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.

 

[gyraf]

..it's really hard to predict from specs the ability to drive the G21 (output impedance does not specify output drive capabilities) - I'd recommend just trying it and see what it behaves like..

 

[jfkelliher]

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?

maybe look at neve eq clones. I think he came up with a stack of inductors for his eq...someone else mentioned inductors...I don't know if the 1073 is "continuous" enough.
You should take this with a boulder of salt because I am just a guitar player with no EE knowledge.

 

[JohnRoberts]

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?

Most equalizers involve RCs... Its far easier to vary the R.

JR

 

[rh001]

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?

Look into "gyrator" circuits. In such circuits, a gain stage is used to amplify the quantity of capacitance so that a small variable cap can become a large cap. Also, if you're into scrounging parts I think some rather largish variable air caps were made back in the vacuum tube and early transistor days and used in audio oscillators so they might have enough variable capacitance for use in audio filter circuits. For instance, this one: https://www.ebay.com/itm/2757683226...uid=h_3YaUgHS0q&widget_ver=artemis&media=COPY

 

[Somkereki]

Yes, this makes me nostalgic, because I was still a child, when we took such 3-stage variable capacitors out of old AM radio receivers. Later, I only encountered such in more expensive FM receivers. The Varicap diodes came later, maybe even now there are 3 identically selected in the factory paper packaging. At the age of 70, it is interesting to think about the changes in technology. I did not use the term development on purpose, since everything is directed towards cheaper manufacturability. I recently repaired a friend's old Quad 66 preamplifier and I was surprised to find a very serious "ringing" at the top of the signal (amplitude 20%) in the case of square signal control. the problem. Also, the phase delay was reduced from 48 degrees to 14 degrees! (in the case of 20 KHz) However, I measured the electrolytic capacitors as good when measured with an HP measuring bridge. Although I have known this problem for a long time, I block all electrolytic capacitors in the path of the audio frequency signal in this way. If it's only 1 - 2 uF, I'll replace it with a Wima polypropylene capacitor. You can really hear it! Back in 1980, I was a sound engineer for rock bands, when my colleague brought me a photocopy of the IEEE magazine, about the sound of capacitors. We talked about it in the hotel rooms. We fought "maniacally" for the best sound.

 

[Somkereki]

I omitted from my previous post that I connected 680 nF polypropylene capacitors in parallel with the 50 uF electrolytic capacitors in the Quad 66 preamplifier. I've been using it for a long time now for good quality sound. Moreover, when I design audio circuits, I only use foil capacitors if possible. In my friend's KEF speaker, I also found bipolar electrolytic capacitors in the crossover. I also replaced those with Mylar capacitors. The result was incredibly better sound!