Low-cost (~$40-60) DIY Stereo E-stim for Audio Input (based on TroniC's MidiStim)

[andyupnaway]

I finished my box, yay! I modified a bit the original build and now it feels great.
This is my final schematic, notice the extra resistors:


The default schematic worked but felt a bit painful, with a constant "static" that got unbearable at the higher levels. After adding the two parallel resistors the static is almost gone and the signal feels very smooth, strong but not painful.
Resistors are inexpensive and this modification barely adds complexity to the build. The quality increase is MASSIVE. I highly recommend this over the original schematic (unless you like pain of course).


@steelhorse545
My audio card was indeed damaged. After trying the estim box with the faulty amp, the left channel had a massive drop in volume. Super annoying and a even bit disorienting, even with the balance corrected in Windows settings. I will have it repaired asap.

I ended up using the box via USB audio interface, configured at 100% volume. Turns out my amp is a bit weaker than expected and I had to raise its volume way up before getting any feeling. Not a big deal but led me to believe that the box wasn't working at all. I believe I was feeling more "distortion" with the USB interface simply because the interface had more power than the broken audio card, but with the extra parallel resistor the noise is gone and that's solved.
Thanks again for the help.

I'm not very well versed in how to wire stuff, when you say these resistors are in parallel, based on the diagram here, one end of the resistor connects to both ends of the purple wire, and one end connects to both ends of black wire, with two resistors total. Or does this mean that each wire gets a resistor with four resistors total. Or is it something else that I'm not getting? lol

use 4 resitors and conect it as in the diagram

Planning on building my box with this suggested mod. Don't want to belabour the issue but want to make sure I've got it right.

Looking at the diagram, it appears that there are total of 4 resistors: (a) 2x10 ohm resistors; and. (b) 2x3.9 ohm resistors. On each channel, the 10 ohm resistor is connected to both the positive feed (after the 3.9 ohm resistor) and negative feed. I don't know much about electronics but not sure what that does.

Now, if when you are saying "use 4 resistors" meaning 4 x 10 ohm resistors (in addition to the 3.9 resistor on the positive feed), inserting them in line on each of the feeds from the amp, then I sort of get that but seems like all you are doing is damping the power of the amp.

Please humor a newbie and confirm. Thanks!

[edger477]

I don't know much about electronics but not sure what that does.

Well you are right, it does "damp" the power of the amp. It makes transformer "lesser" part of the equation, meaning that conductivity of the output matters less - in practice that brings less stinging with lower surface area of electrode (less hotspots) and more consistent output regardless of surface of electrode. I think is pretty mandatory if you make variable surface electrode (I did make one with 2 conductive strips in tremblr receiver - it is... interesting).

Also, some amplifiers require minimum load to properly work, and this resistor ensures there is one, and you get signal on output from lowest volume, instead of being shocked when volume is enough for amp to detect the load.

[andyupnaway]

I don't know much about electronics but not sure what that does.

Well you are right, it does "damp" the power of the amp. It makes transformer "lesser" part of the equation, meaning that conductivity of the output matters less - in practice that brings less stinging with lower surface area of electrode (less hotspots) and more consistent output regardless of surface of electrode. I think is pretty mandatory if you make variable surface electrode (I did make one with 2 conductive strips in tremblr receiver - it is... interesting).

Also, some amplifiers require minimum load to properly work, and this resistor ensures there is one, and you get signal on output from lowest volume, instead of being shocked when volume is enough for amp to detect the load.

Thank you for your response but still not quite certain what is the correct configuration.

Should the box have a total of four resistors (2 x 3.9 ohm and 2 x 10 ohm) ? That is how I read the diagram:

The positive lead on each channel has a 3.9 ohm resistor (two resistors total - one on each channel)

and

Each channel has a single 10 ohm resistor that is connected to both the negative lead and positive lead after the 3.9 ohm resistor on the positive lead. The 10 ohm resister is inserted in the circuits before the negative and positive leads are connected to the transformer.

Is this correct?

Thank you for your patience!

[edger477]

Thank you for your response but still not quite certain what is the correct configuration.

Should the box have a total of four resistors (2 x 3.9 ohm and 2 x 10 ohm) ? That is how I read the diagram:

The positive lead on each channel has a 3.9 ohm resistor (two resistors total - one on each channel)

and

Each channel has a single 10 ohm resistor that is connected to both the negative lead and positive lead after the 3.9 ohm resistor on the positive lead. The 10 ohm resister is inserted in the circuits before the negative and positive leads are connected to the transformer.

Is this correct?

Thank you for your patience!

This does sound correct, but you are not using correct terminology... when discussing circuits before/after terms often don't make sense...

We say that 10 ohm resistor is parallel with transformer (so 2 terminals of transformer are connected to 2 terminals of resistor) - them now making single unit (there are 2 terminals of that unit) which is in series with 3.9 ohm resistor.

I think that picture from ramen is quite clear, I could maybe try to explain how to do it in another way:
1 imagine you connected transformer directly to amplifier
2 then you cut the + wire - then solder the 2 ends you cut to the terminals of 3.9 ohm resistor (now you added serial resistor)
3 then you connect the 10 ohm resistor to the 2 wires of transformer

Easiest is to solder one terminal of each 3.9 and 10 ohm resistors together to one wire to transformer (so 3 terminals connected) then solder the other (free) side of 10 ohm resistor to the other (still free) wire from transformer, along with the - wire for amplifier (again 3 terminals connected here), and solder + wire for amplifier to the remaining free end of 3.9 ohm resistor.

