my goodness
I didn't know what I have started.....
Anyway I have contacted mr. broskie (tubeCAD journal) with the same question.
His answer was something like this (I can not qoute so it will be copy/paste)
"Daniel,
If you build the line amp you sent me, please add two resistor and one diode per channel. (picture is first one)
The 1-Meg resistors protect the power amp from seeing voltage spikes. If the pot loses contact, the grid will be left floating; if the output isn't terminated and if it saddenly does become terminated by the power amp, the power amp might see a 10-20volt spike that could damage it or the speakers. The diode protects the cathode follower at turn-on, when the full B+ voltage would otherwise be presented to the CF's grid, while its cathode was cold and at 0 volts. But I would not build the line amp; instead, I would build something like this picture is second one)
More current and less 100µF capacitor sonic contribution, as the 470-ohm cathode resistor sees a constant current flow, i.e. no AC signal, as the CF, while working in voltage phase with the grounded-cathode stage, works in anti-current phase with it. (picture is third one)
But if you are feeling adventurous, the following line amp has some nice features (4th one)
This circuit eliminates power-supply noise from the output, by injecting the same amount of PS noise at the bottom of the two-tube CF as the top of the CF sees; the result is that the noise cancels. One reader has built this circuit (except that he used 12AU7s) and he loves it.
John R. Broskie
I didn't know what I have started.....
Anyway I have contacted mr. broskie (tubeCAD journal) with the same question.
His answer was something like this (I can not qoute so it will be copy/paste)
"Daniel,
If you build the line amp you sent me, please add two resistor and one diode per channel. (picture is first one)
The 1-Meg resistors protect the power amp from seeing voltage spikes. If the pot loses contact, the grid will be left floating; if the output isn't terminated and if it saddenly does become terminated by the power amp, the power amp might see a 10-20volt spike that could damage it or the speakers. The diode protects the cathode follower at turn-on, when the full B+ voltage would otherwise be presented to the CF's grid, while its cathode was cold and at 0 volts. But I would not build the line amp; instead, I would build something like this picture is second one)
More current and less 100µF capacitor sonic contribution, as the 470-ohm cathode resistor sees a constant current flow, i.e. no AC signal, as the CF, while working in voltage phase with the grounded-cathode stage, works in anti-current phase with it. (picture is third one)
But if you are feeling adventurous, the following line amp has some nice features (4th one)
This circuit eliminates power-supply noise from the output, by injecting the same amount of PS noise at the bottom of the two-tube CF as the top of the CF sees; the result is that the noise cancels. One reader has built this circuit (except that he used 12AU7s) and he loves it.
John R. Broskie
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Hi,
Well, why not call the circuit by its' name?
The first stage is an SRPP direct coupled into a White CF....
As it hapens I already designed half a dozen of those fifteen years ago already and am currently working on one with the ECC99.
A circuit like that can drive the most ridiculous loads; low input impedance amps, headphones (including the notorious AKG 1000), hell even an efficient speaker.
Given a neutral sounding tube (a contradictio in terminis), the only sound you could possibly hear is the one from the passive components.
Have a go at it, you'll love it.
Cheers,
Well, why not call the circuit by its' name?
The first stage is an SRPP direct coupled into a White CF....
As it hapens I already designed half a dozen of those fifteen years ago already and am currently working on one with the ECC99.
A circuit like that can drive the most ridiculous loads; low input impedance amps, headphones (including the notorious AKG 1000), hell even an efficient speaker.
Given a neutral sounding tube (a contradictio in terminis), the only sound you could possibly hear is the one from the passive components.
Have a go at it, you'll love it.
Cheers,
That's not a White cathode follower. Its a normal cathode follower with a CCS load. A portion of the power supply noise is injected into the CCS such that it cancels any PS noise at the output.
A White cathode follower has a resistor above the top valve and the signal developed across this is fed to the lower valve.
A White cathode follower has a resistor above the top valve and the signal developed across this is fed to the lower valve.
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