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

[isidore]

Hi,

Thank you all for your helpful information and comments. I built my own box and it is amazing.

IMG_20220113_120441.jpg

That's a great looking box. Would you mind sharing a few more pictures of it? I'm just doing some planning and I'm curious to see how others have laid everything out. No worries if not, of course. Cheers.

[Electro]

Based on the temperatures I often see my resistors get, if that is PLA, you will likely end of melting your box if you have an extended higher power stim session if the resistors are mounted to the plastic.

[Wen74]

Based on the temperatures I often see my resistors get, if that is PLA, you will likely end of melting your box if you have an extended higher power stim session if the resistors are mounted to the plastic.

We shall see.. I left the sides open for ventilation and the resistors are actually floating in the air. I planned to add heat sinks but so far it has stayed quite cool. Version 2 of the case is already under work...

[Wen74]

Hi,

Thank you all for your helpful information and comments. I built my own box and it is amazing.

IMG_20220113_120441.jpg

That's a great looking box. Would you mind sharing a few more pictures of it? I'm just doing some planning and I'm curious to see how others have laid everything out. No worries if not, of course. Cheers.

Sure, here you go.

https://www.dropbox.com/sh/lpefendkl076 ... UWXda?dl=0

The next version of the case will use separate audio input plug with level potentiometers and a temperature controlled fan. Also embedded labels for the switches and connectors. It will be larger (not necessarily a good thing), have proper ventilation holes and most likely will be printed with PETG or ASA.

Currently waiting for the additional parts to arrive..

[isidore]

Thanks much for the pictures. Very tidy and looking good. Please keep us posted as you move on to version 2!

[ramen]

Hey there. I just started my build and I have found a problem.

The amp (HX-M189) is totally silent. I removed all the other components and still can't get sound out of it. On top of that, the ON light remains lit for a long time after I unplug it. And I mean a long time, like half a minute. Did I just get a faulty unit?
For the record, I poked around the PCB with some wires and I get sound around the input and volume knob, but nothing beyond that.

Perhaps other owners of this amp can clarify if this is normal or if it can be fixed.

An update on this: the amp wasn't simply faulty and doesn't work, it also fried my laptop's audio card in just a few minutes of testing it.
So yeah, turns out this amp is cheap for a reason.

If you are planning to build this ~beware~. Be smarter than me and use an old mp3 player for the initial testing.

[steelhorse545]

Quick question for the smart folks: I'm on my second build, and I've seen two different placements for the triphase switch. One before the transformers and one after.

Does it matter?

Yup. Needs to be after the transformers. You can swap the absolute phase (ie whether they’re both going from positive to negative) on either side of the transformer by adding a switch on one pair of connections to one transformer which swaps over the wires to the rest of the circuit. Just to confuse the issue, if you did add another switch to swap phase direction, that can also have an effect on how the tri phase switch is connected up - ie which two wires are being commoned together, as well as their phase. I guess it’s easier to think of physical (body) connections and what you’re trying to common together.

If you’re seeing a switch on the amplifier side, that could be to change the relative phase, not tri phase - any links ?

Oh, and I think ‘transformators’ are usually found from the same suppliers that sell dilithium crystals and vibranium ;)

The schematic I was looking at was https://drive.google.com/file/d/1_jg2OQ ... sp=sharing

Apologies ! -hadn’t spotted your reply. No, that won’t work. The individual pairs of transformer outputs are floating with no common point of reference. It’s potentially a *really* bad idea, ‘cos it directly connects the -ve of one channel to the -ve of the other ! If those points are effectively ground then it probably won’t upset the amps (tho it’s a long time since I’ve looked at amp designs), but it also won’t have the desired effect either :)

[Gr8M1ke]

Yup. Needs to be after the transformers. You can swap the absolute phase (ie whether they’re both going from positive to negative) on either side of the transformer by adding a switch on one pair of connections to one transformer which swaps over the wires to the rest of the circuit. Just to confuse the issue, if you did add another switch to swap phase direction, that can also have an effect on how the tri phase switch is connected up - ie which two wires are being commoned together, as well as their phase. I guess it’s easier to think of physical (body) connections and what you’re trying to common together.

If you’re seeing a switch on the amplifier side, that could be to change the relative phase, not tri phase - any links ?

Oh, and I think ‘transformators’ are usually found from the same suppliers that sell dilithium crystals and vibranium ;)

The schematic I was looking at was https://drive.google.com/file/d/1_jg2OQ ... sp=sharing

Apologies ! -hadn’t spotted your reply. No, that won’t work. The individual pairs of transformer outputs are floating with no common point of reference. It’s potentially a *really* bad idea, ‘cos it directly connects the -ve of one channel to the -ve of the other ! If those points are effectively ground then it probably won’t upset the amps (tho it’s a long time since I’ve looked at amp designs), but it also won’t have the desired effect either :)

Thanks for the follow-up. I went with the output side and all is well.

[ramen]

Hey there. I finally built this thing and it doesn't work at all, perhaps someone can help me.

I've built the bare minimum components (amp>resistors>transformers) in order to isolate the problem. So far:
- The amp works. I connected some headphones and the output isn't super loud but it definitely works.
- One transformer has a very weak output. I don't have a multimeter but I've tested it with cocodrile clips connected to the headphones; I was expecting to hear loud DC noise or the headphones exploding or something, but it outputs the same audio... except with lower volume than the amp.
- The other transformer seems to be dead. My previous amp was faulty (see above itt) so maybe it fried this transformer too?

