A very nice construction style. Unfortunately 40 year old plastic gets brittle and I broke one just by looking at it the wrong way. So here’s a 3D print model.
EQ section hum
The equalizer is open loop and has no PSRR, so you can’t expect better than 600 uV of hum overall (spec: -80 dB relative to 6V = 600 uV).
If the EQ section picks up more than 1 mV of hum with the 60 Hz slider all the way up and maximum volume, this is due to L601 picking up stray magnetic field from the mains transformer (L601 and C607 are tuned to 60 Hz).
The citation eleven on my desk had two different L601 coils in each channel and I didn’t know which one was the original. But one was wound in such a way that it picked up over 6 mV, i.e. probably a replacement not wound symmetrically. The metal shielding is not effective on magnetic fields and in fact amplifies the field lines as it creates a short circuited winding. So the coil should be magnetically shielded and wound symmetrically.
There is a replacement coil available that is shielded correctly and brought the hum well under 300 uV. I remcomend it over the original, even.
Last time we looked at the amplifier output and found some strange problems. This time we isolate the DSP and verify that in fact, the problem comes from here and is actually worse than previously thought.
Now that it’s clear we need to design a replacement DSP, we need to understand all the system level communication going on. With the help of some probes a logic analyzer, a spreadsheet and educated guesswork we managed to do just that.
The circuit boards for the upgrades have been gathering dust for about a year already. Customers seem to not judge this my highest priority, and fair enough. But fear not, wheels are turning. Next up: upsampling.
If you have absolutely zero idea what a biquad filter or a compressor is and how they work, then I’m not sure why you’re here. But in any case, I sat down with Robert Feranec of Fedevel Academy and we talked about the basics of signal processing.
No video this time, as this was supposed to be a 1-hour job… that ended up on the back burner with all the other projects for 3 months. Paying customers come first…
This is a Quad CD99-II, a friend asked me to check why it no longer read discs. Apparently it had been reading only about half of them for a while, and when that symptom was ignored, it had stopped altogether. Continue reading →
In the early 90s Philips were at the top of their game when they introduced the DSS930, a digital-input only, active DSP loudspeaker far ahead of its time. We got our hands on a pair, investigate the design and the state of these classics and discover a design flaw that has gone unnoticed for 30 years.
Two posts in one year? Is the world ending? Well, yes, but not just today.
I’ve been visited by a pair of handsome Bose 1800 amps over the past weeks. They needed a DC output delay / DC protection relays because apparently that tech did not yet exist in the mid 1970s. Those solid state devices were still a bit scary, I suppose. Continue reading →
Yes, I’m a youtube influencer now. Send me your free swag 🙂
Clearly writing blog posts was not my strength, and not for lack of stuff I have to say… just sitting down and condensing it into a post is a bit 2000. And most audiences want to consume video rather than text.
So, just as a trial run, I took a marginally audio related topic and just filmed my project over the course of a few days, then broke my brain for 2 months on editing the video but finally managed.
Odysee / LBRY
Maybe next time I’ll use that full frame DSLR and Sennheiser lapel mic rather than the $100 gopro clone (what was I thinking?). But the quality over quantity discussion is not relevant since I haven’t posted much of either, so you’ll take it and be grateful, damnit!