Amplifier specs, does they matter?

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

PSAG...no feedback is even better

"The bottom line is it does not seem to be implementation specific. "

With technology, there is a right and wrong way to do everything....and usually many shades of grey in-between.

That's why you hire expensive skilled engineers with great depth of knowledge. Otherwise, anybody could do it.

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.

Yes, specifications are important and do matter, in many ways.

All electronic devices, for example amplifiers are designed and constructed based on specifications first. This is Engineering 101. Design and build an audio power amplifier, with a minimum frequency range of 20 HZ to 20kHZ, power output into 8 Ohms of 250 WPC, harmonic distortion not to exceed 1%, output impedance of x and input impedance of Y. There are other specifications given to the designer, but, these are characteristic.

Specifications with regards to the end user depends on who the end user is. Military useage is different than the average audiophile. For Military, it better meet or exceed the desired specs. For the Audiophile, what exactly do you need to know? 1. Power output over the frequence range, 2 Input impedance, 3. Output impedance, 4 distortion, 5 operating frequency range, 6 load impedance it is designed to handle.

So, yes, specifications are important. If you purchased some relatively inefficient speakers, that have an operating impedance going down to 1 ohm, are you going to pay attention to the specs of the proposed amp? I should hope so. Distortion below 1% is very good in todays standards. But, high distortion is something I want to know about. The amp should be an infinite bus over frequency that amplifies the signal with zero change to the signal. That is what an amplifier is supposed to do. So, yes, distortion ratings are important. However, some manufacturers taylor their designs to a certain sound, read "this adds certain acceptable distortion to the signal" to achieve a certain sound. Too much distortion is just poor design.

enjoy