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

[SniffyWally]

Hi all

I'm building a DIY Stereo E-Stim unit based on Tronic's Midstim, and I have a couple of questions.
Not so "low cost", though. Spent around £220 after buying all the components, wire, connectors, nuts & bolts, enclosure etc.

The Build:

I've opted for 2 x Fosi Audio TPA3116 V1.0G amplifiers (£50 each), and will only be using 1 channel from each, as I wanted individual channel controls rather than having to tweak balance at the source (a laptop with USB audio interface). I would hate having to jump back and forth between box and laptop to tweak balance and level. Individual channel controls is so much better IMHO. This also avoids "one wrong move of the mouse and scream!" issues. I probably could have added pots to control each channel's input level into 1 Amp instead to make it smaller and cheaper, but I wanted to keep any soldering to a minimum. I liked the look of the Fosi volume control, and I liked the idea of going straight into the Amps via the RCA outputs of my USB audio interface, so I chose 2 amps, and will only be using 1 of the 2 stereo channels of each amp.
For the transformers I opted for the Omnitronic ELA-T10 as these were available through Amazon.
I went super overkill on the serial resistors. 2 x 4 Ohm 100 Watts. Yes, you read that correctly. 100 Watts each. I plan to do without a heatsink and mount them to a thin wood sheet on the top of the box, so I wanted a large amount of radiating metal around each resistor to prevent any potential overheating. No parallel resistors or high pass filter arrangement.
The Amps will be fixed to the top of the box at the front, with the heat resistors on a thin sheet of wood further behind (leaving a nice gap for connections in & out of the Amps). Transformers and main connector blocks will be located inside the box (transformers at the back for even weight distribution) with various small holes drilled in the box lid for wiring. External connectors for 'trodes mounted on the top of the box to the right of the Amps. Lever nut wire connectors used throughout, as I find these far more reliable than banana sockets or 3.5mm plugs (dodgy connections = ouch! and more ouch!), with the only exception being the connections to the resistors which I plan to solder.
Quite a large ABS plastic enclosure 300 x 250 x 120 mm to fit all the stuff I need on the top. A bit of a monster.
Everything's ordered and I'm only now awaiting delivery of the transformers before starting the build.

Questions regarding transformer connections:

I would appreciate it if the more experienced builders on Milovana could answer these queries, if possible.

1. The Amps spec a 2-8 Ohm loudspeaker load. TroniC's Midistim schematic shows a 3.6 Ohm series resistor into the 8 Ohm winding of the transformer, which will produce a combined impedance of around 11.6 ohms. With my 4 Ohm serial resistors this will be 12 Ohms. Are there any issues with me connecting a load 1.5 x the specified impedance to a TPA3116 based Amp? Has anyone done this without any issues, or experienced issues when turning the volume up such as "soft clipping" (adding some additional HF content) or "hard clipping" (adding significant HF content)? NB the Amps are powered by 12-24V DC and I will be initially using the supplied 19V 4.74A PSUs for each Amp, but may change those out later (one Amazon buyer commented the PSUs were low quality / unreliable). Should I be looking to change these PSUs for another type?

2. Have other box builders used the 4 Ohm transformer winding instead of 8 Ohms, in order to keep within an 8 Ohm load for their Amps? I understand this will make it a bit more "current driven" (a plus point) but would lower the output power resulting in having to turn the amp up, increasing the power dissipated by the serial resistors (a minus point). I could see 2 options for this:

a) Use the 4 Ohm winding and keep to the 1.25 Watt winding. I calculate the maximum output voltage will drop by 62.5% which is quite significant, i.e. halving the primary winding, and the serial resistor is now using up 1/2 not 1/3rd, of the amp's output. Resulting in turning up the amp quite a bit to compensate, radiating away more heat via the serial resistors. I'm not sure what this arrangement might do to change the voltages / currents passing through the body.

b) Use the 4 Ohm winding but switching from the 1.25 Watt to the 2.5 Watt winding so as to maintain the same turns ratio shown in TroniC's schematic. I calculate the output voltage will drop by 25% rather than of 62.5%, which is far less significant and will only requires a small amp volume increase to compensate for it. Again I'm not sure what this arrangement might do to change the voltages / currents passing through the body.

Have other box builders tried alternative windings like this and could they advise their findings and suggest the best option? Are there any other arrangements that you find work best?

3. In general, presuming you are using TroniC's arrangement of tappings, inputting stereostim source files with peaks normalised to near-maximum levels, and with typical genital / anal trode connections, what level do you typically end up turning the signal source and Midistim amp up to, during a stim session? Has TroniC built in some overhead into his design to cater for low level inputs so with normal level files do you find you have to turn down the signal source in order to give you a good degree of control over the output level via the volume pot?

Answers would be much appreciated.

