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

[stangeorge2020]

First, thanks again for all the shared knowledge in this thread! The parts finally arrived, and I am blown away by how intense this box is. Haven't been able to turn it up past 1/3 with what I've tried out so far.

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Master volume on the left, backwards-wired (oops) Left-Right attenuators on the right. L/R Polarity switches and Triphase switch on top.

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Wiring is certainly not expert-level, lots of room to clean things up here. I also ended up placing the polarity switches backwards, but that's an easy fix. Happened to have a spare 40x20mm Noctua fan from a server project gone wrong, and it works perfectly and near-silently to keep things cool.

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3.5mm outs for tens-compatible connectors and banana plugs for non-standard fittings, holes below for ventilation. Backside switch was going to be a power switch but I noticed in testing that the amp outputs pop when powered off so it became a L+/R+ switch for further customizability without re-wiring during play.

Spoiler: show

I ended up adding feet to the bottom (not pictured) as the attenuator knobs were a little too low for their own good. I opted for RCA inputs as I seem to have more 3.5mm to RCA than 3.5mm male-to-male cables.

After initial testing, I think I'm going to add the parallel resistor pair as I'm noticing a bit of "sharpness" in the signal that I'd like to do away with. I'm just overthinking wattage rating and resistance as there seem to be a few different options. Any input to help me decide would be appreciated!

[Kelvinator]

Balance: I built my DIY stereostim in 2018, based on TroniC's basic MidiStim design. The amp is nice and smooth, but only has a volume knob, no balance or left and right channel controls. I've noticed that my unipolar anal trode was often barely noticible in dual channel or triphase configurations. Last week, I finally bit the bullet and ordered an E-Stim Moaner bipolar electrode from Gary at HappyStim-USA. Initially, I was concerned that with its large surface area, I would somehow need to increase the signal strength when using this new butt trode. I was sort of chasing my tail doing research on the internet today, when I Googled something like "left and right balance on iPad", as I use an iPad as my estim audio player. Duh !! It turns out the audio balance slider control resides in the Accessibility section of the iPad settings. It was there all the time! Surprisingly, the Moaner works best at the default mid balance point most of the time. For some estim files I have to tweak the balance slider left or right to get the left and right signal strengths I like. The iPad balance slider works independently of the iPad volume slider control. Another example of the KISS rule in action (Keep It Simple Stupid).

[MrEigenGrau]

Edit: This appears to be the Bluetooth version of the amp I'm using. But i haven't actually used it.

https://www.amazon.com/Nobsound-Bluetoo ... B07GBPKD49

Edit2: I'm just going to mention that the original, SmartStim tested and recommended Doku/NobSound/Breeze Audio amps are hard to find on Amazon and Ebay. The original amps look like the one in my build post above.
My advice, in lieu of actual stress testing, is to NOT buy the cheapest TPA3116D2 amp you can find but get one with a decent heat sink and a good build quality from a reputable seller. Power them with a double insulated 12v 5A power supply.

So far, no one has posted any bad experiences with the inexpensive amp in the first post in this thread. If this is not the case, please correct me.

Just built this with the amp listed above with some minor modifications. It work quite well, but there are some things to keep in mind:
- When any bluetooth device connects, it assumes the primary source signal.
- The connection pairing code is 0000, so in theory anyone who attempts to connect to your device will start zapping you with whatever signal instead.
- The master volume control dial on the front of the amp does not appear to regulate output.

e.g. You may be in for the shock of your lifetime if your neighbor guesses the pair code and attempts to smoke your pole in a very unpleasant way.

Modifications:
- L&R volume balance using 10k pots.
- 3d Printed the whole case in PETG
- Active fan cooling and aluminum heatsinks for transformers and 4ohm resistors

Other notes:
- There is definitely some voltage trickle on the bass, treble and master volume adjustment knobs that you can feel if you touch 'em. Be sure to isolate the

[stangeorge2020]

So uhh.. My amp fried somehow.

This is the particular model I had ordered:
https://www.ebay.ca/itm/112448797247?ha ... SwlptaQNP-

After months of perfectly fine use, I did a 40ish minute session today, left the box plugged in for another 40 mins while I took a break, then when I went back for round two it felt as if there was a beatmeter going into it with nothing playing. I removed the input cable entirely, and the phantom pulses continued, unaffected by the volume knob on the amp.
Initially I wasn't sure if it was a failure of the amp or the 4ohm resistor (I've noticed them getting a little too warm after long or intense sessions, heatsink is on the way for them) but after taking the amp out of the rest of the circuit and testing it with a speaker, the right channel drops out intermittently, and the left channel is emitting repeating pulses as if it's turning off and on again and again, but 5x as loud as that would normally be. I swapped out power supplies for another known-good to rule out power supply issues, and the symptoms continued. Upon closer inspection, nothing looks off; no popped solder joints on the board, no bulging caps.

