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

[BamaSwitch]

Looking into this further, is it the operating frequency that puts the board out of the safe range?

[hosenguy]

Looking into this further, is it the operating frequency that puts the board out of the safe range?

The frequency of any amp can be limited by auxiliary circuits like tone controls or just a bandpass filter. Sometimes as simple as a capacitor.
My concern is the 120W per channel. This is ludicrous power for stimming if the amp is actually capable of producing it. Think of how a loose connection in the input circuit would pop under worst conditions. Ouch.

Now real science, by me?! If you run this with a 12 VDC power supple it is 50W per channel. For simplicity P+I*E which says the power supply at 12 VDC would need to be 8.33 amp. to provide 100 watts out with NO losses in the circuit like heat. So with a much smaller power supply it would work at lower wattage out and be safe. So I would use a 45 watt 12 VDC power supply and be assured of safety. But you still have to wire your own volume controls and the output is not where they should be.
In the output you would need at least 10 watt controls but 25 watt would be more correct. These size pots also cause a lot of crackle and that won't be good.

Let us know how you get on with this.

[cl13a]

Looking into this further, is it the operating frequency that puts the board out of the safe range?

I did some reading, but I'm still not totally certain of my conclusion (I could be completely wrong!):
http://www.bcae1.com/amplfier.htm
This page says that amplifiers usually have a circuit to boost voltage so the audio output voltage isn't limited to the supply voltage. Under that assumption, an increase in voltage from the original amplifier would be less suitable for estim use (since the 50W amp already is plenty enough).
But if the boards we are talking about do not behave like this (i.e. the most voltage they can ever output is limited by the supply voltage) then the 120W amplifier should put out nearly the same voltage as the 50W amplifier at 100% volume, if you are using the same power supply. The difference in power is because the 120W amplifier can handle more current at the same voltage. I think both these amplifiers are "Class D" amplifiers, which I understand to be limited by the DC supply voltage: https://www.crutchfield.com/S-lgA6rTT9c ... -best.html

So my (again, very possibly incorrect, I spent about 20 minutes learning this) conclusion is that both amps discussed (120W/ch and 50W/ch) will put out the same voltage and consequently the same power when the load resistance and supply voltage are the same, because they are both class D amplifiers. In summary, I don't think I'll fry myself or even feel much difference if I replace my 50W/channel amp with the 120W/channel one. I think the reason the 50W amp is safer is that it is not capable of putting more than 50W per channel through you, but you can limit the power before the amplifier and that works too, as hosenguy suggested with the 45W power supply idea.

[hosenguy]

If you noticed the specs state 50 Watt output when using a 12 VDC power supply and 120 watts when using a 24 VDC power supply. Electronic law says P=E*I. Double the voltage (E) or the amps (I) individually and you double the power.

Therefore as I stated you may as well power this from 12 VDC to be safer. If you short out the output it will go past any wattage listed for audio and output whatever the power supply is capable of. At 12 VDC that is still considered safe. Transformers are the isolation device between you and the power supply. Transformers pass only AC signals so the power supply is not a concern at the trodes from a safety viewpoint (assumed they are double insulated and inspected).

The AC signals at the trodes may be in the neighborhood of 50 to 80 volts AC because we are using the transformers as step-up voltage devices for the audio portion. The 4 ohm series resister does reduce the maximum potential of the amp. (I don't do math after a couple beers so numbers are from distorted memory!)

While I won't advise it, you could use the 120 Watt amp with a 24VDC supply and still be safe by turning down the level on the input BUT there are several mistakes you can make to get way more than you can take. For example you have a low volume .mp3 playing from your phone and you have levels turned up fairly high on the amp when the phone rings or an alert beep is received from your source. Or even wiggling the 3.5mm input connector will provide a significant surprise which may be directly into the amp(?) and bypass the level controls altogether. I hate those jacks! And I am not sure if this type of amp has pop suppression for turning it on or off, but you will know soon enough. Perhaps there is a radio station or some other interference nearby and the amp goes into oscillation? My first DIY would ring after any spike like signal until I fixed the power supply to increase filtering.
An old song I like has the chorus, "There are a million ways to die, leave it to man to invent some more." Substitute "melt a dick" for "die".
A visitor to my house once turned my Sansui amp on and started a record but got no sound, so he turned it up to max volume and then discovered the phono button and pushed that. My speakers literally exited through the grilles. Don't be like those speakers!

I started stimming with a 200 watt (or so) car stereo from Walmart, so I cannot put anyone down for not being safe!
The other statement I made is for you to try whatever you choose and then post how it worked out because there will be a few details not on any spec sheet.

[Tobjac]

I also have a question. I’ve build the Trinic Modistin of from smart stim.... also got the recommended amp... which has 50w*2 but throttles down to 12W per channel when connected to 12V and 8ohm resistance.... I’ve also got the chassis mount 25W resistors.... do they need to be cooled?

The design posted here is also ally the same but just smaller resistors if I see that correctly?

[hosenguy]

I have used 10 watt resisters in a unit I built several years ago and they do not get hot enough for me to need heat-sinks for them.
My amps are KEMO 18 Watt modules, 1 per channel. I cannot max it out unless the input source is real low. Most stimming does not need the full output of the amp.

This build calls for the ceramic type resisters. This chassis type is used a lot (but not a requirement) but may cost more.
https://www.mouser.com/ProductDetail/AR ... a%2FxLg%3D

I believe 10 watts or higher, based on availability, price, and your preference will be fine. If in doubt just check it after about a half hour of stimming. Look for discoloration or wet your finger and tap on it and if it sizzles it is running too hot.
With the 25 watt rating you should not need cooling unless the amp needs it. Does the amp smell hot?
Use good thought process and look for the obvious stuff.