SniffyWally

[diglet]

Hi all

I'm building a DIY Stereo E-Stim unit based on Tronic's Midstim, and I have a couple of questions.
Not so "low cost", though. Spent around £220 after buying all the components, wire, connectors, nuts & bolts, enclosure etc.

...

I measured with an oscilloscope what the effect is for higher values of series resistor. There are no unexpected results for my chinese TPA3116 clone. I know the specs say 2-8ohm load, but it's really 2 ohm or higher.

btw, the 4 ohm winding indicates the inductance of the speaker that should be attached, not the resistance of the winding. The inductance of the transformer is frequency dependent, I measured mine (4ohm) at 16 ohm at 500hz and 27 ohm at 1000hz. Because of this trying to do math with the winding ratio and resistance is complicated, really need to measure it in action.

I use the 8ohm/10w winding because according to my measurements it outputs slightly more (maybe 10%?) power than 4ohm/10w at typical skin resistances and I found myself occasionally clipping the audio. I'm not sure which one is better, you will have to experiment.

The input sensitivity depends on how your amp is tuned. My amp has a potmeter to adjust the amplification, I adjusted it on first use such that maximum volume on my computer is clipping a bit. You may need to open your amp to do this.

I recommend using a 12V PSU, 19V is probably much more than you need. If you find yourself maxing out, you can always upgrade.

[Stefanal]

Hello All!

Thanks for the shared knowledge. After much research i will give it a go to build something. I will go mostly with the original design and some parts from lolol2 tutorial because i am based in europe. I am a complete beginner and have a few questions that maybe some people here can answer.

1. I will go for 2 amps like lolol2 suggested a few posts ago to have separate control over left and right channel without complicating things much. But how will you connect power to both? I got a 12v 5A power supply. Can you just connect the wired to both amps?

2. Is it good to attach the 2mm pins right after the tranformer? Or is it better to first solder the wite to a female 2mm or 4mm plug?

3. I got the tranformer from conrad. https://www.conrad.nl/nl/p/elma-tt-iz18 ... 16104.html. These don't come with preattached wires. Can you just soldet the wire on those hooks? Is there a technical term for this so i can maybe find a tutorial.

4. What is a good way to test these things? I got a multimeter.

5. Can you play the sound from your phone using a usb-c to 3.5mm adapter or will that distort the sound?

6. More from a end user perspective later on: i notice that most anal toys are sold bipolar. Could you also just plug in 1 pin and then another one somewhere else in for example a triphase setup?

I know these are a lot of questions. But hopefully some can help me out . Thanks in advance! If you are dutch (like me). I found reliable dutch webshops where i ordered the parts. So i you want links let me know.

[edger477]

Can you just connect the wired to both amps?

Yes, just connect both + together to supply and both - together to - of supply.

2. Is it good to attach the 2mm pins right after the tranformer? Or is it better to first solder the wite to a female 2mm or 4mm plug?

I use 4mm female plugs on box, and then silicone lab banana cables (I build electrodes with female 4mm plugs too). But it is personal preference.

These don't come with preattached wires. Can you just soldet the wire on those hooks? Is there a technical term for this so i can maybe find a tutorial.

You can solder to these terminals directly, but I would probably get the cold press terminals (this set probably contains a right one but hard to say, would need to measure and find a proper one https://www.amazon.de/-/en/gp/product/B0BC8VNDZ4), because with crimped terminals you can easier change the input/output taps for experimenting and if you need to take box apart for maintenance.

5. Can you play the sound from your phone using a usb-c to 3.5mm adapter or will that distort the sound?

that adapter is just a small sound card... you would have to test how it works, some have ok sound

6. More from a end user perspective later on: i notice that most anal toys are sold bipolar. Could you also just plug in 1 pin and then another one somewhere else in for example a triphase setup?

Yes, and you can also connect both poles of bipolar toy to same wire (combine them into unipolar).

[Stefanal]

Thanks a lot for the answers! I will update when i have finished something

[Stefanal]

Hello, one more question from me.

Is the amp also suitable? I feel like it is very similar to the original tutorial. Different brand but same chip.
https://www.amazon.co.uk/AZDelivery-TPA ... B09PRJQYPB

[senorgif2]

Hello, one more question from me.

Is the amp also suitable? I feel like it is very similar to the original tutorial. Different brand but same chip.
https://www.amazon.co.uk/AZDelivery-TPA ... B09PRJQYPB

that will work. Like you said the chip is the same.

[mini33]

So are all TPA3116's the same?
Also looking at "MidiStim_partsList_2020-06-28.pdf" which i have included as an attachment i see
"> 2x20W RMS @ 4 Ohms, < 2x35W RMS @ 4 Ohms" which i have no understanding of, any help would be apreciated.

[bigfoot58]

Thought I would post a photo of a solder-less device. Just a functioning prototype for now but it might help new folks with the transformer wiring which was the most head scratching part for me. Eventually I will get this thing boxed up and may even add a second amp for separate volume control of the left and right channels.

