere. Before I bought the Fosi amp I did have a TDA7297 15Wx15W amp board but the volume knob eventually broke. Instead of redesigning my whole stim box, I just took it out and added input jacks to it for the Fosi amp. Haven't had an issue with it yet and have had it for 3 years now.

I think the 1 watt lead for the transformer is probably what was needed to make up for the higher resistance value, otherwise you'd probably be maxxing out the amp with the higher load placed on the amp with a higher turns ratio causing the resistor to absorb more of the energy. There was a discussion about using a higher resistance value back in the day on the Smartstim forum and the discussion was mostly saying that the higher load was more likely to cause issues with causing the signal to max out or overheat the amp and trip thermal protection, which probably isn't going to happen with the 1 watt tap but might with a higher turns ratio(lower watt rating) tap, and potentially feel a little different because generally the higher turns ratios tend to feel a little smoother, but if the amp handles it for you and it feels good, it should be fine because it's all a matter of balance.

[LAKE_DUCK_HUNTER]

Has anyone tried to break open a wall wart and use the transformer from those?

[picofarad]

Has anyone tried to break open a wall wart and use the transformer from those?

Recent PS are switching not transformer based.
If you have a transformer based (usually heavier) it is likely well encased, may be potted so breaking open will likely end up breaking leads or damaging the windings.
I guess in theory it could work but the time and risk may not be worth it in the long run. And this has nothing to say how a power transformer (designed for 50-60 Hz) would work in an audio circuit.

[LAKE_DUCK_HUNTER]

I have managed to break one open. I used a hack saw on the glue line to just saw thru. Then used a screw driver to break it open.
Inside I found a small circuit board and a transformer. I can isolate the transformer from the circuit board. I measure the ohms and got something like 1 and 300. I can recall exactly. Might have to give it a try.

[picofarad]

From the amplifier each channel positive goes to a 5.6 ohm 10W resister which then feeds to the 8 ohm lead of a 70V transformer. Each channel negative goes directly to the common on the same side of the transformer. I use the 1W lead for the electrode positive lead and the common lead for negative. Nothing real crazy. Four components inside the box. two 5.6 ohm resisters and two 70V transformers.

Can't remember why I went with 5.6 ohm resistors or why I went with the 1W lead on the transformer. ...

... generally the higher turns ratios tend to feel a little smoother, but if the amp handles it for you and it feels good, it should be fine because it's all a matter of balance.

Got all the hardware today: the Fosi amp, two 4 ohm resistors, and two transformers. So far, I only checked the resistors and the transformers with my Fluke meter. everything seems OK. I now have to rough wire it up with jumpers and see which Tap on the transformer output side works best. Once I am satisfied everything is working, I will get a box to hold the transformers and the resistors.
I will start with the power supply that came with the amp. Its 32VDC and if necessary, I can use either a 24 or 48 Volt PS if the power needs to go down or up from what the 32's range would allow.

[47dahc]

I will start with the power supply that came with the amp. Its 32VDC and if necessary, I can use either a 24 or 48 Volt PS if the power needs to go down or up from what the 32's range would allow.

That's the power supply I'm running. Knew I was forgetting to mention something.

[picofarad]

It works!
Just hooked one channel together. Used blue tooth to feed in the audio from one of the estims here. I had the 4 ohm power resistor in series on the line from the amp to the transformer. I used the 4 ohm tap on the transformer. On the output I used the common and yellow which on my set of transformers is second from the lowest output rating but I believe the second highest voltage step-up tap.
Will want to do more testing but have more gain on both the computer and the amp. Both were maybe 35-45%, so plenty of headroom.
I did use the power supply that came with it, 32 vdc, see no need to change that.

See first post in this thread for component listing.

[darthjj]

Not sure about the need for power resistors both in series and parallel. I have read mixed reviews. I have resistors I can try and I believe this amp has protection circuits which would work with the load the transformers would supply due the resistance/impedance of the primary winding.

I can't promise this is hard fact and not placebo effect, but it definitely seems like parallel resistors reduce the effect of changing the carrier frequency. My first box was built without parallel resistors, and it always felt like lower frequencies became much stronger in intensity. So I whipped up some experiemtal mp3s where I reduced the volume at the same time as lowering the frequency, and vice versa, in order to try to produce a sensation at a constant intensity. But when other people tested it, about 50% agreed with me and the other 50% said that it didn't work at all. After much discussions and speculation, it seemed like the division was caused by some people having parallel resistors and the others didn't.

So I added a pair of parallel resistors to my box, and it definitely feels like the frequency-intensity correlation has been much reduced. Now I can do frequency modulation to somewhat alter the feeling, which adds another dimension to the signals. And I can use content where the signal creator has used different frequencies (probably because they have parallel resistors on their box) without getting shocked.

Since you have the hardware, try it both with and without and see which you prefer.

[picofarad]

Just finished the auxiliary box which has the two transformers and current resistors. The box has for banana plugs for input and four for output. By combining the two common output I get a tri electrode setup otherwise it is two electrically isolated stereo channels.
Since the power resistors are oversized (100-watt rating) no need for any additional heatsinks. I used 3M VHB tape to fasten them to the bottom of the plastic box.

I also got the three pedal Elgato switch, it lets one control the volume on a computer in many ways. I have each pedal programed to do different things, like increase the volume, random (increase, decrease or mute, and one where it ramps up overtime and finally mutes.

I am very happy with my standalone audio amp which connects to the computer via Bluetooth. This gives me another layer of isolation and works well.

aux box with transformers and current limiting resistors

aux box.jpg (506.06 KiB) Viewed 263 times

Milovana with setup

tri electrode setup.jpg (503.75 KiB) Viewed 263 times

amp on aux. box

IMG_6291.jpg (787.3 KiB) Viewed 263 times

screenshot from foot pedal software

IMG_6296.jpg (781.76 KiB) Viewed 263 times

pedal to be used when hands are not free to change intensity (volume)

Elgato Stream Deck Pedal.jpg (617.5 KiB) Viewed 263 times