I'm with you Jimi, I wouldn't use any TL chip if I think it can run in to clip. The thing I don't get is why not fit 1M. MXR D+ is 1M and a 741 although a dirt box is perhaps a poor example.
Too much treble? We have a tone knob on the guitar.
Too noisy? Ermm, as I understand it, the quietest resistor is no resistor.
In series, that's 0 ohms or wire. In parallel, it's infinity ohms or nothing. So 1M or higher even isn't noisy in itself for the parallel fit, but I think opamps have a spec about input noise that's dependent on the source impedance which is lower = quieter. I don't understand the mechanics of how that works but a lower value parallel input resistor will lower the source impedance seen by the opamp - guitar pickup impedance is relatively low but can go higher: 250k if I turn a 500k vol knob on a Gibson with one pup on to half resistance and 125k with a Fender 250k. A 330k input resistor will roughly halve the source impedance then. This is when the TL072 beats others at 1M , it doesn't get so noisy if the guitar is at a high impedance.
I've modded a Peavey to 1M because it was dull with high output 'buckers but I never considered changing the stock input opamp since the other half is in the overdrive circuit.
well, remember, this is supposed to be in an amp that is supposed to be a marshall, so they're likely using the values they chose to get as "close" as they could tonally, as well as in the effective range of the controls. its not a stompbox, so perfection ain't likely. it was built to meet a price point!
i think you're misunderstanding me. when you push a tl into clipping, it clips hard and ugly.
the reason it has more noise/hiss is because its effective frequency range goes much higher than the 741 can. so you're putting out an extra octave or more of treble, above the range of the guitar, which manifests itself as a really loud, annoying HISSSSSSSSSSSSSSS that won't go away.
the only way these amps really sound good is with the gain pegged. below that, they're nothing really special. but when you crank the gain to get the max distortion possible <which cleans up really well when ya roll your guitar back> that extra treble gets distorted and manifests in a most unmusical way.
its not the resistors. that's not the issue. the issue is the amount of gain, and the jfet based chips clipping hard and ungracefully, but most especially the frequency range.
a 741 when clipped acts very differently from a tl071. in the case of this particular circuit, the "worse-er" specs sound way better.
peeps make "laws" that they often don't understand... often based on internet hype and mojo.
who in @#$% wants a "hifi" distortion? that's kind of defeating the purpose.
even a klon(e) sounds better with the bjt based opamps to my ear. warmer distortion, better cleanup from the guitar etc.
i see peeps add tl's cuz the internet tells them they sound better. not based on them LISTENING CRITICALLY to the results. to me, if ya put a tl072 in a toob skreemer, it just sounds even worse than the stock 4558 does. too much glass, too much high end, too hissy. no bueno.
so tho the resistors may make a difference in the biasing of the circuit truly, ultimately it still comes down to the magick jelly bean in there.
the XX58's just plain sound more musical in distortion than the more "hifi" jfet ones.
jfets sound great clean. but they really sound like crap in most cases with dirt. they sound ok til they flatline. and when they do, they make a horrid racket, at least to my ears.
on a side note, i got a reply from marshall.... the ONLY schematic they have is for the one that's already out there.
so far, i've got the input stage and the tone stack drawn up for this specific circuit. its gonna take a while. glaucoma and long traces that twist around in box shapes tend to really mess me up.
rock on!
