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Bass preamp schematic

2/23/2005 3:36 AM
Bass preamp schematic
G'day guys,  
I've been having a bit of trouble with a bass preamp I built and came to the conclusion it's probably more a design problem than a constuction problem  
What I'm after is a solid state bass pre-amp schematic to drive a 300W power amp.  
Features would be boost with at least bass and treble, graphic EQ and a compressor with an overload indicator.  
If there's nothing with everything in it, maybe some parts that could be put together?  
I'm really after some input on SS bass amps you've made and what you were happy with and a link to a schematic if you know of it  
2/28/2005 11:01 PM
Dr. Photon

I have built this circuit with great success:  
The actual preamp portion of it is extremely simple (especially if you remove the clip LED, tone controls, and the tuner out). Solid state amp preamps are very similar to stompboxes, circuitry and topology wise. I modified the schematic a small bit. I also made many substitutions in non-critical areas and used whatever components I had "lying around" (and I amused myelf by trying to see how many different kinds of capacitors I could use!).  
You might consider adding in a simple compression scheme (look at stompboxe schematics), and maybe some diode clippers if you want a small amount of grind.  
I built a +/- 35 volt (no load) power supply with rather over-sized filter caps (had on my desk as paperweights!) powering an LM3886 chip as a 50 watt power amplifier. I installed the whole mess in a surplus 2u rack cabinet from an "LNC power supply", which is a box with some redimentary control logic and some modular linear power supplies which presumably powered the feedhorn amplifiers and whatnot in a TV station satellite system.  
I retrospect, I don't think i needed that shielded cable (but I had a few tens of pounds of it that also went with the satellite system....), and the wiring to the board could have been neater....  
But the 10 turn trimmers (regular would be just fine too) make biassing the fets *very* easy. and they are essential considering that I used different FETs , for which the schematic values were waaaaay off and it sounded like %@$#^.  
3/1/2005 1:53 PM
Albert Kreuzer

Hi Dr.Photon,  
thank you for building my preamp! Nice chassis, too :)  
You are right about the trim pots, JFETs vary from -0.5 to -8 Volts Vgs(off) or even more :(  
And shielded cable can't hurt...  
When you turn up the "Sensitivity" control, you should be able to get a nice growl out of it, also a little tube-like compression.  
3/10/2005 2:32 AM
Pete Re: FETs
A quick question about the BF245's. Any reason you used those particular FETs in preference of J201's, 2N5459 etc?  
Also what are the caps on the bottom of the FET's for (C2 and C6 220uF). Are they for stability??  
3/10/2005 6:37 PM
Dr. Photon

The caps on the bottom (the source, as in the SOURCE of electrons) of the FET are Bypass capacitors. They serve the exact same function as the electrolytic capacitor accross a tube's cathode resistor. Actually, a FET is very similar to vacuum tube. Source = cathode, drain = plate, gate = grid. Have you ever seen the FET preamp pedals where the design looks identical to a Fender (or Marshall) preamp albeit there are JFETs instead of tubes? A FET is even more similar to a pentode (with the screen connected the "usual" way to B+). They are both voltage controlled current sources. a regular bipolar transistor is a current controlled current source (until you play with Ebbers-Moll...).  
The choice of FET doesn't seem to be very critical here. I used some 2N5457's since i had a baggie of 50 sitting around.,4450,2N5457,00.html  
They sound great. My only issue with them is their noise. I can hear some hiss if I MAXX out all teh controlls, but the amp is still silent at settings that would cause an insanely distorted pseudo-squarewave at the output. Perhaps some sockets would be a good idea for the FETs?  
I highly recommend using some trimmers for the source resistor. This makes it very easy to adjust the bias to optimum for any FET. adjusting the bias will change the overdrive characteristics of the amplifier, and if the Bias is way off, the total tonality of the amplifier.
3/11/2005 1:40 AM
Albert Kreuzer

What Dr. Photon said.  
The reason why I took the BF245s was simply because I had them :)  
In fact all small signal N-channel JFETs are very similar if you bias them correctly.Yes, there will be differences in noise and gain, but they all will work. If you want to experiment, sockets and trim pots are a good idea. But double-check the pinout of the FETs, theoretically you can have 6 different layouts (but there seem to be at least 12...)  
3/17/2005 4:56 AM
I used 2N5485's for the FETs. It works but there doesn't seem to be enough gain for me - it won't drive the power amp enough. I'll re-check the values again (I did find a couple of errors), to make sure everythings got the right value. I'm thinking that the impedence of the poweramp and the output of the preamp don't match too well. Because when I've run it with an unity opamp (from a mixer preamp), it seems to run the power amp to full power with no worries. I've been checking out the gains of a few mixer preamps I've run it with before and the gains don't seem to be overly different to this preamp  
That was one of the other grips I had with the other bass preamp I had.  
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