@tomchr I'm one of those who pre-ordered your HPA-1, i was looking forward to it for new year, the delay sucks tho. :/
Thank you for your preorder. Yeah. The delay sucks. Especially as I spent quite a bit of extra money getting the electronic bits together in time. Oh, well. #BeyondMyControl
On the upshot, I have a couple of prototype chassis. They're identical to the final version except in the final version the panels will be anodized after machining, so all holes will be black on the inside. I'll ship one to Amir for measurements once the electronics are in (in a week or two). I'll need to hang onto it as a demo model, though.
You know one measurement i want to see is Transient Intermodulation Distortion on amps, it's supposedly a fixed problem but you never know with some audiphile designs... i see it can be done using a low pass filtered square wave and a sine wave.
Yeah. The challenge is getting the audio analyzer to pick it up. I'll see what I can do for transient measurements.
TIM was not an issue to begin with(for modern transistors). It's basically two more fundamental issues. Slewrate limitation and stability(phase shift, faster slewed front stages )in a feedback amplifier.
As music signal is merely 20khz. It will never trigger TIM.
That's not true actually. If you read the original papers (Walt Jung, et al., AES, 1977), you'll notice that TIM/SID is caused by slewing of internal stages of an amplifier. In a design that has poor TIM/SID (TIM = Transient Intermodulation Distortion; SID = Slewing-Induced Distortion), it's perfectly possible to cause the internal stages to slew, even at audio frequencies. Build a preamp using the uA741 opamp and you'll see what I mean.
Now that we understand the mechanism that causes TIM/SID, we design amps that don't have the slewing limitations. Generally, this means ensuring that the internal stages run at sufficient bias current to drive the various capacitances in the circuit.
Amplifier stability is a different topic. Nyquist covered that back in the day.
To ensure the TIM not being triggered, a simple input low pass filter can completely prevent it.
True. Unfortunately, if your amp has poor TIM/SID when used with audio signals, you need to lower the cutoff frequency of the input lowpass to the point where it cuts into the audio bandwidth. Now the full-power bandwidth of the amp is limited to the point where the amp is useless for audio.
TIM/SID is a solved problem these days. I still think there could be value in a transient test of distortion, however. I suspect it could reveal potential thermal issues and thermal stability issues, in particular in power amps.
Tom
