lm1875 advise

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Hi all.
I'm a total numbskull with electronics and need some help figuring stuff out. My intention is to build a lm1875 amp to run electrostatic headphone transformers as it seems like a decent match and is cheap to build. I ordered a pair of those $2.50 kits from ebay here and plan to use this rectifier with my 28.4 +28.4v 2.67a Avel toroid transformer. I know the output of the transfo is very close to the lm1875 upper limit and that it would prefer 24v, with this in mind, I also have some 40v 3a buck converters that can bring the voltage down to where the amplifier would be happy.

Does anyone foresee any potential problems hooking all this together and having a functional amp or any changes I should consider? I can't afford a different transformer, so the Avel has to stay. I'm trying to keep the whole thing under $20. It's been a good number of years since I last made an amp and I've forgotten most of the little I knew back then and $20 is all I can afford to 'blow' if you know what I mean :bomb:

Cheers :)
 
Hi there,
You describe the transformer as 28.4V/28.4V/2.67A. I assume this means two secondary windings for a symmetrical power supply? if you rectify a 28.4Vac winding you have more than 40Vdc. LM1875 should not exceed +/-25V and operation around 20-22V is better. You will end up with double the voltage. Your rectifier board looks fine if it gets some 2200uF-3300uF capacitors.

Your Buck converters won't be of much help as they are probably only for positive voltages?

Would it be possible for you to find a 2nd hand 120Vac/230Vac converter transformer? Such a thing should not be expensive and it will bring your rectified voltages down to a right level.

Do you happen to have some Darlington NPN and PNP power transistors lying around?
 
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Thanks for the reply.
The transformer is dual 120v primary/dual 28.4v secondary with center tap. I could just use one secondary. The buck converters are inverters, so will do negative voltages. It's possible that using one secondary, rectified and adjusted to +/- 22v should still have sufficient current, if so I can get a pair of inverting boost converters for a couple of $.
I don't have any power transistors here.

Does that sound plausible because I've looked at alternative transformers and they are still outside my current budget
 
You need a tiny inexpensive transformer, specially because job is very light: driving headphones with a few milliwatts of power.
I am quite certain that you can get a 12+12V 1A transformer for way less than that and LM1875 will be *happy* with +/-16V DC rails, specially if driving headphones.

Don´t needlessly complicate this into a Rube Goldberg machine, and to boot wasting ... sorry, ... "spending" ..... more than needed.
 
They are electrostatic and electret headphones being driven through transformers, the power to run them properly is preferably over 20 wpc. Some are incredibly inefficient. These are like the Quad esl of headphones.

Anyway, I've been reading more trying to figure out rectification and it seems that with a center tap I can get full wave rectification with just two diodes. One on the + cycle and one on the - cycle, it cuts the voltage in half so it'd end up about 14v or so.
 
What is the wall-socket voltage where you are? 120Vac or 240Vac?

I'm on 120v.
Just to give an idea of needs, I corrently run the 'stats from a ta2020 t-amp which only gives out barely 10 useful watts at 8 ohms and It's frequently clipping. The lm1875 is an attempt to get a proper 20+ watts. Ideally running it at 24v to squeeze as much as possible from it. I know that it's possible if I can get the psu side sorted out.
 
So, if 28V secondary value is given for two 110VAC primaries connected in series, paralleling the primaries parallel and in phase you will get 14VAC secondaries which are very suitable for LM1875.


As in read it, the other way around:

Primary windings in parallel and phased correctly -> 28Vac.
Primary windings in series and phased correctly -> 14Vac.
14Vac -> 20Vdc, just what is needed.
 
Hey all.
I'm just going to get a rectifier for the lm3886 chip instead.
Amazon.com: NOYITO 25A Dual Power Rectifier Board Power Board AC 0-30V / DC+-0-50V Rectifying Flat Bridge Suitable for TDA7294 TDA7293 LM3886 Amplifier Dedicated Power Board: Home Audio & Theater
NOYITO 25A Dual Power Rectifier Board Power Board AC 0-30V / DC±0-50V Rectifying Flat Bridge Suitable for TDA7294 TDA7293 LM3886 Amplifier Dedicated Power Board

With more input headroom I can use the Avel 28.4-0-28.4v transformer directly and adjust it down to 24+0-24 via the bucks and have enough output headroom for switching to the lm3886 if needed later :)

Sound like a reasonable plan?
 
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