The load impedance as seen by the guitar will have a big effect on the high frequency amplitude response. Scaling down those input resistors by 100X will chop off the high end response of the guitar pretty significantly. Most early amps had a 1Mohm input impedance, and many still do (most guitars are designed for that), but you get less noise if you shrink that down a bit. Reducing it lower than 250K is likely to cause a noticable rolloff at the high end. It's a rare guitar speaker that goes much above 8kHZ, so you do have some wiggle room. I'd use 500K to gnd myself, and get rid of the 68K R's.
The 68K resistors enable a better mixing function when both inputs are used simultaneously, but at the expense of added noise. In a tube circuit they also work with the tubes grid to plate capacitance (miller effect) to form an Rf filter, but I don't think the FET input opamp will work the same way as a tube for Rf filtering. You'd need to check the specs for the input capacitance of the FET opamp and see what you have there. It may be usable. It may vary significantly from one opamp to one of a different part number.
The high source impedance as viewed by the grid of an input tube works with the miller effect (grid to plate capacitance) to form a low pass Rf filter. So the same 68K and 1M resistors load the guitar properly, and does some filtering of Rf. The guitar acts as an antenna and picks up all kinds of noise out of the air, especially in a room with switch mode light dimmers. Since a guitar amp typically has a gain of 1000+ this is a big issue. Rf goes into the circuit and gets "detected" by any non-linear distortion mechanism, which produces sum and difference frequency energy. The difference frequency energy will often be in the audio frequency range, and sound like weird noise. An Rf filter at the input will reduce such noise but not eliminate it entirely.
My experience with toroid power trannys is that they emit almost no field at all, but the wires coming out of them definitely do, so twist the pairs for each winding so to a significant extent the radiated fields will cancel each other out. Keep all wiring as short as is practical, so it doesn't act like inductors or antennas at Rf frequencies.
Personally I always put a passive Rf filter at the input of any amplifying device, because op amps are very good when you don't ask them to do something they can't do well. They generally don't do well with Rf, which the guitar acting as an antenna will supply plenty of. With a guitar amp I'd set the corner frequency of the Rf filter at about 20kHZ. Just a simple RC at the input. The source impedance of typical Fender and Gibson guitars are roughly 10k ohms, so this should be taken into account when you calculate any caps in that region. Since guitar source impedance varies a lot, I'd put in a 10K R in series with a DC blocking cap (0.02uF should work fine for that, -3dB at 16HZ), right at the input, then a 330pF cap to Gnd, paralleled by the 500K input load resistor. This puts a 3dB down point at 24kHZ if the guitar is a 10kohm source impedance. This creates a shelf actually, which gets shallower as the 500K R gets shrunk. The deeper the shelf, the more effective the Rf filter. This network should feed directly into the + input of the opamp. Any volume control should be further down stream, so it turns down noise as well. Always feed a volume control with a DC blocking cap. With gains higher than about 20, you could actually cause some clipping in the opamp, which may not sound good, unless you like transistor distortion (I don't). The dynamic range of a guitar is actually quite large. Driven into an oscilloscope I saw a full volt peak from my strat.
For stability purposes (phase margin - tendency for the opamp to oscillate), you need the cap across the feedback resistor (to create the "dominant pole"), which you have (set that pole to about 20kHZ too for max stability - the lower the better), and it's very important to also have 0.1uF (or thereabouts) caps from the power supply connections to ground within an inch of the opamp (closer is better). This significantly reduces the probability of oscillations. I use ceramic caps for this function, and plastic dielectric caps in the signal path (polypropylene is preferred there). If you have an electrolytic in the feedback network (going to Gnd), put a 0.1uF or so cap across it to insure good performance well into the Rf frequency range.
It may not matter in this case, but it's also wise to put a 200 ohm resistor in series with the output of any opamp, feeding the downstream circuit or cable, to reduce any effects of reactance on the stability of the opamp.
I hope this helps. Sorry if it seems like my cats kept interrupting me while writing this. They did.
The 68K resistors enable a better mixing function when both inputs are used simultaneously, but at the expense of added noise. In a tube circuit they also work with the tubes grid to plate capacitance (miller effect) to form an Rf filter, but I don't think the FET input opamp will work the same way as a tube for Rf filtering. You'd need to check the specs for the input capacitance of the FET opamp and see what you have there. It may be usable. It may vary significantly from one opamp to one of a different part number.
