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In Reply to: RE: PAS3 - Tone Controls Bypassed - Need to connect up to a 50k Ohm Active Crossover posted by aguaazul@gmail.com on February 03, 2014 at 22:42:38
The problem is not the coupling cap but rather the output impedance of the PAS (unmodified). If the Bozak is active it may be possible to increase its input impedance since many times the Zin is determined by an input loading resistor or pot. You should look into that as the first step.
The problem is likely compounded with crossover since many of them have an passive filter right off the input such that they are expecting to be driven by as source impedance of less than 1K. if the Zout is higher, it will change the tuning of that filter.
All that being said - I have on my project shelf now a add on board for the PAS that adds a cathode follower to the stock PAS - this would solve your problem..... however, I have not yet generated _any_ documentation (other than the schematic) for this project so either I would have to modify your PAS or you would need to have more than average skill to install it yourself.
Follow Ups:
"I have on my project shelf now a add on board for the PAS that adds a cathode follower to the stock PAS..."
Interesting news, Joe. Any other new products in the pipeline? Looking forward to any updates to your website.
Regards,
Peter
Thanks Joe. Would removing the 62K Ohm and 510K Ohm resistors in my PAS and adding a 10M Ohm resistor be acceptable? I'm trying the paralleled resistor math here (it's been a while since college.
I am local, how much would be the add on board for you to do it? Also, is it permanent or switchable?
Thanks for your help!
Randall Nerison
No.... loading resistors are used to provide proper termination for the PAS with the stock tone controls to operate correctly when the preamp sees a 100K ohm load. They don't offer more drive current - IOW lower source impedance
There is no way to modify the existing circuit (without changing stage 2 to a CF) to lower the Zout
Dynaco quoted the original release of the PAS as being good into a Z-in of 470K, or, with the 510K resistor removed, into Z-in of 250K. With the x-mod, the PAS was good into Z-in of 100K without changing or removing any resistors.Joe, if by "terminating resistors" you are referring to the 62k and the 510k, they did not change in value when Dynaco transitioned from the original to the x-mod version.
Could you elaborate further?
Edits: 02/04/14 02/04/14
The 62K and 510K resistors were chosen so that the effective impedance presented to the PAS line amplifier output stage (when combined with the load from the power amplifier) is correct for the proper operation of the tone controls.
Joe,how much is the CF mod? I'd drop off and pick up.
Randall Nerison
I haven't given this any thought - lets discuss off forum. You have my e-mail
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