By the way, RV4A and RV4B are simply the two gangs of a dual-gang pot, it's a low pass on one end and a treble shelf on the other, it essentially is a flat response (for guitar purposes anyway) when fully CW and a upper-mid/treble hump when turned CCW, sort of reminiscent of a Tube Screamer in some settings. To be honest, the "boost" channel, which is post-filter by the way, is an experiment, and it is the part of the circuit I'm less convinced about (should I make it pre-filter? Should I provide it with more extensive tone controls?).
Protect the input from overvoltage/esd. Diodes to + and - rails. There's loads of evil electricity introduced into an amp when the guitar lead gets plugged in.
Do you mean antiparallel diodes to ground? It is something that I could do (especially if I use, for example, LEDs) but I would like such an arrangement not to introduce distortion and/or noise. There is far too much variation on guitar/bass output to reliably choose an overvoltage level, some guitars may have peak voltages well over 2V.
That would expose the guitar (and the guitarist) to +/-15V should the diode close.
But why not to ground? Basically what you are proposing is, should you have an ESD through the guitar jack (possibly thousands of volts for a microsecond or so) the diodes would close, with the current essentially going to signal ground (either Vcc or Vdd). With an appropriately chosen LED (one with a sufficiently high V_th) each way, shunted to ground, you'd probably have less noise, since a reverse-biased diode should contribute a (small) amount of noise.
I tried to keep noise contribution to a minimum, since there are already quite a few stages which will bring their share of noise to the table (but if you want treble pre- and de-emphasis, it's inevitable; especially the former, and as a matter of fact I designated that stage as a NE5532 which is a decently low noise opamp - I could do even better but I'm not sure it would be worth the trouble).
Last edited:
Sure, just that you didn't seem sure about that method as you feared clipping from a hot guitar.
See: https://sound-au.com/appnotes/an015.htm
Do keep in mind that I plan on including the Fender-style 68kohm series input resistor, so currents would be very low. The input opamp also has ESD protection.
The point is that when you adjust a pot, the slider becomes OPEN CIRCUIT temporarily after some aging. So what happens to the op-amp: the output produces a spike to the rail, BANG! If the power amp gain isn't high then it's just annoying, but with enough power it could damage your speakers. The difference between theoretical and practical design comes with experience.
That is true of pots exposed to DC. These pots are connected to an opamp output on one side (presumably 0DC), opamp input on the other (slight offset), and ground. There shouldn't be any significant DC. I could add some high value capacitor on the NE5532 side to kill off that slight offset (or I could use other opamps altogether, some opamps are spec'd as OPA2134s, others are NE5532), although I wanted to use as few capacitors as possible (if only because large value = electrolytic, which dry out over time).
I'm wondering why you are using the 5532 anywhere. What's the attraction? Seems the 2134 is superior in all ways. The 5532 Ib is 200nA vs. 5pA and one very annoying 5532 spec is its input resistance "ri" of 300K typ, 30K min! This apposed to 10^13 for the 2134. IOW, say bye-bye to any notion that the 5532 can be treated as "ideal". Also the max input offset voltage is 5mV as opposed to 2mV (albeit the typ = 0.5mV for both) which of course is then multiplied by gain.
The worry is offset build-up
Sometimes with effects pedals - tone-benders, screamers, distortion, fuzz, overdrive, treble boosters, limiters - i see a roll-off the low end at ~100-170Hz with a front-end hi-pass. Reason given is to reduce muddiness. Many effects don't need or want a a lot of the low string freqs.
The worry is offset build-up
Sometimes with effects pedals - tone-benders, screamers, distortion, fuzz, overdrive, treble boosters, limiters - i see a roll-off the low end at ~100-170Hz with a front-end hi-pass. Reason given is to reduce muddiness. Many effects don't need or want a a lot of the low string freqs.
Offset buildup would be eliminated in a few points, since some stages are capacitor coupled.
The 5532 is, simply put, a cheaper opamp and still a very good choice for low impedance audio (low noise, low distortion, good output capabilities). I use the OPA2134 in a few places, but there are at least two places where the 5532 is a fine choice, one of which is the pre-emphasis stage.
As for low-end attenuation, effects do cut the bass, but amplifiers generally have high-pass filters around 60Hz (for guitar) or 30Hz (for bass). This is designed to be a cleanish, Fenderish preamp (still, I'm planning to add a selectable high-pass for guitar). A 20Hz (ish) subsonic filter is included at the end of the preamp, while the pre-emphasis takes care of cutting the low end, while still allowing for de-emphasis afterwards (something you can't really do with a high-pass filter). Also, shelving filters are phase-transparent as opposed to HPFs (not that it matters much).
Would a person be going out on a long limb in saying that probably none of the guitar/instrument amps ever made worried about this loss of pot wiper contact fear?
Btw, do you have the wherewithal to mount the 8-pin SOIC package to a DIP8 carrier/adapter? I get the impression that your choice of two op amps is constrained by the need for pluggability.
Last edited: