[Tutorial] Building your own DIY E-Stim Stereo Device

[steelhorse545]

maybe the transformer is wrong connected and so the volt are to low

Thanks, but no the transformer is connected correctly. It has been working properly for over a year, and it is connected exactly how the other channel is, which works as intended.

In turn, try swapping various connections from one channel to the other. it’s a nuisance but without having test equipment that’s all you can do (other than visual checks). May just be a lousy joint (are you using screw connectors?), but the individual components aren’t particularly fragile or fault-prone.

[BoundSquirrel]

In turn, try swapping various connections from one channel to the other. it’s a nuisance but without having test equipment that’s all you can do (other than visual checks). May just be a lousy joint (are you using screw connectors?), but the individual components aren’t particularly fragile or fault-prone.

Thanks. That's what I was hoping not to do. Everything is soldered except for the amps, which are screw down connectors. I checked all of the connections to ensure that they're sound. I have a multi-meter and checked the resistors; they're reading 5ohm each so they appear good. I've got one more brand new amp- I'll swap that out first to make sure the replacement isn't dead out of the boys. If that's not it, I've got spare resistors to swap in. The only part I don't have extras of are the audio transformers. I would think those are the least likely to fail.

Maybe it's just time to build a whole new box. This one looks like a big giant mess anyway since I've built and rebuilt it about 4 times as I've learned more along the way. it's worked well for 2+years, so maybe I'll just order new everything and try not to zap my junk off with a new build.

[steelhorse545]

In turn, try swapping various connections from one channel to the other. it’s a nuisance but without having test equipment that’s all you can do (other than visual checks). May just be a lousy joint (are you using screw connectors?), but the individual components aren’t particularly fragile or fault-prone.

Thanks. That's what I was hoping not to do. Everything is soldered except for the amps, which are screw down connectors. I checked all of the connections to ensure that they're sound. I have a multi-meter and checked the resistors; they're reading 5ohm each so they appear good. I've got one more brand new amp- I'll swap that out first to make sure the replacement isn't dead out of the boys. If that's not it, I've got spare resistors to swap in. The only part I don't have extras of are the audio transformers. I would think those are the least likely to fail.

Maybe it's just time to build a whole new box. This one looks like a big giant mess anyway since I've built and rebuilt it about 4 times as I've learned more along the way. it's worked well for 2+years, so maybe I'll just order new everything and try not to zap my junk off with a new build.

Cool, checking continuity will help. Actually, just got mine built - made the schoolboy error of using the wrong contact set on the output sockets, and only spotted that by with the ohmmeter. Tho I added a bunch of switches to reverse phase etc, so it’s a bit of a birds nest :/ made worse cos I used some fairly heavyweight speaker cable which is a bit stiff (add your own puns) - I’ll have to rewire it to get it into a decent sized box. But it works ok from initial trials I also added some input pots to control channel volume, and another switch to flip left<>right, but it looks like the (new!) pots may be noisy .. it seems unlikely tbh, tho I’ve got 3 spares to try, so currently just using equal level audio into the amp..now to get my sound sources and vids in the right places, so I can try triphase properly :)

[BoundSquirrel]

Cool, checking continuity will help. Actually, just got mine built - made the schoolboy error of using the wrong contact set on the output sockets, and only spotted that by with the ohmmeter. Tho I added a bunch of switches to reverse phase etc, so it’s a bit of a birds nest :/ made worse cos I used some fairly heavyweight speaker cable which is a bit stiff (add your own puns) - I’ll have to rewire it to get it into a decent sized box. But it works ok from initial trials I also added some input pots to control channel volume, and another switch to flip left<>right, but it looks like the (new!) pots may be noisy .. it seems unlikely tbh, tho I’ve got 3 spares to try, so currently just using equal level audio into the amp..now to get my sound sources and vids in the right places, so I can try triphase properly :)

So, I ordered everything to try Mantrid's build... Except the capacitors apparently. What would happen with that build if it was just the audio transformers with the resistors? What function do the capacitors serve (I ask mostly because they're not in any other build).

So I tried to repair my existing build - replaced the amps, the resistors, and rewired everything. And it doesn't work. The only thing I didn't swap was the transformers, so I believe I roasted them somehow... I got one really sharp spike in one test and now nothing no matter what I do.

So, down until Tuesday while I wait for transformers and more heat shrink butt connectors to arrive.

EDIT: I picked up one of these to show volume and I'm not sure how to connect it.

