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Old 24th April 2012, 04:12 PM   #1
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Default Line stage with u-follower and nfb

I'm planning to build a u-follower line stage with a gain of about 4-5 and a reasonably low output impedance. My tube of choice would be 6FQ7 / 6CG7.
Without any nfb, the gain would be too high for my purposes so I'm thinking of applying nfb as in the schematic below (R5 and R4).

Any opinions/experiences on this topology? I'm a bit worried that the nfb could mess with the nice sound of the 6CG7.

Edit: R6 and C2 represent the input impedance of the power amp + cable.
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File Type: png 6CG7_SRPP.png (14.9 KB, 220 views)

Last edited by Groucho2004; 24th April 2012 at 04:29 PM.
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Old 25th April 2012, 03:37 AM   #2
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I built mine a 12AU7 based closed loop (with nfb) mu follower in mid 90s. It sounds very good. Just a few dB of NFB not a problem.

C4 is way too big use .01uf. You also need to add :
1. a resistor from the junction of R3,R4 to ground .
2. a dc drain resistor (1M) in shunt with R6.
3. Bypass cap across lower cathode resistor R2.
Remember that you need to power up the mu follower first, wait for a minute b4 powering up your power amp if you decide not use an output muting relay.

Art

Last edited by audiohead; 25th April 2012 at 03:43 AM.
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Old 25th April 2012, 10:56 AM   #3
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Thanks for your comments. The schematic was just to illustrate the topology. The final version will have grid stoppers, bleeder resistors at the output, etc.
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Old 25th April 2012, 12:58 PM   #4
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A suggestion: you can put a small Cap in parallel with R4 to adjust the HF gain, and then, to compensate the large capacitive load.

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Old 26th April 2012, 08:13 PM   #5
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Mu followers based on the 6CG7 are capable of very low distortion and excellent noise performance. With NFB the distortion will be even lower.

I would make the following recommendations:

1. Decouple the lower cathode bias resistor. The low distortion of the mu follower is due to the plate load seen by the lower tube being many times its ra. By not decoupling the cathode resistor of the lower triode you unnecessarily increase its ra and hence increase distortion. 220uF should do it.

2. R1 gets multiplied by the top tube mu to form the plate load for the bottom tube. For minimum distortion I would recommend increasing R1 to 10K.

3. The 6CG7 seems to produce the least distortion around 5mA idle current. To achieve this , R2 and R3 should be increased to 1K.

4. C4 can be reduced to 100nF. The 470K (R7) is bootstrapped by the top tube acting as a cathode follower.

5. You should elevate the heaters to around 75V or so to avoid exceeding Vhk of the top tube.



Cheers

Ian
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Old 26th April 2012, 09:41 PM   #6
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I did something similar years ago and the NFB improves the linestage in every way. Better frequency response, lower distortion, lower gain, it's all so much better with NFB.

http://www.quadesl.com/pdf/lineamp.pdf

Sheldon
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Old 26th April 2012, 10:00 PM   #7
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Quote:
Originally Posted by stokessd View Post
I did something similar years ago and the NFB improves the linestage in every way. Better frequency response, lower distortion, lower gain, it's all so much better with NFB.

http://www.quadesl.com/pdf/lineamp.pdf

Sheldon

I am not surprised you needed NFB. The schematic you posted is an SRPP topology which is well known for its excellent drive capability and high distortion. Definitely needs NFB. A mu follower without NFB will beat an SRPP any day.

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Ian
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