What is the signal path -- the red line?

[oldeurope]

moderator Note: thread split from here Hi-end definitive vacuum tube phono pre

Hello Andrea
thanks for sharing.
You wrote “no compromise phono preamplifier”. But I can see some compromises.

First: The cathode bypass cap. It is not in the signal path but it has a strong sonic influence. Signal current causes voltage droppings and these are added (or subtracted) to the input signal between ground and grid.

Second: At high signal frequencies, there is more signal level at the grid of the input triode than at the output triode. Means dynamic loss at high frequencies.

Third: You have transformers in the signal path.

LG Darius

[SY]

It is not in the signal path

Yes it is. So are the EQ caps. So what? They are far closer to "ideal" than the transformers.

[oldeurope]

Hi SY,
try to understand the difference between signal path and current (loops). In the picture the red line shows the singal path, the green line shows the signal current in the output loop.
Comes from Have a look at some commercial schematics. This is the common way signal path is drawn.
I know, in the audiophool word it is different ...

LG Darius

[SY]

Since the expression for voltage at any point in the signal circuit includes terms involving the cathode impedance, the cathode impedance is certainly in the signal path.

[oldeurope]

Hi SY
there is nothing else than tubes, transformers and a 220K resistor in the signal path.
Try to understand #7 .

LG Darius

[SY]

How does drawing a red line eliminate the cathode impedance terms from the expression for voltage at any point along it?

[oldeurope]

The red line shows the signal path, that's all. Drawing the red line does not "eliminate" something. I'm sorry. Please note #7.

LG Darius

[SY]

How does the "signal" know that it doesn't go anywhere else? Is there no signal at the cathode? The bypass caps are perfect? No "signal" goes through the RIAA caps? They're an open circuit?

Drawing a red line doesn't change the flow of charge or take terms out of the transfer functions.

[coldcathode]

Darius,

How does drawing a red line eliminate the cathode impedance terms from the expression for voltage at any point along it?

I am relatively NEW to the hobby but I think that VOLTAGE of the signal being elavated is the Whole POINT of a Preamp.

As I understand all the posts, the goal is a minimum number of components in the signal path. The signal is a sine wave which has FREQUENCY. The voltage or AMPLITUDE changes are irrelavant as long as they are steady or FLAT throughout the AUDIBLE range. Since some changes are REQUIRED (RIAA EQ) then the goal seems to have been met IMHO. The F3 of the bypass caps is 1.1Hz can you hear 1.1Hz?? If so you need to be in the Guiness Book! My math may be a little "fuzzy" but by the time the frequency hits 21Hz (bottom threshold of hearing) The attentuation of the signal level is less than 1/10 of 1 percent.

[oldeurope]

Hi SY
The signal does not know anything. The signal path just shows the way from input to output. There is signal current e.g in the cathode bypass cap. See first in post #4. You can draw the current loop “green line” Voltage droppings caused by the signal current have an influence in the signal. The signal current in the RIAA network causes a signal voltage drop at the 220K resistor. This makes the RIAA eq. Possible.

LG Darius

PS: The underlined/blue words are hyperlinks.