Barry Porter "Net EQ"

[abbey road d enfer]

Last question before finishing up the first draft.

I attached a portion of the schematic


Is this correct?

Not knowing what's connected to R73, it's difficult to say if it's necessary or not. If it's directly connected to the output of an opamp, it's useless, if it is connected to a coupling cap, it's recommended/necessary/despised for DC removal.

RC filter (R73, C71),

No. It would be a high-pass (low-cut or DC removal) filter if C71 was before R73. ATM the HPF is the combo of C71 and R72+R71.

  then a voltage divider (R71, R72), followed by a non inverting opamp (U12), functioning as a unit gain buffer? 

That is correct.

In case it is correct - is the opamps input impedance affected by R73?

The input impedance of the opamp is not dependant on whatever resistor is connected there.  The impedance seen by whatever drives this circuit is affected by R73, although very little. The dominant branch is R72 + R71, for a total of 5.5k.  R73, with 100k accounts for only 5% in the input impedance of the stage.

 

[Gustav]

Last question before finishing up the first draft.

I attached a portion of the schematic


Is this correct?

Not knowing what's connected to R73, it's difficult to say if it's necessary or not. If it's directly connected to the output of an opamp, it's useless, if it is connected to a coupling cap, it's recommended/necessary/despised for DC removal.

RC filter (R73, C71),

No. It would be a high-pass (low-cut or DC removal) filter if C71 was before R73. ATM the HPF is the combo of C71 and R72+R71.

  then a voltage divider (R71, R72), followed by a non inverting opamp (U12), functioning as a unit gain buffer? 

That is correct.

In case it is correct - is the opamps input impedance affected by R73?

The input impedance of the opamp is not dependant on whatever resistor is connected there.  The impedance seen by whatever drives this circuit is affected by R73, although very little. The dominant branch is R72 + R71, for a total of 5.5k.  R73, with 100k accounts for only 5% in the input impedance of the stage.

Thanks for the correction and clearing up the rest.

Gustav

 

[Gustav]

Heres a first draft - I am sure it contains other errors, but I know that...

Some footprints need to be corrected (bipolars, obviously, but also just the size of others, and anything else I might've missed).

Some values are probably wrong here and there.

Connectors for supply nets (nets not verified), and interface pins for relays were not added (use the invoke function for the relay pins)

Changes from the original.

- Replaced in/output circuits with THAT circuits
- Added an MS encoder and decoder
- Separated the shelving filters

Gustav

 

[Gustav]

I tried doing a single sided self-etch-friendly layout of the project, but had to give up and go double sided.

I dont know if its practical to self etch a double sided board, but if it is, there are not a whole lot of holes that need a through hole connection. This could be done using a component terminal as a stud and solder to the pads on both sides.

The project zip contains sch and brd file for 160 x 100mm layout. It loads into hobby Eagle's hobbyist license version, and fits a eurocard for self etch.

MainBPrev1
FilterBPrev1

I cleaned up the schematic drawing and values, but there may still be errors, and the project is untested. (I just realised, I forgot to run a DRC before uploading, but you can do that in Eagle yourself).

(look further down - new file up)

Its not perfect, but you can rip it up and create your own layout, if you have grievances. Please share if you come up with a good design

Next

I started looking into this, because I wanted to play with digital pots and LCDs.

I had set my sights on the AD5204. Data sheet states a 30% tolerance chip to chip, but 1% tolerance within the chip, so I figured using it in gangs of 4 for this would be perfect

Unfortunately, I understood it to be some sort of digitally controlled relay matrix where signal path headroom would not be limited by the 5V max supply voltage. I realised I was wrong, but only after digging into coding and solutions.

Although I have found alternative 30V capable DCPs, I haven't found any quad packages, and for single or dual chips, the relative tolerance between them seems to make it impossible to get perfect, stereo tracking.

Any inputs for a forward direction would be welcome, since I am a little stumped for now.

I'd use Arduino to control it and share it openly. I am looking to learn and share.

Gustav

 

[abbey road d enfer]

Although I have found alternative 30V capable DCPs, I haven't found any quad packages, and for single or dual chips, the relative tolerance between them seems to make it impossible to get perfect, stereo tracking.

In that respect, you would be exactly in the same situation as using a "real" 'mechanical) pot, with an edge. Tracking, when used in potentiometer mode, would be much better.
There are ways to make sure the response of a circuit does not depends on the actual end-to-end value of a pot. But it would mean a redesign of the BPEQ.
Digital pots are perfect for the boost/cut in the BPEQ, but if you want to use them for frequency control, you will need to adjust the capacitor values.

 

[Gustav]

Although I have found alternative 30V capable DCPs, I haven't found any quad packages, and for single or dual chips, the relative tolerance between them seems to make it impossible to get perfect, stereo tracking.

In that respect, you would be exactly in the same situation as using a "real" 'mechanical) pot, with an edge. Tracking, when used in potentiometer mode, would be much better.
There are ways to make sure the response of a circuit does not depends on the actual end-to-end value of a pot. But it would mean a redesign of the BPEQ.
Digital pots are perfect for the boost/cut in the BPEQ, but if you want to use them for frequency control, you will need to adjust the capacitor values.

