a75 75volt rails?

I scaled my A75s up to 58V output and 65 volt front end rails. The issues are component voltage ratings and dissipation. There are also some potential stability issues due to the way mosfet internal capacitances are affected by voltage applied. I haven't had an issue here (although poor layout the first time around :xeye: cost me a rebuild).

I used IRFD220/9220 for the input differentials. I ended up blowing a set due to oscillation ad replaced with IRF610/9610. I had intended it as a temporary replacement, but since they handle the dissipation better and I didn't notice a sonic difference they stayed. Over 65V you're really pushing the IRFD parts too hard, unless you reduce the bias current with potential sonic effects.

Another spot to watch is the current setting resistors for the zeners. You'll want to run them at around 5 mA, so change the values and dissipation ratings appropriately.

To get front end voltage higher than the output rails, the A75 uses a voltage doubler. Of course as the rails rise, doubling them means even more power dissipation in the regulator. A good way to minimize the problem is to use a 30VA or so 0-15, 0-15 transformer with one end tied to each of the main transformer to give you an extra 15V for the front end. Recitfy this and feed it to your regulators set for 10V over your output rails. Again watch the dissipation in the regs.

NP said that the A75 could get away with as few as 6 pairs of outputs. I take this to mean at standard rail voltages and using TO-3 devices. I use 6 pairs of TO-247s but my speakers present a benign load and I have PLENTY of heat sink. At 75V I'd use 10+ pairs and still ensure that you have plenty of heat sink - especially if you want to try to drive 1 ohm loads.

More than 12 pairs and you might run into drive current limitations since the A75 skips that stage. You'd need to increase the VAS current (and deal with that effect on stability) or add a driver stage.

Speaking of heat, I reduced the bias on mine to about 25 mA per device due to excessive room heating. I moved the Vgs multiplier to on top of an output device to help temperature compensation. It seems a bit overcompensated, but better than under. If you leave the Vgs where it is, stay with at least 100 mA per output pair, as NP suggests for thermal stability.

With 10 ouput pairs that will give you around 32 watts into 8 ohms class A, before you go AB - probably plenty for most applications. With 75V output rails that will be about 200W dissipation per channel including front end and regulator dissipation.

As for power output, if your front end is running at 85V, I'd assume that you can get to within 2 volts of your output rails. Assume they'll sag a bit or have a few volts of ripple. So you should be able to swing maybe 5V less than output rails. You can do the math from there, but your 1 KVA transformer starts to look a bit small to power two channels ;)


2005-11-18 1:01 pm
hi thanks for the info.

i have 2 1kva transformers so it will be be one per channel.

you think they are enough?

i was thinking about 10-12 pairs of mosfets per channel.

my speakers are 1.3-2 ohms at 30hz but i have to measure to know for sure.

with these rails is it going to be around 150-200/8 ohm class AB?

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
With the 200V ratings on the 610's and 9510's and the
output devices, I would expect it to work up to +/- 100V.

As BobEllis suggest, a few minor resistor value adjustments
are in order.

As the voltage goes up, the bandwidth tends to increase slightly,
so you have to be prepared to adjust the compensation to avoid
oscillation. Remember - Gate resistors on ALL Mosfets, and
for most stable operation, the folded cascode version gets the

P channel parts in the diff pair and Vas should be Harris or
Fairchild for best performance, but of course the originals were
IR, so clearly they work...

With 75V rails, if you have enough front end voltage that you can swing 70V you'll be pushing 300W at 8 Ohms. 12 pairs of outputs minimum if you intend to push hard at 30 Hz into your 1.5 ohm load.

I am assuming that your transformers are about 55VAC unloaded. If they are 75VAC you're into a whole new kettle of fish as you'll be pushing over 100VDC unloaded.

The A75 articles on the Pass DIY website include excellent tutorials on how to determine the heat sink required and the bias current required for Class A operation into your load. Actually, all of Mr. Pass' articles are excellent learning material.

Yes, one 1KVA 55-0-55 transformer per channel is sufficient. Rule of thumb is the transformer should be rated at 2-3 times the expected dissipation. Look for one of the inrush limiters that use a relay and resistors to avoid blowing your circuit breakers when powering up.

BTW, if using the audioxpress boards, there are a couple of minor errors in the silkscreen. Read through the other A75 threads and youll uncover these as well as learn a lot.

Enjoy it, it is a great amp, and a fun project (My second DIY amp and the first to make it into a proper enclosure).

/emboldened by Papa's approval of my previous post. :cool:/
Don, I used standard values in the CCSs, so around 9 mA total per pair for ~.3W in each at 65 volt rails. Going to higher rails or currents would probably require a small heat sink.

The original IRFD devices are rated at 1W so they should be safe at this level. Mine have been in daily operation for 5+years.