[Bigcars]

I have the following tentatively set up:

1 - Breeze Audio HiFi Class 2.0 Audio Stereo Digital Power Amplifier TPA3116 Advanced 50W+50W Mini Home Aluminum Enclosure amp https://a.co/d/22hG149
1 - 12v 5a 60w double insulated 2 prong power supply
2 - 25W 3.9 ohm resistors
*** 2 - ? resistors (In parallel) ***
12 - terminal blocks
1 - SPST Light Duty Toggle Switch (Trode Power)
1 - DPST Heavy Duty Toggle Switch (Unit Power)
2 - 70V 10W Speaker Line Matching Transformer

On order:
1 - Triphase switch — Twidec/8Pcs Mini Toggle Switch SPST 2 Position 2 Pins ON/Off AC 125V 5A AC 3A 250V with Pre-soldered Wires MTS-101-X https://a.co/d/eR5EvXw
5 - 4mm female banana jacks for L, R, Tri —- 10Pcs Black and Red 4mm Banana Jack Socket Female Adapter Panel Mount Binding Post for 4mm Banana Plug Connectors https://a.co/d/0XkHey9
1 - Project Box — JIUWU Aluminum Project Box PCB Enclosure Case Sand Blast Reinforced Split for Audio Amplifier DIY Power Supply Signal Amplifier, 106x55x150mm (WxHxL) https://a.co/d/fpkpZGS


1. What resistors do I use for the parallel setup?
2. How should I tackle POTs for level and balance with the individual electrodes?
3. Is it easier to swap out the amp for one with these level controls?
4. Adding the triphase switch just joins the 2 black lines to a new common line so I have L, R, Common, correct?
5. Would it be beneficial to add a rechargeable battery pack to this setup?
6. I’m ending up with 2 outlets required which is kind of a bummer
7. I need to accommodate 2 RCA (amp) -> 3.5mm input (stimbox) OR 4 banana jacks (amp) -> 4 banana jacks (stimbox). Is there a preference?
8. I also need to assess the gauge of wire for the power and for the rest of the build as they will be different

[edger477]

1. What resistors do I use for the parallel setup?
2. How should I tackle POTs for level and balance with the individual electrodes?
3. Is it easier to swap out the amp for one with these level controls?
4. Adding the triphase switch just joins the 2 black lines to a new common line so I have L, R, Common, correct?
5. Would it be beneficial to add a rechargeable battery pack to this setup?
6. I’m ending up with 2 outlets required which is kind of a bummer
7. I need to accommodate 2 RCA (amp) -> 3.5mm input (stimbox) OR 4 banana jacks (amp) -> 4 banana jacks (stimbox). Is there a preference?
8. I also need to assess the gauge of wire for the power and for the rest of the build as they will be different

1. None
2. your amp has only one POT... best to use balance in windows for that
3. I did not find any good amps with 2 channels and separate controls, but I built 4-channel box anyway with this one https://www.amazon.de/-/en/gp/product/B087FWWVB6/
4. yes, correct, but with more complicated switches (on-off-on double ones that have 6 wires) it is possible to also make common from red ones (it is same but changes stroking direction)
Since you are using banana jacks for output, you could even do without switch and just get these cables https://www.amazon.de/-/en/gp/product/B07ZVQDH1T/
Or something similar, then you can connect black to black with cable and there is pass-through to connect another one for electrode.
5. If you got a good PSU then no, but it cannot hurt
6. ?
7. I think amp has 4x4mm bananas for speakers not RCA. RCA are for input from PC
8. power from PSU is probably 1.5mm square or less, so that is about 15 awg, and from amp to stim box you can use 14-ish

[breadfan]

Spent the last day reading every word of this entire thread, twice. So with the basic build, long as we're using a TPA3116, we should NOT include the parallel 10-20 ohm resistor??

[Electro]

Spent the last day reading every word of this entire thread, twice. So with the basic build, long as we're using a TPA3116, we should NOT include the parallel 10-20 ohm resistor??

That's correct, it's not necessary and people who who added it generally don't have any difference in the Estim feel of the box either way.

[breadfan]

That's correct, it's not necessary and people who who added it generally don't have any difference in the Estim feel of the box either way.
[/quote]


Guess I'll leave it out on the next build

[Hellme]

Hi,

I want to build also a device with this amp: https://www.amazon.de/-/en/gp/product/B087FWWVB6/

I have this Transformers: https://www.conrad.de/de/p/elma-tt-iz18 ... 16104.html

So i am not shure how to wire the Transformers safe and correct.

1. Do I need the 1k resistor in the Output If I use 16 Ohm, 0,65W setup?

2. Or better use 3.9 Ohm 10W setup in the input line of the transformer?

Cheers
Hellme