SIAM 295b aka Poor Man's W295b

[krabbencutter]

Hey everyone,

I've successfully built a W295b inspired EQ in the spirit of Ian's "Poor Man's" circuits and you can find the schematic (KiCad & PDF) as well as a BoM on GitHub.

My goals were as following: continously variable gain, gryator circuits instead of real inductors, only readily available parts, bipolar supply voltages and of course ICs instead of discrete amps.
I already have a +/-12V PSU from my modular synthesizer so I didn't bother designing one myself. Parts cost without a dedicated PSU should be around $20-$25.

The original W295b did pose a couple of challenges. First of all the high frequency control needs a center tap. To work around this the high frequency control in my design only works one way, but you can use a switch to toggle between boost/cut. Next up I had to figure out the inductance values on the mid band. I couldn't find much on the internet but fortunately it's a German EQ and if we write 700Hz on the front panel you can bet the center frequency is dead on 700Hz. Since I had the resistance and capacitance values I could just calculate the inductance values and arrived at 235mH / 170mH /110mH / 80mH/ 50mH & 30mH. Last but not least the W295b mid band is kind of a proportional Q design, but the bandwidth gets much wider than usual at lower gain values. This is where my design is only "inspired by" the W295b. It's a proportional Q design as well and closely matches the W295b at +/- 8dB but will not be as wide at +/- 2dB and +/- 4dB.

Since I don't have an original W295b at home I could only compare my circuit to Arturia's Sitral-295 plugin and it was pretty spot on.
Let me know if you have any questions or suggestions for improvements

 

[jhaell]

Nice work! Might draw a PCB for this if i have time and you approve. Some transformer in and outputs would be nice.

 

[krabbencutter]

Thanks, of course you can do PCBs! All my stuff is open source

 

[krabbencutter]

To keep in line with my cat themed NYAN 1073 I've decided to rename this project to SIAM 295b (Siemens Inspired Analog Module) =^w^=
I've also added switchable Q-Mod resistors that will limit the maximum gain and widen the Q; similar to the Hi-Q switch in the Neve 1084. This makes it possible to keep the fully variable +/-8dB with a narrow Q and optionally switch to the Siemens' 2/4/6 dB settings.

 

[krabbencutter]

Rev04 of the schematic is online and I've changed some capacitor values. Also here's a direct comparison with Arturia's Sitral-295 plugin

 

[JMan]

This is very cool, great work! At a quick glance, I don't see the power rails on the schematic -- should it therefore be assumed that the +/-12vdc is only being supplied directly to the TL072s and is nowhere else?

 

[krabbencutter]

That's right, there's no bias voltages or similar in the circuit and you need the +/- supply rails only for the opamps.

 

[krabbencutter]

Rev05 of my schematic is now on GitHub. I've added the missing LPF after the Shelving filters and the high frequency noise is much more under control now. The switch position for the gyrators has been moved as well.

My latest prototype works and sounds great and I'm pretty happy so far

 

[TwentyTrees]

I couldn't see your schematic on GitHub - link in the first post is dead for me right now. Great stuff for keeping this going tho, I really like the plugins versions of the W295b I've used.

 

[krabbencutter]

I couldn't see your schematic on GitHub - link in the first post is dead for me right now. Great stuff for keeping this going tho, I really like the plugins versions of the W295b I've used.

Oops, I had a typo in the filename. The link works now.

 

[TwentyTrees]

Oops, I had a typo in the filename. The link works now.

Yep, that's got it. Great work!

 

[vintagelove]

Would love a pcb for this.

 

[MonoCutter]

I designed a pcb for rev4 when krabbencutter released the schematics. JLCPCB produced and shipped them in no time. But then the parcel got stuck in the german customs for weeks. After clearance it was send in my direction via DHL. And somehow they lost the parcel. And after another week of waiting they told me to be patient.

Maybe i should do another pcb for rev5 and see what arrives first.

 

[krabbencutter]

Damn, I hope your PCBs will arrive eventually. Should you do another PCB I'd definitely be interested in helping with the design and since I also live in the Rhein/Main area I'd love to participate in ordering some boards as well.

 

[MonoCutter]

Nice, i'll let you know! Thanks for all your efforts. I'm really excited how it turns out.

 

[tony hunt]

Also in Frankfurt und very interested in a couple of Rev5 PCBs should it happen.

 

[MonoCutter]

It will happen from my side. It might take some time, because i'm working on my master thesis atm.

 

[krabbencutter]

I've uploaded Rev 05c, which consists mostly of housekeeping (additional coupling capacitor, small fixes, partial component footprints, more logical component numbering), as well as a BoM.

 

[MonoCutter]

Finally the now outdated Rev 4 pcbs arrived . As beginner pieces in pcb design, they are not works of art.
Added a relay bypass. Since i have a few different input and output transformers, i wanted to be able to switch back and forth between Audiox's balancing/unbalancing pcb and the transformers.

 

[krabbencutter]

Glad to see they finally arrived! The main addition in Rev5 has been the Sallen-Key filter between the EQ stages. You could easily build that on a small piece of stripboard and add it to your existing circuit with some wires between R16/R18. The SKF reduces the high frequency noise and acts like the Dr1 inductor in the original W295b schematics, which I had ignored until that point. You could also try to increase the feedback capacitor of the Baxandall-OpAmp (C15 in your schematics) to 470p or even higher, which will mitigate the high frequency noise as well.
I also changed the rotary switch for the mid band frequencies to switch at the gyrators' inputs instead of switching the gyrators' ground connections, but it will work either way.
The additional 4M7 resistors can be soldered directly onto the switches and the output capacitor (C34 in my latest revision) is just for good measure to make sure there's no DC offset.