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previous: John In my MXR microamp the feedback loo... -- 3/16/2000 1:40 AM

Re: OpAmp Feedback Loop Question

3/16/2000 10:39 PM
Mark Hammer	Re: OpAmp Feedback Loop Question Yes. The cap sets the low end rolloff so that your op-amp isn't hard at work amplifying DC. The frequency it rools off at is given by 1/(2piRC), where R is in meg, C in uf, and R is the total resistance going from the - pin to ground. Although this rolloff changes as you adjust the gain knob, it changes from a bit under 1hz to a bit over 12hz. In short, you're not gonna hear it. If you didn't have a 4.7uf cap handy, you could safely swap it with a 2.2uf or 1uf, and wouldn't hear any difference in most applications. The small 2.7k fixed resistor may seem redundant with the variable resistor, but it serves to set the maximum gain of the unit. Bear in mind that in this non-inverting configuration, the gain is set by the sum of the feedback resistance (in this case 56k) PLUS the resistance going to ground (in this case, anywhere from 2.7k up to 502.7k), DIVIDED by the resistance going to ground. If you are looking at the same schematic as I am, this means that the microamp is designed to deliver anywhere from around unity gain (558.7k/502.7k=a little more than 1.1), to a gain of around 20 when the pot is turned down all the way (58.7k/2.7k). If the fixed resistor weren't there, it would be 56k/0k...not an enviable position for an op-amp since demanding infinite gain from a 9v battery is, well, pushing it. The good news is that if you want to reduce the 2.7k resistor to 2.2k or 1.8k, and increase the 56k resistor to, say 68k, you can up the maximum gain from 20-ish to 30-ish. Not likely to lead to tonal heaven with a guitar, but if you need to run a cheap mic into a line input in a pinch, there is sufficient oomph provided by a gain of 30 to do the job decently (although the TL061 should probably be replaced by a TL071 for that kind of duty). Since the low-end rolloff depends on that resitor, you want to check that, too. In this case, dropping is to 1.8k, and sticking with the 4.7uf cap, only raises the low end roffoff to 19hz in the worst case scenario.

3/16/2000 10:39 PM

Mark Hammer

Re: OpAmp Feedback Loop Question

Yes.

The cap sets the low end rolloff so that your op-amp isn't hard at work amplifying DC. The frequency it rools off at is given by 1/(2piRC), where R is in meg, C in uf, and R is the total resistance going from the - pin to ground. Although this rolloff changes as you adjust the gain knob, it changes from a bit under 1hz to a bit over 12hz. In short, you're not gonna hear it. If you didn't have a 4.7uf cap handy, you could safely swap it with a 2.2uf or 1uf, and wouldn't hear any difference in most applications.

The small 2.7k fixed resistor may seem redundant with the variable resistor, but it serves to set the maximum gain of the unit. Bear in mind that in this non-inverting configuration, the gain is set by the sum of the feedback resistance (in this case 56k) PLUS the resistance going to ground (in this case, anywhere from 2.7k up to 502.7k), DIVIDED by the resistance going to ground.

If you are looking at the same schematic as I am, this means that the microamp is designed to deliver anywhere from around unity gain (558.7k/502.7k=a little more than 1.1), to a gain of around 20 when the pot is turned down all the way (58.7k/2.7k). If the fixed resistor weren't there, it would be 56k/0k...not an enviable position for an op-amp since demanding infinite gain from a 9v battery is, well, *pushing it*.

The good news is that if you want to reduce the 2.7k resistor to 2.2k or 1.8k, and increase the 56k resistor to, say 68k, you can up the maximum gain from 20-ish to 30-ish. Not likely to lead to tonal heaven with a guitar, but if you need to run a cheap mic into a line input in a pinch, there is sufficient oomph provided by a gain of 30 to do the job decently (although the TL061 should probably be replaced by a TL071 for that kind of duty). Since the low-end rolloff depends on that resitor, you want to check that, too. In this case, dropping is to 1.8k, and sticking with the 4.7uf cap, only raises the low end roffoff to 19hz in the worst case scenario.