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

[steelhorse545]

Solved it! Thank you guys for the help.
Electro was right about the volume. I boosted way up (about 70%) the level in both the laptop and the amp, and it worked. The mp3 player simply won't cut it.

I just did a quick test, but it seems that my (cheap) USB audio interface is super noisy and distorts the sound in a very unpleasant way. When plugged directly to the laptops output the signal felt cleaner, more distinct... and less painful.
Is there anything I can do about that? (other than buying a better usb interface)

What exactly are you using ? - I thought you’d blown the laptop sound output - or were you mistaken ?

Sounds like there’s a bit of a levels mismatch somewhere - I’d have thought the amp ought to accept line level audio - OTOH, a headphones-out ought to be higher output anyway. Is the usb interface headphone or line-out ?

One option would be to use an intermediate (pre)amp, but a better usb audio device might be cheaper -IDK for sure, but the Creative external devices ought to be decent - mebbe worth looked for some s/h older ones perhaps (I’ve an mod sitting on a synology NAS, but check for driver availability for your laptop OS)

Another possibility to reduce the level elsewhere (and hence distortion) - I *think* I’m correct in saying that using the 4 ohm rather than 8 ohm output (assuming you’re also using the 0.625) should give a higher effective transformer turns ratio, amp side to electrode side, and so a larger step up in voltage. Obvs if you try that turn all the levels down first !

[ramen]

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.

[steelhorse545]

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.

Cool ! - that’s really interesting that the parallel resistors helped - I’ve got some (15ish ohm) for my build, but just waiting on the amp. My educated ( or not !) guess is that having the resistance across the transformer defines better what the amp drives into, regardless of how the contact resistance on the other side (ie you) varies according to how you..err.. react ;) and in turn changes the effective transformer load to the amp - if that makes sense ? But the lot I ordered were in a 10-pack, if it needs to be higher/lower than 15 can probably get there with what I’ve got.. the electronics bits box is mostly low power resistors.

Fwiw, I’ve got a bunch of switches to add into mine : swap left/right (at I/p to amp, flip phase (left, right separately), triphase (between either leg of output channel a to either leg of output channel b). Will prob confuse the heck out of me in practise, and may be mostly unnecessary , but at least it’s pretty flexible ! Oh, and separate left and right levels at the amp input too, plus an ‘oh shit, isolate the electrodes switch on the output” :D If it all works ok I’ll post what I did :)

Wasn’t intending to fit the electrode side resistors, but might need some experimentation…

[isidore]

@ramen
@steelhorse545
I'm really interested in your parallel resistors set up. Seems like a very worthy experiment for this schematic. Any chance you want to post a pic of your finished box? I'm always curious to see how others have laid everything out.
And the same request for you, steelhorse545: if you're feeling it, please share a pic of the build with all the switches in place?
Cheers! Appreciated!

[edger477]

@ramen
@steelhorse545
I'm really interested in your parallel resistors set up. Seems like a very worthy experiment for this schematic. Any chance you want to post a pic of your finished box? I'm always curious to see how others have laid everything out.
And the same request for you, steelhorse545: if you're feeling it, please share a pic of the build with all the switches in place?
Cheers! Appreciated!

Hi,

I also have box with resistors parallel to transformer, but is in 3d-printed box and during one of sessions I drove volume too high so they melted the box a bit and are now stick inside... I will try to disassemble it and post schematic, this is a box I used quite a lot and I want to rebuild it, just with better resistors (alu-casing and screwed to metal plate).

If I remember correctly, resistors are made to be in series connected to output, but one with much more resistance than the other (order of magnitude), so that almost all voltage difference is on that one, but it still has more than what electrodes connected to body have, therefore making voltage much less dependent of electrode surface area/lubrication. I am not entirely sure whether they are before or after the transformer, I will update the details (also which resistors are used) once I disassemble it.

EDIT: So I just melted the part that was stick to resistor... it was 22 Ohm resistor which is in parallel with transformer... There are 4.7 Ohm and 22 Ohm in series, and transformer is parallel to 22, so that one dissipates most of the energy especially if there is nothing connected to transformer.

