Systemic Development > Amplification and Preamplification

A Problem With Solid State Devices

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steve:
Even with a 1N914/4148 high frequency type diode with only 1pf of capacitance, when reversed biased
the solid state device bleeds musical information through. A vacuum tube won't.
The bleed thru is not evenly across the band but frequency sensitive.

So can one actually ever achieve linear operation of a solid state component in an analog component?
Not much chance.

Tubes rule.

steve

Nick B:
I’ll agree with your tubes rule statement. Am running a tube preamp and the sound is captivating.

steve:

--- Quote from: Nick B on September 28, 2022, 07:37:30 PM ---I’ll agree with your tubes rule statement. Am running a tube preamp and the sound is captivating.

--- End quote ---

I found out the problem by accident Nick, or I might not have believed it myself.
I have never heard anyone ever discuss solid state in this realm before.

The specs on the diode in no way gives a hint that bleed through is possible.
Only 1 micro micro farad of capacitance is next to zero. Yet the 1n4148 influences the
musical quality. Simply amazing.

There is a lot of science that is never discovered/tested, let alone discussed.

In one of my test designs, it took 5 reversed biased 1n4148 diodes in series to totally
block out the bleed through. Of course it depends upon the surrounding impedances (Z).

Cheers

steve

Nick B:

--- Quote from: steve on September 29, 2022, 01:12:43 PM ---
--- Quote from: Nick B on September 28, 2022, 07:37:30 PM ---I’ll agree with your tubes rule statement. Am running a tube preamp and the sound is captivating.

--- End quote ---

I found out the problem by accident Nick, or I might not have believed it myself.
I have never heard anyone ever discuss solid state in this realm before.

I have worked with not only small signal diodes, but transistors, FETs as well.
There is a lot of science that is never discovered/tested, let alone discussed.

In one of my test designs, it took 5 reversed biased 1n4148 diodes in series to totally
block out the bleed through. Of course it depends upon the surrounding impedances (Z).

Cheers

steve

--- End quote ---

Thanks, Steve. :thumb:

steve:
Thought a little bit more and a couple of reasons why.

1. Being a total solid, unlike a tube with a vacuum as an insolator, there is leakage current
through the junctions. Bipolars have what I would call high leakage currents.
Even capacitors from electrolytics to polys have leakage current.

2. Junction capacitance in transistors can cause problems. A 2 amp hexfet has variable capacitance
up to 25 volts from drain to source etc. At low voltages, the drain to gate capacitance was measured
over 1000pf. That is a .001uf capacitor.

The Miller effect is calculated as gain times drain to gate capacitance. If the gain were 10, the Miller
capacitance could 10,000pf, or .01uf capacitor. Some designers use only a .1uf capacitor for coupling.

Of course small signal transistors should have much less capacitance, but it does not take much
to "transfer" a signal from input to output.

Anyway, another example of science that is never considered.

cheers

steve

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