LED volume indicator

I have one of these for each right and left to get individual volume control.

Amps

The led indicator has power +/-,audio r/l +, and a common audio -. How do I reconcile that I have 2 audio +/- going to a single - on the indicator? If I split the wire to go to both amps, won't that create a short between the amps? Also, should this go immediately after the amps, after the resistor, or on the electrode side of the box? I have no electrical wiring experience except for putting in a few outlets around the house and what I've learned from here so any input would be helpful.

[steelhorse545]

Cool, checking continuity will help. Actually, just got mine built - made the schoolboy error of using the wrong contact set on the output sockets, and only spotted that by with the ohmmeter. Tho I added a bunch of switches to reverse phase etc, so it’s a bit of a birds nest :/ made worse cos I used some fairly heavyweight speaker cable which is a bit stiff (add your own puns) - I’ll have to rewire it to get it into a decent sized box. But it works ok from initial trials I also added some input pots to control channel volume, and another switch to flip left<>right, but it looks like the (new!) pots may be noisy .. it seems unlikely tbh, tho I’ve got 3 spares to try, so currently just using equal level audio into the amp..now to get my sound sources and vids in the right places, so I can try triphase properly :)

So, I ordered everything to try Mantrid's build... Except the capacitors apparently. What would happen with that build if it was just the audio transformers with the resistors? What function do the capacitors serve (I ask mostly because they're not in any other build).

So I tried to repair my existing build - replaced the amps, the resistors, and rewired everything. And it doesn't work. The only thing I didn't swap was the transformers, so I believe I roasted them somehow... I got one really sharp spike in one test and now nothing no matter what I do.

So, down until Tuesday while I wait for transformers and more heat shrink butt connectors to arrive.

EDIT: I picked up one of these to show volume and I'm not sure how to connect it.

LED volume indicator

I have one of these for each right and left to get individual volume control.

Amps

The led indicator has power +/-,audio r/l +, and a common audio -. How do I reconcile that I have 2 audio +/- going to a single - on the indicator? If I split the wire to go to both amps, won't that create a short between the amps? Also, should this go immediately after the amps, after the resistor, or on the electrode side of the box? I have no electrical wiring experience except for putting in a few outlets around the house and what I've learned from here so any input would be helpful.

The capacitor addition appears to set a low frequency cutoff (with the assumption that lower frequencies are undesirable) . In practise, the estim authors are a small number of folks -posting here or whose work is in an archive referenced in (Reddit) r/estim , so maybe it’s not a big concern, it’s a safeguard against bad actors. Omitting ie shorting)the capacitor means there’s no frequency twiddling, ie it’s a flat response.

The other feature of his addiction is some diodes to limit the voltage, not a bad idea but I’m not sure the possible failure modes of psu/amplifier warrant it. I had wondered about adding some to the electrode/output side, but without measuring voltages there it’s impossible to specify anything.

Does that make sense ? - feel free to ask :)

[mantrid]

The capacitor addition appears to set a low frequency cutoff (with the assumption that lower frequencies are undesirable) . In practise, the estim authors are a small number of folks -posting here or whose work is in an archive referenced in (Reddit) r/estim , so maybe it’s not a big concern, it’s a safeguard against bad actors. Omitting ie shorting)the capacitor means there’s no frequency twiddling, ie it’s a flat response.

The other feature of his addiction is some diodes to limit the voltage, not a bad idea but I’m not sure the possible failure modes of psu/amplifier warrant it. I had wondered about adding some to the electrode/output side, but without measuring voltages there it’s impossible to specify anything.

Does that make sense ? - feel free to ask :)

Sorry for the late reply. The Email notification of Milovana seems to be broken.

Together with the resistor the TVS diode limits the power. Since the the output power of the transformer cant be larger than the input power, it does not matter where the TVS diode is located except that it is easier to tailor it if it is before the transformer. For even better protection you can additional add a (poly)fuse in series to the resistor.

The capacitor also protects the transformer against DC and very low frequency currents which are transformed into heat.

The problem with the low frequencies is whether estim authors are aware of the issue and probably not whether they wanna harm you. For estim devices that re-modulate the audio signals these frequencies are neither dangerous nor unpleasant. There exist estim files that try to cause pain using low frequencies: Its reported that this feels stinging. That's a strong indication that this feeling is not just caused by directly stimulating nerves ...

