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Old 25-01-2018, 06:34 AM   #529
Will fap for food
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Join Date: 15 August 2007
Location: Syd City 2000
Age: 28
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I actually had to remember what it did because its been so long since I have ever thought about the switch. What happens is that when the switch is flicked, lets say off to on, you introduce a resistor to go to ground. These two resistors sit on the base and the emitter of the transistor pair.

Now, both the base and emitter *already* have a resistor going to ground so what we essentially do is put resistors in parallel.
So 100k || 100k = 50k ~= 51k in real world parts
And 4.7k || 4.7k = 2k35 ~= 2k2 "" ""

That doesn't mean too much but all it does is change whether more voltage goes to ground at the base or more goes at the emitter. The 100k to 50k changes the transistor bias, the 4.7k to 2.2k changes the gain. It's odd though that the values picked don't give it a substantial difference but that may explain why it's possible that harmonic content may be slightly different as a result of the switch but that's reaching pedantic levels that a lot of original owners argued made no audible difference, hence it's exclusion.

And weirdly enough, you actually made me realise that in the last public schematic revision I may have labelled the switch wrong in the schematic as DPDT when it was SPDT meaning it was not both being switched on, but one or the other. Was that long ago, the difference either way is really minimal but I will definitely check.

I basically just compensated for the switch by choosing one side with the closest part replacement 51k, hence choosing one side of the switch to represent and picking the closest parallel part to minimise part count. It's a few dollars saved each build.

So tl;dr, the switch either turns down the gain, or the voltage bias but in a way that equates to same level and frequency filtering.
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