Noise Reduction in high Gain circuits - Metal Film?

[mooreamps]

Guys, let's try to keep this to the actual topic.



Here's how the parallel triodes thing works: If you connect two triodes in parallel, the resulting tube has twice the transconductance, half the plate resistance, and a square root of 2 times (ie, 3dB) more internal noise, because noise voltages sum uncorrelated, as has been pointed out several times.

All I know is what I am seeing on my oscilloscope. If you're going to be a moderator, and if someone writes a comment like "this is what I measure on a scope", I'd think you have a more open mind about it !

What I am measuring is : cathode resistor in : hash noise on the plate.
Cathode resistor out : nice clean plate signal. Ah, metal film has a slightly lower noise temperature co-efficient than carbon film. I'll double check my data sheets, but that's what I seem to remember.

-regards
-g

[Moose]

So back on topic, something Randall Aiken mentioned,

1) what brands of metal film do you find to be the best in terms of noise? I ask since you said in passing that they are not all equal.

I'm a fan of PrP resistors.

QueitEST? I don't know. Quiet, reliable, transparent, and high quality? To my ears, yes. They're a really good choice for a circuit with a lot of gain.

PrP Resistors

I've slapped them in circuits where I wanted quieter than carbon, and didn't want to screw with it in general, and had no detrimental problems with sound. They didn't make the amps sterile sounding or anything.

[Hardtailed]

In the same vein, I've looked at potentiometer offerings since I happen to be planning a high-gainer right now.

I can't seem to find conductive plastic bigger than 1/4W
On the other hand, it's easy enough to find carbon pots in 2W packages.
Both are roughly the same price.

I have no idea which one is better... small plastic, or big carbon?

[guitician]

Clarostat/Honeywell makes their 380, or 53, or RV4 Series, 2 watt conductive plastic Potentiometers. Conductive plastic lasts longer.

[soundmasterg]

+1

+1


Tee-hee. Because they're told to... These aren't the droids you're looking for...


Actually, that has been studied. Yes, they do!

On a more serious note, re the whole issue addressed in this thread:
1. As a general principle, for any amplifier of significant gain (i.e. over about 10) the first stage noise dominates the noise of the whole unit unless there is a serious flaw in the design or fault in later components in the chain. This is because the noise at the very front end is amplified by all the following gain, and the noise from later stages is amplified by less gain.
2. There *is* a difference in tone quality with carbon comp resistors as well as a difference in noise. However, understanding that the tone difference is because of the voltage coefficient of resistance lets you notice that this is only apparent where the signal excursion across the resistor in question is upwards of 75-200V peak to peak.

Taken together this leads to the understanding that
(a) Carbon comp resistors in the first stage of an amplifier add noise, not tonal quality. You get all the noise, none of the "good" distortion. Mother Nature says that this is a losing proposition.
(b) Carbon comp resistors in later stages add less noise, but not much tonal advantage until the signals get *big*. So it doesn't make sense to pay extra for carbon comp resistors to add some noise, even if less, if they don't get you more distortion-goodie.
(c) Carbon comp resistors work best where the signal swings are biggest and where they are latest in the amplifier. Guess where that one position is...

Phase inverter plate resistors?

Greg

[pdf64]

See
http://www.geofex.com/Article_Folder...carboncomp.htm
'The place to use CC's is where there's big signal - plate resistors, and ideally the stage just before the phase inverter. The phase inverter would otherwise be ideal, with plate resistors carrying the highest signal voltage in the amp, but phase inverters are often enclosed in a feedback loop. The feedback minimizes the distortion the resistor generates.'

[tedmich]

Is there a particular brand/type that you would recommend for best noise performance?

Thanks!

these Charcroft naked Vishay Z foil resistors are supposed to be lowest in noise...
for ~$40 each at Hificollective.com
or try the tantalum Shinkoh 2w resistors for $13 each

seriously I like the Vishay CCF60 series

[Bruce / Mission Amps]

There should be a law against playing Little Wing in public unless you can do it right. That was yet another example of someone doing it very wrong.

Ah, to be sure, it was gawd awful lousy tone, and he pulled some clams but don't be so hard on the guy... at least is out there doing it on stage in front of people.... which I dare say is probably the vast minority of the whiners and tweakers on the NET.
I give him a for getting out there and doing it.

[harry]

these Charcroft naked Vishay Z foil resistors are supposed to be lowest in noise...
for ~$40 each at Hificollective.com

IS THAT A $40 RESISTOR? My word.

I was building a tube mic preamp lately, and I was getting a lot of hiss when used with a ribbon mic. Reducing resistance and/or changing to a wirewound did reduce the perceived hiss in my opinion. This included plate resistors and potential dividers. I changed the cathode resistors to diodes (the schematic called for 0.7V bias - how convenient! I got to use a 1N4148) and I thought I heard a difference, but perhaps not.

(also, what was said about us new EEs is true! I got my MEng in 2008 and I don't think I could've drawn you one circuit from scratch. A block/system diagram maybe.)