160va dual 24v toroid for lm3875?

[jordyyy]

Would two 160va dual 24v toroids from Apex Jr.Home Page be good for the premium dual mono lm3875 from audiosector on 6ohm speakers? Is there a better option for a similar price?

[Redshift187]

24V transformers would give you about 34V rails, that might be a bit high for 6 ohm speakers, what does the datasheet say?

[jordyyy]

Datasheet just has 8ohm specifications:

DESIGN A 40W/8Ω AUDIO AMPLIFIER
Given:
Power Output 40W
Load Impedance 8Ω
Input Level 1V(max)
Input Impedance 100 kΩ
Bandwidth 20 Hz–20 kHz ±0.25 dB
Equations (5), (6) give:
40W/8Ω Vopeak = 25.3V Iopeak = 3.16A
Therefore the supply required is: ±30.3V @3.16A

The build manual from audiosector says:
"When looking over the curves a speaker with a nominal impedance of 4 ohms tend toward 25V rails with a reasonable margin of safety, while voltages above 35V are still well within the range for 8 ohm speakers. This shows that transformers with 18-22V secondaries are well within reason for many common commercial and DIY speakers. A transformer with 25V transformer secondaries can also be successfully with less of a safety factor."

[jordyyy]

I found toroidal transformers with 200va power and 20v or 22v output for $29 each from Antek. I assume these would be more suitable? I really have no idea what any of that means ive just been trying to research a lot.

[Redshift187]

On page 10, graph Output Power vs Supply Voltage has curves for 4, 6 and 8 ohm speakers. 6 ohm shows a max output at 33V before falling off, so less than that is a good idea.

A 22V transformer would give you about 31V rails, which is what I would go with as long as you aren't intending to run 4 ohm speakers with it in the future. That voltage will work quite well with 8 ohm speakers if you want to use those at some point.

[jordyyy]

thanks for the input.

So, 200 va per channel is not overkill? would the 100 va be better? (there is no in between)

[AndrewT]

160VA or 200VA would power a stereo amplifier adequately.

100VA would just about power a monoblock.

160VA for a monoblock is good, but that is two transformers and two PSUs and two chassis. Getting expensive.

[Redshift187]

200VA per channel is plenty and at that price I wouldn't go with 100VA per channel.