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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Hayden J. Nebus on July 19, 2018, 07:53:23 pm

Title: Coherence knee in amplifier TF
Post by: Hayden J. Nebus on July 19, 2018, 07:53:23 pm
Apologies for the shitty cam-on-screen moire, but the below trace is an electronic TF of an amplifier. Can anyone tell me what's going on with the coherence trace? It's solid at Nyquist then it just plain takes a knee. I otherwise believe the analyzer, but I don't have the foggiest notion whats going on with that.

https://drive.google.com/file/d/1hjaK1-pyLO23OWtUpSWroUzil_1IuySB/view?usp=drivesdk (https://drive.google.com/file/d/1hjaK1-pyLO23OWtUpSWroUzil_1IuySB/view?usp=drivesdk)

This is a vintage Altec A500 stereo hifi tube amp I recapped for a buddy. The measurement was made with the amp loaded at 8ohm and the measurement probe in parallel with the load.

I was thinking perhaps I was seeing load nonlinearity reflected back, but I have since worked on a solid state Magnavox of similar vintage, and it measured similarly, unloaded. Both designs are 50 year old class A amps in push-pull with neg feedback, but that's where the similarities end.

Anybody have more of a clue than I do? Normally with electronic TF measutements I would expect solid ruler-flat coherence.