Solid State Guitar Amp Forum | DIY Guitar Amplifiers
Solid State Amplifiers => Preamps and Effects => Topic started by: t0neloc on January 05, 2013, 07:13:09 AM
Hi folks - first post so be gentle!
I need some advice on a suitable really simple preamp/tone control section to go in front of a (low) power amp.
I'm going to make a basic, portable bass amp out of
2 x 7" hifi drivers (good quality and definitely up to the job)
A power amp kit as here -> ebay .co.uk item number 130821174727 . It.s basically a bridged pair of TDA2003 . I couldn't buy the individual parts this cheaply.
Homemade cabinet of an appropriate size and ported (probably).
Battery power courtesy of power tool 12-14v packs.
I've mocked this up to test the speakers by wiring them to an old Vox Busker amp that uses a single TDA2003. It should sound pretty good with a decent range and bass response.
What I need is a simple circuit to build into the amp to act as a preamp with volume and tone control. Two dials is all I really need. And no distortion.
I already have a selection of components including 5xMPF102 and LM386 IC. and would like to use these if at all possible.
I looked at the Tillman FET preamp but it seems it's not suitable for use with the MPF102 - see below http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp6.htm
Any suggestions?
I do appreciate that the question has been asked before, even in the post previous to this one but having followed all the threads I can't find something that is
1. intended/adapable for bass
2. very simple 2 or 3 knob controls
3. uses MPF102 transistors
thanks
Welcome @t0neloc!
For reference, The Who bass rig.
(http://www.thewho.net/whotabs/images/equipment/bass/jae-bassrig2000-2002_whocoll.jpg)
http://www.thewho.net/whotabs/images/equipment/bass/jae-bassrig2000-2002_whocoll.jpg (http://www.thewho.net/whotabs/images/equipment/bass/jae-bassrig2000-2002_whocoll.jpg)
Okay, my 5.5cents worth...
Design rules - general.
1. What have/are other people doing in the field? Genuine research.
2. create an ideal/optimal/full-blooded image/plan
3. compromise it into the time/money budget
Q - Some of the typical characeristics of bass rigs (guitar incl. keyboard/synth).
A - 1.
Lots of EQ; both shelving and Graphic
Compression & limiting
maybe some Fx
Lots of clean power (no overdrive/clipping)
Cab or cabs that can reproduce it.
There's a gotcha with the e-Bay.uk offering; it's rated 20W into 2 ohms; that's four 8 ohm drivers in parallel. Unless you are going to be brave and try automotive boom-box type drivers, finding four or two ohm drivers could be interesting.
Since you intend battery operation reduced power consumption into a higher load impedance for more limited SPL, then "wattless mechanical amplification" arises, the horn, which is vastly more efficient for turning watts electrical into dBSPL.
After building a few, e.g.;
Siamese W-Horn with tweeters, (50W amp).
(http://www.ozvalveamps.org/ampsinaction/siamese1.jpg)
E-Tone 15 inch nominal 200W. Note the two forward facing letter-box tweeters which can provied any required "gar-dang" and poppin' Bootsy-style.
And a classic SS bass rig, Acoustic 360, 361;
(http://i1341.photobucket.com/albums/o743/Roly49/Horn%20loudspeaker%20cabinets%20for%20bass%20and%20synth/acoustic_301_41_zpscff4343b.jpg)
After a lot of calculating and graphing I now feel that the best bang for a given volume of cabinet is a front-radiating, back-loaded J-horn, e.g.;
(http://i1341.photobucket.com/albums/o743/Roly49/Horn%20loudspeaker%20cabinets%20for%20bass%20and%20synth/Fane-J-Scoop_zpsfcdbe786.gif)
Possible to mount a letterbox or up to four bullet tweeters around the driver.
http://www.ozvalveamps.org/cabinets/fane-loudspeaker-book-pages-34-41.pdf (http://www.ozvalveamps.org/cabinets/fane-loudspeaker-book-pages-34-41.pdf)
More here;
http://www.ozvalveamps.org/cabinets.htm#fane (http://www.ozvalveamps.org/cabinets.htm#fane)
The Who system in the heading may well be bi-amped, or tri-amped, (2+2mid+1+1horn); that is the frequency spectrum split up in the preamp stages to provide an active crossover, that only the frequencies you want goes to each driver/cab set (and the higher end is covered by port-tuned direct radiators).
A poor man's active crossover is an stereo upgrade graphic equaliser. {for a minimal rig these often also come free or for $10 at a garage sale, with a large space inside the case and a bit of DC power to mount any other desired electronics, e.g. compressor/limiter. Etc...
So yeah, Hi. :dbtu:
{digging for some MPF102 pre sims.}