Any reason not to use smaller grid stopper at input?

[Alan0354]

Alan, I think you may be missing the point: Typically additional capacitance is not needed with a grid stop over 10K or so in order to block RF, BUT if you want to reduce resistor noise you can, to a certain point (which I discovered in this thread), reduce the resistance value and add a cap to ground to approximately recreate the same frequency break point.

Didn't I just say that?

I don't think thermal noise is ever a problem. Guitar signal is so large, it is in mV!!! Thermal noise ( Johnson noise) is

Vn^2=4kTR(BW)


Where T is temperature in Kelvin ( 298k=25 deg C). Let bandwidth BW=10KHz, k is the Boltzmann's constant which is 1.38EE-23. I worked with signal in uV before I need to worry about thermal noise. Even if you have gain of 1000, it is nothing. Tube input is high impedance, shot noise is minimal.

In^2=2qI(BW)

Where q=1.6EE-19

Let's calculate the thermal noise for a 100K resistor at 25 deg C in 10KHz bandwidth:

Vn=sqrt(4kT(BW))= sqrt( 4X1.38EE-23X298X100,000X10000)=40.6EE-9V =4uV.

[Chuck H]

Even if you have gain of 1000, it is nothing.

Now consider that many modern uber gainers are repeatedly clipping the signal and otherwise have a gain over 50,000. Redo your figures with this in mind and tell us that small amounts of noise at the input grid are insignificant.

[Gaz]

Now consider that many modern uber gainers are repeatedly clipping the signal and otherwise have a gain over 50,000. Redo your figures with this in mind and tell us that small amounts of noise at the input grid are insignificant.

Agreed, and while I'm admittedly envious of your math skills, I know that reducing the resistance of the input grid stopper reduces noise because I spent ten minutes rigging it up in a high-gain build. Honestly, I was amazed at the result, which prompted the topic in this thread.

[Alan0354]

Now consider that many modern uber gainers are repeatedly clipping the signal and otherwise have a gain over 50,000. Redo your figures with this in mind and tell us that small amounts of noise at the input grid are insignificant.

With uber gain, the noise pickup of the guitar is going to swamp the noise. I am not saying you can't hear the noise, but in the environment, does it matter. In fact I exaggerated the numbers, I use 100K ohm and 10KHz BW. From my calculation you can see the signal to noise ratio is 81db or 12500:1. You don't just crank up the amp, turn off the guitar and listen to the hiss, put in the guitar. The hum from even the humbucker is going to be dominant.

As I repeat, I don't disagree you can hear the noise, but does it matter.

[Chuck H]

I agree with you. And IMHE your absolutely correct. But the semantics of the issue are what is in question. I personally don't trouble with any noise that is swamped by the typical guitar noise. Within reason. But the OP in on a quest. And who knows... There may yet be a day when the noise issue is addressed at the guitar level. Then who has the right circuit in their amp??? As designers continue to idealize, things will continue to improve. It's certainly fair to say that with modern cascade designs noise reduction, wherever possible, is a priority. But... My personal amp, which is a souped up power tube clipper capable of gain bordering on modern metal, does exhibit notable hiss and a tad of buzz that is no longer notable once the guitar is turned up. Keep in mind that there are many kinds of player. Some like to turn down the volume control on the guitar for a clean (ish) sound and then crak the knob up for distortion. The general noise level of the amp becomes much more important when the guitar is turned down to a level closer to the noise!

[Alan0354]

I agree with you. And IMHE your absolutely correct. But the semantics of the issue are what is in question. I personally don't trouble with any noise that is swamped by the typical guitar noise. Within reason. But the OP in on a quest. And who knows... There may yet be a day when the noise issue is addressed at the guitar level. Then who has the right circuit in their amp??? As designers continue to idealize, things will continue to improve. It's certainly fair to say that with modern cascade designs noise reduction, wherever possible, is a priority. But... My personal amp, which is a souped up power tube clipper capable of gain bordering on modern metal, does exhibit notable hiss and a tad of buzz that is no longer notable once the guitar is turned up. Keep in mind that there are many kinds of player. Some like to turn down the volume control on the guitar for a clean (ish) sound and then crak the knob up for distortion. The general noise level of the amp becomes much more important when the guitar is turned down to a level closer to the noise!

That is so true. I was actually thinking about this after my last post, if you crank the amp up and turn the volume on the guitar down, then the signal to noise will be out the window. But bare in mind, noise is proportion to the square root of the resistance, so even if you lower the resistance to 1/4, noise is going to be half only. I only have a 20K in series, I can obviously hear the hissing sound if I turn down the guitar and not play. Can it be some other noise source. I am not an expert on tubes and don't know the noise model of the 12AX7.

[Merlinb]

I don't think thermal noise is ever a problem. Guitar signal is so large, it is in mV!!!

We're not talking hifi here. It's not about SNR, it's about the *noise floor.*

Using your example of a 100k resistor generating 4uV of noise (you forgot to include bandwidth in your calculation!), that noise will be amplified by the gain of the preamp.

If the preamp is high-gain, say 1 million times, this will produce 4V of noise at the power valve, causing audible hiss in the speaker, which nobody wants.

Now, your SNR at the *input* of the amp may be great, but if the maximum headroom of the power valve is, say 30V, your SNR at the *output* is a shocking -17.5dB at best! And you have to sit and listen to lots of hiss during 'quiet' passages in your playing.

[Steve Conner]

As I see it, all these considerations only matter when the guitar volume is turned down close to zero. As soon as you turn it up a bit, the resistance of the volume pot becomes the biggest noise source. (the DC resistance of the pickups isn't relevant because of their inductance as JM explains)

I suppose it is still a valid issue if you're trying to design a high-gain amp that can be cleaned up with the guitar volume ( and stays quiet when muted with the guitar volume)

JM: the pickups are Duncan Alnico 2 Pros.

