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10-27-2018, 05:11 PM
Hi Guys
Guitar and bass amps are almost universally under-filtered, making this an easy part of the circuit to upgrade. Where a certain noise level was acceptable in the infant days of electric guitars and related amplification AND caps were expensive then, things have changed in our modern era where caps are inexpensive AND we have options to use equipment that is very quiet; overall, our modern expectation of "quiet" is different.
Decoupling is done in various ways and has myriad benefits, as our "The Ultimate Tone" (TUT) series describes. The projects in TUT3 and TUT5 use a combination of high-node filtering and proportional filtering. It is proportional up to a master preamp node (Vp) from which power fans out to the individual preamp stages. This gives reasonable filtering while allowing each stage to have its own voltage adjustment.Because there are fewer stages of filtering overall, it is highly beneficial to use larger cap values.
Overall, it is easier to use a simple proportional supply with filter nodes for every stage. The only potential complication is where there are multiple signal paths.
Preamps draw constant current, or so nearly constant that sag is a non-issue. using 10uF filters is nonproblematic and lets one use physically small caps of 10mm diameter typically. As a proportional filter this works very well.
The isolation/dropping resistors are usually selected purely for the voltage drop desired and the RC constant falls where it may. For example, 1k + 10uF is a 10ms TC corresponding to a 3dB frequency response of 16Hz. This means that frequencies below 16Hz could interact much more greatly than higher frequencies do, with a possibility of motor-boating if the layout is poor. However, with a Galactic Ground layout there is no concern about this.
Note that in some specific gain stages or circuits, it is important to have a supply filter response that is much lower than the lowest signal frequency expected through the circuit.
In most OEM amplifiers, voltages tend to be rather high in the preamp stages and filtering is quite good. The charge stored is proportional to the voltage and the capacitance, so increasing one or the other will maintain a high energy storage well above what the stage needs to meet signal swing requirements. Fender often has a single 20uF cap supporting several stages where Marshall might have 50uF supporting a single stage or a couple of stages. If it were possible to isolate all other differences and hear what the filter cap influence is alone, which would require a half-ways correction of some layout problems, the higher cap values should provide better suppression of intermodulation artifacts and overall sound nicer.
You can "get away with" all kinds of decisions with respect to R and C values in the supply, along with layout choices, but if you stray too far from the ideal methods the circuit will be unstable or noisy at high-signal levels and if truly awful choices are made, at idle too.
Guitar and bass amps are almost universally under-filtered, making this an easy part of the circuit to upgrade. Where a certain noise level was acceptable in the infant days of electric guitars and related amplification AND caps were expensive then, things have changed in our modern era where caps are inexpensive AND we have options to use equipment that is very quiet; overall, our modern expectation of "quiet" is different.
Decoupling is done in various ways and has myriad benefits, as our "The Ultimate Tone" (TUT) series describes. The projects in TUT3 and TUT5 use a combination of high-node filtering and proportional filtering. It is proportional up to a master preamp node (Vp) from which power fans out to the individual preamp stages. This gives reasonable filtering while allowing each stage to have its own voltage adjustment.Because there are fewer stages of filtering overall, it is highly beneficial to use larger cap values.
Overall, it is easier to use a simple proportional supply with filter nodes for every stage. The only potential complication is where there are multiple signal paths.
Preamps draw constant current, or so nearly constant that sag is a non-issue. using 10uF filters is nonproblematic and lets one use physically small caps of 10mm diameter typically. As a proportional filter this works very well.
The isolation/dropping resistors are usually selected purely for the voltage drop desired and the RC constant falls where it may. For example, 1k + 10uF is a 10ms TC corresponding to a 3dB frequency response of 16Hz. This means that frequencies below 16Hz could interact much more greatly than higher frequencies do, with a possibility of motor-boating if the layout is poor. However, with a Galactic Ground layout there is no concern about this.
Note that in some specific gain stages or circuits, it is important to have a supply filter response that is much lower than the lowest signal frequency expected through the circuit.
In most OEM amplifiers, voltages tend to be rather high in the preamp stages and filtering is quite good. The charge stored is proportional to the voltage and the capacitance, so increasing one or the other will maintain a high energy storage well above what the stage needs to meet signal swing requirements. Fender often has a single 20uF cap supporting several stages where Marshall might have 50uF supporting a single stage or a couple of stages. If it were possible to isolate all other differences and hear what the filter cap influence is alone, which would require a half-ways correction of some layout problems, the higher cap values should provide better suppression of intermodulation artifacts and overall sound nicer.
You can "get away with" all kinds of decisions with respect to R and C values in the supply, along with layout choices, but if you stray too far from the ideal methods the circuit will be unstable or noisy at high-signal levels and if truly awful choices are made, at idle too.
