182MHz Oscillations :-0

[AndrewT]

Thank you.

 

[Bonsai]

Hi Andrew,

I suppose you mean the four 100 Ohms stoppers, right?
With or without the 50nH parasitics no peaking (according the simulation).

Cheers,
E.

I could only get zero peaking by adding a cap from the cascode base to ground. With the stoppers only, there was peaking. I am wondering if you are seeing any peaking with just the 100 ohm stoppers.

 

[PMA]

First Id like to ask if a little oscillation at 200 MHz is really such a big problem, does it affect sound audibly ??

First, it is not "a little oscillation". Bonsai has shown quite high amplitude. Second, the unexpected and unexplained sound differences are often caused by VHF oscillations. It is a big problem, as these oscillations are often difficult to find and measure. If you feed the amp from the generator at a stable frequency and amplitude, the oscillations may not seem important. But, with a complex signal the situation is different.

 

[Edmond Stuart]

small quirks

First Id like to ask if a little oscillation at 200 MHz is really such a big problem, does it affect sound audibly ?? It is true that these small quirks aren't always shown and its not easy for most builders to find with the scopes they have.
[...]

Hi Alex,

I like to add that HF oscillations, even very tiny ones, do increase the distortion. The Super TIS, for example, distorts 3ppm at 20kHz (instead of 0.06ppm) with the parasitic inductances as shown on page 14.

Cheers,
E.

 

[homemodder]

I asked as Ive been aware of this problem especially with bjt LTPs that are cascoded, theres a handbook, I cant recall the author now that advises for compensating of peaking on all cascoded LTPs as a norm. I started using this as norm but I honestly cant say I can hear the effect, although it only costs two small caps and 2 resistors for this. The solution is very much the same as Bonsai solution has for the vas circuit. That book I mentioned shows indepth info on this too.