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

[edger477]

Measure static resistance (with ohmmeter) of input contacts on your transformer.

Some transformer only have 4 or 8 Ohms impendance, but the resistance is way more, so amp does not see any load, and some amps react badly to the point when transformer starts to cause load on amp.

If the transformer has no resistance in range 4-8 Ohms when not connected, then add the parallel resistor, so amp does not have sudden reduction of resistance (and therefore increase of power).

[f12345u]

It's this piece of shit board XH-543. https://cyberpithilo.web.fc2.com/audio/ ... index.html
This goes into it but it's got very high gain, a ton of cross talk, and
"The input sensitivity is too high, maximum output is reached at only 40mV when input semi-fixed volume is maxed out"

I don't think any of my components or wiring is bad. I've double and triple checked it all. I'll recheck it all later again and write it down/take pictures.
I'm going to purchase a different board, and later on modify this one for use in my higher end v2 box.

[f12345u]

I removed and bypassed the onboard pots.

This is my ground loop from the audio inputs to the pots and the audio input center pin. You can't see the wires for the two pots, but I have them mounted the same direction, so the outer top pin (yellow wires) are soldered to the ground for the input jacks and the 3pin connector center pin.
Audio In Ground Loop

This is testing the 3pin connector
Pot Test

Here is an album checking the voltage on the output of the amp.
Voltage Output of Amp

I tested the voltage on the output side of the board playing a 600hz sine signal at 25% laptop volume.

Left channel maxed at 11.17, Right channel maxed at 11.16, both channels moved smoothly through the voltage range from 0-11, neither channel was affected by the other channels pot.

Voltage Output of Transformer

Then I hooked up my transformer negative to the negative on the left channel, and transformer 8ohm, with a series 3.9 ohm resistor that reads correctly with a meter, to the positive.

I placed either side of my voltmeter (AstroAI DM130B) on the outputs from the transfer, negative to C, positive to 0.25Watt.

Playing the same 600hz frequency at 25% laptop volume, the min voltage (pot at 12 o clock) was 0.125vAC, the max voltage was 303.7vAC, moving the pot barely 10 degrees had the voltage at 38.24.

Here is a video of my transformer circuit.
Transformer Ciruit

This amp has 0.8ohm on the input side ( C and 8Ohm leads) and 0.464 on the output ( Output C and 0.25Watt lead)

The two types of resistor are both tested and match expected resistance of 3.9 for the green and 22 for the yellow.

[diglet]

This amp has 0.8ohm on the input side ( C and 8Ohm leads) and 0.464 on the output ( Output C and 0.25Watt lead)

You're supposed to use the common and 10W tap.

Can you double-check the resistance on the output transformer side? The 0.8 ohm on the input side sounds plausible, output side is expected to be around 100 ohm.

On my setup (almost identical board, same resistor values, slightly larger transformer), the peak-to-peak voltage reads 44v at a quarter of the pot turn. But if I use the wrong transformer turn (0.25w), it already reads 164v at a quarter turn which is very similar to your results.

[f12345u]

From left to right, 10w = 40 ohms, 5w = 54 ohms, 2.5w = 80 ohms, 1w = 265 ohms, 0.5w = 0.8 ohms, 0.25w = 0.47 ohms

This is just the leads from the transformer, removed from any caps or the board.

Both brand new Speco t7010 transformers read the same.

Also I don't know what the peak voltage looks like, I'm using a true RMS meter.

[edger477]

0.5w and 0.25w seem shorted

try 1w tap, the tap I use on my transformers has 380 Ohms

[f12345u]

Gonna have to wait till tomorrow, I fried my amp board. Dropped a piece of solder on it on accident and shorted it. I think it'd be weird if both transformers have the same short. Maybe I'll call the shop I bought it from and see if they have more.

[f12345u]

Okay, seems I am a dumb dumb. First, I fixed the board so YAY
Second, I need to go back to first grade and learn to read again. I rechecked the transformers because it wasn't sitting with me right that TWO brand new transformers would be shorted. Here's the updated numbers..
The 0.25 Watt lead reads 0.795 kOhms or 795 ohms.
The 0.5 Watt lead reads 0.467 kOhms or 467 ohms.
The 1.5 Watt lead reads 264 ohms and like the others matches the numbers before.

