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Old 23rd August 2004, 01:23 PM   #1
macka2 is offline macka2  United States
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Default X Aleph Bandwidth & Power Supply

I have noticed today an interesting effect when I use a CRC power supply in the X Aleph (15 volt version with 4 IRFP044) by add 0.22 ohms resisters ( 1 volt across resisters) between the two 30,000 UF Filter capacitors on each rail(30000+ 30000 per rail).

There appears to be a noticable reduction on high frequency bandwidth then using the CRC filtering with a more subude sound stage.

Can any one explain why this is happening or had a similar experience?

Ian
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Old 24th August 2004, 10:34 PM   #2
gl is offline gl  United States
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Hello Ian,

Your question is an interesting one and it hasn't received much response so I'll throw in my two cents.

I think that the added low pass section on your power supply is becoming a significant part of your output circuit at higher frequencies. I apologize for stating the obvious but here are my thoughts.

The Aleph X appears to have it's dominant pole at a fairly low frequency. I recall seeing posts where people (yourself included) found that the circuit rolls off at 20 - 50KHz. The output stage of this amplifier also seems to have a higher output impedance than the typical follower stage.

O.K. - now consider the popular view of a power amplifier circuit as one that simply modulates a large DC current source. If you add a low pass RC filter section to that current source then at higher frequencies this may well interact with the amp. Especially if you have nonideal impedances. For instance if the output impedance of the output stage is higher than usual, or if the power transformer has a higher impedance than usual on the secondary, or if the PS filter caps have a high internal impedance. Is the rolloff an additional 6db?

I've always felt uneasy about having these CRCRC type filters on an Aleph-X power supply. Nelson Pass makes it work by using a huge transformer with many times the required current output. He also uses 75 volt caps for a 32 volt supply. This helps reduce the PS impedance as well. If you use a smaller transformer or less than ideal caps then I think you'll see what you're seeing.

I'd try substituting a bigger transformer - which is probably not what you want to hear. I'd also try fiddling with the R's and C's in the power supply to see what happens and if you can control the interaction.

As long as you're on the line there's a question I'd like to ask you. In the Aleph-X schematic you posted way back when, you had used capacitively coupled inputs with 100K to ground on the input pins and then 2.2uf in series with the input resistors. You had also posted questions to the forum and NP about this circuit. What did you eventually end up using?

Hoping this helps.
Graeme
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Old 25th August 2004, 09:14 AM   #3
macka2 is offline macka2  United States
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Thanks Graeme for your thoughtful response.

At the moment my X Aleph is directly connected to the output of my Bride of Zen Blanced Line Stage with the nominal 10uf coupling capacitors inside the feedback loop(with Hendrik X modification).

Your theory is an interesting one.

Previously I was running 2 pairs of IRFP240 with each with 0.44 ohms source resisters per each side of the X Aleph with a straight 60,000+60,000 filtered supply....No problems.

I then got hold of some IRFP044, changed out the IFRP240's and installed the IRFP044 using 1 pair per each side with 0.22 ohms source resistance.

At this point figured I would improve the supply by spliting the 60,000 into 30,000 +30,000 CRC using 0.22 ohms.

This is when I noticed the rolloff of the highs.

Initially I thought it was the extra gate capacitance of the IRFP044 but I have since bypassed the 0.22 ohms resisters with a shortingl ink and the bandwidth is restored.

Your theory could be as there isa significant voltage drop across the 0.22 ohms of about 1.1 volt an at about 5.2 amps so maybe I am asking too much of the filter capacitors.

I might back of the bias with is a bit too high or use say 0.110 ohms for the CRC and see what happens. Bypassing the local driver PCB may also help if in fact there are hgher than optimum impedanced in the power supply.

Thanks again for your response


Ian
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Old 25th August 2004, 01:35 PM   #4
byteboy is offline byteboy  Netherlands
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Question Interesting phenomenom......