Also found my order for resistors:
22 Ohm
4.7 Ohm

[steelhorse545]

@ramen
@steelhorse545
I'm really interested in your parallel resistors set up. Seems like a very worthy experiment for this schematic. Any chance you want to post a pic of your finished box? I'm always curious to see how others have laid everything out.
And the same request for you, steelhorse545: if you're feeling it, please share a pic of the build with all the switches in place?
Cheers! Appreciated!

Honestly, mine’s a mess :) Well, not so much mess but just a laid out bunch of wiring without a box, and it’s probably a little fragile to keep using it without one. I haven’t really found the extra switches very usefu so far (and maybe they’re not ), in reality it’s been easier to leave the triphase switch(ex) engaged and swap wire <> electrode connections and see how it feels - it’s also not always clear what the order of the triphase connections should be ! I need to rewire with some lighter gauge wire cos the stuff I’ve used isn’t so flexible hence tricky to get into an enclosure. Have the added complication that I want to separate out the amp, and/or add some additional outputs that allow me to drive some speakers as an unrelated audio amp. Also need to sort some power connectors, I’ve got an old canon printer psu doing power duties and it’s just wired into screw terminals, cos I couldn’t work out what the right receptacle was. Sadly one UK-wide high street components store here (a little like Radio Shack -which we also used to have a decade or two ago !) went out of business, and I’ve found it tricky to find high current connector papers looking at spec sheets online.

Oh, interesting point : nothing gets warm, I suspect the psu requirement’s actually pretty low and nowhere near the 2a I have or has been suggested elsewhere, which may make psu connector choice a little easier :)

The other thing is, ive only used square TENS pads so far - haven’t tried CR loops, not sure how much difference that makes. Have some relatively short electrode leads, so need to order some 3.5mm headphone extension cables to get the business end away from the electronics!

Need to revisit the attempt at independent right/left channel level control - when testing as an audio amp, the pots seemed noisy/making poor contact - tried switch cleaner, but maybe they’re just poor quality…but on an ohmmeter they seemed ok. Will have to see if I have some different spares in my old electronics stash.

But at least it essentially works :)

Will try to post a pic at some point, tho not sure it’ll be much different to any others.

And at this point in the evening… perhaps I ought to disappear to do some wa… err.. testing :D Actually, scrub that - I’ve already had a few sessions that have been late at night when I’ve been tired and tbh, not quite into it - still entertaining, but not quite enough - even if HFOs aren’t a guarantee.

(Have noticed it does some -temporary- weird shit to my pelvic floor and lower abs, the former makes it even harder to pee in the middle of the night (even more than at my age with an enlarged prostate), and after a few hours of tinkering a few days ago the following day my lower abs seemed more kinda out of it than a serious core gym session of reverse curls etc.)

[isidore]

@steelhorse545
Thanks for the interim report
I feel you on the problems of adding complexities to this kind of thing--lots of testing required for sure, not always such a bad thing
fwiw I've always preferred (and by a lot) the feel of tens pads to cr loops, but everybody's different. The secret sauce for me has been to use some of that self-adherent hurt-free wrap to keep all the corners of the pads from peeling up, reduces unintended sharp zaps, or at least that's been my experience.
Thanks again for the informative post about your progress with those modifications

[stangeorge2020]

Hi all,

Longtime lurker, first time poster! I got into estim the stupid way years ago (using hifi equip ) before I learned more about the hobby, and now I'd like to join the ranks of those doing things the safe(er) way. I've been going back and forth between this thread and lolol2's thread and I've got two questions.

1. If I wanted to add left/right level pots to this design, like in lolol2's design, would adding them on the 3.5mm/input wires be fine, or should I consider adding anything else to the circuit on top of that? (I am a big fan of the idea of being able to dial back/adjust balance with analog controls rather than software, and use the amp's volume knob to control overall intensity.)

2. Is there a functional difference between putting the load resistors before/after the transformer? I understand the values are different between the two schematics to compensate for their placement, but are they serving the same purpose or is there any advantages/disadvantages to placing the resistor before or after the transformer?

Thank you all for being such a wealth of knowledge. I'm looking forward to posting pics of my completed box in the coming months!

