Till Preamp

[Anti-Idiot]

i've build 2 of the Till Preamp, one for each volume pot (2 humbos), but, since my bridge pickup has lower output than the bridge one, i've put a 10uF cap in parallel with the souce resistor (2.2k). The Output of that (bridge) pickup is greater than the neck one, but when i play clean, the B.PU sound horrible, it clips a lot of signal... sounds like the Strwberry Overdrive... what can i do? take off the cap or the res.?

BTW, merry x-mas

[Paul Marossy]

Lose the cap. Also, you might want to try a trimpot on the source of the FET to dial it in to about half of the supply voltage at the drain. You could also try a smaller bypass cap - something in the neighborhood of 1uF to 2.2uF.

[Anti-Idiot]

Lose the cap. Also, you might want to try a trimpot on the source of the FET to dial it in to about half of the supply voltage at the drain. You could also try a smaller bypass cap - something in the neighborhood of 1uF to 2.2uF.

←

those caps should bypass the 2.2k, right? that would give me a treble boost, right

?

[Paul Marossy]

The 10uF bypass cap on the 2.2K resistor that you currently have on there will boost more of the bass frequencies more than acting like a treble booster.

EDIT: I haven't tried this, but maybe using a small cap instead, like a 0.001uF instead of an electrolytic cap, may give you more of a treble boost effect.

Edited December 25, 2005 by Paul Marossy

[Anti-Idiot]

The 10uF bypass cap on the 2.2K resistor that you currently have on there will boost more of the bass frequencies more than acting like a treble booster.

EDIT: I haven't tried this, but maybe using a small cap instead, like a 0.001uF instead of an electrolytic cap, may give you more of a treble boost effect.

←

no, i just want a clean boost. on till's page he says a low value cap will give treble boost and a large one just give a boost. i think it was my mistake to put the cap in parallel with the 2k2 res. and not bypass it. but i don't know know wut to do... i think i'll follow ur indications and take away the cap and leave the resistor...

[Saber]

Using the 10uF source bypass cap will boost pretty much the whole audio spectrum. The lower you make that capacitor, the higher your cutoff frequency will be so it will end up boosting the higher frequencies only, just like you read on Till's page.

You can put a resistor in series with that 10uF source bypass cap to reduce the boost but maintain the flat response.

Edited December 25, 2005 by Saber

[JohnH]

Using the 10uF source bypass cap will boost pretty much the whole audio spectrum. The lower you make that capacitor, the higher your cutoff frequency will be so it will end up boosting the higher frequencies only, just like you read on Till's page.

You can put a resistor in series with that 10uF source bypass cap to reduce the boost but maintain the flat response.

←

I have also experimented with that circuit, and what Saber says is exactly right.

I'd Keep the 10mfd cap and the 2.2k resistor, and try a 10k trimpot in series with the cap (using just the centre and one outer lug, as a variable resistor). That way, you will get a boost across the whole spectrum, but an amout which increases the lower the value of the trimpot setting. Set the trimpot for the boost you need, without clipping, or instead just try different fixed resistors until you get the right value.

Another issue is that these circuits are sensitive to the type of JFET, and even JFETs of the sae type have significant variation. What type are you using? I adjust the source resistor to set about 6V at the drain, which is halfway from full supply 9v (JFET fully turned off) and the minimum voltage (wth JFET full on) set by the resistors, which is about 3V.

One last idea to throw in - I build these with all resistors x3 in value. It works the same but draws less current, and still has enough of a low output impedance for my purposes.

[Anti-Idiot]

Using the 10uF source bypass cap will boost pretty much the whole audio spectrum. The lower you make that capacitor, the higher your cutoff frequency will be so it will end up boosting the higher frequencies only, just like you read on Till's page.

You can put a resistor in series with that 10uF source bypass cap to reduce the boost but maintain the flat response.

←

I have also experimented with that circuit, and what Saber says is exactly right.

I'd Keep the 10mfd cap and the 2.2k resistor, and try a 10k trimpot in series with the cap (using just the centre and one outer lug, as a variable resistor). That way, you will get a boost across the whole spectrum, but an amout which increases the lower the value of the trimpot setting. Set the trimpot for the boost you need, without clipping, or instead just try different fixed resistors until you get the right value.

Another issue is that these circuits are sensitive to the type of JFET, and even JFETs of the sae type have significant variation. What type are you using? I adjust the source resistor to set about 6V at the drain, which is halfway from full supply 9v (JFET fully turned off) and the minimum voltage (wth JFET full on) set by the resistors, which is about 3V.

One last idea to throw in - I build these with all resistors x3 in value. It works the same but draws less current, and still has enough of a low output impedance for my purposes.

←

i use a 2n5457, and i dont use the preamp as a booster, but as buffer (one for each pickup, making 'em pseudo-active)...

[JohnH]

i use a 2n5457, and i dont use the preamp as a booster, but as buffer (one for each pickup, making 'em pseudo-active)...

←

Cool - that is the JFET type that I use too. I use 6.8k source resistor and 22k drain resistor. With no cap, I get a small signal boost, and very clean results. To balance two such circuits for your two pups, how about putting a 1M trim pot, wired like a volume control, on the input of one or both?

John

Edited December 25, 2005 by JohnH

[Anti-Idiot]

i use a 2n5457, and i dont use the preamp as a booster, but as buffer (one for each pickup, making 'em pseudo-active)...

←

Cool - that is the JFET type that I use too. I use 6.8k source resistor and 22k drain resistor. With no cap, I get a small signal boost, and very clean results. To balance two such circuits for your two pups, how about putting a 1M trim pot, wired like a volume control, on the input of one or both?

John

←

i use 'em like this:

Pickups->Preamp->volume

the whole circuit is attached to the pot (50k), like some EMG stuff. the boost in the neck pickup is quite clean, but in the bridge is overdriven and with an ugly clipping (ugly clipping sucks huh huh huh). i put the additional 10uF cap and the 2.2k resistor in parallel but Till said it was one or another. i think i'll just take off the cap...

Btw, i'll post some picks of the attached volume pot

[Paul Marossy]

Hmm... that's good to know about the resistor in series with the bypass cap. I still think the bypass cap needs to be smaller than 10uF. I think that is boosting the signal by too many dB, and that is why is why you are hearing the clipping - it's boosting the signal too much. You're probably getting 10dB (or more) of boost instead of 3dB or so. I think it should be 3.3uF or less, but that is just my opinion...

Edited December 26, 2005 by Paul Marossy