gain/atten control on passive EQ
« on: January 15, 2011, 11:30:04 AM »
ok, so I'm breadboarding a basic LC Resonant circuit with a pot for gain/attenuation.

The problem is that I can't gat a linear behaviour of the pot although the pot is linear.
In more detail, the pot will increase/decrease much more radically when is rotating to the ends of it's travel.
If the pot is somewher near the middle the gain/att is smoother. Of course this gives me better control in the middle position but I was wondering if this is normal and if there're other eq designs that have the same behaviour.

I have tried other techniques like gyraf's and manley's eqs where there's a switch for cut/boost and the pot is 0 to 15/20 dbs. In this case an antilog pot seems to give the best results but again the behaviour is not so smooth.

I believe that Neve and SSL used to have some similar behaviour in their pan pots but is it normal for EQs and if not are there any workarounds?

well, I hope my description is not very confusing and it makes some sense  :)

thanks
Michael

   


haima

Re: gain/atten control on passive EQ
« Reply #1 on: January 16, 2011, 01:40:30 AM »
can you post a schematic of exactly the circuit you are making? it might make it easier to troubleshoot...

You could try strapping resistors across the pot to modify it's "law" - depending on how the pot is connected - it doesn't work for every application. There's lots of info on the web/forum about this - a quick google search for "modify pot law with resistor" (no quotes) brought up:

http://sound.westhost.com/pots.htm#chg-law

http://web.archive.org/web/20050307111237/www.wavefront.mcmail.com/pot.htm

http://www.geofex.com/article_folders/potsecrets/potscret.htm

BUT that might not solve your problem.. show us a schem and we'll be able to figure it out, hopefully!

Re: gain/atten control on passive EQ
« Reply #2 on: January 16, 2011, 11:14:30 AM »
thanks for the links haima  :)

I have the schematic only on paper at the moment. I'll try to post it if I don't give up on the idea  :)

Re: gain/atten control on passive EQ
« Reply #3 on: January 18, 2011, 09:25:53 AM »
OK, I haven't given up yet  ;D

here's a rough schematic of what I'm talking about. The top section is the one where I have my gain/atten control and it's linear.
In fact, if you notice R6, R7 are used to create a 'fake' center tapped pot.

The problem is that I don't get a smooth resolution. I think is very similar (if not the same) problem that PRR is mentioning here
http://www.groupdiy.com/index.php?topic=42735.msg531605#msg531605

PRR

Re: gain/atten control on passive EQ
« Reply #4 on: January 18, 2011, 10:13:40 PM »
> I have the schematic only on paper at the moment.
> OK, I haven't given up yet   here's a rough schematic


Um...... what a tangled web.

Go back to hand-drawing, skip the details, emphasize the architecture.

PRR

Re: gain/atten control on passive EQ
« Reply #5 on: January 18, 2011, 10:14:34 PM »
Hint: lose the 2K2 parts.

abbey road d enfer

Re: gain/atten control on passive EQ
« Reply #6 on: January 19, 2011, 03:33:11 AM »
The problem is that I can't gat a linear behaviour of the pot although the pot is linear.
In more detail, the pot will increase/decrease much more radically when is rotating to the ends of it's travel.
This is a very common problem. Some manufacturers offer pots with a specific law (the law is the resistance vs. percentage of rotation); it's generally called W or S. I have never seen these available off-the-shelf; one has to order them in several thousands quantity. The most common workaround is using rotatry switches.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: gain/atten control on passive EQ
« Reply #7 on: January 21, 2011, 07:15:13 AM »
thank you gentlemen!

PRR> Could you please elaborate a bit more on the sketch? Being an idiot I'm having hard time to understand a couple of things  ::)
i) the component that goes to gnd is a resistor?
ii) I can skip the 2k2s but I will need a 'middle point' for my LC filter (?)

I'm looking forward to find some time to go back to breadboard and try your suggestion.

abbey road d enfer> It is very hard to find W-law pots indeed. Can I get the same result with something like this?

If everything fails I'll use a rotary switch. The only reason I prefer a pot is for better resolution (not stepped dbs). Not great for stereo matching though...
« Last Edit: January 21, 2011, 07:17:08 AM by warpie »

haima

Re: gain/atten control on passive EQ
« Reply #8 on: January 21, 2011, 07:33:56 AM »
i) the component that goes to gnd is a resistor?

Pretty sure that's the LC (inductor & capacitor in series).

ii) I can skip the 2k2s but I will need a 'middle point' for my LC filter (?)

Not sure I understand you - wouldn't the wiper of the pot form the "middle point"?

Re: gain/atten control on passive EQ
« Reply #9 on: January 21, 2011, 08:35:47 AM »
Quote
Pretty sure that's the LC (inductor & capacitor in series).
thanks! of course... ::)

Quote
Not sure I understand you - wouldn't the wiper of the pot form the "middle point"?
in the schematic I posted before (very messy indeed) the input of LC is connected to  some kind of virtual earth point (consisting by the two 2K2 resistors) and the output goes to the wiper (i.e the input of the LC is not connected to the ground).  

With rotary I think it will look something like this...

(unfortunately I don't have a scanner at the moment)
« Last Edit: January 21, 2011, 08:37:38 AM by warpie »


haima

Re: gain/atten control on passive EQ
« Reply #10 on: January 21, 2011, 08:53:31 AM »
Are you trying to make something like this Neve BA211/A?


Re: gain/atten control on passive EQ
« Reply #11 on: January 21, 2011, 09:21:14 AM »
I wasn't aware of this schematic but it seems very similar topology. I think I'll give it a try too.
thanks haima! all your links have been very useful so far  :)

haima

Re: gain/atten control on passive EQ
« Reply #12 on: January 21, 2011, 09:47:45 AM »
I believe it's the midrange or "presence" control from the 1073 & 1084 - so have a look at the full schematics for these to see how it's implemented and also the schematic for the "amp" section shown above.

The current AMS/Neve company has the full schematics available on their site i think - might take some googling to find it. There's plenty of info on these out there.

abbey road d enfer

Re: gain/atten control on passive EQ
« Reply #13 on: January 21, 2011, 11:37:23 AM »
abbey road d enfer> It is very hard to find W-law pots indeed. Can I get the same result with something like this?
Unfortunately, you want exactly the opposite!
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: gain/atten control on passive EQ
« Reply #14 on: January 21, 2011, 12:32:03 PM »
Quote
Unfortunately, you want exactly the opposite!
Unfortunately, you are right...  :)

...which leads us to the rotary solution again. I might skip the whole idea and try to use a rotary instead.
Shame because I really got into the 'pot solution' mood...  :)


 

Related Topics

  Subject / Started by Replies Last post
4 Replies
1991 Views
Last post November 18, 2004, 12:30:43 PM
by matthias
4 Replies
2736 Views
Last post July 08, 2007, 04:31:07 AM
by matthias
27 Replies
9500 Views
Last post July 15, 2017, 03:43:23 AM
by SV.L
8 Replies
4302 Views
Last post May 04, 2009, 12:33:45 PM
by skal1