Re: Oscillation problems WAS Re: C-Audio schematic?

Eeyore wrote:
> 
> Denny Strauser wrote:
> 
>> Eeyore wrote:
>>> Gareth Magennis wrote:
>>>> "Eeyore"  wrote
>>>>> Gareth Magennis
>>>>>
>>>>>> Anyway, don't MOSFETS, which C Audio mostly used as their output
devices,
>>>>>> suffer from real RF if the PCB etc is not carefully designed?
>>>>> That's one way to make them oscillate ! LOL ! They were fun to tame
when
>>>>> they came out.
>>>>>
>>>>> Good point though. Check the value of the series gate resistors.
Expect
>>>>> anything between 100 and 1000 ohms. Quite possibly ~ 330.
>>>> What kind of frequencies are we talking when they oscillate like
this?
>>> Crikey ! Now you're stetching an old man's memory without digging out
the
>>> Hitachi app notes. Low MHz or high 100's of kHz typically IIRC.
>>>
>>> Enought to be a bugger.
>>>
>>> It most likely wouldn't hurt to 'up' those Rg values by one E6 value
btw.
>> While we're on the topic of oscillation problems, I'd like to ask
>> another question regarding the same.
>>
>> On Yahama 3K, 4K & Midas Venice, I've experienced something that sounds
>> like an oscillation problem. On these consoles, when an EQ gain is
>> turned up the whole way & Frequency is swept while passing a signal,
>> there seems to be some sort of oscillation.
>>
>> What is this?
> 
> Pot mistracking due to lack of regular use. One side of the frequency
circuit goes
> nearly open and it creates an oscillator around that EQ section..
> 
> Whizz the Freq pot back and forth a few times and it'll go away.

I've seen/heard it on brand new Furman PQ-3's, implying I don't think 
it's a worn or dirty pot, more the filter circuit going unstable due to 
the frequency related topology* changing while the gain is high.  I've 
only had it happen while sweeping and with the Q set for less than 1/2 
octave width.  Just keep the gain lower until you find the right 
frequency, then adjust the gain to suit.

*  If you want to go in to poles and zeros I will, but that gets a bit 
pedantic.