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As promised here is a mockup of a solderless very simple estim device. Materials needed 2.0 stereo audio amplifier TPA3116 or TPA3118, two 70V or 100V audio transformer (Speco T-7010), two power resistors 25W 3.9 Ohm, an mp3 device, 1 12V SLA battery or 12V double insulated power brick, Plastic (not metal as pictured!) banana plugs, wire and 4 electrodes.
this device lacks balance control pots, triphase switches, or a kill switch. It also lacks a box! But it will get the job done for less than $100.

A wire connector like this would also be helpful.

Hey thanks a lot for posting this! Really helpful!

One quick question though, the SmartStim guide I saw recommended one of these amps: https://amzn.to/2lyic8q

Are you saying this type of amp would work just as well? : https://ebay.to/2lAzhhZ

Isn't this amp 50w per channel and too strong? I would prefer to use this one as it's $25 cheaper

TDA 3116 is the amp on my set up, it works great. The one you posted is “d2” which I think is design 2 doesn’t appear to have a separate on-off switch instead of built into the volume knob. And I would not get the Bluetooth version. Mine is D1

Can anyone confirm if this is correct?

The banana plugs that insert to the amp from the transformers must have 4mm pins, but the plugs that come from the resistors and insert to the rubber loops or tens pads (for eg) must have 2mm pins?

i was under the impression that i could buy one cable with plugs on each end, cut it in half, and attach the plug end to either the amp or the part going to the body, and then the cut end to be inserted to either the transformer or resistor..

But I'm guessing this wouldn't work because the pin sizes need to be different?

You are right Banana pins are 4mm, TENS pins are 2mm. You can get cheap banana pins with no solder connections just about anywhere. The wires coming off the TRANSFORMER not the resistor go to your electrodes. My early boxes used alligator clips so I could hook up any type of electrode, but if are going to use TENS pads exclusively then putting 2mm pins on makes sense.

Afapp wrote: Mon Sep 09, 2019 1:18 pm You are right Banana pins are 4mm, TENS pins are 2mm. You can get cheap banana pins with no solder connections just about anywhere. The wires coming off the TRANSFORMER not the resistor go to your electrodes. My early boxes used alligator clips so I could hook up any type of electrode, but if are going to use TENS pads exclusively then putting 2mm pins on makes sense.

thanks for the reply, and yes sorry, I meant the wires coming from the transformers to the electrodes. I intend to use both rubber loops and tens pads, and both have 2mm wire pin inputs.

So can I just buy something like this and then cut the wires myself, remove a bit of the outer cable and put attach the ends into the terminal blocks coming from the transformers on my build?

https://www.ebay.de/itm/Electrode-Lead- ... SwQItTydQ6

not all wires are the same on the inside! The smaller and more flexible a wire is the more likely the interior are not easily accessible copper strands but rather plastic coated crap that is a nightmare to work with. I don't know about those TENS pins. But I do know the other end of the wire is a standard 2.5mm mono plug! so you could add 2 2.5mm mono jacks (one from each transformer) to plug those wires into. OR buy a bunch of these you can use them on anything

How are your electrodes configured? I tried a build basically identical to this today and couldn't get anything out of it, but I think my incorrect assumption was that each transformer, how do I put this... I basically hooked up my two electrodes to the same transformer, one to the 1.25 output and one to the 0 output, and didn't have the other transformer hooked up at all. I used these terminal blocks to connect the electrodes to the transformer output. Here's a (sfw) photo (mind that I didn't use it on the blanket; it was contained within a project box.) I figured that since left+right were each both going to each transformer, and that since I was only going to be using two electrodes - one at head and one at base - that the output from one would be sufficient and I didn't have the other hooked up.

Should I be merging the transformer output and connecting both to a single trode? Any quick thoughts on what I might be doing wrong with this setup?

