LM3886 Chipamp -- unwinding a 35 VDC 750VA transformer to . . . (what?) 22v?

[Jimmy154]

Is it possible to unwind a transformer 35volt and 750 VA?
Which value do I unwind it to? I have speakers with 8 ohm impedance drivers, maybe 4 ohm tweeters.

Also trying to put this LM3875 chip amp from chipamp.com together, not 100% sure how. Any good links? chipamp.com is no longer a website.
http://diyaudioprojects.com/Chip/Beast/nigc_kit-users_guide.pdf
https://audiosector.com/wp-content/uploads/2019/01/LM3875-kit-instructions.pdf

I put a chipamp LM3875 kit together 20 years ago. Sounded really good with Eton 8-800 2-ways; LPG tweeter.

I also have 171 VA 28.5 VDC transformer, but it doesn't look as nice.

Should I get a RFI power plug for this amp?
I was hoarding some for years, but I think I threw them out.

I like the finished product to have nice parts.

 

[NanoFarad]

22-0-22v(AC) is fine!
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics

 

[Fusor]

Is it possible to unwind a transformer 35volt and 750 VA?

Please be careful doing this. From personal experience I found removing too much secondary can make the transformer primary work more like an inductor, i.e. a dead short. May I suggest you ask about this in the Power Supplies forum here, specifically Tony Tecson?

[The Wayback archive chipamp]Chipamp Electronics

 

[NanoFarad]

Good advice 🙄

 

[joensd]

The speakers have passive xo?
Use the 28.5vdc transformer. This is an ok all around supply voltage for those chips.
If going active use the 35vdc for the 8ohm woofers and the 28.5vdc for the 4 ohm drivers

 

[rikkitikkitavi]

Please be careful doing this. From personal experience I found removing too much secondary can make the transformer primary work more like an inductor, i.e. a dead short. May I suggest you ask about this in the Power Supplies forum here, specifically Tony Tecson?

[The Wayback archive chipamp]Chipamp Electronics

Removing a section of a transformer secondary reduces coupling between prim-sec, and leads to an increased leakage inductance.

This works in effect as a separate inductance in series with the transformer which leads to a voltage drop when current is passing, and lower secondary voltage.

But it is not a short, unless you damage windings when unwinding.

I am not sure about the 35VDC transformer, a transformer produces AC but is it 35VAC or 24VAC rectified to DC?

The actual removal of windings can be more or less straightforward. I am currently doing it on a 2000VA transformer with 12 secondaries ranging from 5 to 80V, various amperage while trying to save a few. It doesn't help that the transformers outer insulation is varnish coated paper or cotton gauze. But hey, you gotta have a hobby 🙂

 

[Jimmy154]

I unwound too much wire and got down to 21 VDC. Should I leave it or add some wire back? I have 8ohm passive speakers (woofer and mid are 8 ohm). Tweeter is 4 ohm nominal impedance.