Kingston
Well-known member
Because when you can't have the stuff you want off the shelf you have to DIY.
years of work in this one. Turning a lofi compressor into a mastering and all-around flexible unit is not an easy task. Besides the PSU and the general gain staging configuration from the PM670 there's not much here from the original plan. Everything is extended and made optional for front panel. I guess if I started it now I would completely redraw everything into a single PCB per channel instead of making it a clusterF*ck like this one. Every switch is actually a relay so signal paths stay reasonably short, and all signal path wires are shielded. It's a brute force way of doing it but there is practically no 50hz hum peak visible on the FFT noise floor graph. Those lundahl transformers also make frequency response flat to about 50khz! I have no other compressor that can do this.
I guess it could be called the mkII or RM670 (richman). The front panel looks much better than these pictures. I'm a bad photographer. Those four purple top layer panels actually have purple-metallic stickers on them with the text (3mm aluminum slabs bolted on to a white powder coated 4mm aluminum panel). The black text on white background is a transparent sticker. I made sure the stickers can take the heat generated by the unit.
Hey look, it's the infamous "Kingston duct". Laugh all you want, it allows me to take power switch to front panel with absolutely no hum coupled to audio circuits.
I designed two additional PCB's for it. There's one for I/O transformers, mid-side, feedback and sidechain highpass. There's another one for time constants and stereo link. The same time constant network works very well for both scamp and plain modes, switched through relays. They sound quite different to each other. There's some plain RC constants in the network, some parallel trickery. Scamp is not in its original form either, of course I had to tweak it, but I still like the plain mode best. It's fast enough for most stuff, and the time constant "shape" is better.
I completely screwed up with the mid-side implementation. Tried to make it like the one in fairchild 670, but with relays. It's currently disabled and I will have to redo it one day. Can't say I've needed it this far but it would be nice to have. The feedback switch is quite subtle and unnecessary, but I had to try it as well.
Couple of other tricks there, let's see. There's the B+ VAC line that cuts out if heaters die (protection relays). Then there's the completely separate 12VDC supply for relays and other utilities so they don't pollute the audio circuits. It's not visible on these pictures but heater regulator has a massive heatsink outside the case.
What a nasty time-waster of a project! I learned lots, but still can't quite get over the initial disappointment of the stock version. It haunts me how bad it sounded and how inflexible it was compared to all the hype surrounding it. For that reason it's very likely the last kit-pcb project I will ever build. I don't want anyone else to blame for bad sound or design other than me.
Mike
[edit]
I almost forgot. Thanks analag, silent-arts and lolo-m for making all this possible in the first place! Thanks to many others who helped on the projects as well, too many to list here.
years of work in this one. Turning a lofi compressor into a mastering and all-around flexible unit is not an easy task. Besides the PSU and the general gain staging configuration from the PM670 there's not much here from the original plan. Everything is extended and made optional for front panel. I guess if I started it now I would completely redraw everything into a single PCB per channel instead of making it a clusterF*ck like this one. Every switch is actually a relay so signal paths stay reasonably short, and all signal path wires are shielded. It's a brute force way of doing it but there is practically no 50hz hum peak visible on the FFT noise floor graph. Those lundahl transformers also make frequency response flat to about 50khz! I have no other compressor that can do this.
I guess it could be called the mkII or RM670 (richman). The front panel looks much better than these pictures. I'm a bad photographer. Those four purple top layer panels actually have purple-metallic stickers on them with the text (3mm aluminum slabs bolted on to a white powder coated 4mm aluminum panel). The black text on white background is a transparent sticker. I made sure the stickers can take the heat generated by the unit.
Hey look, it's the infamous "Kingston duct". Laugh all you want, it allows me to take power switch to front panel with absolutely no hum coupled to audio circuits.
I designed two additional PCB's for it. There's one for I/O transformers, mid-side, feedback and sidechain highpass. There's another one for time constants and stereo link. The same time constant network works very well for both scamp and plain modes, switched through relays. They sound quite different to each other. There's some plain RC constants in the network, some parallel trickery. Scamp is not in its original form either, of course I had to tweak it, but I still like the plain mode best. It's fast enough for most stuff, and the time constant "shape" is better.
I completely screwed up with the mid-side implementation. Tried to make it like the one in fairchild 670, but with relays. It's currently disabled and I will have to redo it one day. Can't say I've needed it this far but it would be nice to have. The feedback switch is quite subtle and unnecessary, but I had to try it as well.
Couple of other tricks there, let's see. There's the B+ VAC line that cuts out if heaters die (protection relays). Then there's the completely separate 12VDC supply for relays and other utilities so they don't pollute the audio circuits. It's not visible on these pictures but heater regulator has a massive heatsink outside the case.
What a nasty time-waster of a project! I learned lots, but still can't quite get over the initial disappointment of the stock version. It haunts me how bad it sounded and how inflexible it was compared to all the hype surrounding it. For that reason it's very likely the last kit-pcb project I will ever build. I don't want anyone else to blame for bad sound or design other than me.
Mike
[edit]
I almost forgot. Thanks analag, silent-arts and lolo-m for making all this possible in the first place! Thanks to many others who helped on the projects as well, too many to list here.