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Author Topic: Preamp Pedal  (Read 10328 times)

Loudthud

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Re: Preamp Pedal
« Reply #15 on: February 14, 2014, 06:51:27 PM »
The plot below shows various caps I tried. I built a second proto of the second stage only and run it from a 30V bench supply. I even scrounged around and found a MPF102 from a different batch and vendor, no change. The plot is the circuit driving just a 10 Meg scope probe, the AP analyzer is connected to the scope's signal output so vertical scale doesn't mean much but it's still dB. The input was 40mVrms and the output is about 8V pk-pk. You can clearly see the rolloff of 2dB from 100Hz to about 15KHz.

phatt

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Re: Preamp Pedal
« Reply #16 on: February 15, 2014, 02:59:55 AM »
Yep Yep,, I understand that (as best I can) but I don't get too concerned about that kind of detail,, you just tweak the tone circuit to suit the situation.
Heck it's only guitar preamp  :lmao:

As long long as the signal out is bigger than input then it works,, the rest I leave for experts to nut out. :-X

FWIW, Here is how I would approach this design problem.

Tweak that voltage division, I think 4k7 is too low to be useful. (might leave that for the serious teck guys to teckxplain) xP

Next is compensate in the tone tree.
Using the classic fender style it's mostly adjustment of the slope resistor.
(R6 in my circuit)
Obviously an increase of C1 or larger R20 will change that bass response considerably.
I'd likely put a buffer after the output just to be sure it can run into almost anything.

And for the benefit of *GuyB* I've found these discrete FET circuits to be noise prone. and tricky to setup so if you are starting out I'd stick with opamp circuits until you become proficient at understanding design and especially the hidden flaws that lurk.
Phil.
« Last Edit: February 15, 2014, 03:17:22 AM by phatt »

J M Fahey

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Re: Preamp Pedal
« Reply #17 on: February 16, 2014, 10:50:41 AM »
Thanks for posting  :dbtu:
I would not worry at all about the displayed 2dB "loss" , considering bandwidth goes from 100Hz to 20KHz, a 2000:1 range or 8 octaves (1/4 dB / oct?) and would suspect minuscules losses added up along the signal path or even simpler, reduced oscillator output at higher frequencies.
Or all of the above.

What I *am* impressed about is the 70X stage gain  quite uncommon in FET stages.  :dbtu:

Loudthud

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Re: Preamp Pedal
« Reply #18 on: February 17, 2014, 11:05:23 PM »
Thanks for posting  :dbtu:
I would not worry at all about the displayed 2dB "loss" , considering bandwidth goes from 100Hz to 20KHz, a 2000:1 range or 8 octaves (1/4 dB / oct?) and would suspect minuscules losses added up along the signal path or even simpler, reduced oscillator output at higher frequencies.
Or all of the above.

In Reply #8 the J201 is almost prefectly flat at lower gain and bandwidth. That graph was made with the analyzer as a load. The flatness of the generator has been verified even on the JFET side of the Gate stopper. The output still isn't flat driving only a 10 Meg scope probe. It's not a measurement error or equipment malfunction.

adrianhertz

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Re: Preamp Pedal
« Reply #19 on: February 02, 2016, 11:51:09 AM »
Hello everyone, i built the blackface preamp some time ago, used a tda based power amp and run it through a 12x1 cabinet, i recommend using a quality speaker like eminence cannabis or governor which i"ll be upgrading soon...
Also the source follower is necessary to avoid any hum, i didn't change any from the original circuit.

 http://youtu.be/b6STGArMKj8
       

Sorry for the crappy playing...


Sent from my iPod touch using Tapatalk

 

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