[Gr8M1ke]

For the smart people here, does the wattage of the resistor matter?

We always talk about the resistance, but not the other.

Thanks!

[edger477]

For the smart people here, does the wattage of the resistor matter?

We always talk about the resistance, but not the other.

Thanks!

It does matter, if it is too weak it will blow up, and if it is the parallel resistor that fails, you might get your balls fried

If you use recommended ones you should be fine. I am using 50 and 100w because this is just the ability of resistor to dissipate heat, and more cannot hurt (if it is non-inductive resistance).

[Gr8M1ke]

For the smart people here, does the wattage of the resistor matter?

We always talk about the resistance, but not the other.

Thanks!

It does matter, if it is too weak it will blow up, and if it is the parallel resistor that fails, you might get your balls fried

If you use recommended ones you should be fine. I am using 50 and 100w because this is just the ability of resistor to dissipate heat, and more cannot hurt (if it is non-inductive resistance).

Thanks, certainly don't want fried balls!

[andyupnaway]

Have question regarding proper implementation of an output switch. My build will include the two 10 ohm resistors installed parallel to the transformer. I want to install a simple toggle on/off in line switch between the transformer output and the electrodes. Reasoning is that I want to prevent the possibility of momentary spikes that may occur when powering on or off the amplifier. From what I've read, some amps can generate a healthy thump when either powering up or down. I'm thinking that inserting a switch after the 70V transformer output would eliminate this. Prior to powering up the amp, I would switch off the output to the electrodes, turn on the amp, wait a couple of seconds for it to stabilize and then switch on the output to the electrodes. Would do this in reverse when preparing to power down.

Does it make any difference whether I install the switch on the positive or negative lead. Also, any special consideration on the type of switch?

Thanks!

[edger477]

Have question regarding proper implementation of an output switch. My build will include the two 10 ohm resistors installed parallel to the transformer. I want to install a simple toggle on/off in line switch between the transformer output and the electrodes. Reasoning is that I want to prevent the possibility of momentary spikes that may occur when powering on or off the amplifier. From what I've read, some amps can generate a healthy thump when either powering up or down. I'm thinking that inserting a switch after the 70V transformer output would eliminate this. Prior to powering up the amp, I would switch off the output to the electrodes, turn on the amp, wait a couple of seconds for it to stabilize and then switch on the output to the electrodes. Would do this in reverse when preparing to power down.

Does it make any difference whether I install the switch on the positive or negative lead. Also, any special consideration on the type of switch?

Thanks!

I used these: https://www.amazon.de/-/en/gp/product/B07G5KB51M

I did not use them for the purpose you are using them (I tested my amp and it does not produce spike on either on or off, so I do not need switch to disable output), but for polarity switch - these 6-contact switches have 3 positions - connected to one side, to other side and "middle" (connected to none = off). I connected transformer to middle contacts, and electrode to one side with X to the other side, like this:


Same thing just supply = transformer and motor = electrodes.

If you decide to do this, then you get "off" as "extra" when setting switches to middle.

[andyupnaway]

I used these: https://www.amazon.de/-/en/gp/product/B07G5KB51M

I did not use them for the purpose you are using them (I tested my amp and it does not produce spike on either on or off, so I do not need switch to disable output), but for polarity switch - these 6-contact switches have 3 positions - connected to one side, to other side and "middle" (connected to none = off). I connected transformer to middle contacts, and electrode to one side with X to the other side, like this:


Same thing just supply = transformer and motor = electrodes.

If you decide to do this, then you get "off" as "extra" when setting switches to middle.

Perfect! A better solution than I requested. Thank you!

[andyupnaway]

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 .

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!

[edger477]

not necessarily unpleasant but a little bit sharp at times

I would advise to check same signal with speaker(s) or headphones to make sure that there are no distortions or spikes that are not in the played file itself. Not sure if amp has any output volume controls or only one is Bluetooth sound card volume? If it has both, then try using the amp volume and not the sound card one (because if you are reducing volume on sound card, the amp output remains high and is prone to shock you in case of sudden issue with bluetooth or distortion on input).

[andyupnaway]

First opportunity for an extended session this afternoon. Doubt that my TENS 7000 will be getting a lot of use going forward .

Very pleased with the results. Ran it about an hour and didn't observe any excessive heat. The 4 ohm resistor was a little warm to the touch but nothing to be concerned about.

Downloaded a lot of stim files so will be sampling those in the weeks ahead.

Rec'd a project box yesterday so will begin mounting the components. Still using the basic configuration as posted by the the OP. Will be installing switches for polarity swaps and likely the 10 ohm parallel resistors though still undecided on the latter. Will post a photo when completed.

Thanks to the OP and others, especially "edger477" for their input.

[MilovanaFan]

Any one have stl files for a 3d printable case for this project?

[Electro]

Any one have stl files for a 3d printable case for this project?

I hear some people saying their resistors don't get that hot, but I've seen mine at 160f(70c) from time to time and usually see things at least warm to hot feeling even when it isn't a very long session or with high intensity as the intention, and that's with the resistors mounted to an aluminum plate and measuring the aluminum plate temperature. If you printed this with PLA or PETG and didn't have something metal to mount the resistors to with a fan to keep the plastic cool, I'm not sure you'd want to mount these in a plastic case.