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Old 30th August 2004, 06:19 PM   #1
satox is offline satox  United States
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Default Dual Stereo Cathode Folower Setup

Hi,

I am very new to the world of tubes, but I am otherwise a pro audio professional. I have a unique situation for which I would like help:

I have 4 channels of unbalanced audio (0-4 volts max) that I would like to pass through tube circuitry for warmth/etc with a unity gain output. I have decided (after reading many posts) that a "Buffered Tube Cathode Follower" would accomplish this. I have 2 ECC83 (12AX7A) Preamp Tubes (Ruby Tubes brand) that I would like to use for this project (1/2 of each for tube for each of the 4 followers). There is no need for any type of control, as this circuit would be hidden from the casual operator.

Please help me design a circuit for these requirements, hopefully of limited complexity (with great sound though!)-cost is not the main issue. As I am not an electrial engineer (though I can solder and read diagrams), a paint by numbers schematic would be greatly appreciated.

Thanks a ton.
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Old 30th August 2004, 07:14 PM   #2
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FYI, a CF will not add "warmpth" (distortions) as it is a very clean amplifier.

Since you don't need gain, I don't know what else to tell 'ya... a CF in that regard would be ideal, except for the above reason. You could use a low-mu tube (like 12B4) in common cathode and use a voltage divider on the input to reduce gain back to unity.

If you insist on your chosen design, a 100 to 300V supply capable of 5mA will be needed, as well as 6.3V 0.6A or 12.6V 0.3A for heat (AC or DC, doesn't matter at all). If you change your mind, 12B4s are beefier and will need maybe 10mA minimum per - 40, preferably 80mA at 150 to 250V. Things multiply fast with four channels!

Tim
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Old 30th August 2004, 09:11 PM   #3
satox is offline satox  United States
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Default Continuing

Tim,

I appreciate the reply.

p1. I wonder if there is a way to add the required warmth/distortion?A quad gang pot controlling this action for the 4 circuits, would be amazing to say the least. That would be my ideal setup-too much to hope for?-with a fixed level of distortion a second best case. Third choice is a more complicated arrangement, as you suggest later.

p2: From what I understand, I would need four of the 12B4's. As I intend on rackmounting at least the tube half (PSU elsewhere if space is too tight), I wonder if 4 tubes (and related circuitry) would create heat/space problems for me? If not, would the voltage divider hurt the incoming signal-it's a very high grade chain? I'm not committed to the 12AX7A, I just happen to have them on hand.

p3: For power, the 12AX7's look easier to manage, but I'm not afraid of doing what it takes power wise, if more power is needed!

Once I settle on a final idea, I'm going to need help in drawing up a circuit...

Thanks again
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Old 30th August 2004, 10:46 PM   #4
SY is offline SY  United States
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You might consider using a common-cathode amp direct coupled to the cathode follower, then a feedback loop to bring the gain down to whatever works for you. Disadvantage is that feedback will vary a bit with volume control setting, but since you're not shooting for ultimate low distortion... Advantage is that your output impedance will be really, really low. This will take four dual triodes in all.
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Old 30th August 2004, 11:13 PM   #5
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No, you get the exact same problem. CF = mondo NFB for a bit less than unity gain. Your proposal = mondo NFB (depending on type) for around unity gain. Would work okay with a low gain tube, but I don't know of any that aren't old and/or direct heated that have mu = 2 to 10.

Size is no problem, I could fit an entire stereo power amplifier in a rack mount, depending on how much height I'm allowed. This project will be no problem. As for heat, the worst case in my above post was 4 x 12B4, each running 20mA 250V - 80mA 250V supplied = 20W plate supply dissipation plus 6.3 * 0.6 * 4 heaters = 35W total dissipation. If the case is closed up, it might get a bit stuffy; a fan will help.

A distortion "balance" control - interesting idea. The problem is, I think you may need a second stage, because one grounded-cathode stage inverts the signal. Otherwise I would've recommended putting a potentiometer between input and output points (the input to the tube has a voltage divider so its gain is reduced to 1) and using the wiper for output. Thus at one end, it finds only the input signal, no distorted output voltage - at the other, no input, only the distorted output.

Oh- other options - no need for ideal bias current. Could run a 12AU7 at .5 or 1mA, gain would be low (particularly into a small load resistance) and distortion high. It shouldn't be too high as the signal is so small in comparison to the maximum output voltage.

Tim
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Old 30th August 2004, 11:23 PM   #6
SY is offline SY  United States
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Quote:
No, you get the exact same problem. CF = mondo NFB for a bit less than unity gain. Your proposal = mondo NFB (depending on type) for around unity gain. Would work okay with a low gain tube, but I don't know of any that aren't old and/or direct heated that have mu = 2 to 10.
I don't follow you, Tim. If you want, say, a gain of two, you use something like a 12AU7 (plenty of distortion there, especially at low current) or a reall effects tube like a 6ES8, cathode degenerate it, then bring in the loop to get a closed loop gain of, say, 1 to 5 (whatever you want). No DHTs or other exotica needed. What's the problem?
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Old 30th August 2004, 11:46 PM   #7
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Hi,

Quote:
I don't follow you, Tim.
You're not the only one....Chalking it up to remanence.....

Oh, what on earth does mondo mean? Molto, mucho?
Or just being a fan of Mondo Generator?

Quote:
Would work okay with a low gain tube, but I don't know of any that aren't old and/or direct heated that have mu = 2 to 10.
6AS7G and family for octals, 6S19-P for novals...

Quote:
Could run a 12AU7 at .5 or 1mA, gain would be low (particularly into a small load resistance) and distortion high.
Oh yeah, the formidable 12AU7....
So much for tube sound distortion: predominantly even order harmonic distortion is what the doctor ordered, no?

Cheers,
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Old 31st August 2004, 01:18 AM   #8
satox is offline satox  United States
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Thanks for all the replys. The ideas above are great-simplicity is my friend in this. If only we could achieve some kind of consensus, and a plain english version for me to follow!
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Old 31st August 2004, 04:30 AM   #9
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Quote:
Originally posted by SY
I don't follow you, Tim. If you want, say, a gain of two, you use something like a 12AU7 (plenty of distortion there, especially at low current) or a reall effects tube like a 6ES8, cathode degenerate it, then bring in the loop to get a closed loop gain of, say, 1 to 5 (whatever you want). No DHTs or other exotica needed. What's the problem?
True, but most of the gain reduction is performed by distortion-reducing NFB. (There's also the issue of a wider band of distortion products resulting from these levels of NFB, which could be significant at this level -- considering most tubes produce almost only 2nd H, in nearly undetectable amounts of course; but nonetheless.) Of course, it's also debatable how much distortion is needed for "warmth". I personally would jam it full to make sure the end user hears it (this is for studio application right?), to be sure your sensation of warmth isn't just the image fixed in your mind of pretty orange tubes glowing. But not so much that it sounds like crap (>1% THD is it?). Hmmm, this needs research. I think you're going to have to build several models, Satox.

Tim
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Old 31st August 2004, 11:08 AM   #10
Yvesm is offline Yvesm  France
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Hi !
It will probably difficult to obtain a consensus from various poster, and it's a good thing

So here is my contribution :

Click the image to open in full size.

Look at this as no more than a base for experimentation.
This thing has lo gain, no phase invertion and may be build with a tiny cheap 6J6 tube ... among many other.

Regards, Yves.
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