V-type preamp bright switch (high pass filter)

[BassPimp66]

OK, Lords Of Repairs please bear with me.
I'm a wikipedia-educated soldering enthusiast. I know nothing about electronic. I have moded a boss pedal before. That's about it.

I own a Trace Elliot v-type preamp. I love the tone ... but something is bothering me...
With the bright switch disengaged, I'm missing trebles (and the treble knob does work the way I want).
With the bright switch engaged, it's better, but a bit too trebly, and the treble know does not get me where I want.

I checked the technical manual and it explains that the bright switch engages an additional capacitor, making a high pass filter.
After a search on wikipedia, this is what I have found...

my cut-off frequency is defined as Fc = 1/(2Pi RC)

checking the preamp diagram, I spot a 10 MegaOhm resistor and a 150 PicoFarad Capacitor that become engaged with the bright switch.


using formula above, I get Fc = 1/(2Pi x 10 x 10p6 x 150 x 10p-12) = 106

So, I assume the cutoff frequency is 106Hz ... maybe it's a pure coincidence and I got lucky.
But, it sounds within range for cutting low frequencies.

Now... assuming this is correct.

Can I mod the preamp, replacing the capacitor with a 300 PicoFarad?
If I run the same formula, I get a cut-off frequency of 53 Hertz.

All in all, I hope to achieve a shift in the cut-off frequency to make the preamp sound less bright.
So, if changing a capacitor gets me there, houray !!!!
If I burn my lovely preamp due to stupid modding coming from my imagination and some wikipedia links, then game over!

Please infuse me with your knowledge!

Edited December 13, 2012 by BassPimp66

[LukeFRC]

106 seems a bit low. are the sums right?

I would think that there could be other things that I would try first before modding a preamp....
does you cab have a tweeter in it? turn the bright switch on and turn that down (or vice-versa)

[ikay]

The TalkBass link doesn't answer the specific question on modding the cap values but does discuss the function of the bright switch so may be of interest.

- http://www.talkbass.com/forum/f15/trace-elliot-v-type-tube-preamp-question-201756/

[Beer of the Bass]

Changing the capacitor will shift the cutoff point, but the cutoff point is not where you think it is. I think the 10M resistor can be more or less ignored in practice, as it's only there to discharge the capacitor and prevent clicks when the switch is engaged. When the bright switch is on, the 10M resistor is going to be in parallel with the lower side of the volume pot (between the wiper and ground), which will always be 1M or less, so 10M in parallel with 1M is not going to change things much.
The trouble with trying to calculate the frequency from simple R C formulae is that R is affected both by the impedance of the stage driving it and the setting of the volume control. There's a calculator at [url="http://www.ampbooks.com/home/amplifier-calculators/bright-boost/"]http://www.ampbooks....s/bright-boost/[/url] which can be useful here.
From my own experience of changing bright cap values, moving the frequency downwards can actually make things brighter sounding. It might be worth buying several values of cap in the right range and figuring out which one you like best. If you're happy that you can change it cleanly and reversibly, it's about the single cheapest, simplest tweak you can do to a preamp.

Edited December 14, 2012 by Beer of the Bass

[BassPimp66]

[quote name='Beer of the Bass' timestamp='1355482182' post='1898895']
Changing the capacitor will shift the cutoff point, but the cutoff point is not where you think it is. I think the 10M resistor can be more or less ignored in practice, as it's only there to discharge the capacitor and prevent clicks when the switch is engaged. When the bright switch is on, the 10M resistor is going to be in parallel with the lower side of the volume pot (between the wiper and ground), which will always be 1M or less, so 10M in parallel with 1M is not going to change things much.
The trouble with trying to calculate the frequency from simple R C formulae is that R is affected both by the impedance of the stage driving it and the setting of the volume control. There's a calculator at [url="http://www.ampbooks.com/home/amplifier-calculators/bright-boost/"]http://www.ampbooks....s/bright-boost/[/url] which can be useful here.
From my own experience of changing bright cap values, moving the frequency downwards can actually make things brighter sounding. It might be worth buying several values of cap in the right range and figuring out which one you like best. If you're happy that you can change it cleanly and reversibly, it's about the single cheapest, simplest tweak you can do to a preamp.
[/quote]

Thanks for the input and for the link. That thing is brilliant!

[MarkBassChat]

[quote name='BassPimp66' timestamp='1355437488' post='1898537']
Now... assuming this is correct.
Can I mod the preamp, replacing the capacitor with a 300 PicoFarad?
If I run the same formula, I get a cut-off frequency of 53 Hertz.
All in all, I hope to achieve a shift in the cut-off frequency to make the preamp sound less bright.
[/quote]I'm sorry but most of your assumptions are false. First of all, when the bright switch is on, the 10MOhms resistor is not used. The capacitor "works" with upper part of the pot. The purpose of the resistor is to avoid "click" when the switch is turned on. You can replace the capacitor with any other value but when you replace it with 300pF, it will increase high frequency boost so the preamp will sound [u]more[/u] [u]bright[/u]. To make it sound [u]less bright [/u]you have to decrease its value, or just turn the Treble pot down, or, if you have a tweeter in the cab, turn off the tweeter.
And keep reading wikipedia: you are not interested in cut-off frequency in this case. You are interested in high frequency boost, or cut.

Mark

[BassPimp66]

[quote name='MarkBassChat' timestamp='1355489892' post='1899077']
you are not interested in cut-off frequency in this case. You are interested in high frequency boost, or cut.
[/quote]

Is a high pass filter boosting the high frequencies too? I thought it worked by shaving off some lows and making the sound relatively trebly.

[Beer of the Bass]

The cap is used as a high pass filter, but it works by allowing treble frequencies to bypass the volume pot so that the pot attenuates those frequencies less. So, it gives an apparent boost to the highs when the volume control is less than maximum. With the pot turned up full, the bright cap will make no difference.

[MarkBassChat]

[quote name='BassPimp66' timestamp='1355500655' post='1899334']
I thought it worked by shaving off some lows and making the sound relatively trebly.
[/quote]That's not correct. Here you have a high-pass filter with a pot in parallel, which you may consider all-pass filter. So switching the filter on causes that more high frequency signal gets to the output. Low frequency signal is passing through the pot. This is very well explained by Bear of the Bass above. Please read it.

Mark

[BassPimp66]

Thanks guys. I've learned something today.