[Merlinb]

As I see it, all these considerations only matter when the guitar volume is turned down close to zero.

Or when the guitar is at full volume which, let's face it, is most of the time, especially if you're a high-gain player.

[Steve Conner]

No, you didn't understand JM's point. When the guitar is at full volume its output impedance is more than 100k in the middle of the audio band, so its thermal noise contribution swamps that of the first tube stage. The high inductance of the pickups means that their DC resistance can't shunt the volume pot at audio frequencies.

I recently designed a preamp for ultrasonic piezo transducers where I had to deal with the exact opposite effect, the transducer being capacitive rather than inductive. It shunted the first stage's noise current at high frequencies, so I got the best results with a low-impedance preamp designed for low noise voltage at the expense of noise current. I tried all sorts of expensive op-amps but in the end a 2N4401 outperformed them all.

[Merlinb]

No, you didn't understand JM's point. When the guitar is at full volume its output impedance is more than 100k in the middle of the audio band,

But guitar speakers struggle to reproduce anything above 5kHz, so noise above that is largely immaterial. Below 5kHz the pickup impedance is going to be much less than 100k, typically, so even with a 500k pot you're looking at an *average* equivalent source resistance of a few tens of k-ohms. So a grid stopper smaller than 68k really will buy you a few dBs less noise in a high-gain amp, but below 10k you're certainly into the law of diminishing returns.


Even if we forget about the guitar completely, surely if the amp is just sitting on stage being mic'd up, or whatever, you don't want it to produce more background hiss than is really necessary.

[Wombaticus]

But guitar speakers struggle to reproduce anything above 5kHz, so noise above that is largely immaterial. Below 5kHz the pickup impedance is going to be much less than 100k, typically, so even with a 500k pot you're looking at an *average* equivalent source resistance of about:
500k * 100/(500+100) / 2 = 40k.
So a grid stopper smaller than 68k really will buy you a few dBs less noise in a high-gain amp, but below 10k you're certainly into the law of diminishing returns.


Even if we forget about the guitar completely, surely if the amp is just sitting on stage being mic'd up, or whatever, you don't want it to produce more background hiss than is really necessary.

There is no thermal noise associated with a reactance in the first place.

[Merlinb]

There is no thermal noise associated with a reactance in the first place.

Yep, but the reactance affects how much of the pot noise actually gets to the output of the guitar; in other words, how much noise resistance is actually seen by the outside world when looking back into the guitar.

[J M Fahey]

Dear Merlin, please let me repeat that your notion of:

10k is much greater than most pickup impedances over a wide range, so the input capacitance can be as big as you like without greatly loading the pickup.

is wrong and can lead to much confusion.

If somebody else had said so, ... well ... , ... let it pass .... Forum discussions do not change the world anyway ... but you write Books which are read by many, so please let's stop that erroneous concept from the beginning, before spreading and growing.

Here I made a small table, considering a classic pickup, the Gibson PAF.
And why did I choose it ? (before you accuse me again of "pulling it out of the blue")
Well, suppose I choose a Strat pickup.
I guess designing a "Strat only" amplifier is not a very bright idea.
It would certainly compromise the usefulness of the amp.
Designing it for a humbucker will certainly mean that it also works well with a Strat, simple as that.

And why a PAF?
Well, it's a Classic, all others are variations of it, why not go to the source which, by the way, is still valid and widely used?

I'm still using Bill Lawrence's quote of 4.4 Hy.
Other makers' products may vary some, why not? but it does not change the essence of the calculation nor will shift final numbers by much.

Inductive/resistive/total means impedance at the stated frequency.

Frequency------inductive----resistive----total
80Hz-----------2K2------------7K5----------9K7
160Hz----------4K4------------7K5---------11K9
320Hz----------8K8------------7K5---------16K3
640Hz---------17K7------------7K5---------25K2
1250Hz--------34K5------------7K5---------42K
2500Hz--------69K-------------7K5---------76K5
5000Hz-------138K-------------7K5--------145K

As a side note, the pickup has its own internal winding capacitance, which coupled to its internal inductance, causes a resonant peak.
From PAF (pickup) - Wikipedia, the free encyclopedia (which I suggest reading) :
Resonant frequency------------7.715 kHz

and much more important:
Impedance: 319 kΩ at 7.715 kHz resonant frequency

Before you complain "but 7.7KHz is outside the guitar speaker band" let me remind you that that value is with the pickup mounted directly on an LCR measuring bridge.
Add *any* reasonable cable capacitance (even before getting into the amp) and that resonant peak lowers in frequency, well into the audible band.
Add *any* further capacitance at the amp input, and the results are even more noticeable.
Peak frequency can only go down with added capacitance, never "up".

So please, no more

the input capacitance can be as big as you like without greatly loading the pickup

Thanks.

PS: if you spot any error in my Math and offer your own calculations, fine with me

[Wombaticus]

While recognizing the value of real-world experience, it may be illuminating to nevertheless take a look at some results from a reasonably well motivated model.

Check out the following circuit modeling a pickup and an input stage, with typical values chosen.



Here's some results of modeling the Johnson noise spectrum at the grid with this model with, first, a 34K resistor alone, then next with a 10K resistor in conjunction with a 220pF grid-to-ground cap (as shown in the illustration of the model).





Here also are the frequency responses predicted at the grid for each model and for the no-grid stop case (shown up to 2 MHz in this case, as we are interested in RF as well)