FEELER : IT'S ALIVE!!!Finally finished 32C project! (SCHEMATIC & BOM)

rainton · Jun 22, 2016

Oh also I forgot to mention,

I used regular Alpha Dual 50K log pots and did the wafer-swapping trick - it worked!! ;D

It's not that easy though. I destroyed the first pot. The second pot was a success but it took me almost half an hour.
The 3rd -5th pot took me 10 minutes each. And the 6th + pot took me just over 4 minutes each.

So practice will get you there and I think it's worth it, since the Alphas cost only 1.20 Euros each over here if you buy more than 10pcs.
I'll try to open a dedicated thread for it with pics how it's done later this week - I'm sure it'll help a lot of builders - not only for the 32C eq...

AND ONE LAST THING: With the pots soldered directly to the pcbs this EQ will never fit a 1RU enclosure!! At least not for a stereo unit and with a layout as shown on the front panel ForthMonkey made.

The pots will fit the panel, but the edge of the pcbs will be more than 8mm above the top edge of the panel.
So either you do a completely different panel layout or you'll have to separate the pots from the pcbs, handwire them and place the pcbs somewhere in the chassis...

skeeler87 · Jun 30, 2016

So If I expand ForthMonkeys frontpanel design for 2RU will the PCB's fit, or is the spacing of the drilled out holes for the pots wrong as well? Also nice idea for using the HA-4741's! I think I will also do the same thing. Where were you able to acquire those IC's? They are very expensive on ebay.

rainton · Jul 9, 2016

skeeler87 said:
So If I expand ForthMonkeys frontpanel design for 2RU will the PCB's fit, or is the spacing of the drilled out holes for the pots wrong as well? Also nice idea for using the HA-4741's! I think I will also do the same thing. Where were you able to acquire those IC's? They are very expensive on ebay.

I haven't checked actually since after I realized the pcbs would never fit with the pots soldered to them I decided to go a different route , design my own panel etc.

But you can measure the distance between the pots on ForthMonkey's front panel with frontpanel designer and check if it matches the pcbs...
I'll post some pics when I'm done and you'll see what I mean.

I bought my 4741s at Telec-electronics in the Netherlands - 10pcs. For 20£

rainton · Jul 14, 2016

I'm almost done with my stereo 32C EQ.
As mentioned earlier the inserts of my console are unbalanced.
Most of my other gear is transformer balanced, so when the 32C EQ is between two transformer balanced units everything works as it should - however when it's just the 32C connected to an insert I'm running into a problem.

In an unbalanced environment the cold (negative) pin is connected to ground and the 4741 ICs I used don't like that at all.

If ground is not connected it works, but then I have a 60Hz hum obviously.

If I only feed the first IC with the hot (positive) pin I can connect ground without a problem but then I have a signal drop of 4db.

So my options now to make this eq usable in my environment:

1. just connect the hot pin and compensate for the 4db loss elsewhere in the circuit (where would be the best place to do that?)

2.use another opamp to create a balanced the signal before the actual circuit...but on the other hand the actual 32C eq is unbalanced and following ForthMonkey's schematic the first opamp is not existent in the original schematic and is used here to unbalance the signal (?) What happens if Just connect my input after the first opamp? (I'm not so experienced working with ICs )
Or should I change the input resistors to compensate for the 4db loss?

3. use 1:1 transformers on in- and output - the most expensive way, but it would work regardless of what the eq is connected to (electronically balanced or unbalanced equipment)...and depending on the transformers might add some additional sound...

What would you suggest? Any ideas - preferably simple

Thanks
Martin