If you had 1 state variable eq on a desert island.....

[iampoor1]

Hi All

I am looking to build 8-16 "General purpose" 4 band state variable EQ's. I want these to be my general purpose studio EQ's.

I am fond of the urei 545 for its simplicity, availability of potentiometers (all 10K), and tuning range, but I have never listened to them.

Whats your favorite design? If you were stuck on a desert island with 1 type of state variable eq, what would it be?  ;D

 

[Timjag]

Midas XL3, even now when I come back to use it it sounds sweat. Not sure why. There are plenty of studio designed ones, but I dunno the 3 just glows. Could be the preamp before it though!

https://groupdiy.com/index.php?action=dlattach;topic=45134.0;attach=8521

Here's the schematic - I haven't looked at it yet, it'll be funny if it's just the standard stuff considering in how loved it is!

 

[Timjag]

Well just looked at it and it doesn't look particularly different to any other. Hmm I wonder why it sounds so good, could it be the sowter transformers on the mic input?

 

[JohnRoberts]

I would probably use one of the half dozen I have designed over the decades.  ;D

JR

 

[Gene Pink]

If you were stuck on a desert island with 1 type of state variable eq, what would it be?  ;D

A battery powered one. ;D

 

[Bo Deadly]

Unfortunately you don't have the luxury of picking whatever design you want because you can't get the parts. In particular, most SVF designs almost always use 100K dual reverse log potentiometers (eg Calrec PQ1549) which are very difficult to find. At least not decent ones that aren't huge or old and crusty.

However, you can adjust things. For example, Audio Maintenance Limited has 47K dual reverse log. So you can adjust resistors and capacitors accordingly and use op amps with a little more drive to compensate maybe.

Actually the MCI JH 600 uses 50K reverse log. And the UREI 565.

 

[iampoor1]

Midas XL3, even now when I come back to use it it sounds sweat. Not sure why. There are plenty of studio designed ones, but I dunno the 3 just glows. Could be the preamp before it though!

https://groupdiy.com/index.php?action=dlattach;topic=45134.0;attach=8521

Here's the schematic - I haven't looked at it yet, it'll be funny if it's just the standard stuff considering in how loved it is!

Interesting. What sounds so good about it? Yeah looks pretty textbook. 

I would probably use one of the half dozen I have designed over the decades.  ;D

JR

;D

Maybe you will have some thoughts on this John....I have been contemplating what makes different state variable filter designs sound different. Part of me thinks that manufacturers may have slightly different potentiometer tapers, thus the control ranges on some eqs may "feel" better. I am also wondering if part of it comes down to ringing/pcb layout. I wonder if certain opamp types are more unstable being used as integrators. Any other ideas, outside of well, the circuits being different on paper? I know these could be completely off base, just trying to think slightly outside of the box. 8) ;D

A battery powered one. ;D

Well if you could wish for ANYTHING what about one that turns sand into power? Gotta think bigger!  ;D

Unfortunately you don't have the luxury of picking whatever design you want because you can't get the parts. In particular, most SVF designs almost always use 100K dual reverse log potentiometers (eg Calrec PQ1549) which are very difficult to find. At least not decent ones that aren't huge or old and crusty.

However, you can adjust things. For example, Audio Maintenance Limited has 47K dual reverse log. So you can adjust resistors and capacitors accordingly and use op amps with a little more drive to compensate maybe.

Actually the MCI JH 600 uses 50K reverse log. And the UREI 565.

Very true. It would be nice if you could slug potentiometers to get the reverse log law you need in this application (I know you cannot).  I am not against redesigning or modifying designs to fit avaliable parts, there are just so many different state variable filter designs I want to build a few and evaluate them first. Hence looking for suggestions, trying to narrow down the lemons from the golden goose.  ;D

The only affordable dual 100k reverse log potentiometer I am aware of  is this one. I have ordered a few but not used them yet.
https://www.digikey.com/product-detail/en/tt-electronics-bi/P092N-FC25CR100K/987-1764-ND/6155636

 

[Bo Deadly]

The only affordable dual 100k reverse log potentiometer I am aware of  is this one. I have ordered a few but not used them yet.
https://www.digikey.com/product-detail/en/tt-electronics-bi/P092N-FC25CR100K/987-1764-ND/6155636

Actually that's not a bad part. Last I checked I could not find this and I'm pretty good at finding parts. It seems like dual and reverse log pots are actually becoming a little more common. That's kind of a big deal actually. Yay.

Those low watt cheap parts are actually not that bad. They're conductive plastic and small. They have a nice action / feel / drag. My only concerns would be taper and how well the two traces track together. The taper is almost certainly more of a two slope as opposed to a nice gradual logarithmic curve which is not ideal but I very much doubt that any other rlog pot wouldn't be similar. Poor tracking could be an issue but it actually doesn't matter with an SVF as much as you think. Probably just throws off the Q a little bit. But if you're doing one-offs you could just buy 10 and sort for tracking / tolerance.

