Mr. Koren does some really nice work. He's done a lot of research on the PAS preamplifier. Thanks
I think you'll eventually want to get rid of the positive feedback and rework the RIAA.
I think I will do that plus add the cathode follower, but a 12AX7 doesn't make a good follower, from what I understand, is that right?
Well, Audio Research did just that for a number of years, but there are alternatives. The link below is an app for a cf.
One subtlety is that while a cf may have a reasonably low small signal output impedance, it might still not be able to deliver
enough peak current to drive the RIAA network, which could take several mA, depending on component values and signal level.
One function of the cf is to minimize loading of the second va stage, even if it doesn't boost output current drive much.
Cathode Follower Output Impedance Calculator
Last edited:
Norman Koren has addressed that issue….. somewhere in the material he published. I’m not sure if it appears on his website, as I haven’t visited it for sometime. The conventional thinking is that 12AX7 isn’t suited for cathode follower operation; however he showed that at low currents, under 1.5 ma, or so, it is more linear than other popular triodes, such as the 12AU7 or 12AT7. I’m using one of his suggested line stages with a 12AX7 cathode follower to drive a high input impedance (tube) amp, and I’m quite pleased with it. For those requiring more output, he has shown the circuit with a 6SN7 used as a cathode follower, noting that due to the current requirement of that tube, it probably isn’t suited for a PAS, at least without “beefing-up” the power supply.
The problem you have is the power transformer does not have a lot of current. If you are thinking of replacing it, IMO that is the point where you might as well get another chassis and leave the PAS alone.
In the phono, I got rid of that pesky positive feedback resistor and went to a 12AT7, using a cathode bypass cap to get me a little more gain. It worked out quite nicely. I had to rebias things slightly to optimize for the 12AT7.
I move the EQ circuitry to the bottom of the board and get rid of the middle wafer on the selector switch. I hardwire the phono input connector right to the input of the phono section and only switch the output.
For the line stage, for the last 20-25 years or so I've been using a 12AU7, the first half a gain stage, direct coupled to a cathode follower. In either section I use 100K for the plate load and the same value for the cathode follower. Its not the best for the CF by any means, but if you are staying within the confines of the power transformer, it actually works out rather well. It makes about 16 volts or so before it clips. BTW make sure you put a grid-stop resistance in each grid circuit- really helps out, especially in the line section, you can use the volume control as the grid leak resistor without ill effect. I had some switches made up to allow me to build a 23-position volume control instead of the stock part which is pretty junky. I leave all the tone control parts in place, as part of my strategy of not modifying the front panel, chassis or covers.
I've seen the result tank an ARC Ref 3 line section- no contest; if you do this right you have a hellava sleeper that can take on some pretty vaunted competition. Ain't DIY great?
In the phono, I got rid of that pesky positive feedback resistor and went to a 12AT7, using a cathode bypass cap to get me a little more gain. It worked out quite nicely. I had to rebias things slightly to optimize for the 12AT7.
I move the EQ circuitry to the bottom of the board and get rid of the middle wafer on the selector switch. I hardwire the phono input connector right to the input of the phono section and only switch the output.
For the line stage, for the last 20-25 years or so I've been using a 12AU7, the first half a gain stage, direct coupled to a cathode follower. In either section I use 100K for the plate load and the same value for the cathode follower. Its not the best for the CF by any means, but if you are staying within the confines of the power transformer, it actually works out rather well. It makes about 16 volts or so before it clips. BTW make sure you put a grid-stop resistance in each grid circuit- really helps out, especially in the line section, you can use the volume control as the grid leak resistor without ill effect. I had some switches made up to allow me to build a 23-position volume control instead of the stock part which is pretty junky. I leave all the tone control parts in place, as part of my strategy of not modifying the front panel, chassis or covers.
I've seen the result tank an ARC Ref 3 line section- no contest; if you do this right you have a hellava sleeper that can take on some pretty vaunted competition. Ain't DIY great?
You are right on about the current issue, 15mA is not much current for both a line stage and phono curcuit together, considering modern standards that is. You and I are thinking alike, do away with the positive feedback, use a 12AT7 w/ bypassed cathodes , you have plenty of gain, that's of course making the nessessary compensation in the RIAA filter between the stages. I've used the 12AT7 in place of the AX7s in the stock Dynaco stage and it was much quieter even with AC heaters, so you are onto something there!
Yes, using the same resistor in the cathode does help on the current problem when using the 12AU7 as a cathode follower. When it comes to DIY you have to adapt to current standards. I would have to say that for standards that Hafler and others were dealing with at the time, they were spot on in their designs.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.