Something like that.
As DF96 stated "It is amazing what some people will buy/build for audio." 😀
Mona
In many audio circuits it is difficult to avoid adding PSU noise to the output, so extra decoupling is added to reduce this. What is unusual about this design is that it adds PSU noise to the input, where it does more harm. This is a direct consequence of using a dual polarity supply. Why they did this is a mystery. It requires smoothing two rails instead of one (so extra components) yet delivers worse performance. Maybe the designer was more used to SS circuits?
I can only see one positive thing about this circuit: at least it uses an input coupling capacitor, so grid current will be kept away from the volume pot. Many circuits omit this.
I can only see one positive thing about this circuit: at least it uses an input coupling capacitor, so grid current will be kept away from the volume pot. Many circuits omit this.
Perhaps, but with a less then -1V grid voltage, the grid current will charge the coupling-C shifting the bias.
Mona
No, the grid leak resistor will deal with that. There will be very little bias shift. Omit the capacitor and then the bias shifts (a little) with volume control position!
?? 1µF and 470k gives a time constant of allmost ½ sec ! The capacitor is charged by the cathode-grid diode and the bias will shift with the amplitude of the signal.
Even if the C is only .1µ (you never know on a bad copy with that stupid decimal POINT 😡) it's long enough to keep it's charge during the signal.
Mona
So I got the essentially "stock" audio portion breadboarded and got some measurements. The tubes I was able to scrounge up are NOS but not the same brand, one RCA, one Sylvania.
The voltages below are an average of the two channels. Plate resistor is 4.7k and cathode resistor is 200 ohms as per the schematic.
With a B+ of 68v from my bench supply:
Plate to ground: 51.4 v
Plate to cathode: 50.65 v
Cathode to ground: 0.715 v
I probably won't have time to mess with it again until tonight or tomorrow.
The voltages below are an average of the two channels. Plate resistor is 4.7k and cathode resistor is 200 ohms as per the schematic.
With a B+ of 68v from my bench supply:
Plate to ground: 51.4 v
Plate to cathode: 50.65 v
Cathode to ground: 0.715 v
I probably won't have time to mess with it again until tonight or tomorrow.
I have no idea what you are talking about. A coupling cap and a grid leak resistor are the standard way of feeding a valve grid.Ketje said:
This occurs when an input signal overloads the grid and causes grid current to flow.
With Vg=-0,7V you don't need very much signal.
The grid current charges up a dc bias on the grid coupling capacitor. This has the effect of moving the operating point and of clamping the positive part of the input.Producing additional (intended ?) distortion.
Mona
With Vg=-0,7V you don't need very much signal.
The grid current charges up a dc bias on the grid coupling capacitor. This has the effect of moving the operating point and of clamping the positive part of the input.Producing additional (intended ?) distortion.
Mona
Maybe I'm missing it but I don't see any diode between the cathode and grid.
Are you referring to one of the diodes in the PS shown in the schematic? I'm not using that PS - and the FX preamp that's so popular doesn't use it either. Nevertheless, I'm curious about what you're talking about.
He is referring to the intrinsic diode within the tube. If the grid becomes positive relevant to the cathode, electrons can flow from the cathode to the grid so it acts like a diode.
Ahhh. Thanks for the explanation. Yeah, the operating point used in this circuit is certainly not optimal.
I'm guessing it's dictated by the limitations of the type of PS they are forced to use in order to produce a finished product that's so small and sells for $32 shipped. It's probably about as good as you could expect given those parameters.
A lot of owners seem to use it with similarly tiny Class D amps.
I still haven't had time to give it a serious listen with the stock design. After I do I'll set it up with a more optimal operating point.
there are a lot not shown in schematics, stray capacitance, inter electrode capacitance, their effects can be felt....
By the time the grid goes positive, the plate swing is about 15 Volts.
Unless we do something "stupid", like putting the volume control after the amplifer, there's no way we want 15V signals in hi-fi.
(Also THD will be several percent before we reach 15V, but that may be acceptable to some.)
It's an effects box and sold with the denomination "get your tubesound for a bargain". Tubes have no sound of their own but a circuit might suffer from distortion and (while doing so) sound perfectly fine all the same.
High fidelity is an illusion, we are listening to acoustic distortion tied closely to the speaker distortion, ~ 0.5 - 1% at 1W output. A very specific harmonics present on amp output can actually reduce total acoustic distortion. Practical for a single driver or multi amp system, difficult to impossible with multiways.
Some others love the distortion of their speakers to the last drop, they insist on very low distortion amp in order to achieve that goal.
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Yes. With such a low grid bias voltage you would need to keep signal levels low - but you would need to do this anyway because of the gain of the preamp.Ketje said:
It is difficult to see what legitimate role this poor circuit might have in a decent audio system.
How? By amplifying in anti phase? Good luck with that as the level of third and higher harmonics in single drivers are frequency dependent.
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