Triple Line Stage/Crossover

[Stuart Polansky]

For my revamped system, I need to build six line stages. Three per channel, split into three frequency bands. Compromise: twin triodes; octal or noval. (So 12B4 and 76 are out, for instance). Shooting for gain of around 10, give or take. Setup is grounded cathode>filter>DC coupled servo cathode follower. Follower will be a 6SN7 or maybe ECC99.

Suggestions for the gain tube?

Needs to be fairly low plate resistance to drive a medium impedance filter section. Thinking ~1-2K.

Also, GOOD sounding tube!

Thanks for your thoughts!

Stuart

[Roscoe Primrose]

6BL7 or 6BX7? 5687? 6J4/8532? (of course, those are 7-pin miniature single triodes... I've got about 165 8532s)

Keep in mind that in addition to a level control at the input (I assume) you'll need level controls for at least 4 of the output channels as well to adjust the L/M/H levels....

Roscoe

[Stuart Polansky]

Re level controls. plan is for P&G pots controlling overall gain & LDRs for bass and treble trim. problem is going to be input impedance. P&Gs are about 10k, LDRs about 7k. So worst case~2k. Too low. Gotta think on that one

[Roscoe Primrose]

Re level controls. plan is for P&G pots controlling overall gain

Proctor & Gamble makes pots now?!?

Last edited by Roscoe Primrose on December 19th, 2013, 2:54 pm, edited 1 time in total.
Reason: typo

[Stuart Polansky]

Misspelled, but no worries. Go back to sleep. No attenuator is better than whatever YOU are using now.

[Guy]

Should I send you description of a servo-cathode-follower PCB I made? It's set up for a 6SN7, and you'd have one less coupling cap but a -12VDC supply (like maybe a filament supply.)

Actually, the schematic is shown in the Projects thread titled, "Guy's Phono Pre-amp."

Attachments

PCB with components: You supply the socket.

Last edited by Guy on December 19th, 2013, 3:08 pm, edited 1 time in total.

[Stuart Polansky]

Guy, you certainly can if you like Howrver, its worth pointing out that not only do I havr a couple, but I am using one in my phono stage. I will need to buy a couple more.....

[Stuart Polansky]

By the way, I recommend increasing the absolute value of the negative supply, since when I tested mine, I ran into asymmetrical clipping on the negative rail at moderate outputs. Since mine was in a phono stage, at the output, and gain is kind of low, my expected output of ~0.5-1V depending on SUT and cartridge was NOT a problem.

But in a line stage, one which may have to feed a low gain power amp, clipping at ~7 volts I saw as cutting it too close. Theoretical, of course is about 8.5VRMS, but that's with the MOSFET slammed against the negative rail, something I'd like to avoid. In practice, visible clipping showed up around 7VRMS output.

For a line stage, I'd change to a -24VDC rail. Op amp won't have a problem with that, but either the mute relay coil voltage needs to change, or the circuit changed to provide an acceptable voltage to the coil using a dropping resistor.

[Stuart Polansky]

Also worthy of note is that Guy did an excellent job on the board, it sounds GREAT! (No big output coupling cap!) I feed the follower tube with a 0.01uF-0.022uF cap (FAST!) and a 10Meg grid leak (6SN7 @ 7mA/150V on the plate). The mute relay (used to mute output until circuit stabilizes) is also useful as a front panel operable mute feature with a single pole switch not carrying signal current.

It is simpler to use than the Berning/Rankin/Yaniger/Phelps circuit since it doesn't wrap the follower tube in a feedback loop (this corrects at the cathode, not the grid) and has proven to work well using the same B+ supply as the remainder of the phono stage. I know Charlie had some issues with regulators interacting and went to a separately regulated supply just for the servo stage.

I have no idea whether either holds a sonic advantage over the other. I suspect implementation with the right op amps and caps (be nice if they REALLY didn't affect the sound as their respective designers thought!) makes more difference. Charlie did an awesome amount of work figuring out which op amps work best here.

[tomp]

Be wary of using over + or -18 volts DC as the power supply rail voltages on op amps. Most specify an absolute max of 18 volts. To be safe it is better to stick to + or -15 volts. There are higher voltage op amps including a new one from AD that can handle up to +/-50 volts but they are in the vast minority. As always, check the specs first.

Tom