Toroidal VA rating

[Nate805]

Would it be possible to use a 800VA toriod to power two BPA200s (8 total LM3886s), or would that be insuffecient?

how do i calculate such things in the future if I know the voltage of the toriod & the max output power of my lm38XX based chip?

[jacco vermeulen]

Minimum : rail voltage times continuous output current in 4 Ohm.

[pacificblue]

You can power an amp with a transformer that is smaller than the desired amplifier output power. That means to use a transformer as big as the average output power. Peaks will be delivered from the power supply capacitors, which should be accordingly big.

Dr. Linkwitz's Pluto works that way with 3 LM3886 at ± 30 V but with only a 50 VA transformer. In that case the speaker was known to the designer and he could estimate the necessary average power and use 2 x 60.000µF in the power supply. Of course Dr. Linkwitz is also conviced that more than 50 W are not needed at home.

The difficulty is to find out, how much power you use at an average, because that depends on your listening level and on the speakers you use.

Reasonable transformer sizes are 1-3 times the total maximum ouput power of the amp(s).

800 VA will be sufficient for most situations. Bigger can be better in some situations. AN-1192 mentions 2 x 385 VA for one BPA-200 as far as I remember.

[AndrewT]

Hi,
two BPA200 can give a total maximum output of around 400 to 500W.
The transformer can be sized between 400VA and 1000VA.
Some undersizing of the transformer can be compensated by providing more than optimal PSU smoothing capacitance. This works particularly well for normal music signals where transient peaks are often 20dB above the average level being listened too.
However, why a BPA200? is it to power a bass only speaker? This is a completely different situation from normal wideband music. Sustained and continuous output can be demanded for many seconds and this type of duty should be designed for as if it were a PA style amplifier, if you intend using it that way.

Finally, there are better ways of getting 200W + 200W, than using eight chipamps.

[pacificblue]

Originally posted by AndrewT
two BPA200 can give a total maximum output of around 400 to 500W.

Assuming 8 Ohm loads.

[AndrewT]

and 4ohms and 5ohms and 6ohms and 7ohms.

[pacificblue]

So you are convinced they cheated with AN-1192?

[AndrewT]

Originally posted by pacificblue
So you are convinced they cheated with AN-1192?

no,
60W into 8ohms becomes ~240W into 8ohms when BPA is implemented.
60W into 4ohms becomes ~240W into 4ohms when BPA is implemented.

I see BPA200 as just that implementation. It can do 200W into any chosen impedance from 4ohms to 8ohms, it's just a case of adjusting the PSU voltage to suit the load.

[pacificblue]

This is taken from National Semiconductor's AN-1192 (DeCelles and Huebner, June 2004)

[wakibaki]

Originally posted by pacificblue
You can power an amp with a transformer that is smaller than the desired amplifier output power.

...dependent on quiescent power drain (class), maximum efficiency and usage.

Most transformers deliver their best efficiency (lowest waste heat for a given power) at about 80% of rated load, so I'd still want a VA of 125% of average power.

Great big caps and weedy trafos bring other problems in their train, too.

w