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In Reply to: Re: Thanks Charles posted by Charles Hansen on April 9, 2007 at 17:40:24:
The junctions that form all types of transistors: bipolar, MOSFET, and JFET, have capacitances that vary with the instantaneous voltages across them. This means the signal modulates the open-loop bandwidth of a transistor amplifier stage.Running a gain stage from a high supply voltage minimizes this variation. Running a high bias current reduces the effective resistance in series with the capacitance (as does shining laser light on it, ala Edge...).
Tubes don't have this particular problem, but all of them are microphonic to some extent.
Follow Ups:
< < ... minimizes this variation....as does shining laser light on it, ala Edge... > >Let's just say I'm from Missouri on this one. In the first place, Edge doesn't even make this claim. Instead they claim that they somehow use the laser to bias the output stage.
However, I'd love to be proven wrong. Do you have any links to anything that shows that shining a laser on a semiconductor junction minimizes the capacitance variations?
(And even if you did, would it really be a good idea to cut open the hermetically-sealed package so that you could shine the laser on it?)
"However, I'd love to be proven wrong. Do you have any links to anything that shows that shining a laser on a semiconductor junction minimizes the capacitance variations?"I once had a transistor that was light sensitive. Seriously! When a customer would open up a telecom cabinent that contained the rectifier that it was in, the voltage out of a rectifier would mysteriously go up. Opening and closing, shading... led to that one transistor. Perhaps edge got a hold of a batch of them.
The capacitance still varies, but there is less R-C time delay associated with the variation.I spoke with the designer about it at a CES/THE show. It is a way of increasing the carrier concentration without high electrical bias.
The package is there to keep the outside world away from the silicon die. For example, EPROM memory chip packages have expensive little windows in them to allow the UV light, but nothing else, to penetrate. Dust, dirt, polluted air, and humidity will combine to contaminate the exposed surfaces of the silicon die, create leakage paths, and encourage corrosion of the metal leads and bonds. Cutting open a transistor package is not something I would do.
< < Tubes don't have this particular problem, but all of them are microphonic to some extent. > >Plus most of them use steel pins and steel internal parts and are prone to hysteresis distortion from the magnetic fields.
Not to mention the fact that they are wearing out every time you turn them on...
Well, it’s a good thing then that being a high voltage device, the currents are exceedingly low and so are the magnetic fields produced internally. A tube like a 6SN7 can produce VERY low distortion with a spectrum that is low order. Hysteresys distortion is a problem in output transformers associated with Tubes however and they are to varying degrees microphonic.
For that the tap test will tell.Yes they wear out a little bit each time but finding either at a garage sale or attic, the odds of a 20 year old solid state amplifier working are smaller than that of a 30 or 40 year old tube amp working. Plastic transistors are not “air tight” like Pyrex and frit glass, they usually get leaky and fail. Tubes can leak too when the repeated thermal expansion causes micro fractures around the frit and pins.
SS and Tubes are entirely different ways of doing the job, each has totally different strengths and weaknesses from an engineering standpoint.As for the “memory”, the thermal shift is real, I have no idea if you can hear it.
On the other hand, for a Voltage amplifier the cascade approach already minimizes the collector swing on the onput, making the Voltage between the two transistors larger would further reduce the pdis swing and increased the avg pdis.
Heat them inputs up with static bias
Best,
Tom Danley
So right. A tube amp is simple for anyone to fix. It's easier to replace six or eight tubes than it is to find out which of the 40 or 50 transistors inside has failed.
Provided the tubes are good, they can last much longer than a ss amp. Replace the handful of electrolytic caps. every ten or fifteen years and you are all set for another decade. Or you can do the same with a ss amp, although replacing 40 electrolytics in it might be a hassle...
I literally spent 30 years buying one ss amp after another trying to find one that had the sound I wanted. None did. Most of them sounded the same to me, regardless of how much I spent.
"None did. Most of them sounded the same to me, regardless of how much I spent.
"
!
And so are most of us......... :)
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