John Curl's Blowtorch preamplifier

Status
Not open for further replies.
Jump to Latest
#17,066
I use carefully trimmed pieces of old .031" teflon pcb material to make small caps. Stable, not too hard to make and works well. A little pricy:Rogers Duroid

The NPO ceramics do have as much as 10 dB more 3rd harmonic than similar polystyrenes and teflon caps at the .1 uF size. Its not a lot but still significant. The X7R are incredibly bad. I don't have a .1 Mica to try but it would be big enough to not be practical.

For the small values the teflon piston trimmers are really good and not expensive if you can find them Piston Trimmer
 
#17,067
1audio said:
The NPO ceramics do have as much as 10 dB more 3rd harmonic than similar polystyrenes and teflon caps at the .1 uF size. Its not a lot but still significant. The X7R are incredibly bad. I don't have a .1 Mica to try but it would be big enough to not be practical.[/URL]
Click to expand...


Mica is about the worst for DA and not all NPO types are suitable. Polystyrene and similarly constructed foil types are still too inductive for critical frequency compensation applications.
 
#17,071
traderbam said:

I have to agree with John's comments about capacitors. What's the basis for your inductance concern, Glen? Specifically what amount of inductance are you worried about?
Click to expand...


Well foil caps should be generally avoided for frequency compensation for the same reason that they are avoided for supply rail bypassing. And if someone is overly concerned with DA then mica is a no-no.

Cheers,
Glen
 
#17,073
1audio said:
I use carefully trimmed pieces of old .031" teflon pcb material to make small caps. Stable, not too hard to make and works well. A little pricy:Rogers Duroid

The NPO ceramics do have as much as 10 dB more 3rd harmonic than similar polystyrenes and teflon caps at the .1 uF size. Its not a lot but still significant. The X7R are incredibly bad. I don't have a .1 Mica to try but it would be big enough to not be practical.

For the small values the teflon piston trimmers are really good and not expensive if you can find them Piston Trimmer
Click to expand...

DA != distortion, anyway are we talking the difference between 140dB and 130dB? My mic circuit used 680pF mica and 22Meg carbon comp on the input and was easily -110dB thirds at 12V p-p.
 
#17,076
syn08 said:
If you can measure any DA difference in, say, 47pF NP0 vs. polystyrene I am ready to pay and eat a dozen each for breakfast.
Click to expand...

Would you mind ingesting a couple of Bybees as your forfeit instead? It's just that polystyrene caps are really good and world supplies of polystyrene dielectric appear to be limited, according to LCR.
 
#17,078
scott wurcer said:


DA != distortion, anyway are we talking the difference between 140dB and 130dB? My mic circuit used 680pF mica and 22Meg carbon comp on the input and was easily -110dB thirds at 12V p-p.
Click to expand...

More like -160dB to -150dB.

I would be cautious of the 22M carbon comp resistor. That will have a high excess noise and less that stabe value. The need for a biasing resistor for the mike capsule requires such a resistor but I have spent many hours troubleshooting a noisy resistor like that in an HP400H voltmeter. Caddock has some good options for that application.
 
Status
Not open for further replies.
Top Bottom