Aleph-X PS question

[leadbelly]

So I wanna build myself a frugal Aleph-X. The main area to cut cost is the use of surplus EI transformers (2 x 12VAC) and surplus caps for the PS.

The thing that scares me is how do I tweak out any PS caused hum if it comes to that? As I understand it, a pi filter is not easy to achieve because a very low dcr inductor is needed due to the high current draws involved. So, what kind of tweaking can I fall back on if hum turns out to be an issue?

[nobody special]

Leadbelly,
I am doing exactly as you describe- cheap surplus trannies and caps.
I will warn you now...
count on your transformer voltage being lower than advertised, and your dc voltage- ac voltage ratio being right around 1:1.
I bought 4 13.5V@20A transformers. Unfortunately, I found out that the 13.5V is the unloaded value. Depending on regulation, I am hoping to get 12-13V out of them, DC. Ideally, it would be more like 15-16V. I will eventually replace these with some higher voltage toroids.
A lot of the noise will depend on how much current you are pulling out of the transformers. I will have a 7A constant current out of each of mine. I have found that the only way to really get the ripple down there is to use a pi filter. As you said, you will lose a lot of voltage this way. I plan to use 2mH 14ga. air core inductors. These should be about .31ohms for a drop of a little more than 2 volts.
If you haven't done it yet, download the psu designer from duncanamps.com. Play with it for a while, and see how different factors (transformer regulation, capacitor ESR, series resistance, etc.) affect the outcome. This will give you a little more of an idea of what you want to do. Hope this helps.
-NS

[Eric]

I'm currently assembling my Pass Labs a-40 amp and have noticed some hum at the speaker that is the result of power supply ripple. My original configuration was toroid -> bridge -> caps (24,000uF per rail, surplus caps). When I placed a CRC pi-filter in the power supply, the hum was gone, and the speakers were absolutely silent!

The new power supply configuration is toroid -> bridge -> 24,000uF cap -> 0.30 ohm 5w wirewound resistor -> 24,000uF cap. The resistor gets a little warm, but certainly not hot and it drops less than one volt for each rail.

[nobody special]

The new power supply configuration is toroid -> bridge -> 24,000uF cap -> 0.30 ohm 5w wirewound resistor -> 24,000uF cap. The resistor gets a little warm, but certainly not hot and it drops less than one volt for each rail.

CRC's can be effective, but a choke is more effective, given the same series resistance. I looked into doing a similar thing, but found the difference in ripple would be about 80mV (or more) with a CRC compared to 8mV (or less) with an inductor of a similar resistance. That's 10X better!
My supplies will consist of 12V surplus tranformers, bridge pack, 10 X 6800µF caps, 2mH inductor (.31 ohm), 10 X 6800µF caps. Should be about 5mV ripple.

[Eric]

True, using a choke may reduce measureable ripple to a greater extent than a simple resistor in series with the caps. However, once the hum is gone from my speakers (as it now is with the resistors), I don't really care what measured value it has It is possible that speakers with greater efficiency may require something more than a simple resistor, though - I'm clearly speculating now.

Additionally, I can buy a resistor for each of my rails and not spend more than $1. Inductors that can handle the currents needed in a power supply are

[leadbelly]

You're scaring me off here. To my thinking, having to buy big inductors to make a quiet PS will negate the cost savings of a surplus transformer in the first place.

Is there something else I could look at, like maybe a higher order filter using just R's and C's? Or a capacitance multiplier circuit?

[Eric]

leadbelly: Looks like your jumping over board before you know for sure that the ship is sinking

Here is what I would suggest. Gather all of your parts and hook them together on the table top and give it a listen (don't assemble the parts into a custom chassis just yet). It may turn out that you don't need anything other than a few caps to get an amp you are happy with. If that's the case, then you are done! If, on the other hand, you have some noise that seems to be coming from power supply ripple, try the cheap solution first: put a resistor in series with a second cap to make a pi filter. Chances are, this will remove any audible ripple induced hum that you have. If not, then get your inductors (after making sure that your hum is not coming from some other source, such as induced by RF emissions from your power supply)

This is why its important to hook it all up for the first time with the parts spread across the table top, not crammed into a chassis. If you have hum, you can play with moving wires around, moving transformers closer/further away to see if that cures your problem. Then, when you are happy with the sound, fabriate your chassis. This way you don't end up in the situation of not having room to add extra components, should they be required.

[nobody special]

Eric,
Good advice.

[Eric]

ns: Thanks for the compliment! Unfortunately, like all good advice, I learned this the hard way...

[audionut]

You can also roll your own inductors using a home made form. I made a form out of a 1" broom stick handle (the core) and two thin slabs of 4" square wood to make the sides. This was all bolted together with a long bolt that goes into the chuck of a variable speed electric drill. I takes about 150" of #14(I think) of enamaled wire wound onto this form to get around 2mh with DCR of .5 ohms. Once wound, unbolt the bolt, the sides come off, and punch out the wooden core. I used nylon ties to hold the coils. Cost of wire is between $10 and $15. per coil. All the hum went away when I put the coils between the capacitors in my aleph 3's. Well worth the effort.

I like to think that the inductors also keep out the high frequency hash. I also like to think that the coils add a bit of warmth to the amps.