aletheian-alex
Member
- Joined
- Dec 16, 2006
- Messages
- 24
This is not an informative thread but rather one that is merely for everyone's amusement. Of my thousands of tinkering experiments... I post very few on public forums, but I thought the Neve-heads on this board might get a kick out of this one and maybe kick-start some discussion about similar experiments that other people have tried but have not bothered to talk about. I have seen much cork-sniffing BA284 tinkering (which I really love because it saves me time and money trying out rare and exotic parts, and I don't have golden ears to try it myself anyway) but not much dirty grass-roots stuff. This one is dirty, low-fi and heretical, but FUN and startlingly enough... it didn't smoke on power-up!
To keep my mind off the blues from crushing the poverty of professional musicianship and my subsequent 30th birthday, I have been engaged in the heretical activity of tweaking the BA284 modules.... not IMPROVING the modules (I would not be that presumptuous) just tweaking for fun... swapping transistors, changing op points and feedback ratios, rail voltages, headroom, etc.
This version tickled me in particular, so I figured I would toss it up here. I don't have a schem, just a pic of the circuit on perfboard. The design parameters gave me a lot of leeway: A guitar preamp to feed a line input stage or amp/Hi-Z mixer input, etc.... unbalanced 1Meg input and unbalanced output that could feed high input impedances (10k, 50k, 1Meg... that sorta thing).
I took the BA284AM, schem and rifled through my various junk boxes for suitable-enough parts. I ended up swapping the BC183C BJTs for thoroughly-tested, high-hfe 2n4401--a nice, common, cheap little low rbb transistor of which I have dozens-- and tried a few versions of the output driver... this one is a cheapo mosfet: I have piles of 3055's and near equivalent metal case-BJTs that I have tried, but I wanted to give FETs a whirl this time, which was a bit of a problem, but I got it ruunning.
Since the output is for Hi-Z, an L01166 equivalent is not neccessary except to load the output stage, which can also be done with a choke since the output is basically a tiny inductive-load parafeed, AC coupled circuit driving a high Z load (and I certainly don't need the full 60mA of idle current) but all my REAL chokes had too high a DCR, so looking for a suitable load inductor. I rifled through junked wall warts until I found one that had a low-ish DCR and decent inductance (not as critical with the high-ish low freq response of a guitar) under a reasonable current load range, and I used that for the inductive load on the output transistor.
The circuit values were tweaked to give reasonably-close DC performance, as well as some AC performance swing, gain and feedback tweaking and an eye to the limited bandwidth and relative signal amplitude of a guitar.
All the typical externals were wired onboard. The topology is FET/BJT constant current input buffer->potentiometer in place of the input pad stepped attenuator->BA284 circuit-> output from "F". The pin "K" gain boost circuit was wired onboard with a 100uF cap in series with a range-limiting resistor in series with a trim pot wired as a variable resistor. Resistors are Rat Shack crap, and the caps are a mix of old polystyrenes, a few Tants pulled from power supplies, and a mish-mosh of various salvaged electrolytics of varying quality... I didn't knock myself out and measure the DCR/DCL, I just used caps from switchmode supplies and said "meh... good enough for perfboard".
Anyway, I have to test and tweak, but since it didn't smoke on startup and actually functions, here is a pic for your amusement:
To keep my mind off the blues from crushing the poverty of professional musicianship and my subsequent 30th birthday, I have been engaged in the heretical activity of tweaking the BA284 modules.... not IMPROVING the modules (I would not be that presumptuous) just tweaking for fun... swapping transistors, changing op points and feedback ratios, rail voltages, headroom, etc.
This version tickled me in particular, so I figured I would toss it up here. I don't have a schem, just a pic of the circuit on perfboard. The design parameters gave me a lot of leeway: A guitar preamp to feed a line input stage or amp/Hi-Z mixer input, etc.... unbalanced 1Meg input and unbalanced output that could feed high input impedances (10k, 50k, 1Meg... that sorta thing).
I took the BA284AM, schem and rifled through my various junk boxes for suitable-enough parts. I ended up swapping the BC183C BJTs for thoroughly-tested, high-hfe 2n4401--a nice, common, cheap little low rbb transistor of which I have dozens-- and tried a few versions of the output driver... this one is a cheapo mosfet: I have piles of 3055's and near equivalent metal case-BJTs that I have tried, but I wanted to give FETs a whirl this time, which was a bit of a problem, but I got it ruunning.
Since the output is for Hi-Z, an L01166 equivalent is not neccessary except to load the output stage, which can also be done with a choke since the output is basically a tiny inductive-load parafeed, AC coupled circuit driving a high Z load (and I certainly don't need the full 60mA of idle current) but all my REAL chokes had too high a DCR, so looking for a suitable load inductor. I rifled through junked wall warts until I found one that had a low-ish DCR and decent inductance (not as critical with the high-ish low freq response of a guitar) under a reasonable current load range, and I used that for the inductive load on the output transistor.
The circuit values were tweaked to give reasonably-close DC performance, as well as some AC performance swing, gain and feedback tweaking and an eye to the limited bandwidth and relative signal amplitude of a guitar.
All the typical externals were wired onboard. The topology is FET/BJT constant current input buffer->potentiometer in place of the input pad stepped attenuator->BA284 circuit-> output from "F". The pin "K" gain boost circuit was wired onboard with a 100uF cap in series with a range-limiting resistor in series with a trim pot wired as a variable resistor. Resistors are Rat Shack crap, and the caps are a mix of old polystyrenes, a few Tants pulled from power supplies, and a mish-mosh of various salvaged electrolytics of varying quality... I didn't knock myself out and measure the DCR/DCL, I just used caps from switchmode supplies and said "meh... good enough for perfboard".
Anyway, I have to test and tweak, but since it didn't smoke on startup and actually functions, here is a pic for your amusement:
