I spent understanding now how high-impedance inputs can have noise problems, and how to reduce those by isolation. That didn't have any impact. I also looked closely at what I was setting my gain to (near unity), and realized that my estimates on the pickup voltage levels were way too low. So I think I don't actually need nearly as much gain as I thought I did. I tested this by just running the pickup through the output stage, and seemed to get good results. I'll run it through the four-track tonight so I can listen more carefully.
While this makes the final amp substantially simpler, it also excises my solution to the tone problem. Perhaps I'll have to stick a conventional second-order filter in.
Monday, October 29, 2012
Thursday, October 4, 2012
v1.2
I tried series feedback on the v1.1 model, and started picking up BBC World News without any other improvement. So maybe those spice model freakouts were accurate.
I thought a little bit more about a long-tailed differential pair, and decided I could use one after all. I had originally rejected it because one needs a current source and/or mirror for it to have good amplification. But I have plenty of gain due to my multiple stages, so don't need the LTP to have much umph. In addition, because the LTP has a non-inverting output, I can use just two following stages and so not increase my overall transistor count at all.
It did seem more convenient to use a virtual earth to offset the input and feedback. The spice modeling looks really good. Hopefully it will work in practice too.
I see that I mention a volume control, but didn't draw it in. I figure to place it at location C, so that I can turn down the volume and still keep distortion. That doesn't seem to work, though. It does reduce the amplitude, but introduces its own unique kinds of distortion. So maybe an in-line resistor in the headphone would work better, but according to spice that distorts as well.
I thought a little bit more about a long-tailed differential pair, and decided I could use one after all. I had originally rejected it because one needs a current source and/or mirror for it to have good amplification. But I have plenty of gain due to my multiple stages, so don't need the LTP to have much umph. In addition, because the LTP has a non-inverting output, I can use just two following stages and so not increase my overall transistor count at all.
It did seem more convenient to use a virtual earth to offset the input and feedback. The spice modeling looks really good. Hopefully it will work in practice too.
Monday, October 1, 2012
Noise
This is without any negative feedback. Something is going wrong with the shunt feedback I initially tried, so I took it out for this recording.
Subscribe to:
Comments (Atom)