Best shelf filter for signal to noise ratio?

[Tubis]

Best shelf filter for signal to noise ratio?
Passive or active? Or, it makes no difference?
As illustrated:

[booj]

The first one will boost highs
The second one will cut lows

[Tubis]

That was not the question.
The signal level will still be the same at the other end.
And both circuits will boost the highs.
In the first the highs get at shortcut around the resistor and in the second the cathode bypass makes fore more high gain.

Nothing else in the circuit has changed, the 470k/470 splitter is in both circuits making it have the same frequensie respons and the same total gain.

The question is if the signal to noise ratio is the same.

[Robert M. Martinelli]

Hi Tubis,
IMHO the S/N ratio with the given values will be the same. With the exception of hum, which is not the issue here, noise in tube amps is mainly thermal noise, to get a better S/N ratio you could try to lower the resistor values at the input voltage divider, as the resistor equivalent noise is the result of the equation nv=sqrroot(4KTR) where K is the Boltzmann' s constant, T is the resistor' s absolute temperature ( in degrees Kelvin ) and R is the resistor value, so, all the other conditions being the same, using 220K resistors ( and changing the caps accordingly to keep the same frequency response ) will cut the resistors' thermal noise more than half, still it won' t load the preceding stage significantly...

If noise is your main concern I would put special attention in choosing the tubes, using "special quality" preamp tubes, like E83CC, 7025, ECC803 and the like.

Hope this helps

Best regards

Bob

[Tubis]

How about current noise? Like through the plate resistor? Isnt that a big part of the noise? If the gain in the high freq is amplified greater isnt more current flowing trough the plate resistor?

[Robert M. Martinelli]

Of course there' s room for varying the plate resistor as well, but IMHO it' s better to reduce noise as much as possible at the input ( where impedance is higher ), otherwise it will be amplified by gain over the whole bandwidth ( being thermal noise it can be found over the whole spectrum ). 12AX7s have a very low plate current, so usually the plate resistor cannot be lowered much, values between 100 and 220 K are quite common, I have rarely seen stages with 12AX7s with a plate resistor under 100 K, that' s the value I found in your schem, so I left it that way....also, lowering the plate resistor values reduces the headroom, so, as usual, it' s all about compromise....and experimenting.
OTH I think the tube itself it' s rather important noise source, so a good quality tube is mandatory to reduce noise.

[Tubis]

Thanks. I think the s/n picture is beginning to clear up. I think i gonna do some calculating on the thermal noise and the current noise to better understand where it all happens.
The tube stays JJ as i found them the best, sound, price and noise.

[Robert M. Martinelli]

Hi again Tubis,
since you' re going to use JJ tubes, I strongly recommend their ECC803, this version of the venerable ECC83 has lower noise, spiral filament for low hum, a longer plate and special countermeasures have been taken to avoid microphonic noise, furthermore, they're are more rugged than "regular" ECC83 having a thicker glass envelope ( they also have gold pins, but I leave such things to "audio philosophers", as I live in the real world ), more importantly, they only cost a little more than a regular ECC83, so your wallet won' t be hurt....
Give' em a try!
Best regards
Bob

[Tubis]

Well i tried an ECC803 and it didnt sound as good as an ECC83S in an overdriven position.
The noise isnt realy a huge problem, i just want to find ways to shrink it as much as possibly.

[Steve Conner]

The second circuit will be noisier because it throws away signal and then amplifies again. The first one has no more attenuation than necessary.

If you designed them both for a 6dB shelving treble boost, the second circuit would be 6dB noisier. However, the way you drew it the second circuit will treble boost more than 6dB, making it noisier still.

If you're designing a high-gain amp, you can say to a first approximation that all of the noise will come from the first triode stage, and neglect the contributions of all the others.