Why Is High Feedback Considered Bad In Audio? In Simple Terms

Chopper type opamps don't have 1/F noise, and have drifts measured in nanovolts per degree C. They are good for some low-level stuff.

I can't recall any cases where tubes are still sensible, except for very high power, microwave, and optical applications (PMTs, microchannels, night vision stuff.) Any more?

John

AFAIK they're still unbeatable for high voltage low current switching where the off-state capacitance matters. I used an 811A in a plasma measurement some years back--compared with transistors it was amazing. I'd probably have had to use a spark gap otherwise--not very appetizing.

Cheers,

Phil Hobbs

This is sort of a vacuum tube...

ftp://66.117.156.8/Kry_Danger.jpg

ftp://66.117.156.8/Kry_Guts.jpg

This is a little krytron, but it will still switch a megawatt in a couple of nanoseconds.

John

My next-door lab neighbour has a few really old ones of those. Cute little devils--he had to have a security clearance to get them, back in the day.

Cheers,

Phil Hobbs

Remember the RCA "New Vista" TVs? ;-)

Cheers! Rich

This one showed up in the Los Alamos Thursday junk sale by accident.

John

Ok that's some weird filter, but I guess you have a point there. It isn't a Bessel though!

Chris

litude

Chopper opamps do sort of have a 1/F noise. It is displaced from the zero frequency up to the Nyquist. This can lead to an increasing noise with frequency character

Do GDT lightning arresters count?

Microwave power, say above 3 GHZ and 10 W they still have turf.

Yep. Say didn't Win have a high voltage ramp into some huge cap where he ended up using a tube? I think it was about a year ago maybe more.

And since lots of lowpass filters don't overshoot, they don't ring, so there must be highpass filters that don't ring.

This makes less and less sense all the time.

John

A 1B3 makes an interesting 30KV amplifier. You vary the filament voltage to control plate current.

I used to use a neon sign transformer to charge big oil caps, through a 1B3 "controlled rectifier", with a flashlight battery and filament rheostat (long plastic shaft!) allowing manual charge rate adjustment. Amazing I'm still alive.

John

I know you can't make a high pass that is linear phase. [Highpass in the true sense, i.e. zeros at DC/] A highpass filter has no delay at infinite frequency, but has delay near it's corner. There may be a highpass/allpass combo that makes a net filter that is linear phase. Of course, linear phase doesn't always translate into no ringing.

One thing to investigate would the Linkwitz-Riley (sp) class of highpass filters. My recollection is they take an optimal delay and subtract a Bessel to get a highpass. The idea was to get a smooth response at the crossover.

You could make X-rays with that setup, if you aren't careful or do want to.

Chris

Actually, the CD player thingy cost $4.95. The strip club puts "Audio Supplies" on the credit card bill.

That's my story and I'm sticking to it.