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Old 16th Sep 2021, 5:59 am   #36
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 18,311
Default Re: Advice on this linestage build

Spice is learnable. It's an answer to all sorts of problems rather than being a problem in itself. It is a fair step in learning it, but you don't have to absorb the whole thing in one go. You just need to do something simple and learn how to not turn on all the things you don't need. They'll wait til you have a use for them later.

The problem is that there are many half-assed simulators which look lots easier, and divert people from starting with a full-blooded one. They fairly soon hit the limitations of the 'easy' ones and get bored with them or frustrated. Some go on to look for and use a slightly more elaborate simulator and find they have to learn it from scratch.

It makes sense to curse a pox on all their houses and go learn one that will grow with you. If only it didn't look more difficult than it is....

Back to Baxandalls:

Peter Baxandall wrapped an existing sort of passive tone control around an amplifier such that its pots could swing between attenuating the input through to reducing the feedback and revving up the amplifiers gain. He'd created a circuit with fairly symmetrical capabilities. You could arrange one to be capable of as many dB of boost as it had of cut, and you could choose those limits. Rather neat, and it out-performed the passive version sufficiently that it became the dominant circuit for many years.

The passive circuit cannot give gain, so you must choose how much pseudo-boost you want with the pots screwed fully clockwise, and that is also the minimum loss you can have with it all set to flat.

Baxandall's active circuit has its limitations, too. It cannot provide more gain in the peak boost condition than the raw amplifier it uses can manage. Worse, the raw amplifier probably needs enough gain in hand at peak boost, that there is still enough feedback around it to help with its distortion and to control its output impedance. You can get a bit more gain per valve than you can get per transistor. Doing a Baxandall with the usual 12dB or so of boost doesn't look too bad with a good gainy valve, but the impedances you need to run fight you. With care, you can make something reasonable, but anyone doing distortion measurements with the pots much away from flat will help fuel the audiophile belief that tone controls are the spawn of the devil (FX: Thundercrack in the distance). Doing one with a single transistor is appreciably more difficult. Do-able it seems, but actually worse-performing.

But transistors have an ace, they are cheap and you can easily use two or more. You can have enough gain to make a good Baxandall without bad impairments at much boost, and you can avoid driving yourself down into the noise. You could have multiple valves, but the costs and heater power discourage people. Multi-transistor amplifiers are also availably in handy, easy to use, OP-amp form. You don't have to stick to the Baxandall curves, You can have parametric EQ if you want, like a zillion mixer desk channels around the world. You can even have, shhh whisper, a graphic equaliser (FX: BIG thunder crack!). Of course, that's being silly and going too far, but you can have one in your system. It doesn't need to be connected, just on power and lit up. It will then radiate its audiophile repulsion field and protect you from nuisance.

David
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