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In Reply to: RE: Which leads to the question. Is it better to go for maximum bandwidth or sufficient bandwidth?... posted by Cleantimestream on July 21, 2017 at 02:47:04
General rule of thumb for measuring equipment is 5x the measured bandwidth. So to accurately measure a 20khz square wave, your scope needs at least 100khz of bandwidth.And I would say today, an audio amplifier that goes out to 100khz is good but also the reasonable limit. Going beyond the 5x rule is just wasting money and additionally inviting stability issues you don't need.
A popular example is the HK Citation power amps with their video pentodes in the driver section. But there was a very good reason for this. Remember that the Citation uses nested feedback loops. Stu Hegman needed video bandwidth to get the gain he required and keep phase shift low at the same time. But as you can see these were somewhat complicated audio amplifier circuits for the day. The complexity is in the math of selecting those component values.
Edits: 07/21/17Follow Ups:
My bad.
I was thinking almost exclusively of speakers and the desire to go far beyond 20k. This IS DIY Tube.
Am well aware the 'usefulness' of wide bandwidth in output transformers and agree 100k is excellent and in some cases {yours mentioned} required. Writing of output transformers, Silk and Hashimoto are the equal {better?} of many highly revered ones.
The Mind has No Firewall~ U.S. Army War College.
I think the video pentode was just an easy choice driven by output impedance, gain and the required plate dissipation rating required to get the first two. For gain, gm and load resistance rule, and with output impedance, the load is set...which drives plate dissipation requirements well beyond the usual signal pentodes.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"General rule of thumb for measuring equipment is 5x the measured bandwidth. So to accurately measure a 20khz square wave, your scope needs at least 100khz of bandwidth."
Not to nitpick your comment, but for accurate visual analysis of square waves, 10X is a better rule. We have scopes where I work with bandwidths of 1.5 gHz, and we make (among other things) devices that output TTL-level square waves to beyond 200 MHz. I've tested 80 MHz square waves (one of our "standard" frequencies) with both a 400 MHz scope and our 1.5 gHz scope. There's a marked improvement with the higher frequency instrument.
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