I've double-checked all the connections and even looked for non-insulated spots even when my box is made of plastic lol.
Not sure what else to try. Any ideas?

[JBK]

Hey there. I finally built this thing and it doesn't work at all, perhaps someone can help me.

I've built the bare minimum components (amp>resistors>transformers) in order to isolate the problem. So far:
- The amp works. I connected some headphones and the output isn't super loud but it definitely works.
- One transformer has a very weak output. I don't have a multimeter but I've tested it with cocodrile clips connected to the headphones; I was expecting to hear loud DC noise or the headphones exploding or something, but it outputs the same audio... except with lower volume than the amp.
- The other transformer seems to be dead. My previous amp was faulty (see above itt) so maybe it fried this transformer too?

I've double-checked all the connections and even looked for non-insulated spots even when my box is made of plastic lol.
Not sure what else to try. Any ideas?

It will be easier to troubleshoot this for the experts (and find your mistake or the defective part) if you take a clear picture of all the components connected together.

[steelhorse545]

Hey there. I finally built this thing and it doesn't work at all, perhaps someone can help me.

I've built the bare minimum components (amp>resistors>transformers) in order to isolate the problem. So far:
- The amp works. I connected some headphones and the output isn't super loud but it definitely works.
- One transformer has a very weak output. I don't have a multimeter but I've tested it with cocodrile clips connected to the headphones; I was expecting to hear loud DC noise or the headphones exploding or something, but it outputs the same audio... except with lower volume than the amp.
- The other transformer seems to be dead. My previous amp was faulty (see above itt) so maybe it fried this transformer too?

I've double-checked all the connections and even looked for non-insulated spots even when my box is made of plastic lol.
Not sure what else to try. Any ideas?

There’s so many unknowns about exactly what you’ve got that it’s hard to say ;)
Are you using a tpa3116-based board (like your original, iirc) ? They’re intended for speakers of 4-8 ohms , but your headphones could be anything from 8 to 80 (or even 600 for some) - so if they’re not 8 ohm, then that could explain the low volume at the amp output since there won’t be sufficient current to drive them (and is the rest disconnected when you try this ?).
(The other thing is the transformer config - even with a human connected ! - is proabably going to be some other than 4-8 ohms, so not sure how the 3116 will react if there’s nothing connected to the final output)
Headphones on the amp output - presumably the two channels sound about the same level ?

The amp side of the transformer is intended to be lower voltage, higher current, whilst the other (output) side is higher voltage, lower current - so it may be that connecting headphones there (personally I wouldn’t, but if you’ve not blown them so far.. ! ) would also suffer from a similar effect as with the amp only - it’s high voltage, but low current , but the relatively low resistance of the headphones connected here means the voltage across the headphones still drops, so even less current in the headphones. But this is into ‘unintended use’ territory, so it’s hard to say exactly how the volume will compare to the amp side
BUT you expect the two channels to be consistent- what happens if you swap left and right at the amp outputs ? - does the low volume remain wrt the transformer, or the amp output driving it ?

Hopefully the above made some sense -it’s as much a brain dump for me ;)

[ramen]

Thanks for the replies.

Yeah, using the headphones as a testing device was a bit of a desperate move, but since I wasn't getting any feeling at all...
I haven't tried swapping the transformers from one channel to another, but their volume is roughly the same (I discovered a bad solder point and now both of them seem to work, albeit at low volume).
I just checked and the headphones' datasheet and they are 24 ohm, in case that helps troubleshooting.

I also tried swapping the PSU cable in case I was supplying -12v to the amp. No change in either case.

Here's the current setup, not sure if it helps since there's only a few components. The outputs I'm using are the Common and 0.65W of the transformers.

[Electro]

Ramen,

If you connect speakers directly to the output of the amplifier(with the resistor and transformers not a part of the setup), that amplifier should have enough power to be uncomfortably loud for the ears, which for headphones would probably melt/damage the voice coil in the headphone speakers. If that's not happening, perhaps the volume is set too low at the input or volume not turned up enough somewhere?

In your picture it looks like the resistors and transformers are connected in the right places to where I'd expect you to feel something. Maybe others with a Visaton can chime in, but the 8 ohm .625w tap setting would be about 45 turns? ..which could be a little high but I'd still expect it to work but you might be limited for how much max power you'll get out of it, the 1.25w setting would be 32 turns if my notes are right, which I think would match your resistor value better.

What sort of device do you have connected to the input of the amp? Also you said this amp fried your laptop? Are you sure the amp isn't still fried and that you might be working with bad equipment?

[ramen]

[snip]

Thanks for the reply.

The previous amp was faulty, this is a new one of the same model. The amp volume output isn't massive, but it isn't quiet either... indeed it gets a bit uncomfortable at max volume.
I tried it with an old mp3 player at high volume (but not at max, it had a safety cap), and also with my laptop. Didn't get any feeling either way.
At first I tried the 1.25w output of the transformers fearing that the 0.65w would shock me, but there was no difference.

[ramen]

Solved it! Thank you guys for the help.
Electro was right about the volume. I boosted way up (about 70%) the level in both the laptop and the amp, and it worked. The mp3 player simply won't cut it.

I just did a quick test, but it seems that my (cheap) USB audio interface is super noisy and distorts the sound in a very unpleasant way. When plugged directly to the laptops output the signal felt cleaner, more distinct... and less painful.
Is there anything I can do about that? (other than buying a better usb interface)