Welcome, Guest. Please login or register.

September 28, 2020, 12:29:55 PM

Login with username, password and session length

Recent Posts

 

Author Topic: Simple bass preamp section to add onto power amp?  (Read 3788 times)

t0neloc

  • Chipper
  • *
  • Posts: 1
  • Chip Points: 0
    • View Profile
Simple bass preamp section to add onto power amp?
« 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
« Last Edit: January 05, 2013, 08:27:28 AM by t0neloc »

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: Simple bass preamp section to add onto power amp?
« Reply #1 on: January 05, 2013, 11:14:18 AM »
Welcome @t0neloc!

For reference, The Who bass rig.

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).

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;



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.;


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

More here;

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.}
 
If you say theory and practice don't agree you haven't applied enough theory.