How to reduce 'blow'??

[Gaz]

i've built a hum free amp, but get a the normal 'blow' sound when volume is up. how can this be reduced? i understand that this is partly due to thermal noise form resistors. i am currently using carbon film, but am aware that flameproof are quieter. where would be the most beneficial place to try flameproof resistors? plates?

also i have heard about 'snubber' caps across solid state rectifers to reduce "switching noise." could anyone describe what switching noise might sound like?

thanks,

alex

[Albert Kreuzer]

How to reduce 'blow'??

Easy - just add some 'suck'

Seriously: Find out where the hiss comes from. Usually the first stage is most sensitive. Does the noise vary with the volume/tone controls?

Higher value resistors tend to hiss more than low values, especially when they're in the signal path (e.g. the 470k voltage divider in many Marshall amps). Other candidates are the 1M input resistors from grid to ground and the plate resistor of the first stage.

Then replace any suspect resistor with a metal film type.
If this is a high gain amp, you probably will never get rid of the 'blow' completely, but you should be able to reduce it significantly.
Don't forget to try different tubes. Some are noisier than others.

Cheers,
Albert

[Robert M. Martinelli]

Hi,
to reduce "blow" or "hiss", you must be aware of the factors being involved, in this case, as you correctly stated, they're mainly due to thermal noise, and, talking about resistors, the Equivalent Noise for a resistor is En=Sqrroot(4*K*T*R).

By analyzing the above equation we can see that, as Albert said, the greater the resistor's value R, the higher the noise. K is the Boltzmann constant, while T is the resistor's temperature in degrees K.

The Boltzmann constant cannot be changed ( they don't call it "constant" for nothing ), the only other thing that you can do is to lower the resistors' operating temperature T,( thus reducing the En ) by doubling the resistors' power rating ( e.g. use 1W ones where 0,5W ones now are ).

And, yes, I agree that metal film resistors help a lot, as they manufacturing methodology and the materials they're made of make for much "quieter" resistors ( their "thermal noise" behavior is the same, but they have a better NF when it comes to "spot" and "contact" noise )

Now, about the "snubber caps"....we dealt with 'em in the past, so a small search would help, I use a 10 nF 1000 V cap across each rectifier diode, though values of 22 or 47 nF are not out of question.

Hope this helps

Best regards

Bob

[Chiamp]

I heard something about carbon type resistor should be larger in W size to be quieter but metal film is actually better when smaller in size any truth to this ?

[Robert M. Martinelli]

I heard something about carbon type resistor should be larger in W size to be quieter but metal film is actually better when smaller in size any truth to this ?

AFAIK the "the bigger ( W ) the better" rule applies to both types, because a bigger wattage resistor will always work at a lower temperature if compared to a smaller wattage one ( all other factors being the same ) thus lowering the En ( thermal noise ) as well. Carbon will always have a worse NF when it comes to "shot/contact" noise.

The quietest resistors are the wirewound ones, but higher values ( which are pretty common in tube amps ) are practically impossible to manufacture, furthermore they also have a partially inductive behavior due to their mfg. methodology, so my vote always goes to metal film, which is second only to wirewound in the "less noisy resistors" scale ( telling the truth, the recently developed "foil resistors" should be next to "wirewound" ones in terms of low noise performance, but, since I didn't have the chance to test them yet I' ll stick to metal film for now ).

HTH
Best regards
Bob

[Gaz]

thanks!