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Pass HPA-1, what do we know?
Pass HPA-1, what do we know?
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Old 12th January 2019, 04:45 AM   #51
Hikari1 is offline Hikari1  United States
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20x2 is not unusual for an interconnect. 17x2 is, which is what I thought you were using. But whatever sounds good to you go for it. I’ve certainly tried a lot of interconnects, home brew and otherwise.
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Old 12th January 2019, 10:16 AM   #52
JeffYoung is online now JeffYoung  Ireland
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Pass HPA-1, what do we know?
I should think 120 feet would still be within reason.
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Old 12th January 2019, 04:50 PM   #53
lae2 is offline lae2  United States
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Originally Posted by JeffYoung View Post
I should think 120 feet would still be within reason.
Thank you for suggesting as much. I am so pleased with the HPA-1/XA25 that I am thinking of putting a second set in another room - 18X24. An XA25 on a wall shelf at the far end and an HPA-1 at the near wall. About a 35 foot RCA interconnect. But first, I must extend an addition. Adjusting to a new (to me) house.

As a general comment. My migration toward Pass is largely attributable to DIYAudio. In my past, I built DC coupled instrumentation amplifiers fed by 30M input impedance. My noise problems have been local radio stations and failing ballasts 50 ft down the hall. And ground loops. Hence, I strive for single point grounding. Perhaps not necessary for audio work. Not a fan of circuit boards. I have had about 300 ft of instrumentation cable on the shelf that I now use for audio work - about 10 bucks a foot. Whatever is left over, I want in the casket.

The HPA-1/XA25 combination is more black than anything I have not heard. Haunting.

Last edited by lae2; 12th January 2019 at 05:04 PM.
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Old 15th January 2019, 03:57 PM   #54
JeffYoung is online now JeffYoung  Ireland
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Pass HPA-1, what do we know?
I got a bit of time to start on the front plate of my DIY HPA-1.

Copying the Pass Labs version isn't an option because my mill is the old-fashioned kind (I don't even have digital readout), and the setup charge for anodising single units is too high (which means an off-the-shelf volume know). But I am trying to keep it in the same vein.

Pass HPA-1, what do we know?-progress_frontplate-jpg

Cheers,
Jeff.
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Old 15th January 2019, 04:35 PM   #55
WalterW is offline WalterW  Netherlands
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Pass HPA-1, what do we know?
Did you do that yourself? With a drill? How did you make the sides of that panel?
Can you give us more information?
Wow I thought only expensive CNC's could do this?
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Old 15th January 2019, 11:51 PM   #56
JeffYoung is online now JeffYoung  Ireland
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No, not a drill. An old 1950's Bridgeport milling machine. The sides were done with a 45 milling cutter (basically a big countersink), and the groove was done with a ball-end end-mill.

I cut it to size first on my table saw with a 120-tooth carbide blade.

Pass HPA-1, what do we know?-bridgport-jpg
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Old 21st September 2019, 09:49 PM   #57
JeffYoung is online now JeffYoung  Ireland
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Pass HPA-1, what do we know?
I'm still machining the case, but it's far enough along that I started stuffing the board:

Pass HPA-1, what do we know?-board-1-jpg
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Old 21st September 2019, 10:04 PM   #58
Zen Mod is offline Zen Mod  Serbia
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Pass HPA-1, what do we know?
thread title could be now - Pass HPA-1, now we know too much!

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Old 21st September 2019, 10:31 PM   #59
JeffYoung is online now JeffYoung  Ireland
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He he.... let's see if it makes sound first.
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Old 23rd September 2019, 03:35 PM   #60
JeffYoung is online now JeffYoung  Ireland
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Pass HPA-1, what do we know?
OK, let's talk about matching.

The input JFETs I assume should be matched as closely as possible (given the constraints of N:P matching). Presumably Idss is a good enough proxy for gm.

The cascodes are just "voltage umbrellas", so I'm guessing no matching required there?

The current mirrors are going to mirror the current through the JFETs, so I'm guessing don't need to be matched top-to-bottom. However, I think the transistors within each current mirror need to be matched for hFE in order to get a 1:1 mirroring?

The servo will zero the offset, so it seems the output MOSFETs don't need to be matched. But will matching them make their distortion profiles more consistent or anything?

Thanks,
Jeff.
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