[edger477]

1. If I wanted to add left/right level pots to this design, like in lolol2's design, would adding them on the 3.5mm/input wires be fine, or should I consider adding anything else to the circuit on top of that? (I am a big fan of the idea of being able to dial back/adjust balance with analog controls rather than software, and use the amp's volume knob to control overall intensity.)

For this goal, if you can find product like this should be perfect: https://www.amazon.de/-/en/Sennheiser-H ... B000H0K8VY

This is not available anymore, and I can't find similar product available, but you can just add them inside your box.

2. Is there a functional difference between putting the load resistors before/after the transformer? I understand the values are different between the two schematics to compensate for their placement, but are they serving the same purpose or is there any advantages/disadvantages to placing the resistor before or after the transformer?

The difference here would only be affecting edge cases (sudden impulses surrounded by no signal etc) due to difference in signal before and after transformer caused by its internal inductance... Most of schematics I saw place them before because you don't need to keep transformers loaded all the time.

On my latest box I left them out because I am afraid of potential that parallel resistor burns out... it could be very painful/dangerous spike when all that current hits the transformer.

[steelhorse545]

1. If I wanted to add left/right level pots to this design, like in lolol2's design, would adding them on the 3.5mm/input wires be fine, or should I consider adding anything else to the circuit on top of that? (I am a big fan of the idea of being able to dial back/adjust balance with analog controls rather than software, and use the amp's volume knob to control overall intensity.)

For this goal, if you can find product like this should be perfect: https://www.amazon.de/-/en/Sennheiser-H ... B000H0K8VY

This is not available anymore, and I can't find similar product available, but you can just add them inside your box.

2. Is there a functional difference between putting the load resistors before/after the transformer? I understand the values are different between the two schematics to compensate for their placement, but are they serving the same purpose or is there any advantages/disadvantages to placing the resistor before or after the transformer?

The difference here would only be affecting edge cases (sudden impulses surrounded by no signal etc) due to difference in signal before and after transformer caused by its internal inductance... Most of schematics I saw place them before because you don't need to keep transformers loaded all the time.

On my latest box I left them out because I am afraid of potential that parallel resistor burns out... it could be very painful/dangerous spike when all that current hits the transformer.

The sennheiser volume control seems to be available on eBay, they’re about £20/$20. If you’re driving them from a headphone output they should work ok, if driving them from line-out -which expects a far higher impedance (ie not headphones), it may not work so well (ie attenuating the signal significantly).

As regards resistor positioning, I’m not sure it’s an inductance issue as such. With resistors on the output side, if the body/contact resistance changes, that will affect the current and also what the amplifier thinks it’s driving into + which probably won’t be the ideal of 8ohm or whatever the amplifier expects. Picking an appropriate resistor value relies one some else getting it right with some knowledge/measurement of how these setups work - but in particular (AFAIA) means the amp load is somewhat undefined.

Using resistors on the amplifier side : a resistor in series with the transformer means that the amp will ‘see’ a minimum load of the series resistor, even if you shor out the pad pins - that resistor is typically about 4ohms, which many amps will handle.
Adding a resistor in parallel to the transformer means that if there’s a complete pad disconnect, the load the amp sees will be the series resistor in series with the transformer parallel resistor, similarly if the body/pad resistance connectivity changes, the amp load isn’t solely dependant on that.
The net result is that the serie/parallel network should give a more defined load to the audio amp. - perhaps worth bearing in mind that the cheap/cheerful tpa3116-based amps might need a range of impedance load, since they’re class D amps. Not sure if needed in practice with the particular design.

Regarding parallel resistor burning out - simply increase the power rating if you’re worried about it. For reference, I’ve got a 4r7 series resistor (15W) and 15r parallel resistor (5W) - nothing gets even warm. It might be a little different with another transformer (but prob not), unless you’re nuking the fuck out if your junk :D

[edger477]

The sennheiser volume control seems to be available on eBay, they’re about £20/$20. If you’re driving them from a headphone output they should work ok, if driving them from line-out -which expects a far higher impedance (ie not headphones), it may not work so well (ie attenuating the signal significantly).

