I feel good about giving out these specs: 1.0 W (20 Ω), 1.0 W (32 Ω), 250 mW (300 Ω). SNR: 132 dB. Residual noise & hum: 1.25 uV RMS (20 Hz - 20 kHz, A-weighted). Channel separation: 95 dB (limited by the 1/4" phone connector). Multi-tone IMD residual (AP 32-tone, 100 mW, 300 Ω): <-140 dB (ref. 100 mW, 300 Ω). Output impedance: 35 mΩ (1 kHz).
The THD+N is below the measurement floor of the APx525 (and likely also below that of the APx555). I was able to build a 1 kHz notch filter and use a precision oscillator to tease out the THD. The first measurement below shows the harmonic spectrum with the amp providing 10 mW into 300 Ω. Note, however, that you need to add a correction factor to the harmonics to compensate for the attenuation of the notch filter at the harmonic frequencies. I added the EQ curve for you in the second plot (which is why the noise floor looks all wonky). The THD comes in at about -133 dBc (225 ppb or 0.0000225%). I feel good about saying <-130 dB for now.
The 60 Hz hum in the THD measurement is from the setup (the precision oscillator power supply to be specific), not the amp.
I'm still working on improving my THD measurement setup. My attempt of world domination the Baxandall/Cordell/Christiansen way didn't quite pan out. At least not at the first attempt. I have another precision oscillator coming. It's supposed to be able to get THD below -150 dBc, so there's room for improvement in the measurements below.
I will be exhibiting at
RMAF in the "Headspace". I'll be collaborating with Ryan Redetzke of Redscape. If you're in Denver Sept. 6-8, feel free to swing by to chat and listen to the HPA-1 prototype.
I will open up for preorders before I leave for RMAF - so around Labour Day (Sept. 2nd). You'll be able to place your order here:
www.tomchr.com. Currently the site is password protected. I'll remove the password once I'm closer to done with it. I hope to have the amps built and shipping by mid-November.
I expect the preorder price to land around $549 and the final price around $649-$699. I need to get some assembly quotes before I can promise anything regarding the pricing, but those are my estimates based on past experience with assembly cost. The preorder discount is
ONLY available until the amps come back from assembly. Once I have amps in stock, the final price will kick in. I will
NOT have sales aside from this preorder sale.
Tom