Bandwidth question?

I have never been a big fan on Mhz frequency response especially in an amplifier. Having a bandwidth that high can cause major problems including parasitic oscillation, rf interference etc. This can cause the top end to sound hard, grainy, and strident. This is especially true if the amp uses global feedback. To answer Kijanki, ten times the bandwidth is just a rule of thumb but it is something I like to see especially in a preamp. Personally I find -3dB down @ 200Khz to be sufficient. Also, when talking about amplifiers there are differences in specs. between power bandwidth and small signal bandwidth. By the way, most transformer coupled amplifiers are not capable of ultra wide bandwidth frequency responses.

Orpheus10. That's all part of this. Sine waves don't really tell you much about the bandwidth of an amp or preamp since all your getting is the fundamental frequency without the harmonics. I tend to look at square waves. That tells me a lot about the amp or preamp I'm looking at. The reason is, a 1Khz square wave shows you not only the fundamental, but the harmonics as well. A perfect 1Khz square will tell me that this piece of gear is flat from 100Hz to 10Khz or ten times the fundamental frequency in both directions. It can also shed some light on the interesting things some manufacturers do with their output coupling transformers to make it seem like the transformer has more high frequency bandwidth then it really has. So for example for perfect 20kHZ square waves, an amp or preamp needs to have a bandwidth out to 200Khz. Sad thing is, it really won't tell you how the gear under test is going to sound. For that you need the best test instrument in the world, your ears.

I watched the video and here is my interpretation of the performances. What he calls slant is actually called tilt or phase shift and it's measured in degrees. The Mac and the Dynaco look similar at maybe 20-25 degrees of tilt. This indicates that the low frequency response is rolled off. Possibly -3dB at 10-15 Hz. The Dynaco does a better job as the tilt is linear at low frequency. Therefore it's rolling off smoothly. The Mac on the other hand has tilt that is not linear at low frequency. At very low frequency the Mac frequency response might actually rise indicating some instability possibly caused by a coupling cap or power supply issue. High frequency response is similar on both amps with a bit of overshoot and only one or 2 cycles of well damped ringing. Some manufacturers do this so the amp sounds like it has greater high frequency response then it actually has. The Fisher looks like it has some major issues as it's high frequency response is not linear. It probably dips then rises and dips again. Either the amp has coupling cap issues, feedback cap issues, and/or the output coupling transformer is not properly compensated for. When the frequency response or any amp becomes non linear, i.e. rises and falls dramatically the test is pretty much over since that non linear response in not useable. I normally use sine waves to determine -3dB down points of an amp or preamp under test and confirm it with square waves.