Phono stage for 2 7F7s (stereo) plus buffer? - diyAudio
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Old 19th August 2008, 03:37 PM   #1
GordonW is offline GordonW  United States
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Default Phono stage for 2 7F7s (stereo) plus buffer?

Got a chassis here... only have room for two three octal or loktal sockets for the phono stage.

I'm thinking 7F7, after checking around everywhere I could think, due to the much-lower capacitance (compared to any octal I could find) and good-looking plate curves. With probably a 7F8 or 7N7 as a cathode follower, in the output...

Anyone got any pet circuits that would work? I've already looked at the Hagerman Cornet circuit and Bob Danielak's DC-coupled design (modified RCA, apparently), subbing 7F7s for the 6Sl7s. If I used Bob's circuit, I would add a cathode follower buffer to the output.

Any other designs I should be looking at, that can use a total of three loktal twin-triodes for the entire stage (including buffer)?

Thanks!

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Gordon.
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Old 19th August 2008, 03:45 PM   #2
GordonW is offline GordonW  United States
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Here's the schematic for the Bob D. DC-coupled design.

I would probably spring for diode-generated bias, at least in the first stage, to get rid of that huge cathode cap... it's just under two volts needed...

Click the image to open in full size.

BTW- it looks like the Hagerman Cornet (from the notes on his drawings) only has 36dB gain?! That's awfully low. Need at least 40-46 dB or so, IME, to work well with most moving-magnet cartridges (i.e, to match the output levels of most CD players, tuners, tape decks, etc... between .5 and 1v line output with 5mv input). Here's a link to the schematic for that:

http://www.hagtech.com/pdf/cornetoctal.pdf

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Gordon.
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Old 20th August 2008, 01:12 AM   #3
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Gordon,

IB in the high mu/high RP types is too low for reliable LED operation. A tweaked RCA circuit, using grid leak bias in the 2nd gain block, improves bass extension and eliminates a cathode resistor bypass capacitor. Look here. A 'SL7 and 2X 'SJ7s will yield all the gain you could possibly use. Buffer the 'SJ7s with ZVN0545As.
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Old 20th August 2008, 07:39 PM   #4
GordonW is offline GordonW  United States
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Quote:
Originally posted by Eli Duttman
Gordon,

IB in the high mu/high RP types is too low for reliable LED operation.
I was thinking that myself.

Of course, I can probably find a couple of fast-recovery diode models, that I can connect in series, that won't mind the low current...

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Gordon.
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Old 21st August 2008, 12:16 AM   #5
SY is offline SY  United States
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Take a look at my measurements in the 12AX7 thread. That was with an LED and 0.8mA with a comparison to the same setup with the LED running 8mA. No real difference.

If it bothers you, you can bump up the LED current by running a resistor from the B+ to the LED-cathode junction sized to give you whatever current you want. That won't affect the tube's operation at all.
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Old 21st August 2008, 03:14 PM   #6
GordonW is offline GordonW  United States
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Quote:
Originally posted by SY
If it bothers you, you can bump up the LED current by running a resistor from the B+ to the LED-cathode junction sized to give you whatever current you want. That won't affect the tube's operation at all.
This was what I was thinking of.

Probably something like a 100K resistor... that would get the total current up into the 4ma range. Should be able to be done with a 2w resistor, if my calculations are correct...

Oh, one LED question- would there be any benefit to bypassing the LED with a good high-quality film cap? Possible high-frequency diode noise shunt?

Thanks...

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Old 21st August 2008, 03:17 PM   #7
SY is offline SY  United States
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No, no real benefit. The dynamic impedance of the LED is on the order of a few ohms. FWIW, I use LEDs to bias each stage of my MC phono preamp with no noise problems.
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Old 21st August 2008, 03:23 PM   #8
GordonW is offline GordonW  United States
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Given the low impedance of the LED... SY, have you ever used more than one channel's worth of tubes, on one LED? I'm thinking that the constant-voltage aspect of the diode would make "crosstalk" between the two pretty much nil? This would put more current on the single LED, making it more "stable", in even a worst-case condition...

Sorry for the piecemeal questions... the thoughts are coming a bit at the time this morning...

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