Edit: For the record, I'm using an AccuRelief Tens Supply Kit for my output. I pulled one of the trode wire pairs apart, clipped off the plug end, and trimmed some of the rubber off the far ends so I could get at the wire. Plugged that into the output from the terminal blocks. I also trimmed down the pads to just the little circular bit in the middle where the wires snap on.

Further edit: Parts Express 70W 10V line matching transformer. It's wired up as follows: Red banana plug into grey 8ohm lead, black banana plug into black "C" lead. Outputs on the C and the 1.5W leads.

some troubleshooting advice BTW I am in no way an engineer or an expert but I have built many of these devices:
Your audio input or mp3 player volume must be turned all the way up 95-100%
Don't connect 2 wires from the same channel
hook up the other transformer or a small speaker to the other channel
don't cut the TENS pad leads these wires are very small. Instead stick a small solid copper wire in and attach with alligator clips, or clip 2 alligators to the 2.5mm plug on your TENS pin wires
sometimes you need to use a different tap on the transformer primary side. For that transformer the primary taps continue over to the secondary side. I think the orange wire (70V 1.25W) is the proper one to use not the red (25V-1.5W 70V-10W) tap. I'm going by Parts Express picture for colors. Your pic shows you connected to a blue wire.
Also you may have the transformer backwards 8 Ohm/ Common secondary side goes to the Amp, 1.25W/Common primary goes to the trodes. A closer pic of both sides of the transformer would help.

I think you're right about the orange wire vs the blue. I do have the secondaries connected to the amp though, at least. I'll try balancing by connecting something to the second line out ports too. I'm not sure about using copper and alligator clips for the electrodes though, there are two outputs to one plug, and I kind of need them separated so I can connect them to different leads off of the transformer.

just for quick testing to see you got it working. you can wire to electrodes a dozen different ways that are stable after you have a working setup.

Rewired everything twice, switched to better terminal blocks and got rid of all the electrical tape, still nothing. Maybe I got bum transformers from amazon?

https://imgur.com/a/8VnxUx6

To address the troubleshooting points:
I had my (tablet) cranked to 100% volume. Was using VLC to play the MP3.
Had both transformers hooked up
Used a different brand of leads with an exposed 1/8th jack; did not cut the leads (though not 100% sure I used the alligator clips correctly)
Using the orange wire (70V 1.25W)
8Ohm/Common going to amp, 70V 1.25W lead going to electrodes

What am I doing wrong?

Edit: Maybe my banana cables are bad? When I slice them open and remove some of the insulation I don't get a single wire but a lot of really small, fine wires.

Further edit: Got it working. Was using the wrong output tap - needed to be the .62 (purple) tap. Any recommendations for tens cables? Related followup questions:

1) Is it safe to trim unused taps off the transformer or should I just tape them back with electrical tape?
2) The leads I'm using are configured with single 2.5mm jack to two 2mm outputs (see attached.) I have two of these for a total of four 2mm outputs. If I rig L+ up to one and R+ up to the other using alligator clips, I don't get any feeling on my arm from the pads. Is this expected?
3) If I wanted these leads to support a single electrode per (so I'd need two different cables for L+ and L-, four cables total, and four different ports on the box) is it safe to cut one of the leads off, or should I find specific 2.5mm->2mm single wires?
3.5) I picked up four of the 2.5mm female ports at a local electronics shop. They've got three terminals on the wiring end (see photo.) Which of the three terminals should I connect the output taps from the transformer to?

Testing on my arm or leg never worked well for me. I think the skin is thicker there and so the resistance is higher or more nerve endings? In any case stimming there vs stimming your junk is just different.
The 2.5mm jacks you bought are STEREO not mono. They are made for this your mono TENS lead looks like this in the stereo plug there are 3 contacts L,R and ground. you may get it to work though because the tip and middle are L,R and the bottom is ground

As for types of trodes I don't like TENS pads because they tend to fall off. I use a stainless steel shackle for the head A triple ring for the base and a custom "electrified" aneros or a Zeus torpedo plug for the prostate metal trodes last longer. conductive rubber gets hot spots over time.