The high source impedance as viewed by the grid of an input tube works with the miller effect (grid to plate capacitance) to form a low pass Rf filter. So the same 68K and 1M resistors load the guitar properly, and does some filtering of Rf. The guitar acts as an antenna and picks up all kinds of noise out of the air, especially in a room with switch mode light dimmers. Since a guitar amp typically has a gain of 1000+ this is a big issue. Rf goes into the circuit and gets "detected" by any non-linear distortion mechanism, which produces sum and difference frequency energy. The difference frequency energy will often be in the audio frequency range, and sound like weird noise. An Rf filter at the input will reduce such noise but not eliminate it entirely.
My experience with toroid power trannys is that they emit almost no field at all, but the wires coming out of them definitely do, so twist the pairs for each winding so to a significant extent the radiated fields will cancel each other out. Keep all wiring as short as is practical, so it doesn't act like inductors or antennas at Rf frequencies.
Personally I always put a passive Rf filter at the input of any amplifying device, because op amps are very good when you don't ask them to do something they can't do well. They generally don't do well with Rf, which the guitar acting as an antenna will supply plenty of. With a guitar amp I'd set the corner frequency of the Rf filter at about 20kHZ. Just a simple RC at the input. The source impedance of typical Fender and Gibson guitars are roughly 10k ohms, so this should be taken into account when you calculate any caps in that region. Since guitar source impedance varies a lot, I'd put in a 10K R in series with a DC blocking cap (0.02uF should work fine for that, -3dB at 16HZ), right at the input, then a 330pF cap to Gnd, paralleled by the 500K input load resistor. This puts a 3dB down point at 24kHZ if the guitar is a 10kohm source impedance. This creates a shelf actually, which gets shallower as the 500K R gets shrunk. The deeper the shelf, the more effective the Rf filter. This network should feed directly into the + input of the opamp. Any volume control should be further down stream, so it turns down noise as well. Always feed a volume control with a DC blocking cap. With gains higher than about 20, you could actually cause some clipping in the opamp, which may not sound good, unless you like transistor distortion (I don't). The dynamic range of a guitar is actually quite large. Driven into an oscilloscope I saw a full volt peak from my strat.
For stability purposes (phase margin - tendency for the opamp to oscillate), you need the cap across the feedback resistor (to create the "dominant pole"), which you have (set that pole to about 20kHZ too for max stability - the lower the better), and it's very important to also have 0.1uF (or thereabouts) caps from the power supply connections to ground within an inch of the opamp (closer is better). This significantly reduces the probability of oscillations. I use ceramic caps for this function, and plastic dielectric caps in the signal path (polypropylene is preferred there). If you have an electrolytic in the feedback network (going to Gnd), put a 0.1uF or so cap across it to insure good performance well into the Rf frequency range.
It may not matter in this case, but it's also wise to put a 200 ohm resistor in series with the output of any opamp, feeding the downstream circuit or cable, to reduce any effects of reactance on the stability of the opamp.
I hope this helps. Sorry if it seems like my cats kept interrupting me while writing this. They did.
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Thank you for that passage!
I am against any limiting of harmonic frequencies. It takes the edge off the tone. In some music, you need to "cut through" the other instruments, especially on lead. Besides, when the harmonics that are produced by tube distortion are attenuated, you may as well go solid state!
My test speakers are a 10" "woofer" (80s vintage stereo), and a Jensen 5" MOD series guitar speaker; starts to fall at 15kHz. Here's the graph. http://www.jensentone.com/speaker/mod_5_30
We don't always need all that high end shrill, "buzz" or whatever you want to call it that most people find discordant. Fact is, you may need it sometimes. Moreover, it can be tempered with reverb or echo.
So far nothing is picked up with DiMarzios on the 100k with the 68k grounded at the input. When all tube, removing the 120p cap from V1 made a receiver out of the unit. I experimented with a 10k gridstopper and adjusted the cap accordingly with the same result as the "normal" 68k and the appropriate cap. The amp is consistent and there are no wierd noises. I need to check all this with a 20 foot guitar cord. I am using a 10 foot for these tests.
I made a "ghetto" tranny from an AC line regulator, which uses an autotransformer. That has about 12 wires coming out around the periphery. It's regulation is poor, but I could not afford a real one. I found the answer at Antek, but to avoid a custom unit, I have to buy two. One for the 12.6V and the HT, and another for the SS power amp, which requires +/-35V. However, this may work out good, since they are both in phase, they could be stacked with the output wires facing each other. Not perfect cancellation as would be if they were matched in VA, windings, and load, but it beats shielding. Now that the amp is under control, the 60 cycle hum is less pronounced anyway.
I want to keep the line regulator, and it will balance the weight if the other two are on the input end. If I have to fully shield the input jacks, it's no big deal.
It did, and cats are permitted, within reason, to do anything they want!
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