3D "AIR" EQ - "Night EQ" PCB's Complete!

[sneakthief]

Did you know you can approximate a reverse log pot using a dual-linear pot?

Check out the schematic on the right:

"Square Law Attenuators

The figure below illustrates another way to approximate logarithmic attenuators using dual linear pots. In the first circuit, we get a nice log/audio taper. It is actually a square law, but it has a nice "feel" to it. The second circuit is an inverse sqaure law attenuator (notice it's the configuration of the first circuit flipped upside down), and is probably the more useful of the two because it approximates the reverse log taper - and reverse log pots are hard to find.

Use similar rules about keeping the load resistance RL 10 times the value of the pot to minimize loading effects.

For the square law attenuator, The equivalent resistance presented by the network (as seen by Vin) varies from Rpot || RL when the pot is at 0% rotation, to Rpot when it is at 100% rotation.
For the inverse square law attenuator, The equivalent resistance presented by the network varies from Rpot || RL when the pot is at 0% rotation, to Rpot / 2 when it is at 100% rotation."

- Courtesy Scott Bernardi: http://home.comcast.net/~sbernardi/elec/og2/partsub_pots.html

 

[clintrubber]

[quote author="sneakthief"]Did you know you can approximate a reverse log pot using a dual-linear pot? [/quote]
Cool info :thumb:
Let's now check if it can also do a rev-log rheostat (which is what is needed for the Green & Air-EQ).

 

[clintrubber]

[quote author="sneakthief"]Or six of these (2x24) for 8 euro each?

http://www.uraltone.com/kauppa/product_info.php?products_id=1077

The only catch is that they supposedly won't fit in a 1u rack.[/quote]
Hmm, too bad they don't have single decks... 2channels*6switches*8Uri's = impressive...

But I doubt a switched alternative could be much cheaper than this, unless a 12-pos would be enough (don't think it will).

 

[clintrubber]

[quote author="clintrubber"][quote author="sneakthief"]Did you know you can approximate a reverse log pot using a dual-linear pot? [/quote]
Cool info :thumb:
Let's now check if it can also do a rev-log rheostat (which is what is needed for the Green & Air-EQ).[/quote]
Haven't checked it, but I believe there's no real reason why we can't skip the rev-log-pot.
I.s.o. the 500k rheostat (2 terminals) use a single log pot (three) & keep the 5k62 and/or adjust.
Or i.s.o. the single log perhaps a dual-lin configured as shown above. The reason for this might be the possibly/assumed lower tolerances of lin-pots (from sample to sample).

For an alike inverting topology, I recall a parametric EQ (state variable filter) that "should have used" rev-log 'floating variable resistances' (rheostats) as well for the freq. SVF's usually do.
This one didn't, it used log-pots. It works.

I realize this suggested change might still have some flaw somewhere (please debug) and/or might feel like it won't sound any longer like the real thing.
I doubt the latter, but feelings are feelings.

Bye,

Peter

 

[clintrubber]

This may be a PAQ (Previously Asked Question), but I don't see how the air-band can be boost only. It sums just like the others. Are the values of R34 & R35 correct ?

 

[wmb]

This looks like a very interesting project. Does anyone have even a rough estimate of the cost of building a couple channels less case, pots and connectors? Also, how many boards can be powered by the power supply board being offered with the main pcb.

I'm (very) slowly putting together a couple 1081's and would like to build something less expensive and with a shorter turn around. After reading all 22+ pages a couple channels or more would be interesting if the boards are less than $100 per channel to stuff.

Cheers, wm

 

[Harpo]

[quote author="clintrubber"]This may be a PAQ (Previously Asked Question), but I don't see how the air-band can be boost only. It sums just like the others. Are the values of R34 & R35 correct ?[/quote]
Reason for boost only is the labled 2.5kHz shelf band ( for real it is a 1.25kHz shelf, check fo -3dB=1/(2xPI()xRxC), makes even more sense, as the 2.5kHz is also availiable in the air section ). Assuming this filter section with it's pot centered to +/-0dB you can't lower the amplitude of any frequencies above in the air section as this section is also part of the summing node.
The values of R34 (500k rev log) and R35 (5.62k) are correct, giving you an inverted AV varying from 0.1 to 10 (AV=-1 x R30 / (R34+R35)). Switching on the air band to any selectable frequency will always add an at min. -20dB shelf level on top.
I don't believe the rev.log trick with a dual lin mentioned above will work as a rheostat. you'll have to make it 50k at center, 500k full ccw. Please let me know better.

