Grid current

[Leo_Gnardo]

As far as buzz is concerned, I'm thinking about a guitar amp in a studio where the sound engineer puts a mike up against the speaker and monitors it through headphones (before the guitarist even plugs in) and says 'I can hear a buzz from that amp. Get me another one!'

That's where the producer says "I hear a whine from the engineer, get me another one!" These days easy enough to clean up any track with ProTools or whatever similar program you're using, unless the engineer's lazy and inclined to whinge instead of work, then should be sent looking for another job.

[Chuck H]

That's where the producer says "I hear a whine from the engineer, get me another one!" These days easy enough to clean up any track with ProTools or whatever similar program you're using, unless the engineer's lazy and inclined to whinge instead of work, then should be sent looking for another job.

It's common in the studio to mute tracks bussed to the final mix when that track isn't doing anything. Like a manual noise gate. Well, it used to be manual. Now you can program the timing (much better). I've been in a few studios and seen it done. I have very little experience as a recording engineer or producer, but I remember doing this myself on the few occasions I've been in that role. Any studio tech not willing to do this, leaving track noise integrity to the musicians, is lazy and misguided. Being a musician and idealizing music recordings are two different things. Guys that have learned to negotiate what's acceptable on a live stage have no clue how to manage what's acceptable in a studio. Not their job. That's what we used to rent studio time for. The equipment AND the techs. Now we can do it all at home. For better or worse

[Enzo]

Yes, we are splitting hairs, this is the last I'll say on it. Malcolm, the whole remove the cap and consider the 8k thing was ONLY to demonstrate what the cap is doing in that circuit at that point. it was never remotely a design suggestion. Of course there would be no reason to leave the 8k if we removed the cap, but we were not changing the circuit, we were only considering what spurious noise and signal would be present without the cap, and thus we would know what the cap was doing. The point was to show the relative amounts of ripple reduction versus the amount of crosstalk signal reduction performed by the cap. That in turn tells us what the primary purpose of the cap is at that point, decoupling or ripple filtration.

[Malcolm Irving]

Thanks for clarifying Enzo. I understood that your hypothetical removal of the cap was just to explore what it was doing in the circuit and I totally agree with what you are saying.
My thinking was that the 8k and 16uF together form an RC filter. If you remove either the R or the C you don't have a filter anymore.
Another reason for having the 8k is to drop the voltage so that the preamp can run on lower B+ than the power stage. If we designed an amp where we decided to run the preamp on the same voltage as the power tube screens we could get rid of the 8k type resistors and all the caps would combine in parallel to one big capacitance, with a lot of plate load resistors radiating from it. Would that give the necessary decoupling to avoid LF oscillation (I'm not sure - I'm just asking)?
I suspect the RC filters are actually needed for the decoupling function.
No need to prolong the discussion though - I've already conceded!