Input impedance of a driver circuit for “VU” magic band level indicator?

[rock soderstrom]

Hi guys, I'm currently working on a circuit board for EM84 indicator tubes that will function as a pseudo VU meter at preamp level.


Here is the circuit, as you can see a transistor (and the voltage doubler) provides the adjustable signal gain to reach the -22V the EM84 needs for full scale.



My basic consideration is that the input impedance of the driver circuit should be rather high in order not to load the amplifier circuit unnecessarily and to keep the THD low. Is that correct so far?


The circuit shown uses a BJT transistor such as a 9014 or BC549, which as far as I know have rather low input impedance. Wouldn't a JFET transistor be better?

What do you think?

 

[ruffrecords]

The input impedance is approximately R1 in parallel with R2 in parallel with Q1 beta times R4. Assuming Q1 beta is over 100 then the input impedance will be a little less that 100K

Yes, a FET can give you a much higher input impedance but you could achieve a similar effect by bootstrapping Q1

Cheers

Ian

 

[rock soderstrom]

The input impedance is approximately R1 in parallel with R2 in parallel with Q1 beta times R4. Assuming Q1 beta is over 100 then the input impedance will be a little less that 100K

Yes, a FET can give you a much higher input impedance but you could achieve a similar effect by bootstrapping Q1

Cheers

Ian

Thank you Ian! I will test to what extent this makes a difference. My test setup is ready, I made the PCBs this morning and then assembled them over the day. Thanks to the "Ruud PSU" I was up and running quickly - everything seems to be working. I'll continue tomorrow...

 

[ruffrecords]

Looking good. My very first tape recorder had an EM84. Robuk RK4.

Cheers

Ian

 

[rock soderstrom]

Looking good. My very first tape recorder had an EM84. Robuk RK4.

Very similar to the Grundig TKs.

Here a short video with the EM84s in action. Right side with 100nF "release" cap looks way better than the 470nF on the left.

View attachment 1000031693.mp4