The voltage across the C and 0.25W lead and 10W lead at max volume are 433 vRMS and 70 vRMS respectively.

With all this said, what is a SAFE maximum voltage for these estim boxes to output. I don't mind if the very very top of the volume knob hurts a tiny bit, but I don't want to use this without knowing what the safe upper limit of voltage is.

[diglet]

There isn't really a safe voltage. There is a safe amperage, about 50mA, but you can cause damage with fewer amperes than that if the frequency is too low or high.

On my stimbox, which uses the 10w and 4ohm tap, I measured these values:

With open contacts:
91vrms highest volume setting before the waveform starts distorting (about 40% of the pot)
137vrms highest volume setting, heavy distortion.

With 440 ohm resistor:
22vrms highest setting before significant distortion happens (40% of the pot)
41vrms highest volume setting, heavy distortion.

I didn't notice a 4ohm tap on your transformer. The default build uses a winding ratio of 16, I think that's 8ohm + 2.5w on your transformer. With 8ohm+10w the winding ratio is 8? If I have that right, it makes sense that you measure around half (70vrms) of what I see on the 10w winding.

[edger477]

With all this said, what is a SAFE maximum voltage for these estim boxes to output. I don't mind if the very very top of the volume knob hurts a tiny bit, but I don't want to use this without knowing what the safe upper limit of voltage is.

When considering safety, the most important safety limit is current. The most widely accepted limit for estim (below the waist) current is 50mA.

Now, the reasons we use transformers are:
1. galvanic insulation (both outputs are not connected to inputs in any way)
2. transforming amp output to reduce current and increase voltage

Our perceived sensations come mostly from voltage, that is why most often we take the lowest power tap on transformer (since it has most voltage and least current). Voltage on transformers without any load is not really meaningful value since 0.25w tap already has 795 Ohms internal resistance, so if you shorted its outputs, max current that it could produce would be determined by its power (0.25w) which means current x voltage = 0.25. So Ohm's law says that U = R x I, and in our case 0.25/I= 795xI which when solved (I = square root of 0.25/795) which is around 17mA. This is maximum current we could expect from lowest tap (it will be much less since body resistance is far far from 0).

To check what current you really have, you would need to connect output to a resistor of 600-1000 Ohms (this is approximation for body resistance) and check voltage (we can calculate current then based on Ohm's law). With load, your 0.25w tap might have lower voltage than the 0.5 since its power is very low, but maximum safe voltage with 1000 Ohm resistor would then be around 50V for 50mA "safe" current (0.05A x 1000 Ohm).

When I tried to submit answer I see that digled already mostly answered, but since I already wrote all this, I will post it too.

[f12345u]

Thank you for the information. Here's my build. It's almost finished, but I'm waiting on some WIMA caps to replace the input coupling capacitors on the amp board with some better ones and some 10uF chokes. The changes I made to the board dropped the gain output of the amp board down to +20db, as well as removing cross talk within the op amp. I also removed and bypassed the shitty onboard pots, which cleaned the sound up from the board a ton.

Before making these changes, my pots would only be at about 1/2 before maxing out the voltage when I played from my laptop at full volume (1vAC across gnd to signal line), and the pots would influence one another. Now I have the full range at max volume :D

This is only the second thing I've built, it's kind of ugly inside but I'm overall happy with it. I'm still a bit scared to use it because of the shock that I got, but I'm hoping it'll make for many fun times :D

[edger477]

Nice when you have knowledge how to turn shitty amp board into a decent one.

[f12345u]

So with the 10W leads, the box feels great, I CANNOT take it up past about half way without it being overwhelming but

With the massive heat sink and fan, stays very cool. I put on a 600hz sound at max volume for an hour and barely any heat was coming out of the vent.

I just need to play with electrode placement and how to estim, first time with it lmao.

[edger477]

have you tried less powerful leads? I use the lowest powers on my box, tried next and did not feel as good.

[f12345u]

yeah lmao, the 0.25w leads shocked the hell out of me. Too much voltage I guess.. It's good like this :D very good.

*Edit* This has been in the back of my head for a bit. I don't rightly understand how I could have gotten such a shock from this. The current would have been less, and I tested the voltage with the 1k ohm resistor and it did massively drop into a "normal" range. It's got me worried about my build's safety. I've used it probably a bit too much since finishing it up , and it WORKS, but I'm starting to think about tearing into it again.