[steelhorse545]

The capacitor addition appears to set a low frequency cutoff (with the assumption that lower frequencies are undesirable) . In practise, the estim authors are a small number of folks -posting here or whose work is in an archive referenced in (Reddit) r/estim , so maybe it’s not a big concern, it’s a safeguard against bad actors. Omitting ie shorting)the capacitor means there’s no frequency twiddling, ie it’s a flat response.

The other feature of his addiction is some diodes to limit the voltage, not a bad idea but I’m not sure the possible failure modes of psu/amplifier warrant it. I had wondered about adding some to the electrode/output side, but without measuring voltages there it’s impossible to specify anything.

Does that make sense ? - feel free to ask :)

Sorry for the late reply. The Email notification of Milovana seems to be broken.

Together with the resistor the TVS diode limits the power. Since the the output power of the transformer cant be larger than the input power, it does not matter where the TVS diode is located except that it is easier to tailor it if it is before the transformer. For even better protection you can additional add a (poly)fuse in series to the resistor.

The capacitor also protects the transformer against DC and very low frequency currents which are transformed into heat.

The problem with the low frequencies is whether estim authors are aware of the issue and probably not whether they wanna harm you. For estim devices that re-modulate the audio signals these frequencies are neither dangerous nor unpleasant. There exist estim files that try to cause pain using low frequencies: Its reported that this feels stinging. That's a strong indication that this feeling is not just caused by directly stimulating nerves ...

Strictly speaking the TVS diode limits voltage, which results in power/current limit, but as an engineer I’m allowed to be pedantic ;) - and I’m curious about a few things you’ve mentioned..

What’s your primary design intent ? Limit in the case of amplifier(etc) failure, poorly constructed audio file, EMI effects, or something else ? (Obvs diode/clamping voltage choice will depend on which side of the transformer it’s on.)

Audio amplifiers don’t normally/ever have a non-zero quiescent/dc output or are capacitively coupled : otherwise it wastes power, plus since they’re driving loudspeaker coils then having a constant dc drive, if not undesirable for the speaker then it also limits the possible cone excursion.
Back in the dawn of time.. well, valve amplifiers :) class A amplifiers typically have an audio power transformer sitting between anode and the supply rail - there’s always a dc current. Actually, same for transistor amps, I remember drawing load lines for both. I gather there’s an effect called hysteresis distortion - maybe worth considering for audiophiles, but in this application I wonder if it’s worth worrying about.
I guess if you were driving a (linear) amp with something that was pwm- like then there might be a dc offset, whether that’s significant is another matter…but there’s probably going to be capacitive coupling into/within the amp anyway, which I suspect would limit/omit any effect.
Or have I missed something ?

[mantrid]

Strictly speaking the TVS diode limits voltage, which results in power/current limit, but as an engineer I’m allowed to be pedantic ;) - and I’m curious about a few things you’ve mentioned..

To be pedantic too, I wrote:

Together with the resistor the TVS diode limits the power.

Furthermore I mentioned:

For even better protection you can additional add a (poly)fuse in series to the resistor.

Audio amplifiers don’t normally/ever have a non-zero quiescent/dc output or are capacitively coupled : otherwise it wastes power, plus since they’re driving loudspeaker coils then having a constant dc drive, if not undesirable for the speaker then it also limits the possible cone excursion.

All common amplifier classes (B, AB and D: all of them consist in something like a H-Bridge with two push-pull drivers) can deliver DC output, (Class A always as a DC offset but isn't used in prcatice.) Of course, that is never desirable for an audio amplifier and in practice that should be avoided by AC coupling the input. I would only expect DC currents that can kill the transformer in case of a malfunction. If you connect a loudspeaker you probably can live which that risk. But if your connect you crown jewels it may be worth it to spent one or two EUR/USD in extra safety.

But, to be pedantic again, I wrote:

The capacitor also protects the transformer against DC and very low frequency currents which are transformed into heat.

So both, DC and low frequencies (outputting them is the aim of the amplifier) are transformed into heat which may burn the transformer and cause shortcuts. That's especially an issue for the small HF-transformers from Digikey I recommended in combination with capacitors.

Because someone wanted to use that transformer without capacitor, I felt compelled to write another warning (knowing that people don't want to hear it...)

[BoundSquirrel]

The capacitor also protects the transformer against DC and very low frequency currents which are transformed into heat.

So both, DC and low frequencies (outputting them is the aim of the amplifier) are transformed into heat which may burn the transformer and cause shortcuts. That's especially an issue for the small HF-transformers from Digikey I recommended in combination with capacitors.

