[sdiy] basic inverting op amp question 101

[Barry Klein]

Both deal with a high dynamic range input signal where noise and amp saturation are critical design concerns. The detector circuitry geek hangout is geotech1.com. They are always experimenting with circuits to reduce signal to noise for deeper target sensing. A guitar amp redesign would likely benefit from op amps with faster slew rate and lower noise and also sharing the gain across the dual stages.
Another area that you guys might find worth investigation is medical equipment design. Check out the book Design and Development of Medical Electronic Equipment Instrumentation by David Prutchi and Michael Norris.


> On Jan 1, 2020, at 3:30 PM, Adam Inglis <21pointy at tpg.com.au> wrote:
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>> On 1 Jan 2020, at 4:11 am, Barry Klein <barryklein at cox.net> wrote:
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>> Makes me wish (more) I could go back and redesign guitar amp stuff I did at Music Man years ago. Now commonly done in better pulse induction metal detector designs.
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> Are you saying metal detector technology is relevant to guitar amp design? How so?
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> Adam
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>> Barry
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>>>> On Dec 31, 2019, at 7:20 AM, Gordonjcp <gordonjcp at gjcp.net> wrote:
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>>> On Mon, Dec 30, 2019 at 02:03:34PM +0000, Tom Wiltshire wrote:
>>>> At the simplest level, it’s a trade-off between current and noise. Lower value resistors generate less noise, but use more current (and provide lower input impedance) and higher value resistors do the reverse. So you tend to see “compromise” values around 10K-100K. 1Meg is getting “big” and 1K or less is getting “small”.
>>>> 
>>>> Years ago I had the same question and since the internet didn’t exist yet, I did some experiments on my breadboard with a 741 and discovered that 1M upwards was noticeably noisier, but that anything between a few K and a few hundred K didn’t really seem to make much odds.
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>>>> Tom
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>>> And this is where it gets really weird - like Monty Hall Problem weird -
>>> because despite what you'd think, two opamps cascaded for -10x gain will
>>> be quieter than a single opamp with 100x gain.  Say you've got 10k input
>>> resistors, for x10 gain you need 100k feedback resistors which are
>>> rather better than ten times quieter than 1M - and you're only
>>> amplifying the noise by 10x.  With a single stage the resistor is
>>> noisier and you're multiplying that by 100.
>>> 
>>> This assumes not-terrible opamps, of course :-)
>>> 
>>> -- 
>>> Gordonjcp
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