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Old 21st January 2013, 01:29 PM   #41
Loren42 is offline Loren42  United States
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Just so everyone is on the same page, here is the PAS 3 (Preamp Abomination System 3) schematic. Well, it is everything from the selector switch back.

The output is DC blocked with a 1 uF cap, but notice the EQ section is not buffered and set up as an output impedance of 500KΩ.

Eli's FET buffer or even a DRV134 is needed to isolate the high EQ impedance and provide a line driver for the long cable run to the power amp.

I think a shielded, twisted pair cable (i.e., Belden 1800F) is a solid choice for the cable run if the shield is only grounded (connected) at the power amp.

The shield will provide a Faraday cage for the twisted pair and no signal is carried on the shield.

Belden 1800F has a capacitance of 13 pf/ft.

Click the image to open in full size.
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Old 21st January 2013, 06:06 PM   #42
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The cable shield should be connected to the preamp chassis.

Capable line driver or not, modest cable capacitance is a good thing. Mogami W2549 from MarkerTek is reasonably low capacitance/ft. MarkerTek is also a good source for the Canare F-10 RCA plug, which is quite decent and modestly priced.
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Old 21st January 2013, 06:25 PM   #43
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Eli Duttman View Post
The cable shield should be connected to the preamp chassis.
Got it.

Quote:
Originally Posted by Eli Duttman View Post
Capable line driver or not, modest cable capacitance is a good thing. Mogami W2549 from MarkerTek is reasonably low capacitance/ft. MarkerTek is also a good source for the Canare F-10 RCA plug, which is quite decent and modestly priced.
I already have Belden 1800F.

In regards to the FET buffer I have some questions:

1. What is the B+ voltage for the FET?

2. With regards to the 12B4 circuit, were you suggesting to use the FET circuit with that as well or suggesting that the 12B4 is an alternative?
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Old 21st January 2013, 06:59 PM   #44
smbrown is offline smbrown  United States
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Unless I've missed it, I'm not seeing much mention of the power supply changes needed to support the 12B4 besides one post from Eli. Do remember Loren42, that the PAS supply can not keep up the the demands of the 12B4 and buffer.
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Old 21st January 2013, 07:20 PM   #45
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by smbrown View Post
Unless I've missed it, I'm not seeing much mention of the power supply changes needed to support the 12B4 besides one post from Eli. Do remember Loren42, that the PAS supply can not keep up the the demands of the 12B4 and buffer.
For the time being I can pull the tubes from the phono section. As long as the heater supply appears the same, but I think the 12B4 draws 2X what a 12AX7 does, so it may be a problem.

I am gathering parts for a DIY preamp build since the more I get into this the more that seems to be the better route. Easier to do as little as possible to the PAS to make it PASable.
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Old 21st January 2013, 07:34 PM   #46
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Quote:
What is the B+ voltage for the FET?
Bridge rectify the O/P of an inexpensive Triad N-68X isolation trafo, with 4X 600 PIV Schottky diodes. CLC filter. Regulate, if you like. Anything above 135 V. rates to be OK. You can easily pull 200 mA. from a N-68X. So, the B+ PSU you build will be more than sufficient to power the buffer or the buffer plus 12B4s.

Quote:
With regards to the 12B4 circuit, were you suggesting to use the FET circuit with that as well or suggesting that the 12B4 is an alternative?
Your application requires a balls to the wall line driver and that means the FET buffer. Its O/P impedance will be well under 100 Ω. Combine that with drain current, at "idle", around 20 mA. and you have a "winner". I'd have no qualms about the 12B4 driving a 3 ft. cable into your 10 KOhm load. The 12B4 has the ability to charge and discharge significant cable capacitance. However, I don't know if it's up to the task of simultaneously driving both significant cable capacitance and a low impedance load. I have no doubts, whatsoever, of the FET's ability to get the job done.

BTW, Belden 1080F has over 3X the capacitance/ft. compared to Mogami W2549. W2549 is quite reasonably priced. I strongly suggest you rethink your decision.
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Old 21st January 2013, 07:50 PM   #47
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Eli Duttman View Post
Bridge rectify the O/P of an inexpensive Triad N-68X isolation trafo, with 4X 600 PIV Schottky diodes. CLC filter. Regulate, if you like. Anything above 135 V. rates to be OK. You can easily pull 200 mA. from a N-68X. So, the B+ PSU you build will be more than sufficient to power the buffer or the buffer plus 12B4s.



Your application requires a balls to the wall line driver and that means the FET buffer. Its O/P impedance will be well under 100 Ω. Combine that with drain current, at "idle", around 20 mA. and you have a "winner". I'd have no qualms about the 12B4 driving a 3 ft. cable into your 10 KOhm load. The 12B4 has the ability to charge and discharge significant cable capacitance. However, I don't know if it's up to the task of simultaneously driving both significant cable capacitance and a low impedance load. I have no doubts, whatsoever, of the FET's ability to get the job done.

BTW, Belden 1080F has over 3X the capacitance/ft. compared to Mogami W2549. W2549 is quite reasonably priced. I strongly suggest you rethink your decision.
I am confused. Are you saying the FET B+ should be ≤ 135 VDC?

The Belden 1800F was cited as <13 pF/ft conductor to conductor (26 pF/ft conductor to conductor & shield):

http://www.mcmelectronics.com/conten...s/24-14680.pdf

Is this not good enough? I already have this cable. Should I not use it and buy the Mogami?
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Old 21st January 2013, 08:53 PM   #48
smbrown is offline smbrown  United States
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The original PAS power transformer is only good for about 15ma and the circuit you're discussing will be drawing 40ma if you disconnect the phono board. That's a bit of an overload. Different transformer needed, IMHO.
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Old 21st January 2013, 09:20 PM   #49
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by smbrown View Post
The original PAS power transformer is only good for about 15ma and the circuit you're discussing will be drawing 40ma if you disconnect the phono board. That's a bit of an overload. Different transformer needed, IMHO.
Yeah, it would be nice to get a PS upgrade...

Tubes4HiFi has a regulated board with two toroidal transformers for about $140. A little pricy for a PAS, particularly if I am going to build my own at some point. Probably just keep milking what I have until then.

Dynaco PAS3 upgrades & modifications & parts
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Old 21st January 2013, 09:25 PM   #50
smbrown is offline smbrown  United States
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Okay, just be prepared for a "milk down" at some point... :-)
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