vox boost plus MOds

[wyn]

Hi, ime making this at the moment and have changed the input cap 22nF and the output to 180nF.

what does the output cap do? i cant hear any difference from 100nF to 180nF.

i also changed the transistor to a 2N5551 but have used a 2N3904 as well. If i use a germanium transistor i get a more gritty sound. (matsushita 2SD252H)

heres the schematic

https://plus.google.com/u/0/102729661034733473192/posts/eMgd8eHz9PZ?pid=6226582134952697634&oid=102729661034733473192

[amptramp]

Increasing the capacitance extends the bass response.  The large increase in input capacitance has more of an effect in extending bass response and also tends to reduce noise since the noise at the amplifier input is loaded down by the guitar connected to the input.  But these changes mean it is no longer a treble booster - it is more of a full range booster.

[wyn]

what does the output cap do? i cant hear any difference from 100nF to 180nF.

[wyn]

i ended up using a 2,2nF input for the treble sound.

If i want to be able to switch between 2 different caps on the input, whats the best way to do that?

[mth5044]

With a switch of course!

SPDT, input to middle lug, cap on either side. Connect the other end of the caps together and then back into the circuit.

[wyn]

thanks mate!

[wyn]

  can you draw it up so i can see it?

[mth5044]

If you give it a try, we'll let you know if you've drawn it right!

Center of the switch is common and it switches between the two outer lugs. Think about what the circuit looks like in both positions.

[PRR]

> what does the output cap do? i cant hear any difference from 100nF to 180nF.

It blocks the DC of the transistor stage so it does not upset the bias of whatever you plug this in-to.

It is picked with consideration of the input impedance of the next stage (pedal or or amplifier).

We usually don't know the input impedance of what it will be plugged onto.

Generally we take a low guess, and pick the cap "large" for that input impedance at the lowest frequency of interest.

(In this case of a "treble booster" we might pick not-large, because a little bass cut is similar to a treble boost.)

Amp and pedal input impedances run over 1Meg to under 100K, maybe 50K.

0.1uFd (100nFd) with 50K is full bass to 33Hz. This is far below the lowest note on guitar. This "passes everything", which is usually what we want.

0.18uFd (180nFd) with 50K passes down to 18Hz. This also "passes everything". The difference at 82Hz, even 64Hz drop-tune, is hard to measure and insignificant on stage.

The 0.18uFd costs a few cents more, and since there is no audible difference, is a "waste" of a few cents. If you build a million of these things you will need to watch your pennies. In DIY work and part buying, the difference in cost may be zero (all caps of a range 75 cents each) or may be utterly unimportant in your overall fun/dollar balance. In very tight builds, the difference in size 0.1uFd and 0.18uFd may be a factor.

A special-case: *this* circuit's input impedance is clearly less than 22K, and probably under 18K. With one of these into another, the bass-cut with 0.1ufd will be more like 90Hz. This slightly shaves your bottom note. With 0.18uFd the cut starts down at 50Hz. But with a double treble boost, the exact level of your lowest notes may be un-important.
_____________________________
This circuit is designed for the 0.6V B-E drop of Silicon transistors. If you use Germanium, with 0.1V-0.2V drop, the circuit is "mis-biased". It will work but overload easier, with a different sound. Change 100K to 143K (150K) to get bias back to as-designed. Try values of 50K to 1Meg for extreme mis-biasing. 

[wyn]

you guys just made me do some late night soldering!!

found this and it worked.

http://www.diystompboxes.com/smfforum/index.php?topic=99394.0

[wyn]

https://plus.google.com/u/0/102729661034733473192/posts/DyCnofgajiS?pid=6226780485417661746&oid=102729661034733473192

[wyn]

cos i dont know how to upload pics!!

[wyn]

my own drawing

https://plus.google.com/u/0/102729661034733473192/posts/EMAjRr6QzA5?pid=6226784461422087378&oid=102729661034733473192

[mth5044]

Exactly right! Good job!

There are various iterations of what you are doing, like using an SPDT on/on/on with three caps in parallel arrangement for three different values, as well as putting two caps on a pot to blend between them (which only kind of works). They way you did it is great though.