So, I've ordered another (as I can't find a different board design of same power output on the sites that I know of that will ship to Canada) and I'm just hoping the next one doesn't die... And having a sad month since it'll take that long for the new board to arrive. Any ideas what might've caused this? Or know of a better quality board that I could get shipped to Canada?

[JakofClubs]

My guess as to some potential causes for failure are (a) the amp go too hot with that small heatsink, (b), something might have shorted out or, (c) I read that some of these amps don't like to be ran with no load at all. The fix for (c) is have a resistor across the + and - outputs, sometimes called a smoothing resistor, so the amp sees a continuous load. Seems like people have been using resistors in the 10 to 20 ohm range.

From the forum, it looks like a lot of people have been using amps that look like this one without issue, so it's possible that you just got a bad one. Has anyone else had one of these amps fail? If so, please let us know.

[edger477]

Has anyone else had one of these amps fail?

I had one fail, that was 12V when I plugged in the laptop power supply instead of the 12V I had... :)

Since then most of the amps I bought are accepting 12-24V so there should be no new explosions :)

[arig4711]

So uhh.. My amp fried somehow.

This is the particular model I had ordered:
https://www.ebay.ca/itm/112448797247?ha ... SwlptaQNP-

I have bought four of these amps last year. I think they are not very good. There are too strong resistors on the audio input (not as recommended by TI) with too much noise / too much resistance and two of the amps fried (also somehow). The other two are used in a quad stim box with 5V (as an alternative to PAM8406), what seems to be pretty reliable.

I have good experiences with this amp:
https://www.aliexpress.com/item/1005002 ... 9145%21sea

[ABucketSausages]

Hey everybody, I'm new here, thanks for welcoming me, also thanks, dear mods, for the invite!

I'm a fairly fresh DIY stimmer, built a MidiStim a couple months ago, used it a couple times, it's amazing.

Unfortunately its amp (this one: https://amazon.de/dp/B07S243WL6) kicked the bucked recently, so I've already asked over at Reddit if a particular amp is suitable. Since a response is long in the coming, I just ordered it yesterday. It arrived today. First tests are promising. Haven't had the time yet to actually use it for stimming, but I could already make a short test on my lower arm muscles. I can feel the stim and am looking forward to my first stimming session.

The only remaining question that I have (for now) is, how safe is that thing, the amp in particular, used for our rather unusual purpose?

I'm using a double insulated 12V power supply with a CE sign, certified for the European (German) market. So I guess that thing's already pretty safe. Still wondering if something can go awry with the amp itself.

It's this one: https://amazon.de/dp/B082VD22BK

Cheers!

[senorgif2]

Wondering If someone could help me out here. I built one of these months ago was working fine until a few days ago, when I lost the ability to send signal from the right channel of the box. The other side seems to be working OK still. I'm not sure how to trouble shoot really, I checked for continuity on a few wires, but that doesn't work going through a transformer and 4 ohm resistor. I want to know if its the transformer, the amp or something else I haven't thought of. Any help is appreciated.

EDIT: Took a closer look at the Amp board, and it looks like one of the chip inducers got blown. I say that because there seems to be hole in the corner that i can just barely see through, and some kind of solidified liquid on top, like a drop of water but it doesn't wipe away. That doesn't seem good to me lol, and it was the same channel that stopped working.

[senorgif2]

Hey everybody, I'm new here, thanks for welcoming me, also thanks, dear mods, for the invite!

I'm a fairly fresh DIY stimmer, built a MidiStim a couple months ago, used it a couple times, it's amazing.

Unfortunately its amp (this one: https://amazon.de/dp/B07S243WL6) kicked the bucked recently, so I've already asked over at Reddit if a particular amp is suitable. Since a response is long in the coming, I just ordered it yesterday. It arrived today. First tests are promising. Haven't had the time yet to actually use it for stimming, but I could already make a short test on my lower arm muscles. I can feel the stim and am looking forward to my first stimming session.

The only remaining question that I have (for now) is, how safe is that thing, the amp in particular, used for our rather unusual purpose?

I'm using a double insulated 12V power supply with a CE sign, certified for the European (German) market. So I guess that thing's already pretty safe. Still wondering if something can go awry with the amp itself.

It's this one: https://amazon.de/dp/B082VD22BK

Cheers!

