Internetfolklore: weerstand in serie met PrePIMV?

[HaroldA]

Jullie weten onderhand wel dat ik alles geloof wat er op het Internet staat en ik kwam laatst dit stukje (http://www.sonicdeli.com/ThunderTweakWeb/plexi_mods.htm) tegen:


11) our friend Master Volume
Now that another triode of gain has been added, which is a lot, we need a way to control how much signal hits our splitter. Otherwise any little turn of the gain knob would produce truly stupid amounts of volume. This is where the master volume comes in. Looking at the 2204 schematic we can see, after the treble knob, there's a 1M master volume control feeding the input of the splitter. They call this a "pre-phase inverter"-type master. I like it a lot because I can drive the preamp as hard as I feel the need and have control over how much signal hits the input of the splitter and balance the preamp/poweramp overdrive. When you think of it this way it gives a finer control over the whole amp's tone and feel.

Some folks don't like this type of master because it sounds thin at low volumes or it doesn't sound the same at every volume. The easy fix is to add a 100k-220k resistor between the 1M pot and the input of the splitter. Boom, a master volume that sounds great at all levels! My turret boards even have a spot for this resistor!

Nou ben ik natuurlijk geneigd om dit direct uit te voeren op al mijn MV amps, maar toch wringt dit een beetje. Ik heb al een hele hoop schema's bekeken, en of ik heb er helemaal overheen gekeken, of dit wordt helemaal nergens gebruikt? Ik heb alleen dit stukje (https://sites.google.com/site/yourtubeamp/mods-and-maintenance/-master-volume-controls) kunnen vinden:


Kevin O'Connor has a good explanation in The Ultimate Tone about how this affects the tone of the amp. When the preamp signal is attenuated, some treble is lost. The more attenuation, the more treble that is lost. A series resistor of about 100K can help reduce this effect, but you'll lose a little volume.

Daar wordt een 100k aangeraden om het verlies in hoog tegen te gaan, ten koste van een beetje volume, maar dat kun je weer oppeppen door je master hoger te zetten ;)


Is een 100-220k weerstand in serie met de wiper van de volumepot onzin, of eigenlijk best een goed idee? Heeft er al iemand ervaring mee?

[HaroldA]

Is een 100-220k weerstand in serie met de wiper van de volumepot onzin, of eigenlijk best een goed idee? Heeft er al iemand ervaring mee?

De eerste reactie is al binnen:


old trick..... 180k: better Tone in both low gain setting and high gain setting.... in the middle quite right isn't it...

[HaroldA]

En de 2e:


People may like the sonic results, but the explanation is silly and doesn't match what actually happens. The fact of the matter is that adding such a series resistance only accomplishes a worsening of the loss of treble! This is true even when the volume is full-up. For instance, with a typical Marshally type LTP setup (w/o) NFB, you would get a bit more than 27dB of gain at 10KHz if you could feed it with something with an output impedance approaching zero -- and that'll decrease down to about 22 dB as the output impedance of the driving circuit increases to 250K (maximum output impedance of a divider made from 1Meg pot). On the other hand, with a 100K added series resistance after the divider, that will vary at 10KHz from about 25.5dB down to approximately 20.5 dB - also about a 5 dB decrease as source impedance increases. All you're managing to do therefore is decrease volume just a hair at all frequencies (a dB or two at mid-band), and since there is even more volume loss at high frequencies it makes the problem treble loss problem permanent regardless of pot position. It's worth pointing out also that in practice the source impedance of the circuit driving the divider is never actually zero, so there is even less spread than suggested by this simplistic example. This doesn't do what you think it does folks.