Variable capacitors

[RoadrunnerOZ]

I’m surprised no-one ever made a liquid electrolyte variable capacitor - bit of engineering as far as a liquid sealed tank for a 3 gang but in those days there was a lot of fine engineering just to make an air gap tuning cap and things were big. They could have made them smaller using liquid maybe larger values suited to audio……..

 

[Somkereki]

Well, my opinion is primarily based on an A-B test, as with all audio equipment. I do have quite a few good instruments, for example Neutrik A-2 D Audio Test & Service system, THD, IMD, phase etc. for measurement, as well as several generators, oscilloscopes, 6.1/2 digit HP multimeter. But the most important is the comparison test with switching. Fortunately, I have a friend with a very serious recording studio and several trained musicians. (I was a sound engineer in many rock and jazz bands when I was young.) I am 70 years old now. I always listen to their opinions. My "obsession" is that noisy things always sound bad. Maybe I have a prejudice about LF... and TL071 etc. from times. I remember building a 24-channel mixing desk for my band in 1980. Then I selected all the parts according to their sound. For example, there was the LM301 in both a plastic case and a metal case (military) version. Both were made by NS. The metal casing (M) had an incredibly nicer sound. Although according to the catalog data sheet, only the operating temperature range is different. My test material was a recording of jazz music recorded with a professional microphone from a Uher Report tape recorder, the power amplifier Crown DC 300 A The monitor speaker is a custom-built Isophone KK-10 dome tweeter. Also, the square signal (and sawtooth signal) transmission from the function generator. Checked with an oscilloscope. At my previous job, I learned what the various transmission distortions of a square signal mean. Transients constantly appear in the music, and very rarely a clear tone. That's why I wrote that I already bought several types of OP series FET input operational amplifiers, but I haven't had time to test them yet. (Unfortunately, my Chronic Lyme disease takes a lot of time, but I'm not young either.) For example, I recently had to repair the ATC monitor (active) speakers of my studio friend. I'm already retired, but my hobby is the same as my job, I can't stop. This makes it easier to bear the severe pain caused by my illness. I wish everyone good health!

 

[poppaflavor]

..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..

Thanks very much, I'll do some exploring and report somewhere here how it goes.

 

[vintageampfixer]

If you really want to use a variable capacitance, then a Varactor (varicap diode) might be an easier route to take because they are at least solid state, somewhat more reliable, significantly smaller and indeed readily available. That said, their capacitances still tend to be in the range starting with a few 10s of pf ... and to quote JR:

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