Transformer for LM3875

[hayden123]

Hi there,

I'm looking for a transformer to power an LM3875 based amp driving 6ohm speakers. However, I'm not sure what transformer to buy since the only toroidal transformers I can find in New Zealand are either 18V or 25V at 225VA. According to the datasheet for the LM3875 on the National website, 25V would be too high since it would result in 25*1.4=35V rails which wouldn't work too well with 6ohm speakers. Would 18V be too low or should I import/custom-make a 22V transformer? I'm fairly new to this so any help would be much appreciated.

[Nordic]

Are the speakers measuring 6 ohm, or specified as 6 ohm... ?Many 8 ohm speakers have a series resistance of about 6 ohm... however the impendance is still 8 ohm.

If this is the case 25V will work nicely and you can even go higher... remember the chip itself also drops a few volts.... just don't use too small heatsink.

"To determine the maximum supply voltage the following
parameters must be considered. Add the dropout voltage
(5 volts for LM3875) to the peak output swing, Vopeak, to get
the supply rail value, (i.e. + Vopeak + Vod) at a current of
Iopeak)." from datasheet.

[daveiacov]

Hi
check out this site
http://www.plitron.com/shopping/shop...=Surplus+Stock
Dave

[DocLorren]

Hi,

The 18 volt toroid will work perfectly fine as well. At 225VA you will have a bit more current at your disposal compared to the 25V version, not that it will help outright power but it's nice to have for heavy transients. I do run my GCs on a 22v 300VA toroid and with my reasonably efficient speakers (91db/w/m) it has plenty of grunt. Let the efficiency of your speakers and your listening preference decide between 18 or 25v. The GC itself won't mind (or change in character) too much.

[hayden123]

Hi everyone,

The speakers are specified as 6 ohm. They're wharfedale 9.1's. To be honest, I don't know what they measure as I haven't bought them yet. I plan to buy an extra pair of 9.1's to go with this amp I'm building when I buy a wharfedale home theatre speaker package. The heatsinks I'm using are pretty big.

Thanks for that link. I had a look there but it seems shipping to NZ would be near impossible. I had a look around some other sites though and found http://www.amplimo.com which has 300VA, 22VAC transformers for NZ$135 including shipping which isn't too bad really. Is there a lot of difference between toroids or should I just go for the cheapest price I can get? I could get the 225VA, 18VAC toroid in NZ for NZ$104 but I'd rather pay a bit extra if it gets me a better transformer that is closer to the voltage I want.

[AndrewT]

Hi,
I don't know the Wharfedale range but the 9.1 is fairly recent.
The specified 6ohm impedance is the average over the whole audio band, but the 9.1 could well be a 4 to 8ohm speaker using a 4ohm bass/mid driver and 8ohm treble driver to obtain an efficient matching of sensitivities.

If the speaker really is using a 4ohm driver then the amplifier should be designed for that duty not for the 6ohm rating.

[troystg]

Get two of the following from Steve @ Apex.jr and do dual mono amps.

http://www.apexjr.com/miscellaneous.html



The last one in red....


Chia Yu Co Ltd
Input: 2 X 120Vac 50-60Hz 3" Dia X 1 1/2" Tall
Output 42Vct 21V- 0 - 21V
$12.50ea

EDIT: I actually use ONE of those for STEREO pairs of computer speakers.. Would not push one on a stereo pair for Hi-Fi use but 2 in mono config would be fine.

[hayden123]

That's a good point about the impedance. I'll ask wharfedale if that is the case. Buying the speakers first would involve putting the amp on hold for about a month while I get the money together.

I couldn't find a VA rating for those transformers and judging by the size compared with the other two in red, they must be under 100VA. Would they provide enough current for the amps?

[troystg]

They are ~80VA which is fine for music. Unless you plan on DJ'ing with them then I'd advise against them AND a single chip chipamp.

If you were using the LM3886 then it might be tight, but the lower powered LM3875 in not a concern as long as you use one Toroid PER chip.

PS, I usually add a pair of those toroids and a pair of 3875 chips to plate subwoofer amps for sat /sub systems for friends.

[alleycat]

I got my toroid from Harbuch in Sydney. They're great to deal with and they'll wind whatever toroid you like.