Another BPA 300 Build thread...newb perspective

[pacificblue]

Originally posted by peranders
2 x 30 VAC toroid transformer means 30 VAC at full load meaning more than 42 VDC and certainly more at idle state. 2 x 24-25 VAC would I recommend and at the most 2 x 28 VAC.

Funny, usually it's me advocating lower supply voltages.

At idle the IC withstands ±47 V. When loaded the voltage will sag to much less than the permitted 42 V DC. So 30 V AC is safe with high enough impedance speakers.

Originally posted by peranders
Remember that we talk about deciBells in output power (you won't notice the difference) but you will run the amp much more safe and it will also run cooler.

That is true. So, if power is no issue, but heatsinking is, why build a BPA in the first place?

[peranders]

Originally posted by pacificblue
That is true. So, if power is no issue, but heatsinking is, why build a BPA in the first place?

Indeed! It's probably better to use LME49710 etc and a discrete output stage.

Originally posted by pacificblue
At idle the IC withstands ±47 V. When loaded the voltage will sag to much less than the permitted 42 V DC. So 30 V AC is safe with high enough impedance speakers.

I think no design engineer would judge the situation like you'll do. What happens if you experience slightly higher mains voltage?

[pacificblue]

Originally posted by peranders
I think no design engineer would judge the situation like you'll do.

The authors of AN-1192 were not engineers?

[peranders]

The app note is a bit contradictive. The recommend max +- 37 VDC but thet have a 2 x 30 VACrms transformer.

[tryonziess]

Halo6,
From experience I will say that a transformer with 20-22 volts will make you much happier in the long run. If you read the original thread and the information at Shine7.com you will note that Alexw88 makes several statements regarding the heat output from this design. I used 625va 25-0-25 transformers in my first BPA 300 amp and it gets hot as hell. Like Peranders said a few extra volts at the mains and you are adding still more heat. I am speaking from past experience. The 25+ volt trannies will work you just do not need them. The LM3886 transistors, of which there are only 3 per side, do not offer a tremendous amount of surface area for cooling. Most 300 watt amps have twice as many output devices for this power level.
You are buying very nice quality components from what I have read just do not make the "more volts is better mistake" This little chipamp has plenty of grunt with 28-30 volt rails. It is a balanced/bridged configuration which means the speakers see 4 times the output as conventional power and a ground speaker connections.

Tad

[juluska]

Originally posted by tryonziess
Halo6,
From experience I will say that a transformer with 20-22 volts will make you much happier in the long run. If you read the original thread and the information at Shine7.com you will note that Alexw88 makes several statements regarding the heat output from this design. I used 625va 25-0-25 transformers in my first BPA 300 amp and it gets hot as hell. Like Peranders said a few extra volts at the mains and you are adding still more heat. I am speaking from past experience. The 25+ volt trannies will work you just do not need them. The LM3886 transistors, of which there are only 3 per side, do not offer a tremendous amount of surface area for cooling. Most 300 watt amps have twice as many output devices for this power level.
You are buying very nice quality components from what I have read just do not make the "more volts is better mistake" This little chipamp has plenty of grunt with 28-30 volt rails. It is a balanced/bridged configuration which means the speakers see 4 times the output as conventional power and a ground speaker connections.

Tad

What about this transformer. I will build the bpa300 in stereo integrated configuration, only one enclosure and passive preamp:
http://www.antekinc.com/AN-8420.pdf

[peranders]

tryonziess, you have a good point that the LM3886 sees only 4 ohms and this will make the use of 30 VAC even more wasted.

[tryonziess]

Those antek transformers are a very good deal. I have bought about 10 of them and had no trouble to date. They have really good service and the item usually arrives in 3 days.
Another option is to use two completely separate power supplies. One for each channel. You then can buy a couple of 500 or 625 va units and have superior separation. They come with auxiliary windings for controlling relays or whatever. Nice little option.
Have you explored the possiblity of bending the LM3886 leads straight out and mounting parallel with the pcb. I did this and it worked out just fine. No 90 degree elbow to cause layout problems. Just mount everything to the heatsink, nice and flat. You can even run the legs up from the bottom to allow for the standoffs. You then can solder in the small caps on the top. It looks real neat.
Tad

[pacificblue]

Originally posted by peranders
The app note is a bit contradictive. The recommend max +- 37 VDC but thet have a 2 x 30 VACrms transformer.

Not contradictive at all. The supply voltage must remain below ±47 V at idle and below ±37 V at nominal output. The maximum supply voltage of ±42 V at nominal output takes care of the IEC-permitted mains fluctuation of +10/-15 %. A well designed power supply with a 2 x 30 V transformer fits all those conditions spot on.

The BPA application takes those ICs to their limits, therefore it takes accuracy to implement it. And for the same reason it makes little sense to build it, if you do not really want to take the application to its limits. You don't need the additional 6 dB? So build a single chip amplifier instead.

Originally posted by tryonziess
Alexw88 makes several statements regarding the heat output from this design.

Of course he does. He has no fans on the heatsinks and the pictures on his homepage show no air intakes at the bottom and much too small and wrongly distributed air outlets at the top.

Originally posted by tryonziess
This little chipamp has plenty of grunt with 28-30 volt rails.

Exactly, so why go for the BPA in the first place? A single IC with that voltage will deliver ~30 W into 8 Ohm and ~60 W into 4 Ohm. If that is not enough, different speakers will give a better return on investment.

[HaLo6]

Awesome! This thread is getting some great feedback. I will tone down the trafo to a 20-22 VAC... I rarely drive my speakers that loud anyway...