[Tobjac]

I just checked yesterday while trying it out... the resistors get what I would call slightly warm. Not more... the amp doesn’t get warm at all... so I guess I’m fine :D

[hosenguy]

To satisfy my own curiosity I purchased the exact BOM from Parts Express above to test out.

I put it together today and tested it for an hour and a little more tonight. It seemed a better idea than watching the election results.

The results are that it has plenty of power for me. I ran it at half volume with some files I have adjusted. My usual stim box would be at about the same level to get the same results. The feeling was just a tiny bit sharper than my usual box which has a little different output resister value and arrangement. I will use this box again as it is a good as any I have tested in the past.

I found that the volume control has an off/on switch. The amp does not pop or produce any spike when turned on but makes a fairly pronounced one when turned off. It is not suppressed.

The volume control knob is smaller than I like but I guess it could be easy to get used to. The audio input is close to the volume and I will use a right angle 3.5mm plug in the future to keep it out of the way of the volume.

During my use this evening there was only a small bit of warmth from any part of the assembly. 10 watt resisters were cool to touch and the heatsink on the amp was barely warm. Transformers were cool as the room temp.

I know there have been comments about using a bare board without a housing and after looking at the parts I agree that some housing will be necessary or it could be accidentally shorted out and damaged.

In order to test this safely I 3d printed the lower half of a housing I esigned and mounted the parts to it. I will post the files to print your own housing in a few days, after I correct a couple measurements for mounting holes and complete a couple optional layouts for the front and back panels. I was able to completely build this without any soldering after adding 2 single terminal splice blocks.

I think this is a good beginner's stim box and provides a substantial experience. With a couple adds it could easily become a more advanced box.
Stay tuned.

[cl13a]

To satisfy my own curiosity I purchased the exact BOM from Parts Express above to test out.
....

Big kudos for validating the ultimate budget BoM!
It's a shame that the Parts Express amplifiers are out of stock though, but at least the transformers are on sale to $3.00 from $5.19. The thread has been viewed >2100 times as of this post so perhaps there are many lurkers buying up the amps and getting into E-stim!

[boundstimmer]

I once built myself tronics midistim, but with a different amp (lepy 2x 20W). Worked fine and now I want to rebuilt the device, but I don't have the amp anymore. But I got an car radio (kenwood kmm 264 with aux input, more power than lepy) that I don't need anymore + the 12 v power source I once used for the lepy amp.
Is this safe? I couldn't think of a reason why it should not be safe, so I'm asking you guys.

Thanks in advance.

I also use resistors with more resistance...because that's what what I got in the store back then, resistors under 4 ohm weren't avilable. The ones I use got about 5 ohm, never gotten significantly warm ( i tend to stim at higher powers cause I'm into that bdsm torture stuff). Compared to the 3R7 ones ... you just need to turn up your volume a litte bit. Just in case someone wanted to built a box and can't find the recommended resistors, you can use other ones too (i guess).

[JakofClubs]

I just built a version of this with the "bonnie" volume controls from this thread: https://www.smartstim.com/forum/viewtop ... 06#p232406

At my stimming levels, the amp heatsink barely gets warm. The 25w 3.9 ohm resistors are just room temperature, but they are attached to a large aluminium plate, which seems to be overkill. The from panel is welded to the bottom plate which makes it conductive to audio negative, which probably isn't a good idea in hindsight. I did a separate input for the volume controls, but they affect the built-in input as well to some degree. This has a lot more "bite" than a 2B or ET-312, in a good way.

Spoiler: show

Close up of volume controls

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IMG_20210117_151026570.jpg (353.09 KiB) Viewed 5553 times

Bonnie volume control

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Edit: After some more playing around, I've found that some files, like the web-teases, are much better with stereostim. Some other files are better with 2B or ET-312. Bottom line is that different files work better with different boxes.

[mangoman]

you're doing rocketscience to jerk off

[DV8ion]

Hi

Just to be clear, this works good with many of the e-stim webteases on milovana?

Interested to try but never got over the pricetag of the commercial ones just to try it not knowing if I would like it.

[JakofClubs]

Hi

Just to be clear, this works good with many of the e-stim webteases on milovana?

Interested to try but never got over the pricetag of the commercial ones just to try it not knowing if I would like it.

Very much so. It feels better (to me) than the 2B or ET312. It's like the difference between listing to music on a transistor radio versus a quality home stereo. You feel subtle sensations missing in the commercial boxes.

I liked it so much a did a cleaner, minimal design with the recommended amp from SmartStim.com

Spoiler: show

midistim_inside.jpg (833.67 KiB) Viewed 5445 times

midistim_front.jpg (646 KiB) Viewed 5445 times

[phoopha]

Hi

Just to be clear, this works good with many of the e-stim webteases on milovana?

Interested to try but never got over the pricetag of the commercial ones just to try it not knowing if I would like it.

Very much so. It feels better (to me) than the 2B or ET312. It's like the difference between listing to music on a transistor radio versus a quality home stereo. You feel subtle sensations missing in the commercial boxes.

I liked it so much a did a cleaner, minimal design with the recommended amp from SmartStim.com

Spoiler: show

midistim_inside.jpg
midistim_front.jpg

Any chance of some more detailed information on what your using and how you did your build?