It looks just about the same as the unit used in the instructions that started this thread. Should be perfectly safe, as long as you follow e-stim rules of thumb. E.G. don't set up circuits that pass through the heart or brain. Now can you still hurt yourself with it? yes. If you turn it up too high too fast it will hurt. If you play painful signals, it will hurt. If you use it too long and too high you can get really irritated, especially with your more sensitive bits. None of that should cause any lasting damage, however. I can say that using my unit that is build to spec of the second option at the start of this thread, I have turned it all the way up, gradually, and nothing terrible has happened. I don't recommend using it full blast for too long, as you get desensitized to it, and it will dry out your skin.

Should be noted that the one I use is a 50x2 watt amp, this one you linked is 15x2 amp. You might find it under-powered if anything.

[senorgif2]

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

[Spielers]

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

[JakofClubs]

> Should be noted that the one I use is a 50x2 watt amp, this one you linked is 15x2 amp. You might find it under-powered if anything.

A 15x2 watt amp might use the same amplifier chip as an amp advertised as 50x2 watts, but the 15x2 rating is powered by 12 volts and the 50x2 at 19v. They might also be rated at different ohm loads.

[andyupnaway]

First post in these forums. Right off, want to thank the webmaster for running this site and also the OP for starting this thread. Found my way here from a reddit posting regarding stereo estim. Very useful information.

I have been stimming for 10+ years using a TENS, usually achieving HFO, but have always wanted to try stereo estim after reading about it on “SmartStim” when that website was active. Because of my success with the TENS, I didn’t feel the need to invest in a commercial box so seriously considered putting together a “Davey Box” but never got around to it. Even though I have some electrical experience and can solder, I know very little about electronics so even the Davey Box seemed a little daunting. I have read thru the thread and have studied the wiring / circuit diagrams. This project looks much simpler and also low cost.

I was ready to purchase one of the mini-amps (more on that) but had to ask the question is it really necessary? Several years ago, upgraded all of my home audio systems and have an excellent used Sony receiver on a closet shelf gathering dust. Only reason for replacing was the lack of digital / HDMI connectivity and multi-channel audio processing. The Sony is stereo only, mid 90’s vintages with working remote. Although not audiophile quality, it is upper level enthusiast grade with a true 100W X 2 RMS output (class AB amplifier). And, with the remote, no need to ever touch. I know it is over kill for this project but with careful volume control, so what. Is there any reason why I shouldn’t use this? Due to the amplifier power, should I use different resistors/components in the estim box?

If there are good reasons to stick with one of the mini amps, here are two that I am considering. Open to other suggestions if neither of these is suitable.

This one is based on TPA3116 based board for around $20. Like the Bluetooth capability. Not crazy about the output connection…would prefer banana plug sockets but, for $20, not bad. Plan on using a 19V 4.5A universal laptop power supply.
https://www.amazon.com/dp/B099W7YPJ8/

The other goes for around $40 which is high but comes with a remote control and a power supply. It is based upon AC6922A amplifier chip. Are these suitable? Seems like TPA3116 boards are preferred but may be due to low cost?
https://www.amazon.com/gp/product/B0868LMPNP/

The case and knobs of the mini amps appear to be metal. Should I be concerned about touching the mini amp? Will the external power supply isolate me from the “mains” power (power supplies indicate they are double insulated)? Suppose I could wrap the knobs in electrician tape but want to keep it unmodded of possible.

For the estim box, planning on mounting the resistors/transformers on a PCB. Would welcome recommendations on suitable box that will simplify mounting the PCB as well as mounting terminals and output jacks.

Thank you for your guidance. Looking forward to rigging this up.

[edger477]

Is there any reason why I shouldn’t use this?

The reason would be the fact that these receivers have 220V cable going in directly and an internal transformer to whatever DC it is using (probably 16-30v). If something fails, i.e. a capacitor in power supply (that is only thing I imagine is probable to happen due to age), there could potentially be current spike that goes to the speaker outputs before whole unit dies. Unit might be "safe" under those circumstances because speakers might or might not survive - if it is in warranty manufacturer could easily afford to cover speakers cost - they did not need to prevent a spike that could injure/kill someone who is using the unit for estim because it is not designed for it :)

When I was a teenager, I used similar unit too and survived, but now that I have family who depend on me I don't consider 50 euros for amplifier + double-insulated power supply too much of a price for insurance against sudden death.

Also, long time ago I used amp that had identical housing to your second one - it had very unpleasant spikes in output, and I had to throw it away. But it was long time ago and it might have been based on different chip. One I am using now is based on TPA3116 and it is fine.