Because someone wanted to use that transformer without capacitor, I felt compelled to write another warning (knowing that people don't want to hear it...)

This makes sense to me. This is the important part. Thank you.

90% of the technical talk goes over my head. I have no idea what an H-bridge is or how the conversion from AC to DC works so I'm pretty well lost there. That means that I skip most of it. Warnings in simple terms are much more easily followed - 'low frequencies and DC power can melt the small transformer which can zap your junk' is easy for us non-electrical engineers to get.

In my case, I had a tiny little box working great with the transformers you recommended. I was really proud of it due to it's small size and easy of portability. And then 1 channel fried on the 3rd use. No idea what happened but I get nothing.

So, I went back to the large components in a giant box that's not going anywhere. It works just fine and apparently reduces the risk of burning up my junk.

[edger477]

Hey @mantrid I see you did a lot of engineering in this area, so if you would care to comment on this? If I am adding capacitor to existing midstim, aside from small signal distortion, is there any issue if using standard electrolytic ones (I have those on hand)? I am planning to try out 330uF for about 8-12 ohms in transformer + resistor?

Also, what about output? With 3-phase in theory you could have current going out from one transformer into other (because they can be phase-shifted on same frequency), would it make sense to add capacitors there (on all 4 terminals), and which ones? I think there electrolytic ones might not be suitable as voltage is higher.

[steelhorse545]

Strictly speaking the TVS diode limits voltage, which results in power/current limit, but as an engineer I’m allowed to be pedantic ;) - and I’m curious about a few things you’ve mentioned..

To be pedantic too, I wrote:

Together with the resistor the TVS diode limits the power.

Furthermore I mentioned:

For even better protection you can additional add a (poly)fuse in series to the resistor.

Audio amplifiers don’t normally/ever have a non-zero quiescent/dc output or are capacitively coupled : otherwise it wastes power, plus since they’re driving loudspeaker coils then having a constant dc drive, if not undesirable for the speaker then it also limits the possible cone excursion.

All common amplifier classes (B, AB and D: all of them consist in something like a H-Bridge with two push-pull drivers) can deliver DC output, (Class A always as a DC offset but isn't used in prcatice.) Of course, that is never desirable for an audio amplifier and in practice that should be avoided by AC coupling the input. I would only expect DC currents that can kill the transformer in case of a malfunction. If you connect a loudspeaker you probably can live which that risk. But if your connect you crown jewels it may be worth it to spent one or two EUR/USD in extra safety.

But, to be pedantic again, I wrote:

The capacitor also protects the transformer against DC and very low frequency currents which are transformed into heat.

So both, DC and low frequencies (outputting them is the aim of the amplifier) are transformed into heat which may burn the transformer and cause shortcuts. That's especially an issue for the small HF-transformers from Digikey I recommended in combination with capacitors.

Because someone wanted to use that transformer without capacitor, I felt compelled to write another warning (knowing that people don't want to hear it...)

Semantics, eh ? ;)
I guess it depends on one’s frame of reference: voltage if it’s primarily to prevent transients reaching the user, power if it’s primarily to prevent high current in/fusing of the transformer in a fault condition.

The short answer I was mainly looking for : in the event of an amplifier/psu fault driving dc into the transformer, fusing it and any unwanted effects downstream.
I suspect that for many of the transformers I’ve seen listed - ie 10W - low freq heating in the transformer isn’t likely to be much of a concern, but I guess it subtracts from any power handling headroom with the small ones you’re using (aside from any undesirable effects on the user).
Edit : Not sure how one would work out the power rating for any TVS diode since it’s an unexpected fault condition and the current could be sustained, but I guess that’s where adding a fusible component would fit in.
Thanks for the reply.

[InfamousPlantain]

Hi All, been following the latest few posts and as I lurk on this and other threads, it occurred to me that people are setting up modifications in the same spot. I'm hoping someone can help explain the purpose of the parallel resistor method found here: viewtopic.php?p=283515#p283515 ? How does it compare to the low pass filter method here? Does the parallel resistor method offer any protection at all?

It sounds like the low pass filter is more of a safety thing but the parallel resistor is just an improvement on the feeling thing? Is it possible to use both methods? Thanks!

[edger477]

It sounds like the low pass filter is more of a safety thing but the parallel resistor is just an improvement on the feeling thing? Is it possible to use both methods? Thanks!

