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lochness 19th June 2008 01:19 PM

variable capacitor volume control
 
I've seen it done in REALLY old radios (never looked at how the circuit worked though). I was wondering if any of you Circuit Geniuses knew how they did it and if it could be applied to a tube stereo amp.
I've tried resistive and inductive so now I want to try capacitive.

SY 19th June 2008 02:03 PM

Really? Can you give an example? I've worked on a lot of old radios and never seen a capacitor tune the volume.

BL21DE3 19th June 2008 03:13 PM

SY,

Check out post #5 on this forum for some info on capacitor volume controls.

http://www.radiomuseum.org/forum/old..._markings.html

Regards,
Ewan

kevinkr 19th June 2008 03:51 PM

One of my side hobbies is building regenerative detector based breadboard sw radios and restoring old SW radios. The capacitor shown in that circuit varies the coupling between the input coil and the antenna and provides RF attenuation, this is pre-detection so although it works a lot like a volume control it really isn't. From an RF standpoint this isn't optimum either as it has a relatively negative impact on receiver noise floor. In my radios it is there solely to prevent overloading and squeeging in the regenerative detector. It was sometimes used in inexpensive, and early DIY radios in lieu of an additional resistive volume/gain control to reduce complexity and save cost - it is a sub optimum approach as explained above.

A cap in this location was also often used to reduce the antenna's influence on the Q of the input coil, and in regenerative receivers to prevent the antenna from loading down the tuned circuit to the extent that the detector would no longer oscillate. The optimum value differed for different antennas and received frequencies.

The circuit would not work well in audio applications because of the change in Xc with frequency, plus the load impedance presented to the previous stage and the source impedance presented to the succeeding stage would vary as a function of frequency. Two variable caps in series would create a reactive divider, but good luck in emulating a logarithmic response to changes in volume setting. IMHO This is a total non starter..

Incidentally the comments at the end of the thread about AVC might be inaccurate at best, my early 1932 vintage Atwater Kent 310 chassis is a superhet, and has AVC as well as a "shadow" tuning meter. Interestingly to get it going again all I needed to do was replace 2 electrolytics in the power supply. Everything else was still good !? Wish I had the rest of the radio.. :bawling:

coresta 19th June 2008 06:16 PM

Hi ! Have a look at this very scarce monitoring amp from ALCATEL built for the former ORTF , the french public broadcast agency : item on ipe 110260930583 ... the volume pot is a progressive loudness control , very efficient and lovely sounding for late listening sessions :)

SY 19th June 2008 07:35 PM

Ewan, educational, many thanks!

Eli Duttman 19th June 2008 11:55 PM

In addition to the info. Kevin provided, I've seen regenerative circuits that used a variable cap. as the feedback level control.

Regardless of method, tweaking the positive feedback up until just shy of oscillation was the goal.

Anybody for a "spud" (1S5) battery powered AM/BB radio? :D

EC8010 20th June 2008 08:40 AM

You could use a variable capacitor as an audio volume control. Think of an inverting op-amp but replace the resistors with capacitors, one variable. Now replace the op-amp with a high-mu triode and use Cag as one of the capacitors. Make the other variable. I'm not saying there won't be all sorts of hum problems, mind.

Tweeker 20th June 2008 08:56 AM

Anyone seen/have examples of variable capacitors used in tone stacks?

Wavebourn 20th June 2008 04:10 PM

I saw a pad switch in condenser microphones, where additional capacitance added by a switch to the capsule makes a "capacitive voltage divider". Though, input resistance of the following stage is around GigaOhm, such capacitive pad is almost frequency independent.

Quote:

Originally posted by EC8010
You could use a variable capacitor as an audio volume control. Think of an inverting op-amp but replace the resistors with capacitors, one variable. Now replace the op-amp with a high-mu triode and use Cag as one of the capacitors. Make the other variable. I'm not saying there won't be all sorts of hum problems, mind.
...utilizing Miller capacitance for good. ;)


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