Variable capacitors

[C.B. - Boudio]

Something I've been lately thinking about: would it be possible to use a variable capacitor, as found in older radio's and tuners, to make an equaliser with a stepless variable frequency? (usually the mids, then)
Or are there too much disadvantages to even consider it?

 

[RuudNL]

Most variable capacitors as used in (AM) radio receivers are 500 pF max.
So if you want to use them in an audio circuit, this means that you have to use very high impedances!

 

[gyraf]

There is a company doing this, the audiotales 1951.

if you want continuous tunable, it's probably better to tune inductance (variac)

Jakob E.

 

[C.B. - Boudio]

Most variable capacitors as used in (AM) radio receivers are 500 pF max.
So if you want to use them in an audio circuit, this means that you have to use very high impedances!

That's something I thought about as well.

There is a company doing this, the audiotales 1951.

I'll look into that.

 

[sahib]

Here is a simple example.

 

[poppaflavor]

There is a company doing this, the audiotales 1951.

if you want continuous tunable, it's probably better to tune inductance (variac)

Jakob E.

Very interesting, I didn't realize that was the tech behind the device. Do you know which feature utilizes the continuously variable capacitor? I need to learn more about it; from some videos I thought the Aircap and Barytone were 'just' on/off toggle while the knobs were potentiometers.

 

[gyraf]

https://soundandmusic.ch/wp-content/uploads/2021/11/MasteringEQ_open800x436.png

..looks like the variable capacitor is for the light-blue to dark-blue unspecified control for the shelving high band, slightly grandiosely named "Aircap". Probably controls shelving frequency, i.e. the cap in series with the viper in bax circuit.?

 

[Barry Farmer]

Something I've been lately thinking about: would it be possible to use a variable capacitor, as found in older radio's and tuners, to make an equaliser with a stepless variable frequency? (usually the mids, then)
Or are there too much disadvantages to even consider it?

With the traditional radio-style variable capacitors, you won't be able to get the values required for working in the audio spectrum. Plus you'd have to tool-up for making them. And anyway, why?

 

[Matt Syson]

I suppose a carefully designed physical layout of parts and FET input op amp could be used with decent results or even a 'bootstrapped' BJT input op amp once you get your head around the bootstrapping. It is mainly the high impedance aspect (with the probability of stray capacitances upsetting things) that make it tricky. For the really adventurous, try it in stereo with a pair of mechanically ganged variable capacitors!

 

[poppaflavor]

https://soundandmusic.ch/wp-content/uploads/2021/11/MasteringEQ_open800x436.png

..looks like the variable capacitor is for the light-blue to dark-blue unspecified control for the shelving high band, slightly grandiosely named "Aircap". Probably controls shelving frequency, i.e. the cap in series with the viper in bax circuit.?

Nice! Thank you! I hadn't seen any internal pics, and think you nailed it here.

I've actually got one of the 1951 (and a 1954) on order now being built, and your observation has me even more keen to explore this device. Will hopefully be just a few more weeks until it arrives.

I've heard some pretty tricky things about its pairing with other devices in line, and the output impedance of 500 ohms will take some consideration.

Along these lines, I believe you might be the perfect person to give me a leg up on where to start with this follow-up question...

I have your passive Gyraf G21 (hands down my most favorite and intriguing piece of gear so far).

Would it be futile for me to attempt to put your G21 directly after the 1951, given that the 1951 has a 510 ohm output and according to the manual the G21 has a 500 ohm input?

Or is this okay because It seems to mimic the older style of matching impedance (600:600)?

I'm hoping to use the G21 as a final shaper and clipper after this EQ, but I'm not sure if I need an intermediate solid state active device between them.

I realize I'm probably just going to have to try it, but I was curious if you had any insights into this pairing.

Apologies if this question veers too far from the intent of this thread, I can message you instead if that's more appropriate.