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Yes,
This is all about high gain, high sustain preamps.
You have a gain stage and then knock down the gain with an inter-stage attenuator, add more gain with another stage and knock it down again with another inter-stage attenuator. This build up / knock down adds harmonic content and determines the ability to sustain a sound level.
The inter-stage attenuator between the 1st and 2nd stages is either divide by 5 or divide by 2 depending on whether that 100K is switched in or not (switching it out is the gain boost).
Eventually you get enough gain such that the signal level into the next stage is high enough to attempt to swing the grid positive and you get a clamping action due to grid rectification current PLUS you get blocking distortion because the coupling cap between the stages charges up as a result of that grid rectification current. That extra series resistor, (R15 for example) is to limit the current into the grid and hence prevent blocking distortion. That resistor also acts as a grid stop: and rolls off the high frequency so a cap is wired across it to restore some of the high frequency response. Note that this resistor was not required into the 2nd stage as the signal level was not high enough at that point (not enough gain ahead of that stage) to cause grid rectification current.
The final gain stage goes into saturation first, then the one preceeding it etc. You get sustain as stages come out of saturation, the ones closest to the input first and the final gain stage last.
You will note there is some frequency shaping on the inter-stage attenuators. The harder you overdrive the more higher order harmonics are generated and harmonics past the 5th are more objectionable with 7th and 11th (I think, don't quote without looking it up) being musically "quint", that is, bearing no musical relationship to the original note and are therefore very objectionable even in small amounts. In early stages you may want a little treble boost to emphasize the harmonics created (cap across the top arm of the attenuator) but further down the preamp chain you will want some treble roll off to limit higher order objectionable harmonics.
In general the grid stop used to limit grid current, and hence stop blocking distortion, will give you the required treble roll off, up to a value of say about 82K to 100K. Above that value you MAY get better blocking limiting but you roll off so much treble that it is necessary to add those "treble tweak" caps across the series resistor. I've never found it necessary to use larger than 47K for that series resistor (so I did'nt need a treble boost cap) but then I've never designed a shreader "uber gain" preamp either, although I have done 4 stage preamps.
What worries me a bit about these circuit posted above is that they are likely to be high noise. Noise in high gain preamps is largely contributed by random fluctuations of small grid currents. To control it (shunt it to 0V) you want to see a lowish value of impedance/resistance to 0V when looking away from the grid back toward the preceding stage. The inter-stage attenuator actually helps this but the series grid current limiting resistor makes it worse.
So do I have the arrogance to look at that preamp example above and think to myself that I could have done it better, particularly with respect to likely noise performance, yeh I do (but then I've been wrong before).
Cheers,
Ian
This is all about high gain, high sustain preamps.
You have a gain stage and then knock down the gain with an inter-stage attenuator, add more gain with another stage and knock it down again with another inter-stage attenuator. This build up / knock down adds harmonic content and determines the ability to sustain a sound level.
The inter-stage attenuator between the 1st and 2nd stages is either divide by 5 or divide by 2 depending on whether that 100K is switched in or not (switching it out is the gain boost).
Eventually you get enough gain such that the signal level into the next stage is high enough to attempt to swing the grid positive and you get a clamping action due to grid rectification current PLUS you get blocking distortion because the coupling cap between the stages charges up as a result of that grid rectification current. That extra series resistor, (R15 for example) is to limit the current into the grid and hence prevent blocking distortion. That resistor also acts as a grid stop: and rolls off the high frequency so a cap is wired across it to restore some of the high frequency response. Note that this resistor was not required into the 2nd stage as the signal level was not high enough at that point (not enough gain ahead of that stage) to cause grid rectification current.
The final gain stage goes into saturation first, then the one preceeding it etc. You get sustain as stages come out of saturation, the ones closest to the input first and the final gain stage last.
You will note there is some frequency shaping on the inter-stage attenuators. The harder you overdrive the more higher order harmonics are generated and harmonics past the 5th are more objectionable with 7th and 11th (I think, don't quote without looking it up) being musically "quint", that is, bearing no musical relationship to the original note and are therefore very objectionable even in small amounts. In early stages you may want a little treble boost to emphasize the harmonics created (cap across the top arm of the attenuator) but further down the preamp chain you will want some treble roll off to limit higher order objectionable harmonics.
In general the grid stop used to limit grid current, and hence stop blocking distortion, will give you the required treble roll off, up to a value of say about 82K to 100K. Above that value you MAY get better blocking limiting but you roll off so much treble that it is necessary to add those "treble tweak" caps across the series resistor. I've never found it necessary to use larger than 47K for that series resistor (so I did'nt need a treble boost cap) but then I've never designed a shreader "uber gain" preamp either, although I have done 4 stage preamps.
What worries me a bit about these circuit posted above is that they are likely to be high noise. Noise in high gain preamps is largely contributed by random fluctuations of small grid currents. To control it (shunt it to 0V) you want to see a lowish value of impedance/resistance to 0V when looking away from the grid back toward the preceding stage. The inter-stage attenuator actually helps this but the series grid current limiting resistor makes it worse.
So do I have the arrogance to look at that preamp example above and think to myself that I could have done it better, particularly with respect to likely noise performance, yeh I do (but then I've been wrong before).
Cheers,
Ian
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