Newbie Q: Amp selection for high power/low Z

Ladies and Gents,

Searching has proved fruitless. I fear I must ask you a question you've been asked before, but always in that non-translating, round about fashion.

Which Pass (or perhaps Pass/Thagard) amplifier would be best suited for a new DIYer with a demanding set of speakers to drive?

I have a pair of Thiel CS3.5's. The little beasties want more power than I have. I'm sure that most of you are familiar with their like - 225W RMS, 4 ohm impedance, woefully poor sensitivity...

I have the magazine articles/instructions for the A75 and have entertained the thought of a bridged pair. I confess that I know very little of the Alephs, but the X-clones look promising. Additionally, I have the project books from Old Colony and G. Randy Slone. Some of the featured units could work in my application, but I have doubts. Most amplifiers of 100WPC or more claim to adequately drive low impedance loads. My listening tests do not support these claims.

Any Suggestions? TIA.

This is going to boil down to a question of bias current. That doesn't sound so bad...until you get to the implications. Lots of bias will require lots of output devices so that the load on any one of them doesn't get too bad. Generous heatsinking. A hefty power supply, not necessarily in terms of voltage, but in terms of current. Then comes the question of whether the front end, as designed, can properly drive the output stage.
I'm assuming that you play loud. Have you considered biamping? It makes a huge difference in apparent power as well as sound quality. Other than that, I'd say to begin by looking for a design that can take a lot of bias, as that's your answer for a low Z speaker.


Thank you for your replies. I must say I am honored that two thirds of the Pass "monster (tm)" dream team have come to my aid. Needless to say, I pay embarassingly close attention when I run across posts by either of you.

I reviewed the service manuals for the Aleph 1 & 1.2 today. What would recommend the A75 over either of these?

BTW, do you know of anyone using heat pipes on their class A amps? I have some ideas that could be quite effective with large, columnar radiators (6' tall, or so). I need to pick up a few more books, but rejecting a KW or so each would not be unrealistic using ammonia or acetone as the working fluid.

Know of anyone with an CNC EDM set-up?

Embarassed, you say?
Huh, you ain't half as embarassed as I am. Nobody's supposed to pay attention to what I say, I'm just the ghost of circuits past. When you hear a rattling of interconnects in the night, pay no heed, it's just me.
All right...take two.
I can understand the temptation to go for all the 8 ohm watts you can get, hence the 1.2
...but... (you knew this was coming...there's always a but...)
if you're looking at a low Z load, you don't need to focus quite so much on the voltage swing as the current capability. Remember that--assuming there is sufficient current available--an amp's wattage will double when you halve the load impedance. Ergo, a 100W Aleph 2 becomes a 200W amp into a 4 ohm load
you're going to need to boost the bias current a bit in order to get that 200W, as the stock Alephs (except the 3) did not have sufficient bias to double into a 4 ohm load. You can go at this in two ways: either increase the bias on the stock 2's six pairs of devices, or add another pair or two of devices.
Just for fun, let's say 200W into 4 ohms. Pardon me whilst I rub Ohm's magic lamp and, presto!, the answer pops out. Give yourself a target of around 8A output and keep in mind that the output stage will deliver twice its bias current, hence your total bias will be ca. 4A. Eight pairs of devices will be ideal, using the same .47 ohm Source resistors. The front end can handle eight pairs just fine. Ta da! A custom 200W (into 4 ohms) class A Aleph.
I did a thread on water-cooled Alephs once upon a time that might be of interest, especially as you will be dissipating the better part of 400W heat per channel.
When it's time for the hot dog roast, call me.


Thank you for the intel. Now that I know that there could exist an Aleph that fits my needs, I'm left with three questions:

How are the availabilities for parts, PCBs, etc. for the Aleph series?

Which (Aleph or A75) would present the most reasonable project for the competent but inexperienced builder?

Why did Mr. Pass recommend the A75 over any of the available Alephs?

I trust that I would need to fab' the boards for the A75(s). The Aleph could do the job in a single chassis. Still too many variables.

BTW, how about the heat pipe question? I noticed the Thermacore vapor chamber heatsinks while looking around. Adapt this concept to a columnar radiator and you're in business. With regards to the output MOSFETs in question, are we looking for thermal equilibrium or a particular, steady, threshold minimum temperature?


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
AtomicDog said:
Why did Mr. Pass recommend the A75 over any of the available Alephs?

I trust that I would need to fab' the boards for the A75(s). The Aleph could do the job in a single chassis. Still too many variables.

The Alephs will not like a 2 ohm load, delivering less power at
greater distortion. The A75 will laugh at such a load. I personally
prefer the folded cascode version.

I recently found an A75 board (empty) on the back shelf. You
are welcome to it if you pay attention to any errors posted about
the original boards. Email me your address.


Autographs are great for those who are into them.

I would much rather talk with someone whom I respect, over a series of bottles of wine (or pitchers of margaritas, for that matter). Even good music is an afterthought in the company of a true thinker.

Going back to the A75, I saw Dr. Thagard on Discovery Science last night. Nothing really ever is accidental... He's another one for my dinner party list. Keep your autographs, I will have something created by two individuals that intrigue me.