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PostPosted: 11 Sep 2009, 20:18 
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Joined: 06 Jun 2008, 14:03
Posts: 272
Location: Tulsa, Oklahoma
Ok let me try to explain this thing. I have a st-70 in my shop and use a passive volume control I built and it works great, here is the question.

If I run my tuner or gemsound mixer direct to the amp it sounds, well funny but if I run ether of them through the passive it is smooth like a tube pre amp is in the system, could this little passive be adjusting the impedence from the solid state to more equily match the tube amp?


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PostPosted: 12 Sep 2009, 06:48 
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Joined: 13 Jul 2009, 20:01
Posts: 92
Location: Space Coast Florida
Yes, it sounds like a mismatch condition. Check the specs on your equipment.


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PostPosted: 12 Sep 2009, 07:51 
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Joined: 06 Jun 2008, 14:03
Posts: 272
Location: Tulsa, Oklahoma
Thats probly the reason the passive thing is helping the sound, That is also the reason I am on a hunt for a pas 2 or 3 to restore not only to have a tubed pre but to be able to have more inputs + a phono pre built in.


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PostPosted: 12 Sep 2009, 12:15 
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Joined: 13 Jul 2009, 20:01
Posts: 92
Location: Space Coast Florida
The PAS tube preamps are very, very touchy when it comes to output impedance. The tone control circuit is the last thing in the signal chain and directly drives the output.

There is no buffer stage after the tone control components. I assume that a final buffer amp was never employed due to costs, but if the impedance strays much beyond the 500K Ω it is designed to drive you will have asymmetric frequency response in a big way.

Other than that I like my PAS preamp a lot.


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PostPosted: 12 Sep 2009, 22:32 
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Joined: 06 Jun 2008, 14:03
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Location: Tulsa, Oklahoma
Well it will be pushing a dynaco st-70 that I rebuilt so it should be ok.


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