New prototype - 550 style EQ

Haha and yes, that is a plywood face plate. It makes it easier to mock up different layouts.

Indeed.

Yep - fully discrete stepped EQ using a bridged T filter design, fully symmetrical passive cut and active boost. The boost is handled by two socketed discrete op amps. The whole audio path is discrete, including a third DOA for the balanced input. Transformer balanced out. 

I did some testing with a single unit on a bench, and it's nice and quiet (less than -100dB A weighted noise and 0.013% THD at something around +15 dBu in and out).

Sent some drums through it - kick, overhead, room, and snare files. For each one I have the output of the DAW, the EQ set flat, then each band doing something by itself, and finally all three bands in. I wasn't trying to get a particular sound as much as just play with it a bit.

www.dropbox.com/sh/nw1smokcxp3g4oy/AAAa6umkqLcnTrRjfbUHa5iha?dl=0


I've been thinking about adding a 12 dB boost option, but I haven't decided to include it or not. You think that would be useful?

The steps are 2,4,6,8,12, symmetrical boost and cut. At zero the filter is completely switched out, not only centered, so frequency response is perfectly flat. The same caps are switched between the feedback loop of the op amp for boost and the passive circuit for cut, so the EQ moves are reversible.

Q is peaking, yup, so broad at low gain and tight at high. I have mapped out the response plots, but I haven’t gotten them polished up yet.

The toggles on there are to go between shelf and bell. 6 frequencies per channel.

There’s no reason it would be difficult to add a band, but I’m out of room on the 1RU as it is haha.

Yes, super cool.

No i miss read your thing of saying 15dB I think.

Your gain steps are great wouldn't change them for this type of eq.

So you're talking output gain boost? Wouldn't be useful really unless it also cut. So +/- 12dB trim. Which would be a bit of a shame as I'm not sure you can fit another stepped grayhill in there and everything else is nice and stepped.

For the nerd-inclined, here's basically how it works.

The two resistor banks are resistor ladders on the boost and cut side of the gain switch. When you change the gain you vary connection on the common of the switch (A1 or A2) across the resistor ladder between A1 and B, or A2 and B.

When you cut, the capacitor bank is connected across B and A2. When you boost, it's between B and A1. When you're set flat the caps are disconnected, so there's no filtering at all. Switching frequencies changes the values of the caps.

For both boost and cut the Q (and also the center frequency and gain) is set by the resistor to ground at the bottom of the T (which is also switched).

wiz I plan to eventually offer them as assembled units. I’m close to having a few products ready for sale, but metalwork is completely kicking my butt right now. This would be the next one up.

Not sure if you know it, but check out Newark when you're looking for film caps. Occasionally you can find certain ones on "sale" for crazy cheap. I usually stock up on them when they are.

Yes - in fact what prompted me to consider it was that I often found myself using the gain feature in several EQ plugins. It's easier in that regard to make the EQ come back to unity gain, especially when doing aggressive cuts. On the other hand, none of my hardware units have this feature and I don't ever recall wishing they did. So I'm not sure. I'm thinking about leaving it out.

I can change the Q by changing the value of the resistor ladder to ground. You could just as easily make it or narrower or broader. My favorite EQs all have a Q of around 1.2, though.

That level of customization is something I'd like to be able to offer - different op amps, transformers, center frequencies, and sure, why not, different Qs if desired.

When ready please let us know price point ? thx !

I was kind of toying with the idea, actually. Just need to put a piece of metal behind it to support the weight on the rack ears.

I mean.. sure... why not?