sunsvc said:
The whole idea behind the use of a grid stopper is to form a first order LPF with the series stopper resistor and the parallel Cmiller + Cin + Cstray. What I do is to o'scope the output to see what the "problem" frequency is. where I'm at, that's the 700KHz noise coming from a 50KW BCB xmtr about 30 miles away. To keep that out, I shoot for a grid stopper that'll give a cutoff at ~70KHz.
Schematic for preamplifier with ECC82 (SSassen)!
Hello SSassen!
Where is the power supply of the preamplifier scheme?
If you have opportunities to give the show here on this site!
A big thank you!
Alright, I spent the last two days optimizing the preamplifier. I initially tried the math approach, ie. trying to calculate exactly all the required component values etc. This however proved to be rather difficult; distortion and linearity did not seem to be consistent with my calculus. Hence I simply measured these parameters with RMAA and continued doing so with every component change I made and gradually improved the preamplifier.
The above schematic is the result of this painstaking process. As you can see the current for the first and second stage has increased considerably from the original Velleman schematic. The current for the voltage amplifier is now 6/2200 = 2.7mA and 152.5/2700 = 5.6mA for the cathode follower. Below are the results for the frequency reponse, noise level and the THD for both channels driven at 6Vpp output into a 22K load.
The frequency response needs some explanation, what you're looking at is the curve of my Audigy 2 ZS soundcard, the actual preamplifier has far greater bandwidth, but what's important here is to show that it does not affect the frequency response in any way, it is linear up to about 80KHz. These scores are acceptable for a tube design (as far as I can see). I'm sure they can be improved upon with other tubes and a different schematic, but this is as good as I got them without having to completely hack up the Velleman PCB.
Best regards,
Sander Sassen
Hardware Analysis - Computer hardware news, reviews and forums that provide help and assistance with problems.
Hello SSassen!
Where is the power supply of the preamplifier scheme?
If you have opportunities to give the show here on this site!
A big thank you!
Last edited:
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