Studer 900 EQ clone for API 500 series

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rbuskov

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Update: The boards for the first prototype have arrived...

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I’m building a API 500 series clone of the channel EQ from the Studer 900 mixer. I had the opportunity to play around with a Studer 900 desk a few years back and building this EQ has been on my todo list ever since.

As other Studer designs this is an active EQ and as I recall it, an exceptionally smooth and musical sounding one, even at extreme settings. I’m still trying to wrap my head around the finer details of the design, but it appears to be a Multi Feedback Filter of some sort (perhaps people smarter than me can comment).

The EQ has four bands with gain and frequency pots. HF and LF bands can be switched between shelf and bell, while HMF and LMF band have selectable high/low Q. The gain is +/- 15 dB for all bands and I intend to use pots with centre detent for these.

I added a line receiver and driver from THAT Corp for balanced in and out. Nothing special here (I took the circuit straight out of the data sheet) but it should outperform most other electronic solutions. I thought about using Lundahl transformers instead but I somehow doubt the benefit would justify the cost. I might do a listening test once I have the first prototype built though.

If everything works out as planned, I will be selling these both as assembled units and DIY kits. I will post in the White Market when that time comes but until then, I will be sharing my progress in this thread.

So far, I have redrawn the circuit in Kicad and simulated it in Spice. Next step is to design the first version of the PCB and order a few pieces for prototyping.

I have attached the redrawn schematic to this post.

/Rasmus

 

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Ricardus

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I could see myself building 6.

I don't think the Lundahl solution is needed. The THAT chips are the way to go... as long as we can get them!  :D  I couldn't source 1646's for a couple months. Mouser was out and I needed some for my SSL preamp builds! Fortunately they're back in stock.
 

rbuskov

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Ricardus said:
I could see myself building 6.

I don't think the Lundahl solution is needed. The THAT chips are the way to go... as long as we can get them!  :D  I couldn't source 1646's for a couple months. Mouser was out and I needed some for my SSL preamp builds! Fortunately they're back in stock.

Cool. Yes, I also noticed that the 1646 was out of stock at Mouser for a couple of months, but I was able to find them elsewhere. Hopefully the Covid situation will not affect component lead times and availability more than it does now...

/Rasmus
 

rbuskov

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The prototype boards have arrived and are looking good...

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Hopefully I can find an evening or two to build one of these bad boys during the week. Note that the edge connectors on the final boards will be hard gold plated (I'm just keeping the cost of the prototypes down).

The construction of the module will be similar to the SSL 500 series stuff, like this module:

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The switch extenders will be 3D printed (see this thread for details) and the potentiometer samples I have ordered should arrive next week.

I also have a quote for some Sifam push-on knobs that look really nice, but unfortunately the lead time is not quite so pretty (8-10 weeks) so I might look into other options.

I will design the front panel once I have all the other stuff figured out.

/Rasmus
 

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Kevin

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Because the mainboard does not sit in the orignal 500 position, he put another board for the 500 connection
ok so you do the orignal ssl style design ;) so you have to print many plastic parts to extend that.
which bending of resistors you used. it looks like 7,5mm and not 10mm
 

JMan

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rbuskov said:
The prototype boards have arrived and are looking good...

Hopefully I can find an evening or two to build one of these bad boys during the week. Note that the edge connectors on the final boards will be hard gold plated (I'm just keeping the cost of the prototypes down).

The construction of the module will be similar to the SSL 500 series stuff, like this module:

I also have a quote for some Sifam push-on knobs that look really nice, but unfortunately the lead time is not quite so pretty (8-10 weeks) so I might look into other options.

I will design the front panel once I have all the other stuff figured out.

Exciting stuff!
 

rbuskov

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Ricardus said:
Why is the edge connector a separate board?

As Kevin already explained, the main board is not in the normal position for a 500 module, but rather centred horizontally (when the module is plugged in).

This way, you can put 9 mm pots and push buttons (with extenders) on either side of the board and make the front panel design more symmetric. Also, with 4 pots on each side of the main board there should be no need to use screws to hold it together with the front panel.

Thomann has some nice pics of the SSL module, which is where I got the idea: https://www.thomann.de/dk/ssl_500_series_611_eq.htm

/Rasmus
 

rbuskov

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Kevin said:
which bending of resistors you used. it looks like 7,5mm and not 10mm

Good catch! Yes, those resistors foot prints are 7.5 mm wide. I didn't really think about it... I personally prefer 7.5 mm (easier to work with when you lay out boards) but I realise that most DIY projects use 10 mm (easier to work with when you assemble boards).

I'll change the resistor foot prints to 10 mm in the next version of the board.

/Rasmus
 

john12ax7

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7.5mm is too close for lead spacing for 1/4w resistors.  Bending leads too close to the body can cause internal damage to the resistors, the bend should happen in the metal lead,  not where the lead meets the body. 10mm is often the minimum recommended by manufacturers.
 

rbuskov

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john12ax7 said:
7.5mm is too close for lead spacing for 1/4w resistors.  Bending leads too close to the body can cause internal damage to the resistors, the bend should happen in the metal lead,  not where the lead meets the body. 10mm is often the minimum recommended by manufacturers.

I'll change it to 10 mm.
 

rbuskov

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I'm having trouble sourcing the the pots for this project. The manufacturer is working on reduced capacity due to covid19 and it's not clear when they will be able to deliver.
 
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