Question on packaged amp and use of transformers for DIY stereo estim

[picofarad]

I've read through most, if not all, postings here on DIY stereo stim. I've been using a large stereo amp that is connected to the mains. I want something more portable, not connected to 120vac but rather can be battery powered.

I saw this one which seems to be a fairly basic audio amp and being able to "broadcast" via Bluetooth gives me another point of isolation (i.e., I am not physically connected to my desktop computer). https://www.amazon.com/BT20A-Bluetooth- ... B07BQC7GNL This is a Class D amplifier. My current amp is a Class AB amplifier.

I want a device that can deliver painful shocks not just the happy estim signals. I assume this 100 watt per channel amp should be able to deliver, if not please comment.

I will use outboard audio transformers if I can understand the wiring and the purpose. I know they can bring isolation. It appears that they are wired in reverse in most of the diagrams here. Is it that way to stepup the voltage? If so, what is the ideal transformer winding ratio? e.g., 1:10

Does one need current (in series?) resistors? and why?

I have a pretty strong background in electrical and network matters but am getting a bit lost here so any technical pointers would be greatly appreciated.

[picofarad]

BTW, the stereo amp I am currently using (referenced in the original posting) is a Hafler P1500 Trans-Nova Professional Power Amplifier 75W /CH @ 8-Ohms

[Electro]

You shouldn't need more than a 50w class D/T amp to produce painful shocks. Using the recommended turns ratio usually will do that with a 50w amp but if that's hitting a limit moving one tap over should bring you there. I used the recommended tap for my Specco T7010 (blue version) of 0.5w and only ran into maxxing it out with intense hour+ long sessions and with the 1 watt tap, I've never turned the volume to a point where it maxxed out and I wanted the volume any higher and that includes running multiple hours of estim and sessions involving pain.

The topic of making it battery powered is a different thing though, you'd definitely want one of the class D/T amps because they are more efficient, these TPA3116 amp chips can crank it out in my experience as long as the amp has a reasonable heatsink and it doesn't take much, just be sure it actually has one on the amp chip, which it seems almost all amps do. The issue with running it battery powered is that the recommended resistor size and transformer tap setting we use with a 12v power supply was designed with intention to bring the most comfortable stim and also cause the amp to max out before causing serious intensity beyond what you'd ever want even with a long intense session and more importantly to limit max current to limit the chances of injury in case the amp went rogue or an unintended volume change or rogue stim audio went into the amp that it would limit that output to a certain degree. It's not uncommon to be pulling a double digit wattage from the amp although I wouldn't expect that you'd be pulling 50 watts but perhaps <30 watts regularly, so your battery would need to be sized appropriately for this and the input voltage of the power supply or battery, if it sags under load or as the battery voltage drops as it is discharged, it will change the maximum output the rig can produce. Similarly if a 3s lithium ion battery is fully charged at 4.2v per cell, you'll be slightly above 12v and it will be stronger than when it's at it's nominal voltage, but going to another cell with a 4s pack would be more powerful than a 12v power supply when when the battery is approaching a 3v per cell cutoff. These are things to consider, it's possible some safety factors will be ignored or stretched to use a battery based DIY setup and generally the plans for modern Stereostim setups like Tronic's midistim were intended to be used with a 12v fixed voltage from a double insulated power supply with a 2 prong plug.

The amp you linked to would be the same as a 50w amp when connected to a 12v supply and it does appear to be a suitable amp for the purpose, I also use a cased/packaged amp similar to this but mine doesn't have a bluetooth input. For what it's worth I feel safer using a 12v fixed voltage power supply versus a battery powered unit where the voltage could be variable or where I'd be tempted to use a transformer configuration that would output more power than I'd want to handle in order to make the battery last longer when a UL listed double insulated 12v power supply with the rest of the safety factors put in place seems like a safer option to me. The only times I would feel less safe is when there is lightning outside, in that case I unplug or use the DG Lab Coyote instead that night.

[picofarad]

Electro,
Thanks for the thorough reply. I did go back and re-read the posts on building a stereo estim device. I believe I understand why the outboard transformers are installed backwards. My only remaining question are the specified transformers large enough so they don't get saturated? It seems the wattage rating is fairly small.
I also found it very confusing that some of the posters were obviously referring to voltage but continued to refer to it as "power".

The other thing I noticed about these Class D amps is that they have a single volume control, no balance or separate volume control for each channel. With some of the Webteases, using the three-electrode arrangement, the ability to balance the sensation is quite useful.
picofarad

[Electro]

Electro,
Thanks for the thorough reply. I did go back and re-read the posts on building a stereo estim device. I believe I understand why the outboard transformers are installed backwards. My only remaining question are the specified transformers large enough so they don't get saturated? It seems the wattage rating is fairly small.
I also found it very confusing that some of the posters were obviously referring to voltage but continued to refer to it as "power".

The other thing I noticed about these Class D amps is that they have a single volume control, no balance or separate volume control for each channel. With some of the Webteases, using the three-electrode arrangement, the ability to balance the sensation is quite useful.
picofarad

Current is what the nerves feel, voltage is more a function of the resistance. If the resistance is higher such as with dry skin, small, bad, worn out electrodes, the voltage will be higher to get the same current and underlying stim effect which is less comfortable or potentially even underisable stingy sensation(also an unrelated issue with frequencies too low) and painful in a not good way. Generally we stim with 50mA at a peak level and many commercial boxes have designed that as their max. So the actual power to the electrodes isn't high at all. We aren't driving loudspeakers via a public address system, so we aren't using the transformers in their usual configuration and I'm not entirely certain on this part, but I don't think the wattage spec correlates directly to how we are using them.

Tiny transformers saturate easily, the specced ones don't seem to. There used to be a bunch of oscilloscope graphs on Smartstim and lots of lost information when that went down about the why, how, and reasoning behind all of the choices behind the various designs of Stereostim plans.

[diglet]

I have an elma TT IZ1892 transformer. Last month I tried my best to drive the transformers into saturation in order to measure the transformer properties, I wasn't able to. Only when I removed the series resistor and drove the transformer with many times more current than you will see in normal operation, was I able to spot some saturation. It's not a concern with these big transformers.

[picofarad]

diglet,
Thanks for the info on the transformers. I am going to get a pair of these then.
https://www.parts-express.com/70V-10W-L ... er-300-040