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Old 11th May 2002, 02:00 PM   #1
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Default 5687 long tail pair

The circuit is from http://www.anidian.com/audio/construct/p3a_adden2.shtml

It looks interesting and easy to built. I was hoping to built the circuit but the current source value and circuit layout wasn't indicated in the website. I hope Tiroth answers this thread.
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Old 13th May 2002, 01:57 PM   #2
tiroth is offline tiroth  United States
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Nick,

I used the "preferred current source" found on Rod's page.

http://sound.westhost.com/ism.htm

Scroll down a bit to find it. (the actual current source is the part above the "output" of course) This is a pretty common design--you'll need to run it from a negative supply of -20V or more, and use the complements to the transistors shown.

Rod gives the formula for obtaining the current:
I=0.65/Re

Some measurements on your chosen transistors (I used BC546) are in order, but you know that about 55-65 ohms is right for Re. The resistor to ground should be 100K.

I'm still refining this circuit, but it does work well, and with 5% resistors and no attempt at matching provides 24dB CMRR. Omit the current source and CMRR drops to 3dB in this configuration.

===

This circuit was developed with the aid of quite a few people on this board. The thread where much of the discussion occurred is "Plate follower and applications".
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Old 13th May 2002, 03:09 PM   #3
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Default Thanx

thanx for the help. Really looking forward to built this circuit. By using a LTP with tubes would the circuit be imprinted with the tube like sound.
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Old 13th May 2002, 03:57 PM   #4
hifi is offline hifi  Sweden
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would it be possible to draw an negativ out from the other tube? to make the preamp balanced?

/micke
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Old 13th May 2002, 05:28 PM   #5
tiroth is offline tiroth  United States
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hifi:

Certainly!

NickC:

The circuit was designed for transparency. The "tube sound" will come through more and more as B+ and bias current are lowered--more and more harmonic distortion will result.

For instance, running the circuit on my bench supply with B+ of 30V and bias of 3mA resulted in harmonic distortion of about -48dB, almost pure 2nd harmonic to the noise floor of -110dB. Although one might suspect this sounds awful it really isn't as bad as would be expected. (I still recommend building the circuit as described)
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Old 13th May 2002, 05:43 PM   #6
hifi is offline hifi  Sweden
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hello,

ok, I thougt it was some audiophile reason not to make it balanced.

I got some and e88cc (telefunken goldpins) that I think I will try to design a LTP for them with current sorces. btw should the currentsorces be Mosfet or BJT based ? any prefereces there?

Or maybe IŽll just rescale the bride of son of zen for tubes.


Anybody knows where to find a Pspice library with e88cc?




/micke
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Old 14th May 2002, 12:31 PM   #7
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I am quite new to this tube stuff. 5687 tube is used in your site, what would it differ from the 12au7, ax7. I not sure on how to load up the characteristics of the 5687 on the circuitmaker program. I taught might use this tube as can't simulate for the 5687
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Old 14th May 2002, 01:04 PM   #8
tiroth is offline tiroth  United States
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12AX7/12AU7/5687 are all quite different, with steadily decreasing mu and Rp of ~60k/7k/2k. Using the 5687 helps to maintain a low output impedence.

You cannot directly substitute one for another. There are plenty of circuits published for 12A?7, including the LTP circuit in the "plate follower" thread.

Or, you could use the student version of Pspice. The simulation models work with that without modification.
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