[FullRangeMan]

Hi Folks,
Wonder IF the traditional Mu Follower Circuit is best suited for Preamps, Power amps input stage or both??
What do you think??
Gustavo

[Steve]

Hi Folks,
Wonder IF the traditional Mu Follower Circuit is best suited for Preamps, Power amps input stage or both??
What do you think??
Gustavo

Hi Gustavo,


It depends upon some factors such as output impedance of the preamplifier, miller capacitance of the input stage etc.

From my white paper here are some figures concerning distortion and high frequency response. (100:1 to 10:1 refers to the input impedance (Z) of the amplifier divided by the preamplifier output Z, using separates. General conditions as some high distortion tubes may have greater distortion than indicated.)

Decreasing the ratio from 100:1 to 10:1 results in virtually no increase in harmonic distorion. As a general example, if the total harmonic distortion at 2v rms output measures .01% (-80db) using the 100:1 ratio, changing to 10:1 raises the distortion by approximately .012% to -79db, a rise of approx 1db. It could be more depending upon the loadline and tube distortion in general.

Let's check for any high frequency response advantages vs preamplifier output impedance (Z). If one uses a high capacitance interconnect cable (IC) and include amplifier input capacitance, say 250pf of total capacitance, and the output impedance (of the preamplifier) rises from 100 ohms to 2000 ohms, the high frequency response drops approx 0,4db at 100,000 cycles per second, and approximately .015db at 20khz.

Using a 50pf interconnect cable results in less than 0,02db drop at 100khz, and virtually zero at 20khz. By the way, everyone knows that capacitances should be minimized with interconnect cables. (However, rarely, a longer IC with higher capacitance is neccessary as there is no choice.) Actually a major portion of the loss in high frequency response is due to the volume control resistance/input tube capacitance relationship.
 

If the capacitance is, say 125pf total, the FR will drop off only approximately 0.0075db at 20khz.

Sometimes one just has to use a Mu follower such as extended ICs etc.
However, a mu follower type etc does add another stage, with all the associated parts, which will degrade the musical signal further. One doesn't get something for nothing/no perfect stage.

Hope this helps. Cheers.

[JohnR]

hi Gustavo, fwiw I built a mu-follower headphone amp a number of years ago, and last I heard it was still in use and very valued by the friends to whom I gave it. I found the circuit on the site that has a bunch of tube headphone circuits on it... I forget the site but the circuit was by Eric Barbour who I assume many will recognize

[FullRangeMan]

Hi Gustavo,


It depends upon some factors such as output impedance of the preamplifier, miller capacitance of the input stage etc. From my white paper here are some figures concerning distortion and high frequency response. (100:1 to 10:1 refers to the input impedance (Z) of the amplifier divided by the preamplifier output Z, using separates. General conditions as some high distortion tubes may have greater distortion than indicated.)

If the capacitance is, say 125pf total, the FR will drop off only approximately 0.0075db at 20khz.

However, a mu follower type etc does add another stage, with all the associated parts, which will degrade the musical signal further.

Hope this helps. Cheers.

Thanks Boys for the input, very illuminator.
IF I get the idea, this tube circuit depends on the tubes impedances to work properly.
Excuse my ignorance again, I understand what it capacitance, but what is Miller Capacitance in tubes??
Gustavo

[Steve]

Thanks Boys for the input, very illuminator.
IF I get the idea, this tube circuit depends on the tubes impedances to work properly.
Excuse my ignorance again, I understand what it capacitance, but what is Miller Capacitance in tubes??
Gustavo

Hi Gustavo,

An example will probably be the easiest to understand. Say we have a gainstage, tube V1 and it is connected to gainstage tube V2.

The output of V1 will see some capacitance from

1) output wiring capacitance to ground and other objects called stray capacitance
2) output capacitance of V1 itself, plate to cathode etc
3) capacitance from input of V2 input capacitance, grid to cathode, grid to filament etc
4) Miller effect (capacitance); The grid to plate capacitance (Cgp2) of V2 magnified by approximately the gain of V2.

Stray capacitance may be some 13-20pf. Output capacitance of V1 maybe 10pf. Miller capacitance of V2 depends upon the gain, but could be some 30 to 150pf. Just general numbers listed mind you, to give you an idea.

Ct, total capacitances combined above with impedances involved limit the high frequencies.

John R gave an excellent example of a useful Mu follower.

Hope this helps Gustavo.

[FullRangeMan]

Thanks Steve for this detailed explanation.
Looks Miller is the inner capacitance between the tube elements, and with others parts and tubes.
Best Wishes, Gustavo

[Steve]

Thanks Steve for this detailed explanation.
Looks Miller is the inner capacitance between the tube elements, and with others parts and tubes.
Best Wishes, Gustavo

Hi Gustavo,

Sorry I may not have been as clear as I should have. The Miller capacitance is the grid to plate capacitance of the tube multiplied by approximately the gain of the stage. No other elements of the tube are involved in a triode's Miller capacitance (for simplicity sake). The Miller capacitance "shows up"/influences at the input of the tube. So tube V1 sees the Miller capacitance of the input of V2.

The other capacitances (stray capacitance, output capacitance etc) I mentioned above just show other sources of capacitances so one has an overall picture of all the capacitance sources. But these are not figured into the Miller effect/capacitance.

I hope I was a little clearer. 
Take care 

[FullRangeMan]

Oh, very clear now , Thanks for this lesson Steve, you are a good teacher!! Congratulations.
Thanks again, Gustavo

[Steve]

Oh, very clear now , Thanks for this lesson Steve, you are a good teacher!! Congratulations.
Thanks again, Gustavo

Your welcome Gustavo.

Have a great weekend.