 

[Dave_B]

Peter are you planning on making the PCB artwork available for those who'd like to roll their own boards?

 

[dissonantstring]

Hi everyone,
I've contacted Colin (aka slenderchap on the forum from audio maintenance) and he says that he can get Omeg 470K rev. log pots with center detent available for all of us who are about to build the Night EQ (thanks to Kevin H. and Peter C.)

Colin says that the pots are 10% so they should provide a very close value to the required 500K. He would like to know if we would prefer 16mm or 20mm sized and also if we prefer PC mounting or solder lug termination.
I want to thank Colin for his help in sourcing this somewhat diffiuclt to find pot with center detent! :thumb:

I'm thinking that since most seem to be thinking of a dual channel in a 1U chassis that a 16mm pot with PC mount would be best, but I'd like to hear what others are thinking.

Kind Regards,
Grant

 

[peterc]

The Lorlin is 27mm in diameter, the P20 Omeg is 15mm from the shoulder on the solder pins to the shaft centre, & the P16 is 12,5 mm from the shoulder to the shaft centre. So either would work.

The board will accomodate the PCB mount omegs, pin spacing is 5mm.

I estimate about $25-00 per channel for components excluding PCB & pots.
That is based on what I paid for my prototype channel in South African Rands converted into USD.

I will make the PCB pattern available for home etching, lets get the PCB's done first.

Peter

 

[Purusha]

Definitely 16mm but with solder lug termination. I am in the process of making the front panel design
for a batch of Night cases and it's just gonna be too much crowded if all pots are soldered to the
185mm wide PCB.

I think that a nice front panel design is more important than the commodity of avoiding soldering a few
extra wires. At the end you will anyway look only at the front panel design and not into what is inside

:wink:

 

[Greg]

I hope this doesn't start some big debate... but if the board acommodates PCB mount OMEGs I'd prefer PCB mount over solder lug.

 

[dissonantstring]

o.k. one vote for solder lug termination and confirmation that either 16mm or 20mm pot size fitting the PCB from Peter (thanks Peter!) and one vote for PCB.

obviously solder lugs would accomodate various front panel layouts and we are talking about 6 of those pots plus a rotary switch on the front panel for each channel (not to mention a power switch) for the full version.
so, any more votes for either pcb or solder lug terminations?

Peter, which inductor would work best with your PCBs or will any do?

cheers,
grant

 

[Greg]

Also keep in mind that it's still quite easy to solder to PCB mount pins if someone still wants to run wires to the PCB. It's much harder to go the other way.

 

[dissonantstring]

ah, excellent point greg!

 

[khstudio]

x

 

[peterc]

Peter, which inductor would work best with your PCBs or will any do?

I did not use any inductors on the prototype! But I have used inline inductors (they look like resistors) in other projects & they worked well. This is what the board is laid out for.

PCB pins are fine for flying leads, what I do is put a bit of heatshrink over the solder joint to get the extra mechanical strength. "I hate flying leads..."

Peter

 

[khstudio]

[quote author="peterc"]

Peter, which inductor would work best with your PCBs or will any do?

I did not use any inductors on the prototype! But I have used inline inductors (they look like resistors) in other projects & they worked well. This is what the board is laid out for.

Peter[/quote]

I used the inductors on mine as well but I'm still not sure if they're needed or not :? It was mentioned that they may have been added because of the "Switching Power Supply" (which we will NOT be using)... but I can say they didn't hurt the sound in or out to my ears... may need test equipment to really see what they're doing.

 

[sonicwarrior]

[quote author="Purusha"]I think that a nice front panel design is more important than the commodity of avoiding soldering a few
extra wires. [/quote]

Well I hate wiring extra wires if there is the possibility to PCB-mount the pots.

I will only buy the cases if they hold a stuffed board with PCB-mount pots.

 

[fucanay]

I ordered the solder lug version from small bear. There doesn't seem to be a whole lot out there without having to get involved in another omeg group buy, which didn't turn out so good last time. Not to mention, that soon there is going to be a nationwide shortage on 500k reverse log pots, thanks to this eq.

I don't know about the shipping to the rest of the world, but I got 12 of them shipped for $27USD. Not too bad for the costliest parts in the project.

Matt