Don't like the prices I see on transformers. Are SMPS a cheaper way to go?

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Plecto

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Hi. I have become quite fond of making power amplifiers and I've recently made a LM1875 amp. For that design I bought a 100VA transformer, but I see that these can become quite expensive For my next project I wan't to use 2xLM3886, but the cheapest suitable transformer I see on digikey cost like 70USD! I was wondering if it's worth to start looking into SMPS?

I have very little experience with this though. I toyed around with a boost converter a while back and got it to work, but I found it hard to make it usable for anything. When it comes to stepping down 230VAC to something like 80VDC I'm almost clueless. I've looked at buck converters, but as I have understood, these non-isolated designs are bad in case of a failure so a transformer should be used instead of a simple inductor? Could anyone help me by pointing in the right direction?
 

If you only buy several transformer, price is expensiver.
If you buy mass products, price is cheaper, because transformer is customized normally,raw materials for transformer has MOQ requirement, normally MOQ is 5K for raw material in factory.

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If you only buy several transformer, price is expensiver.
If you buy mass products, price is cheaper, because transformer is customized normally,raw materials for transformer has MOQ requirement, normally MOQ is 5K for raw material in factory.
 

Again its a matter of quantity. I bought a 15V 4A replacement battery charging PSU for a lap top recently, cost £10 (16$?), So 5 of these would give you 75V@ 4A for £50 (80$?). You need to fiddle the mains distribution and You would be lucky to get exactly the voltage you want.
Frank
 

There is the technique of:

(1) Running mains AC directly into a full-wave diode bridge, giving you 340 VDC. (This will drop as you add a load.)
(2) Put that through a flyback converter at high frequency. This uses a smaller (and cheaper) transformer.
(3) Install a voltage regulation circuit which will deliver 80 VDC for all combinations of supply and load.

It gives you isolation from mains AC. It involves more work.

Seeing Digi-Key's high prices, I'd avoid them and go with less expensive mail-order houses such as All-Electronics.
 



This is too much for 100W transformer, in my country 100W (Torus) is 27eur with all taxes.

Check on this site:
https://www.tme.eu/en/katalog/#id_c...444,101,449,445,74,456&used_params=101:24545;



Maybe you choose some special transformer from Digi-Key?


Always look to use standard isolated transformer primar/secundar, avoid usage of autotransformers.
 

I believe I need at least 200VA for a LM3886 amplifier. Each can deliver something like 50W and it's around 50% efficient. The cheapest 200VA power transformer (I'm looking in the right directory) at digikey is 50USD, but it has only one secondary coil. The cheapest suitable one is this one: https://www.digikey.com/product-detail/en/182P22/182P22-ND/454578 which cost 73USD.

I'm going to look at tme.eu and all-electronics and see if I can find some cheaper ones. A BIG transformer in an amplifier always makes the amp look powerful so I'm not that eager to turn to SMPS
 


Okay. tme.eu seems to have a very limited range of center tapped transformers while allelectronics.com only has 13 different transformers in total and all of them are meant for 115VAC and not 230VAC.
 

I have bought transformers from mouser before. Mouser seems generally more expensive than digikey. A 100VA transformer is about half as expensive on digikey while the 200VA transformer was only slightly cheaper at digikey.
 

I have thought about getting second hand transformers, but it's very hard to get the exact VA rating along with the right voltage, no to speak of center tapping, multiple secondary coils etc. Taking transformers out from second hand equipment can also be difficult since these are in my limited experience rarely marked I am also looking to sell amplifiers in some close or distant future, if that I happens I need to have a constant source of psu's.

What's wrong about making SMPS's?


I have looked the flyback converter and I believe I understand the concept behind it. What I don't understand is how the feedback works. This has to be isolated, so how can the output voltage be compared to the input voltage if their grounds aren't connected together?
 

I'm afraid that your product cant be competitive on market with price if you plan to sell that. Just metal case of your aplifier will cost almost half as finished product in shop.

You can try with SMPS why not, but I think that is smart before you enter in deeper expenses to read other people expirience with that.
 


Feedback is done through an optocoupler, based on the brightness of the internal LED.
 

Plecto said:
I have thought about getting second hand transformers, but it's very hard to get the exact VA rating along with the right voltage
Click to expand...

You can get a slightly higher value of VA, no problem to have more power available



You cannot just go out and sell lm3386 amps (by the test cicuit of the pdf), times are tough, and its not hard to get a lawsuit (ie from a customer who claims that got electrocuted)

What's wrong about making SMPS's?
Click to expand...

smps are quite more complex than typical transformer, do require more knowledge than of making an amplifier, plus equipment (maybe oscilloscope) if you are innovating
 

I'm afraid that your product cant be competitive on market with price if you plan to sell that. Just metal case of your aplifier will cost almost half as finished product in shop.
Click to expand...

The LM1875 amp I recently built cost me like 55usd including a self made plexiglass box and all. A LM3886 amp will cost a little bit more, but not that much if I can find a reasonable price for the transformer. To find an equally powerful stereo amp on the marked here in Norway you would have to pay something like 350usd.

I'm not thinking of selling these anytime soon. I would have to make 100% I know what I'm doing, that it's safe, durable and all that. I'm not looking to make a living out of it either, it's just for fun really.

smps are quite more complex than typical transformer, do require more knowledge than of making an amplifier, plus equipment (maybe oscilloscope) if you are innovating
Click to expand...

I got an oscilloscope for christmas actually, a Rigol DS1052E. It's no harm in learning about SMPS, that's why I made this thread really. I will naturally start with small PSU's and then maybe make one that's big enough for a power amplifier. Is there a flyback circuit I could look at? Just the simplest form of an isolated SMPS? I've come up with this schematic involving a PWM controller. Am I on the right track?

https://www.circuitlab.com/circuit/673yp4/isolated-smps/

Don't mind the component values or the pinout of the IC.
 

One SMPS compromise might be to get a PC PSU like this :- https://www.quietpc.com/zmx00-gs, £33 for 450W. Strip the 12V 24A windings rectifiers smoothing cap and replace with 80 V components with an added linear stabilizer (LM 317). You get a lot of bits and pieces fro your money ! case/fan inverter/mains connector. Also standard case and connections so alternatives are available.
Frank
 

Hi chuckey. Do you mean that those components are 12v rated and if replaced by ie 80v followed by regulator, r will output a constant 80 v ?
 

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