Help with adjustable CF line out on Bogen EF86 mic pre

[tubby]

Hi all,

I´m trying to add an adjustable line out to the MIC2 channel of a Bogen MXM mixer/pre.
Pls find schematic attached for the Bogen unit.

So far I´ve disconnetcted the Vol 2 pot so the signal is not unnecessarily attenuated and added a simple
AC coupled CF (1/2 ECC83, 280V on anode, 100k cathode to ground).
The CF grid is connected directly to pin 6 (anode; at ~100V) of the EF86 on MIC2 channel.

Now when I scope the CF´s output on pin 3 I get a nice clean signal which slightly compresses with larger signals but never clips
So far so good.
As soon as I add a load (25k pot to ground, in order to make the line out level adjustable) to the CF´s output via a coupling cap
(e.g. 100nF) the signal is at once clipped hard on the bottom part of the curves at all except very low drive livels, see pic attached.

What´s going on?

[Helmholtz]

AC coupled CF (1/2 ECC83, 280V on anode, 100k cathode to ground).
The CF grid is connected directly to pin 6 (anode; at ~100V) of the EF86 on MIC2 channel.

There's a contradiction here.
I guess you mean a DC coupled CF.

It would help to see a drawing how you connected the CF and the pot.

What is the intended application?

[tubby]

There's a contradiction here.
I guess you mean a DC coupled CF.

It would help to see a drawing how you connected the CF and the pot.

What is the intended application?

Correct, it is a DC coupled CF.

Attached you'll find a drawing.

Intended application is an adjustable line out to use with an audio interface for recording purposes.

[Helmholtz]

Attached you'll find a drawing.

What is the cathode voltage? Does it change when you disconnect the 0.1µ coupling cap?

[tubby]

What is the cathode voltage? Does it change when you disconnect the 0.1µ coupling cap?

I made some more measurements:
- Cathode voltage is 60V; it does not change when i disconnect the coupling cap (no signal applied).
- The output from the CF starts to clip on the bottom half of the waveform when the 1 kHz sine input signal exceeds 165 Vpp
(much higher than expected), so I guess the CF runs out of headroom?
- Ideally the CF should be able to cope with drive levels of 200Vpp. Could a solution be attenuating the signal between the EF86 and the CF?

[Helmholtz]

I made some more measurements:
- Cathode voltage is 60V; it does not change when i disconnect the coupling cap (no signal applied).
- The output from the CF starts to clip on the bottom half of the waveform when the 1 kHz sine input signal exceeds 165 Vpp
(much higher than expected), so I guess the CF runs out of headroom?
- Ideally the CF should be able to cope with drive levels of 200Vpp. Could a solution be attenuating the signal between the EF86 and the CF?

Still trying to understand the strange and low ouput signal in your scope pic.

- With a cathode voltage of only 60V, negative going signal will be clipped at (-) 60V. Moreover it means that the EF86 plate voltage can't be higher than 60V either.
How does the CF grid signal look?
- Where is the 200Vpp number coming from? What output level do you actually need?

One of the problems of your design is loading the CF with 25k. This causes a very steep AC loadline which limits output swing.
Ideally the load should not be smaller than the cathode resistor value.
If you only need a couple of volts at the output you could wire a 100k resistor in series with the 25k pot.

A DCCF is a tricky and non-linear crcuit. If you don't need the compression/distortion from the DCCF, I would go with an AC coupled CF which provides better linearity and offers more freedom for design.

[tubby]

Still trying to understand the strange and low ouput signal in your scope pic.

- With a cathode voltage of only 60V, negative going signal will be clipped at (-) 60V. Moreover it means that the EF86 plate voltage can't be higher than 60V either.
How does the CF grid signal look?
- Where is the 200Vpp number coming from? What output level do you actually need?

One of the problems of your design is loading the CF with 25k. This causes a very steep AC loadline which limits output swing.
Ideally the load should not be smaller than the cathode resistor value.
If you only need a couple of volts at the output you could wire a 100k resistor in series with the 25k pot.

A DCCF is a tricky and non-linear crcuit. If you don't need the compression/distortion from the DCCF, I would go with an AC coupled CF which provides better linearity and offers more freedom for design.

- I measured the EF86 plate at ~100V.
- The CF grid looks undistorted.
- The 200Vpp is the peak signal I can expect at the CF's grid when I use a large diaphragm mic for input.

Thanks for the input, I've rewired to an ACCF which definitely makes more sense here.
Circuit works fine now!

[Helmholtz]

- I measured the EF86 plate at ~100V.

That's strange. With a DCCF the EF86 plate voltage and the CF cathode voltage (60V) should be essentially the same (within 2V).

- The 200Vpp is the peak signal I can expect at the CF's grid when I use a large diaphragm mic for input.

Clean 200Vpp at the CF output requires the cathode voltage to be larger than 100V.

Circuit works fine now!

Congrats!

Did you find an explanation for the strange scope signal of post 1?

[tubby]

That's strange. With a DCCF the EF86 plate voltage and the CF cathode voltage (60V) should be essentially the same (within 2V).



Clean 200Vpp at the CF output requires the cathode voltage to be larger than 100V.



Congrats!

Did you find an explanation for the strange scope signal of post 1?

I did find the coupling caps (.1, .0015) on the EF86's anode were all leaking DC. That might have messed with the CF?

So far I'm not really happy with the experiences I had with CF's driving heavy loads, like inputs of digital power amps (~ 8k input impedance).
I have tried some designs from Merlin B's / Valve Wizard's book which are supposed to be able to drive those loads, but they were always clipping on me on the bottom half of the waveform.
Maybe a FET source follower would be better suited?

[Helmholtz]

So far I'm not really happy with the experiences I had with CF's driving heavy loads, like inputs of digital power amps (~ 8k input impedance).
I have tried some designs from Merlin B's / Valve Wizard's book which are supposed to be able to drive those loads, but they were always clipping on me on the bottom half of the waveform.

What tube did you use? An ECC83 won't do because it can't provide sufficient current for low impedance loads.
Merlin typically shows an ECC82, which should work ok.
You could even wire the 2 triodes in parallel for lower output impedance. But your power supply must be able to deliver enough current.

Still not clear how much clean output signal you need.

http://valvewizard.co.uk/accf.html

[tubby]

What tube did you use? An ECC83 won't do because it can't provide sufficient current for low impedance loads.
Merlin typically shows an ECC82, which should work ok.
You could even wire the 2 triodes in parallel for lower output impedance. But your power supply must be able to deliver enough current.

Still not clear how much clean output signal you need.

http://valvewizard.co.uk/accf.html

Hmmm, I used an ECC82 as in the schematic in Merlin's book. I probably must've had a mistake in there somewhere.
Interesting idea with paralleling two triodes! I'll try that.

For the application in the power amp mentioned in my last post the input impedance is 8k
and max differential input voltage is specified as +-12V.