I now got this headphone preamp
It has 4 outputs, I am going to use it to separate channels (so 1st for left, 2nd for right), plus I can now use both of my boxes with same signal, but I need to figure out what kind of electrode setup could this drive :)

[stangeorge2020]

Thanks for the input! I'm going to go with in-box pots for input volume controls, to keep things cleaner on the outside wires.

Regarding the parallel resistor, what's the recommended wattage? Could I use a ceramic 10W or would you need a higher wattage? I think I'm going to omit it for the initial build, but considering ramen's report of 'static' I figured I would think about what I'd need in case I find myself in the same boat.

[baektje]

I am US based and putting together a build - I think I've pretty much found everything in stock, but I have a few questions.

First, just compiling here my list of links in case it is helpful for anyone:
Amp: https://www.parts-express.com/TPA3116D2 ... ol-320-699
Transformers: https://www.amazon.com/gp/product/B0002 ... VVSP&psc=1
Resistors: https://www.parts-express.com/Dayton-Au ... stor-006-4
Power Supply: https://www.parts-express.com/12-VDC-5A ... ug-120-056
Power Jack to Screw Terminals: https://www.amazon.com/gp/product/B07C6 ... 91Z5&psc=1

Also planning on including the 10Ohm resistors across the transformer inputs to smooth out the signal.

Now to my questions:
1. What are the bullet connectors for? Is the intent to wire the transformer outputs to female bullet connectors? And then plug in purchased electrodes to those female bullet connectors?
2. lolol2's design includes pots on the input - I'm assuming if I don't go that route I just control the left right balance/overall volume at the source (my computer).
3. lolol2's design includes a triphase switch, which supposedly simplifies the wiring? I don't see a huge difference on the output side of things. What does the triphase do? And if I wanted to include, how would I wire it up to this design?

Thank you in advance!

[edger477]

Now to my questions:
1. What are the bullet connectors for? Is the intent to wire the transformer outputs to female bullet connectors? And then plug in purchased electrodes to those female bullet connectors?
2. lolol2's design includes pots on the input - I'm assuming if I don't go that route I just control the left right balance/overall volume at the source (my computer).
3. lolol2's design includes a triphase switch, which supposedly simplifies the wiring? I don't see a huge difference on the output side of things. What does the triphase do? And if I wanted to include, how would I wire it up to this design?

Thank you in advance!

I had bullet connectors on my first electrodes and used it for first DIY box, but recently I built new one with 3,5mm mono output for each channel so I can use commercial tens cables and ones from 2B. I think this is something that you should decide for yourself, what is best for you.
For any custom electrodes I now plug them into WAGO connection clamp along with the 2mm pin from the cable.

Regarding potentiometers on input, these need to be before amplifier, so for me was very inconvenient because I built only midstim box and used standard connections to amplifier (to keep it more modular), and as amplifier does not have separate left/right volume control then you have to use balance on PC. However, I have 2 boxes and I want to try stimming on 4 channels, so I got this headphone preamp that I will use to control them separately. It is stereo preamp for 4 headphone pairs, but I will use only left output from channels 1 and 3, right output from channels 2/4, so when one box is connected to outputs 1/2 I can separately control left/right.
This has additional benefit of keeping it modular, and I use commercial product as a step in the pipeline for this, without complicating the design.

Triphase switches are something I will be adding to my box. And switches for polarity change. You can do all of that on cables, but I want the comfort of testing how do different configurations feel in an instant, while also being able to skip WAGO connectors whenever I use pads or electrodes with commercial connectors (because I normally connect triphase through WAGOs and use them also to reverse polarity/change common).

I want to use these switches for both polarity reversal and thriphase, as I can make one be triphase switch with off in middle, connecting black/common outputs of transformers on one side, and connecting reds on other side, and then 2 more switches (one for each channel) to serve as normal/off/reverse.


These are just quality of life upgrades that allow to quickly test many different configurations, while reducing amount of cables at same time.

[JakofClubs]

@ramen
That extra pair of resistors is a great upgrade! If feels much better. I used 15 ohm resistors because that's what I had on hand.