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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
Got it.
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?
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.
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.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.
BTW, Belden 1080F has over 3X the capacitance/ft. compared to Mogami W2549.
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/content/ProductData/Spec Sheets/24-14680.pdf
Is this not good enough? I already have this cable. Should I not use it and buy the Mogami?
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
Okay, just be prepared for a "milk down" at some point...
Too funny!
Yeah, so far it has been running without fail for a few years now.
No, the FET buffer needs not less than 135 VDC. Getting 160 V. with a cap. I/P filter from SS bridge rectified "120" VAC is easy. However, if you regulate the rail, some drop has to occur.
As the other posters have indicated, the PSU in the PAS can't support more than that circuitry already present. For Pete's sake, Mouser charges all of $11.20 for a Triad N-68X. Being frugal is good. Being cheap to the point of "shooting yourself in the foot" is bad.
Hey, I just noticed I have a PS transformer rated a 325-0-325 VAC & 12 VAC @ 2.5 Amps sitting in a drawer.
I don't know the current for the HV secondaries, but it has to be more than the PAS transformer as the transformer is bigger than the stock PAS.
Does anyone know if this transformer suitable for a replacement for the stock unit?
If so, it might also be a source for the 160 VDC for the FET.
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