Thanks again for the input.

I found some pots that should work. I'll create a thread in the lab if I get anywhere with it and specific questions pop up, since these issues are not related to the NetEQ per se.

Hope it turns out to be as fun as I think it'll be (although, I am aware frustrations will arise).

Gustav

 

[Gustav]

Attached,

rev2 of the eagle files for the alternative layout I did.
LTSplice model of the EQ

I boiled it down to the 4 bell filters, changed the Q a bit, and added the MS (hasn't been tested yet).

I think thats it for now. I tried to make sure all Frequency and Q selectors ran on identical resistor ladders (thats a matter of taste and degree of OCD, of course).

Gustav

 

[rafafredd]

I see some NetEQ units with lowpass filters. What circuit are you using for this? The original unit has only hi pass...

 

[rafafredd]

Bump. Anyone?

 

[trashcanman]

Attached,

rev2 of the eagle files for the alternative layout I did.
LTSplice model of the EQ

I boiled it down to the 4 bell filters, changed the Q a bit, and added the MS (hasn't been tested yet).

I think thats it for now. I tried to make sure all Frequency and Q selectors ran on identical resistor ladders (thats a matter of taste and degree of OCD, of course).

Gustav

Thanks for these. I just thought I'd mention that there's a broken connection on one of the diodes at the right output.

 

[Gustav]

Thanks for these. I just thought I'd mention that there's a broken connection on one of the diodes at the right output.

Thanks for the feedback. I just checked, and there is a dead-end trace, but that should be removed, not connected.

Gustav

 

[rafafredd]

Hey Guys... someone must have the answer to this:

"I see some NetEQ units with lowpass filters. What circuit are you using for this? The original unit has only hi pass..."

I posted it up there, and it was completely ignored.

Gustav, did you took the lo pass out of this?

 

[Majestic12]

Hey Guys... someone must have the answer to this:

"I see some NetEQ units with lowpass filters. What circuit are you using for this? The original unit has only hi pass..."

I posted it up there, and it was completely ignored.

Gustav, did you took the lo pass out of this?

I have added the LPF circuit myself since there is no such thing on the original pcb design.

The circuit is a simple active sallen-key filter based around an opamp and inserted just before the output stage of the EQ.

http://images.google.de/imgres?imgurl=http%3A%2F%2Fwww.changpuak.ch%2Felectronics%2Fimages%2FSK-LOWPASS.png&imgrefurl=http%3A%2F%2Fwww.changpuak.ch%2Felectronics%2Fcalc_08.php&h=229&w=508&tbnid=whLyqCt5mWCDoM%3A&docid=WBSt7khrSv_2bM&ei=eB8YWLeQFoH0UomukJAJ&tbm=isch&iact=rc&uact=3&dur=1880&page=0&start=0&ndsp=42&ved=0ahUKEwi3rMmx4YbQAhUBuhQKHQkXBJIQMwghKAMwAw&bih=1072&biw=1920

 

[rafafredd]

I see...

So, you are actually switching cap values??

 

[Majestic12]

I see...

So, you are actually switching cap values??

No, resistors R1 and R2 get switched to change the cutoff frequency

 

[skidmorebay]

Harpo, or anybody in the know,
Just wondering if I want to have 12 frequency selections for each band, can I just pick 12 of the values I want from Harpo's spreadsheet? Or, do the resistors add up in series in a way that wouldn't allow this to work?
thanks,
JS

 

[Harpo]

maybe use my previously linked Excel-file "Porter_NetEQ_X_freqstep.xls" in order to overwrite the content of corresponding cells 'wanted step positions' in column F and change FI. 23 with 12 for your wanted 12 positions ...
same goes for center frequency range from / up to, just have an eye on parts limits and maybe adapt frequency setting cap value to archieve this ...
couldn't be so hard to do.

 

[skidmorebay]

Perfect, thank you.

 

[skidmorebay]

Anyone see any issue with substituting a THAT 1200-series line receiver, and 1600-series line driver for the input and output?
I assume you would inject the output of the lines receiver at C7 (after? or before?), and take the input of the line driver from either R24 or R25...

I've got Gustav's boards. I would use a couple of the small utility pcb's that DIYrecordingequipment.com sells for the THAT chips.

 

[Majestic12]

Anyone see any issue with substituting a THAT 1200-series line receiver, and 1600-series line driver for the input and output?
I assume you would inject the output of the lines receiver at C7 (after? or before?), and take the input of the line driver from either R24 or R25...

I've got Gustav's boards. I would use a couple of the small utility pcb's that DIYrecordingequipment.com sells for the THAT chips.

I quote myself:

3. THAT 1XXX series chips as in and output work just fine. I already did this in a prototype channel of the BP and it can easily replace the complete circuit blocks without any noticable change in sound.

You can insert your signal right after C7 and put the output stage directly after R24. You don't need all those DC blocking caps in front and after the THAT chips.