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-   -   Got me a Nak PA-7 amp... (http://www.diyaudio.com/forums/pass-labs/221368-got-me-nak-pa-7-amp.html)

mark_1 11th October 2012 04:49 PM

Got me a Nak PA-7 amp...
 
and I'm quite impressed with its sound presentation, build quality and looks. Way superior to amps I have previously owned.

But it does have an odd issue. I have twice now reset the bias to spec at 40 mv with no load at the o/p and the i/p shorted as per the service manual. And in that condition, the heatsinks get nicely warm.

However, with speakers hooked-up and preamp connected, the heatsinks are noticeably cooler, even after/while playing music for an hour or more.

I'm a little baffled by this. With my limited test equipment (i.e. DMM) I do plan to check that all of the o/p transistors are operating properly.

I'm not looking for extensive fault finding, maybe a bit of guidance, and some thoughts on why it would operate as I indicated.

By the way, it does have the protection circuit boards, and DOES NOT have VR101/201 to set the o/p offset (which is 12.4 and 24 mv L&R).

Cheers, and thanks in advance for any responses!

Mark

Zen Mod 11th October 2012 06:00 PM

how high is bias , exactly ?

Old'n'Cranky 12th October 2012 12:03 AM

um, if it doesnt have vr101/201, how are you setting bias and offset ?
vr101 is used for dc offset (0 +/- 100mv)
vr1-2 is used for bias (40mv between tp 101/102 for the left board)
perhaps a pic of that top edge of the amp boards where the pots are supposed to reside will help us..

PS, Hi ZM ;)

AudioSan 12th October 2012 12:10 AM

Quote:

Originally Posted by mark_1 (Post 3197876)
and I'm quite impressed with its sound presentation, build quality and looks. Way superior to amps I have previously owned.

But it does have an odd issue. I have twice now reset the bias to spec at 40 mv with no load at the o/p and the i/p shorted as per the service manual. And in that condition, the heatsinks get nicely warm.

However, with speakers hooked-up and preamp connected, the heatsinks are noticeably cooler, even after/while playing music for an hour or more.

I'm a little baffled by this. With my limited test equipment (i.e. DMM) I do plan to check that all of the o/p transistors are operating properly.

I'm not looking for extensive fault finding, maybe a bit of guidance, and some thoughts on why it would operate as I indicated.

By the way, it does have the protection circuit boards, and DOES NOT have VR101/201 to set the o/p offset (which is 12.4 and 24 mv L&R).

Cheers, and thanks in advance for any responses!

Mark

that is typical class A/B. its cooler while playing. until you reach a level:)
same goes for classD too. classA only gets cooler the more power you drag out.

AudioSan 12th October 2012 12:12 AM

Quote:

Originally Posted by Old'n'Cranky (Post 3198334)
um, if it doesnt have vr101/201, how are you setting bias and offset ?
vr101 is used for dc offset (0 +/- 100mv)
vr1-2 is used for bias (40mv between tp 101/102 for the left board)
perhaps a pic of that top edge of the amp boards where the pots are supposed to reside will help us..

PS, Hi ZM ;)

VR101 and VR1-2 can have been replaced with resistors:)

mark_1 12th October 2012 01:36 AM

Thanks guys for your answers, I appreciate you taking the time.

Zen Mod, the bias is set at 41 mv each channel.

Old'n'Cranky, I double checked the pots and they are VR101 for dc offset, and VR102 for the bias. VR201 & VR202 are for the right ch. So with VR102 & VR202 I can set bias; without VR201 & VR202 I can't adjust dc offset. ;) you had me triple checking what I had said.

AudioSan, hmm, I guess that means there is more energy going to the speakers than up in heat. Or I'm just not cranking her up enough!

I've only had this beauty a short while, so there is much more to learn and appreciate yet.

Cheers.

AudioSan 12th October 2012 01:53 AM

mark. at idle. all power(bias) is going up in smoke. but when you play. it use that power to drive the speakers. there is some going up in smoke even then. but how much depends on the amp. and volum:)

mark_1 12th October 2012 02:00 AM

That's cool AudioSan! Pun intended :p

AudioSan 12th October 2012 02:16 AM

at 41mV pr ch. i guess its a class A/B amp:)


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