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Re: Ultrasonic AC for DHT Filament

Posted by Triode Kingdom on March 7, 2003 at 07:55:00

In Reply to: Re: Ultrasonic AC for DHT Filament posted by dave slagle on March 6, 2003 at 18:05:40:

> but how does this explain AC sounding
> better? there is no common mode rejection from the line?

When AC is used, the common mode path doesn't include semiconductors. I was suggesting that might be the reason for the difference in sound.

> when you refer to different regulators sounding
> differently, are you referring to devices, or concepts???
> that is LM317 vs LM338 or current vs voltage regs?

Concepts mostly, although I've heard of people who claim they can detect differences between regulators. I haven't personally tried so many ways to do this, due in large part to my dislike of diode junctions in this location. Those junctions might not be relevant, however, if they were somehow isolated from the filament at audio frequencies.

> many prefer the current regs to voltage regs, and this
> fits into the ac current path logic, since the current
> source provides a high impedance (like a series choke)
> and the voltage reg provides a low impedance (big cap
> across the filament) each would behave differently to the
> signal currnet.

In its simplest form, placing a capacitor across the filament affects only differential currents. The addition of a ground at one terminal changes that to include common mode current, but now we've also modified the current path for the audio. The problem is, there are two common mode paths - one desired, the other undesired. The undesired path is the one between the filament and the AC line. The desired path is between the filament and the common (return) terminal of the amplifier's power supply.

> looking at batteries, we have the perfect isolation from
> the AC source, but we do not have the benefits of the
> single current path for AC since the battery behaves as a
> cap across the filament... and batteries like caps are
> nonlinear, and i just don't find it to be a good practice
> to put 1/2 of your signal current through a nonlinear
> device, either put all of it through it or none of it.

If the battery floats and the filament has a resistive "centertap" network to ground, little or no signal current will flow through the battery.

> the thought of two audio quality chokes in the filament
> legs makes me ask "why do we need to put one in the
> ground leg?"

Neither filament terminal in my amplifier is grounded. Each terminal connects through a resistor to ground. When I use DC, that doesn't change, I float the supply. This allows me to maintain a small amount of degenerative feedback, and also prevents the applied DC voltage from impacting grid-cathode bias. It's a common topology for 211/845 amplifiers.

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