All SVF are pretty much the same. They're not going to sound that different. Not unless you think op amps sound different (I do not). As long as the center frequency, boost / cut and Q are the same, which is entirely determined by various resistors and capacitors, they will sound exactly the same.

Personally I think I would look at the circuit in Figure 10.34 of Small Signal Audio Design by D. Self. I always seem to reach for that book first when I'm not just copying a classic design. It's supposed to be 6dB quieter which is a nice plus seeing as how SVF are not the most transparent type of filter since it's a relatively complex circuit.

 

[john12ax7]

Those BI pots at digikey are good,  but the shafts are really long,  so it looks a bit funny unless you cut them.  If there is interest I might order a batch of 100k dual reverse log and make them available,  as I need them too.

For EQs I like the SSL black,  doesn't have the color of Neve or API,  but they work so well for mixing.

 

[ruairioflaherty]

Is the requirement for tracking or mixing?  Or both?

 

[iampoor1]

Actually that's not a bad part. Last I checked I could not find this and I'm pretty good at finding parts. It seems like dual and reverse log pots are actually becoming a little more common. That's kind of a big deal actually. Yay.

Those low watt cheap parts are actually not that bad. They're conductive plastic and small. They have a nice action / feel / drag. My only concerns would be taper and how well the two traces track together. The taper is almost certainly more of a two slope as opposed to a nice gradual logarithmic curve which is not ideal but I very much doubt that any other rlog pot wouldn't be similar. Poor tracking could be an issue but it actually doesn't matter with an SVF as much as you think. Probably just throws off the Q a little bit. But if you're doing one-offs you could just buy 10 and sort for tracking / tolerance.

All SVF are pretty much the same. They're not going to sound that different. Not unless you think op amps sound different (I do not). As long as the center frequency, boost / cut and Q are the same, which is entirely determined by various resistors and capacitors, they will sound exactly the same.

Personally I think I would look at the circuit in Figure 10.34 of Small Signal Audio Design by D. Self. I always seem to reach for that book first when I'm not just copying a classic design. It's supposed to be 6dB quieter which is a nice plus seeing as how SVF are not the most transparent type of filter since it's a relatively complex circuit.

Yes, I missed them in my initial search too. I think digikey has only recently started stocking them.

I just found mine and played around with them for a little bit. Very nice feel. Feels much better than a 2$ part. As john mentioned the shaft length is a drag.....
I wonder when the last time a "true" reverse log pot was manufactured, or what it would cost today. Actually, maybe ignorance is bliss.  ;D

I think some opamps sound different, but that is due to nonliniarities, and the fact that some opamps are just not great (lm741 hehe). I am not a cork sniffer when it comes to this stuff tho.

Cool, I will check that out, I forgot he had a circuit published. I rather like his HF shelf circuit.

Is the requirement for tracking or mixing?  Or both?

Both, but more on the side of mixing. I rarely track more then 4 channels at a time. I figure I can get more outboard EQ's for tracking if I can get some really nice Mixing eq's. What would your suggestions be if I was focusing on tracking, versus mixing and vice versa? Curious to know!

 

[pucho812]

I used to own a 545.... they sound nice when working...  I sold mine a while ago.

 

[ruairioflaherty]

Both, but more on the side of mixing. I rarely track more then 4 channels at a time. I figure I can get more outboard EQ's for tracking if I can get some really nice Mixing eq's. What would your suggestions be if I was focusing on tracking, versus mixing and vice versa? Curious to know!

I can't imagine using outboard EQs in a modern mixing workflow unless you are working on only your own projects?  (Everyone expects recall these days).  If you are mixing only your own work then congrats!

The best EQ I've used is the Porter, the one I use daily was modded by a friend with simpler input and output stages and great parts.  It's only 4 band with 6 frequencies per band.  Yes an SSL EQ can carve things up like a Christmas turkey and some tones can only be made that way but that's not my tonal preference.  Sontecs, GMLs etc all work fine too but don't excite me. 

Have you seen Bruno2000's console style version of the Porter?

 

[Gold]

The  Barry Porter Net EQ was a result of an early online contest for the best SVF  design. It’s a winner. 

 

[iampoor1]

I used to own a 545.... they sound nice when working...  I sold mine a while ago.

Thanks!

I can't imagine using outboard EQs in a modern mixing workflow unless you are working on only your own projects?  (Everyone expects recall these days).  If you are mixing only your own work then congrats!

The best EQ I've used is the Porter, the one I use daily was modded by a friend with simpler input and output stages and great parts.  It's only 4 band with 6 frequencies per band.  Yes an SSL EQ can carve things up like a Christmas turkey and some tones can only be made that way but that's not my tonal preference.  Sontecs, GMLs etc all work fine too but don't excite me. 

Have you seen Bruno2000's console style version of the Porter?

Basically. My small studio/workshop does some commercial work, but only very simple mixing projects. I do mostly tracking for some clients I have long standing relationships with. The projects I have mixed have been 1 at a time...nice and simple. