DIY Stereo Stim with amp,transformers and maybe resistors

[picofarad]

Hi,
While I have built electronic devices from kits several times I want to keep this simple. After reading almost all the post in this froum I think I understand things.
Please let me know if the following makes sense or I am missing something. (I will be using this with the teases which have an element of torture/punishment/pain so I want to make certain it has enough power to accomplish that).

• will be getting the input from one of the teases here, it will run on my PC which has both BT as well as a sound card with a line out jack.

• I'm looking at this amp on Amazon. It comes with a 36VDC power supply but since it is rated for max power using a 48VDC power supply I might try that one. https://fosiaudio.com/products/bt20a-pr ... -amplifier

• I would use a pair of transformers like these: https://www.osdaudio.com/sp70t-20w-8-oh ... depot.html

• Not sure about the need for power resistors both in series and parallel. I have read mixed reviews. I have resistors I can try and I believe this amp has protection circuits which would work with the load the transformers would supply due the resistance/impedance of the primary winding.

• And if I read everything correctly the transformers would provide: isolation, voltage step up, and would be wired backwards or opposite of what they are generally wound, using the 4 or 8 ohms on the amplifier side and the highest wattage on the secondary or load side.

Thanks!

[edger477]

You seem to have figured everything out already. I built several DIY (there is even a thread here about 4-channel box I built), and only comment is that this amp is too powerful for estim. You'd probably have better results with 24V since you will be able to use larger portion of volume pot (not to the max for sure but maybe 40%). Or just use weaker amp.

[picofarad]

You'd probably have better results with 24V since you will be able to use larger portion of volume pot (not to the max for sure but maybe 40%).

Thanks for the reply.

I think I might have a 24vdc power supply. I believe it comes with the 36vdc supply. And if I don't fry the amp and if it doesn't work, I'll use it to power some bookcase speakers I have.

[Hestius]

You seem to have figured everything out already. I built several DIY (there is even a thread here about 4-channel box I built), and only comment is that this amp is too powerful for estim. You'd probably have better results with 24V since you will be able to use larger portion of volume pot (not to the max for sure but maybe 40%). Or just use weaker amp.

Would this one work better in your opinion?

https://fosiaudio.com/collections/2-cha ... 1696&_ss=c

Or did I misunderstand which item you were objecting to?

[edger477]

You seem to have figured everything out already. I built several DIY (there is even a thread here about 4-channel box I built), and only comment is that this amp is too powerful for estim. You'd probably have better results with 24V since you will be able to use larger portion of volume pot (not to the max for sure but maybe 40%). Or just use weaker amp.

Would this one work better in your opinion?

https://fosiaudio.com/collections/2-cha ... 1696&_ss=c

Or did I misunderstand which item you were objecting to?

Ye, looks better. Actually I used it (well it was branded differently, as "Breeze Audio" - https://www.amazon.de/-/en/gp/product/B07KPSYHWF, it is now powering two Edifier speakers), it was still very strong, I never got to 50% of volume. I am now using this one for estim, at 12V (so half power) https://www.amazon.de/-/en/gp/product/B087FWWVB6/

And I also use serial resistors (4 Ohm, + 4 Ohm transformer leads, so practically only half power goes to transformer, other half is converted to heat in resistor), and still I only get up to about 50% of volume on amp, maybe 65 for some large internal electrodes.

And yes, I don't use parallel resistors, these are not needed with modern amps.

[picofarad]

And I also use serial resistors (4 Ohm, + 4 Ohm transformer leads, so practically only half power goes to transformer, other half is converted to heat in resistor), and still I only get up to about 50% of volume on amp, maybe 65 for some large internal electrodes.

I have never understood why these resistors are used. Any explanation or citation would be much appreciated. I can get a pair rated at 100watts for about $6US. If I do need these resistors is 100-watt rating OK? Or do I need to find a set of resistors, so I have 200-watt rating?

The amps both of you cited seem very good. I'm likely to stick with the one I cited. If I do use it for a pair of bookshelf speakers and feeding it with BT from a phone, it would ideal.

I do note that I could also use an 18vdc power supply (it's rated from 18-54 vdc and comes with a 36vdc 5amp PS. I never like taking any amp over 50% on its volume control.

Finally, any comments on the transformer I selected? It is larger (20-watt vs 10-watt) than what most of the DIY projects use for the transformer.

Thanks!

[edger477]

I have never understood why these resistors are used. Any explanation or citation would be much appreciated. I can get a pair rated at 100watts for about $6US. If I do need these resistors is 100-watt rating OK? Or do I need to find a set of resistors, so I have 200-watt rating?

The amp's power is halved when driving 8 Ohm load compared to 4 Ohm load. So serial resistor reduces max power that can go through transformers, gives you better range for volume control pot on amp (since you can dial it up more), while you can still use the best windings ratio on transformer (the 4 Ohm one).

I use 25W resistors, they are more than enough, but best to find ones with metal casing that you can screw to metal build plate so they don't get too hot.

[diglet]

The resistance of the transformer ranges from 0.5ohm to 20 ohm depending on what's attached to the other side and transformer saturation. The amp will blow itself up if you try to drive a load less than 4 ohm (exact value differs per amp). A serial resistor is needed to prevent the load from ever dropping below 4 ohm.

[picofarad]

I can get a pair rated at 100 watts for about $6US.

The amp's power is halved when driving 8 Ohm load compared to 4 Ohm load. So serial resistor reduces max power that can go through transformers, gives you better range for volume control pot on amp (since you can dial it up more), while you can still use the best windings ratio on transformer (the 4 Ohm one).

I use 25W resistors, they are more than enough, but best to find ones with metal casing that you can screw to metal build plate so they don't get too hot.

OK, I'll get the 100watt rated 4 ohm resistor, it is in a metal heatsink. I will put it in series on one of the legs feeding the transformer. I will use the 4 ohm tap, is that right?

[edger477]

OK, I'll get the 100watt rated 4 ohm resistor, it is in a metal heatsink. I will put it in series on one of the legs feeding the transformer. I will use the 4 ohm tap, is that right?

It is. You are overkilling it but it will work :)

[picofarad]

Thanks again.
I've ordered the amp. a pair of transformers and the 4 ohm resistor. I'll let everyone know how it works when I get it setup and tested.

[47dahc]

You seem to have figured everything out already. I built several DIY (there is even a thread here about 4-channel box I built), and only comment is that this amp is too powerful for estim. You'd probably have better results with 24V since you will be able to use larger portion of volume pot (not to the max for sure but maybe 40%). Or just use weaker amp.

Would this one work better in your opinion?

https://fosiaudio.com/collections/2-cha ... 1696&_ss=c

Or did I misunderstand which item you were objecting to?

This is the exact one I use and depending on estim file or restim settings, I'm usually around half volume +/-.

[picofarad]

Would this one work better in your opinion?

https://fosiaudio.com/collections/2-cha ... 1696&_ss=c

Or did I misunderstand which item you were objecting to?

This is the exact one I use and depending on estim file or restim settings, I'm usually around half volume +/-.

This is good to know. Are you using transformers and if so, what leads are you using on each side?
As to resistors what values and configuration?
Finally, what is the voltage of your power supply with that amp?
Thanks!

[47dahc]

Would this one work better in your opinion?

https://fosiaudio.com/collections/2-cha ... 1696&_ss=c

Or did I misunderstand which item you were objecting to?

This is the exact one I use and depending on estim file or restim settings, I'm usually around half volume +/-.

This is good to know. Are you using transformers and if so, what leads are you using on each side?
As to resistors what values and configuration?
Finally, what is the voltage of your power supply with that amp?
Thanks!

From the amplifier each channel positive goes to a 5.6 ohm 10W resister which then feeds to the 8 ohm lead of a 70V transformer. Each channel negative goes directly to the common on the same side of the transformer. I use the 1W lead for the electrode positive lead and the common lead for negative. Nothing real crazy. Four components inside the box. two 5.6 ohm resisters and two 70V transformers.

[47dahc]

From the amplifier each channel positive goes to a 5.6 ohm 10W resister which then feeds to the 8 ohm lead of a 70V transformer. Each channel negative goes directly to the common on the same side of the transformer. I use the 1W lead for the electrode positive lead and the common lead for negative. Nothing real crazy. Four components inside the box. two 5.6 ohm resisters and two 70V transformers.

Can't remember why I went with 5.6 ohm resistors or why I went with the 1W lead on the transformer. I also had two 10 ohm resisters somewhere in the mix but took them out. I'm sure I read it somewhere. Before I bought the Fosi amp I did have a TDA7297 15Wx15W amp board but the volume knob eventually broke. Instead of redesigning my whole stim box, I just took it out and added input jacks to it for the Fosi amp. Haven't had an issue with it yet and have had it for 3 years now.