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Old 26th November 2006, 10:00 PM   #1
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Default Single Stage Line Amp (Diff Amp) Tube?

Hello!

New thread, new topic, new project and some questions from my side.
I'd like to build a preamp with an single line stage. The preamp should be balanced to drive my Aleph-Xs. So building a diff amp would be the right way. Now the question is which tube should I use?
I've two types here on my desk: ECC81 and ECC99, a friend can give me some ECC83s, so what do you think, is any of these tube adequate for building such a diff amp or have I to get another tube? But because I have them I'd like to use them.
Is one of these tube adequate to build a constant current source?


Raphael
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Old 26th November 2006, 10:06 PM   #2
anatech is offline anatech  Canada
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Hi Raphael,
Are you going to use a cathode follower to lower the output impedance?

I would use a solid state CCS. Use a red LED for a voltage reference and a suitable NPN transistor. It's quiet and has a high impedance. I don't think cascoding the CCS would buy you much.

Someone else will have to speak to those tube types, the ECC83 is the only one I'm familiar with.

-Chris
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Old 26th November 2006, 10:24 PM   #3
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Hello,

I was dissuaded from using cathode follower. Instead of that I should use plate loaded tubes only.
Maybe paralelling two tubes on each side of the diff amp can help to lower the output impedance.

Raphael
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Old 26th November 2006, 11:09 PM   #4
anatech is offline anatech  Canada
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Hi Raphael,
I would definitely use a cathode follower, or some type of transistor as a follower. You need to provide a path for noise current and drive the capacitance from both the cable and input of the amplifier.

The other alternative is to use some kind of circuit (op amp, buffer IC) to provide the current you need.

If you want to double up on the tubes to lower the plate resistance (bad choice), you are further ahead to use a different tube to begin with.

-Chris
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Old 27th November 2006, 12:03 AM   #5
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Raphael, the design of a line stage depends to a large degree on the amp that it will drive.

The first thing to consider is the input resistance. Tube power amps tend to have fairly high input resistance of at least a few hundred kohm. That works well with a typical tube line stage that might have an output impedance of 10kohm or so. On the other hand, many SS amps have a much lower input impedance; maybe only 10k. That will not work well with a line stage with 10k output impedance. Even a cathode follower with a theoretical output impedance of a few hundred ohms will fall on its face when faced with a 10k load (unless you use a power tube that can deliver a lot of current.)

The other thing to consider is the signal voltage that the power amp requires. If the amp only needs 1 volt rms for full power and you are using a CD player that puts out 2 volts rms it makes no sense to put 50x gain in between them; the 2V from the CD player becomes 100V and then you need to attenuate it down to something like 0.5V under normal listening conditions. That can't possibly be good for the sound quality.

What are the input impedance and drive requirement of the Aleph-X?

Getting back to your choice of tubes. The ECC81 has a mu of around 60 and a plate resistance in the neighborhood of 10k. That means you would get a gain of something like 40x or 50x and an output impedance of around 10k. The gain of a long-tail pair circuit would be half that at 20x or 25x. Besides that, my personal opinion of the tube is that it sounds lifeless, but that's just my opinion; others will disagree.

The ECC83 has a mu of 100 and a plate resistance of 60k. There's no point in talking about this tube as it is completely inappropriate for this application.

The ECC99 has a mu of 22 and an rp of about 2.3k. In a LTP circuit the gain might be around 8x and the output impedance around 2k ohm. You could use this to drive an amp with a 10k input impedance, though it might be pushing things. It should have no trouble driving short interconnects. On top of that, my opinion of the tube is that it sounds far better than the other choices by a large margin. I really like the tube.

A line output transformer could be used to lower gain and output impedance at the same time, but they are not free. If you want to build with what you have (or can get easily) then there might not be much point in talking about them.

-- Dave
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Old 27th November 2006, 12:13 AM   #6
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Hi Dave,
I am a little curious. What is your view of using a cathode follower with a little feedback (say - 6 dB or so)? That way you can have a really low output impedance and reduce the impact of variations between tubes of the same type.

-Chris
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Old 27th November 2006, 12:16 AM   #7
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http://www.diyaudio.com/forums/showt...56#post1064156

ECC99 or ECC88 can be handy as anode followers for toob susy

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Old 27th November 2006, 11:13 PM   #8
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Chris, I suppose there are ways that cathode followers can be put to good use, but I think there are a lot of places that they really don't work as well as we might hope. The major problem is that they tend to run out of current long before their low output impedance would seem to indicate.

As an extreme example, an ECC83/12AX7 with a CCS under it should have an output impedance of something like 1/gm = 700 ohms. But, running at 1 mA of bias current, how much AC current can it really deliver to a load? 0.5 mA rms over a 50 V rms swing is an extremely generous estimate. Let's see: 50V/0.5mA=100k ohm. wow! Despite the 700 ohm output impedance, a 100k load is pushing things. I admit, that's an extreme example, but I hope you get the point.

I might not have answered your question; I'm not entirely sure how to apply additional feedback to a cathode follower....

===========

Choky, that is a very cool circuit. Very simple. I like simple. As obvious as it might seem, I have never tried applying feedback from plate to grid like that. Could be that a small amount will not suck the juice from the sound. I (or AudioAngel) might just have to give it a try.

-- Dave
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Old 28th November 2006, 12:50 AM   #9
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Hi Dave,
I was just looking for a way to shield the gain stage from the outside world while also reducing the non-linearity of the follower. I assumed that you would run it at a higher current as well.

-Chris
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Old 28th November 2006, 11:38 AM   #10
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I run a balanced line stage myself. I've tried cap out (russian teflons) and transformer out in Push Pull and prefer the transformer. People have tried toroid mains transformers here with good results - a cheap option. You could use 110-110 primary and use the centre point for your HT, and 110-110 or 55-55 as secondary, depending if you want step down.

For volume control, you can build a simple shunt attenuator from a 12 position rotary switch and resistors - easily done and great quality.

Leave plenty of room for valves - you may want to upgrade it to octals or even UX4 sockets for Directly Heated Triodes. If it were me I'd cut out four holes for UX4 sockets and have a sub-frame with whatever sockets you want to use right now. that way you could eventually use some really good vintage tubes.

Have a look at this page for a design
http://www.raleighaudio.com/MI%20articles.htm
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