At least one company claims that their really-fast negative feedback loops eliminate the problems traditionally associated with negative feedback. Maybe the situation has improved to the point that it can potentially do more good than harm.
Amplifier specs, does they matter?
For solid state designs, the manufacturers boast about their signal to noise ratios, total harmonic distortions, slew rates, frequency responses, and many others. Meanwhile, the makers of the tube amps praise the liquidity and musicality of their designs. Obviously, amplifiers with tubes don't measure nearly as well as solid state amps. So, do any of these specifications really matter?
- ...
- 39 posts total
Nelson Pass published an excellent article on distortion and feedback: http://firstwatt.com/pdf/art_dist_fdbk.pdf The bottom line is it does not seem to be implementation specific. About 50 years earlier, Norman Crowhurst had some similar comments on feedback and showed that harmonics can be added up to the 81st harmonics, with attendant intermodulations at the point of feedback (the feedback node). This document is an excellent read: http://www.pmillett.com/tubebooks/Books/crowhurst_basic_3.pdf |
Mapman, there is nothing wrong with use of negative feedback but it has to be done right. Using deep global NGF to achieve better spects at the expense of the sound is what I'm against. Why would anybody need DF=1000 if choke in series with the woofer limits it to less than 100 anyway. Speaker impedance, mostly resistive, already poses limitation. I can understand output impedance 10x lower than 4ohm to provide good damping (DF=20@8ohm) but above it it is just nonsense. Class D amplifiers have very high DF at low frequencies but it wasn't intended IMHO. It comes from the fact that speaker is always shorted between two low impedance points (GND, VCC) by very low impedance Mosfets switches. Some negative feedback, used to linearize output, reduced output impedance even more. Extending frequency response to >100kHz might be a good thing to avoid phase shift in audio range (poor summing of harmonics) while reducing THD to fraction of percent should be enough. Good design involves quality components combined with very linear topology. Frequency limited input circuit should be followed by extremely fast output stages. In short it should be very linear and fast to start with instead of fixing it with NGF. It should be just enough of negative feedback to reduce THD, IMD to fraction of percent (only few times reduction). Bandwidth will increase but you need to reduce it at the input back to one that amp had before feedback was applied. This will prevent TIM completely but amplifier has to be fast to start with. |
- 39 posts total