• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Simple Cap-Coupled CF Advise Needed

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I built the following to place between CD analog outs and preamp....B+ value is not as noted (170), the cathode resistor is 10K.


TubeCad results indicate that the circuit should be -.343 dB, actually it is down more like 3-5 dB. It is very clean sounding, but the -3-5dB is problematic.

When I removed the the 1 meg resistor from input to ground, the excess loss of gain is reversed, however a nice buzz is audible through my Klipschorns.

Concerning the input stage...

What is the purpose of this high value resistor?

Can/Should a lower value resistor be substituted?

How does this resistor reduce gain?

always ready to learn more about the tube...

I don't care for this ciruit...but thats just me :D

Above the Low frequency pole formed from the input cap and circuit resistance.... You have an equivelent input resistance of 315K ohms...... What is the output resistance of your CD player??
Also...no need for the 100K output resistor, unnecesary loading of the follower, since this 100K will be in Parallel with whatever you plug into..... Use 1Meg....
I prefer to to self bias in this situation....

The source in this case was a Decware modded Sony 685 (text book grounded cathode output stage taken off the DAC), and the resultant output impedance is @ 18K at 5volts. See 685 output stage below:

An externally hosted image should be here but it was not working when we last tested it.

I have built 2 other CF buffers without the 1 meg to ground and no buzz. This one, however, still has the "intestines" of the power supply outside the chassis and the grounding scheme is temporarily jumpered to a common chassis and earth ground. I just wanted to make sure there were no serious issues before I stuffed it into a smallish box, 4"x 6 " x 2".

I am getting +30v dc on the grid and +33.5 dc at the cathode.

It may be that the 460 meg resistor to ground before the 1000 input resistor is not grounding properly in this temporary configuration causing some buzz.

Steve Eddy, if you really want to hear something ear splitting, (on the buffer) disconnect the ground to the caps accidently with your power amp at full volume!

I'll have to check if there is DC showing up on the input side of the cap.

Thanks for the input on the input,

Doing some quick hand calculations on this circuit.....
Your input source being 18K and your output load being another 100K.....
Your input loss is -.5dB and you low frequency POLE is at 1Hz at the input cap...

Your ouput loss is -.42dB and the output cap POLE is a little below 1Hz.....

So total insertion loss is -.92dB, when ciruit is operating normaly.

Try examining the by-pass cap for the plate of the follower...If this cap is "whacky" then it could cause degeneration and loss of Gain...

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