Hi, those two are "orthogonal" concepts - meaning they are not interfering with each other, but additional resistor in parallel with transformer increases "load" through capacitor so cutoff value might be moved to higher frequency or you might need larger capacitor, but I forgot hot to calculate those... college was long ago, and even if I google the formula, I am not sure below what frequency I want to filter out so research/testing is necessary

Anyway, parallel resistor can be used with capacitor... it is just additional load on amplifier that ensures amplifier works, as some transformers have too low resistance when there is no signal, there might be no output from amplifier on low volume (amplifier does not sense that speaker is connected), then suddenly you get zapped when you increase volume above threshold where transformer becomes actual load and amplifier "wakes up". I would guess that lack of parallel resistor can also cause signal to feel very "spiky" and unpleasant with small transformers if their resistance is going outside amplifier's design.

I have 2 midstim boxes now, one with and one without parallel resistors and I don't see significant difference, but I use large ELA-T10 transformers from Conrad that already have 4 Ohms of static resistance.

This parallel resistor also makes the voltage on the output less dependent on the load connected (basically you have less voltage drop when you use electrodes with larger surface area), might be useful for some scenarios, i.e. if you make venus/tremblr receiver lined with conductive rubber, so your electrode surface area is variable.

[InfamousPlantain]

I see.. that does clear things up a little. Thanks!

[mantrid]

Hey @mantrid I see you did a lot of engineering in this area, so if you would care to comment on this? If I am adding capacitor to existing midstim, aside from small signal distortion, is there any issue if using standard electrolytic ones (I have those on hand)? I am planning to try out 330uF for about 8-12 ohms in transformer + resistor?

330µF are okay. Cuttoff frequency (-3 db, about 30% reduction of voltage) would be about 110 Hz. I would use 100µF (cutoff frequency about 350 Hz),

You have to use a bipolar audio capacitors with a voltage of at least 16V (if amplifier is powered with 12V). Normal unipolar electrolytic capacitors won't work.

Also, what about output? With 3-phase in theory you could have current going out from one transformer into other (because they can be phase-shifted on same frequency), would it make sense to add capacitors there (on all 4 terminals), and which ones? I think there electrolytic ones might not be suitable as voltage is higher.

Not required. In 3(or more)-phase setup current between the electrodes is mixture of the two input signals. If these signals contain no low frequency competents, such components cannot appear is the mixed signals.

Semantics, eh ? ;)
I guess it depends on one’s frame of reference: voltage if it’s primarily to prevent transients reaching the user, power if it’s primarily to prevent high current in/fusing of the transformer in a fault condition.

Schematic is there: viewtopic.php?p=288013#p288013

Power that reaches the transformer is U*I=U^2/R. Thus, if R>0 and U is limited, power is limited. Peak pulse power dissipation of the 1.5K... TVS diodes I use is 1.5kW. In order the withstand continuous over voltages a fuse should be added in series to the resistor (not in the schematic because that idea was stolen later from someone in this thread or in this forum)

I'm hoping someone can help explain the purpose of the parallel resistor method found here: viewtopic.php?p=283515#p283515 ? How does it compare to the low pass filter method here? Does the parallel resistor method offer any protection at all?

It sounds like the low pass filter is more of a safety thing but the parallel resistor is just an improvement on the feeling thing? Is it possible to use both methods? Thanks!

edger477 already answered it. Just a few additions.

Of course, the parallel resistor wont change how current stimulates the neurons, but there are some side effects.

1. Parallel resistor makes the output less current controlled and more voltage controlled. That can also be achieved by choosing a lower series resistor value of by choosing a smaller winding ratio (I would go the latter way). Whether the estim signal is current controlled or voltage controlled matters when the contact surface changes. With current control reduction of the contact surface can cause high voltages and high current densities which can feel stinging. With voltage control the opposite happens: The signal feels less intense if the contact surface is reduced. If you experience that problems, you should change you electrode setup.
   • Electrode surface should be as large as possible
   • Contact resistance should be as low ass possible (use salted water based lube)
   • Resistance of the electrode should be low. Thats relevant for conductive rubber which has a quiite large resistance. If that material is used, it should have something like a metal core.
2. Parallel resistor can cause clipping if the amplifier cant deliverer enough output power. In that case increasing volume has less effect and (harsh) signals that play with volume variation become less harsh.
3. In 3-phase setups signals are mixed differently. But it's IMHO impossible to predict whether this feels better or worse.

Both concepts, parallel resistor and hi-pass filter can be combined. It does not matter where the parallel resistor is located, before the capacitor, behind the capacitor or even behind the transformer (in that case you need to increase the resistor value.)