Why did you change to a CRC filter in the power supply in the first place? Was it because of hum being audible?

Maybe increasing just the capacity (no CRC) could improve this situation?

What transformer did you use? (Torodial/EI? Regulation (Ri of secondairy winding) would be interesting to know too.

Did you notice any other change in sound between IRFP044 and IRFP240?
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Old 25th August 2004, 04:43 PM   #5
MikeW is offline MikeW  United States
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Short out the resisters and see if your bandwidth comes back. If not then it is because of the change to the 044's.
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Old 25th August 2004, 06:08 PM   #6
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I would be very surprised if it were the power supply, assuming
that the caps are in fact good.
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Old 25th August 2004, 11:06 PM   #7
macka2 is offline macka2  United States
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I tried reverting back to the straight 60,000 uf with the 044's and it seemed okay.

But whats even more odd is that my other bigger X Aleph on 25+25 volt rails has CLC wit 2 mh 0.070 ohm chokes with the same capacitors 30,000+30,000.....no signs of the highs fading there. But it uses IRFP240's so who knows....another diy mystery.

I will wait a few days and do some more tests when my winter buggs gone........Nelson I miss your Northern California climate...not a hint of a sniiffle when I was over there!

If I could swing it I'd be over there again now.

Ian

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Old 28th August 2004, 03:49 AM   #8
macka2 is offline macka2  United States
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Well I had a good play around with it this morning.

Using the IRFP044's I decided to add an additional 15,000 to the amp end of the CRC*. I think that helped with the stereo imaging a bit. Shorting the 0.22R resister did not change much with the extra filter capacitors but I still felt the presentation was overly rich compared to what I have been accustomed to.

I then changed over one channel to the IRFP240's with the same power supply conditions.(Grey's orginal schematic)

Nelson comments about gain devices effecting the sound were borne out here. In comparisions and I am still assessing this, the IRFP240 appears to have a somewhat thinner presentation and more billiant high frequency response. The IRFP044 is richer sounding with more body in the midrange but otherwise less linear .....The jury is still out but I tend to think that while using only one pair per side of each, neither is better than the other and the IRF240 perhaps offers more lattitude to use in various numbers for the right tonal balance. I doubt if I would use the IFRP044 for a tweeter amp in a bi amp setup and more likely the midrange.

Mean while back at the ranch I think the IRFP044's were masking my subjective impressions overall. I plan to investigate CLC filtering to see what happens there as I will be using the amps for a horn loaded mid and tweeter in my 4345 monitors (JBL 2308 /2307 / 2425 & 2405).

I just tried the extra capacitors again, they certainly help and looking at Nelson's photographs of the AX 200 there appears to be an extra pair of 25,000 75v capacitors and the amp end of the supply.


Thankyou all for your comments.


Ian
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Old 28th August 2004, 06:18 AM   #9
EUVL is offline EUVL  Europe
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Ian,

May I suggest another (not costly at all) experiment for you :

Try to replace the IRFP's with a lateral FET (e.g. 2SK1529), and change the gate resistors (starting value for 4 FETs per channel 1k). You can tune the sonic balance by just changing the gate resistors values up and down.

Patrick
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Old 28th August 2004, 01:36 PM   #10
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I also noticed a similar thing when I added 6mh torodial chokes into my X supply. So much so tnat I shelved the X's for now and added the chokes to my pair of Aleph 2's in which a dramatic improvement in the noise floor was achieved and with out the disappreance of any hf response. It actually also made the A-2's sound quite a bit better overall just at the loss of a couple of volts on the rails. I was using 240's in my X's to begin with. Never have used the 044's.

I thought it odd that the hf disappeared literally overnight from my system and now I may have to drag the X's back out and go through the whole process again and listen with and without, and this time take measurements and look at a square wave or two. The strange thing was that the hf rolloff was so noticable that I thought the tweeters on my Dyns had failed for some reason. Puttong the 2's back in the system confirmed they were ok.

Mark
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