Choke power supply for Pass A75


2002-07-09 12:45 pm
It's been a while since I built any amps (when I started a family strangely enough). Making a start on a PassDiy A75 using Holco 0.1% resistors, Black Gates, etc. The intention is to build it as two monoblocks, with a sufficiently large PS to enable biasing into lower impedances.

I see a lot of the PassDiy projects using chokes (inductors back in Blighty) in their power supplies and need advice on a suitable design for this amp. From my box of hoarded bits and pieces I have found a pair of ILP 30V+30V 500VA transformers which I intend to use as a starting point.

Quality is my focus on this project, and recommendations for good suppliers of necessary parts would be gratefully received.



2002-07-03 7:00 am
Because the iron (metallic) cores saturates with internal currents and air doesn't react like this. The only advantage for the non-air core inductors (used as a choke in your case) is the space saving.

As you mention it, if the goal is quality, don't mess with iron... Solen mades nice and cheap ones. I use the litz type along with their "FAST" capacitors in my speaker crossover designs with very good results. Many gauges/prices. You may also want to check Goertz or CFAC inductors. Much more expensive but worth the look. ;)

You may also wind your own with very good results.

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Since we're talking about a power supply choke here,
the linearity is not a big consideration, but dissipation
in the copper wire is, which is why a good iron core
inductor is of value. Some of your better iron or ferrite
inductors from Zalytron or MCM will take 6 amps DC or
more before you start seeing some saturation, and
work quite well.
Zappa said:
Because the iron (metallic) cores saturates with internal currents and air doesn't react like this.

The best way is to use an iron cored choke that has a small airgap. Without an airgap the core saturates very easily with dc. The vast majority of the energy held in the choke is in the magnetic field across the airgap. Easy to calculate the energy in caps and chokes. Is measured in Joules (= 1 watt for 1 second). For a cap is 0.5 x Farads x volts x volts = Joules. For a choke is 0.5 x Henries x amps x amps = Joules.



2002-07-09 12:45 pm

I've had the A75 boards for just under two years and had already started getting the parts together. I also have no experience of single ended designs and, until recently, was not aware that the Aleph designs were so freely available. A project for for future perhaps.

My present system is Linn Sondek LP12 with Helius Orion II arm (ATF5 cartridge), Naim 32.5, SNAPS, NAP160 and Spendor Prelude speakers (not upgraded since children arrived!!!). I recognise that system needs upgrading and DIY is the best way to acheive this.

Once the A75s are done, I shall probably build the Aleph P as a replacement for the Naim pre-amp (although I am toying with the idea of buying a secondhand 32.5 and running my system fully dual-mono whilst I upgrade the spare Naim boards with Holco resistors, etc). Has anybody else tried this? Is it worth it?

I am sure most, if not all, of you are already converted to Passlabs 'sound' but would appreciate any feedback on how people feel the A75 compares to the Naim sound.

By the way HP, I've seen your posted projects, you build some excellent chassis. Nice work!

I'm aiming to construct my own amps using some 8" aluminium channel (6" too tight for transformer) for top and bottom. BIG heatsinks to be bolted directly to channelling, fixings therefore reduced to a minimum and top surface therefore free of screwheads. Possibly using a third piece between power supply and amp board as a mounting for power mosfets.
I have built 3 pairs of A75 monoblocks since 1993. And as good as A75 is, it is not a match for Aleph. A75 is more forward sounding with more pronounced high frequencies and overall more SS sound. Aleph, on the other hand. is more subtle, more texture revealing, more liquid and more 3 dimentional. One might say more tube sounding. One might also say that one amp is more suitable for certain type of music than the other. My main interest in music is industrial stuff, in the likes of Front242 and Apoptygma Berzerk, and I still prefer Aleph.;)
I have another 2 sets of completed A75 boards and the best price I could get for them was $50. A75 is using also twice as many parts as comparable Aleph (not to mention voltage doubler or regulated PS for front end). Also the hardest part and most time consuming of any construction is the chassis building. I am seriously consider modifying some of my A75 amps into Aleph or Aleph X.
So it's only my suggestion, but I would not waste my time on building A75 now, when Aleph schematics are around, not to mention Aleph X from GRollins. Those amps are way easier to built than A75 and apparently better sounding too.


2002-07-09 12:45 pm
The A75 can't be too bad if you've built three of them. They do have some positive features I hope. What do (or did) you like about them?

My existing system tends towards 'softness' due to the arm and speaker combination. This is definitely not a problem caused by the Naims. Good as my amps are, I feel that the forwardness you describe may actually be of benefit. Comments from any existing or previous Naim users could of value here. Anybody care to reply?

P.S. I have enjoyed the 'tube' sound before. Used to run a Musical Fidelity preamp though a pair of original Quad IIs. Lovely!!
Persig said:
The A75 can't be too bad if you've built three of them. They do have some positive features I hope. What do (or did) you like about them?

The low level detail is really good. It's just that Alephs sound seems to be more refined. But still, A75 is good amp. When somebody borrows my Alephs I still enjoy listening to A75. I guess I enjoy them both.;)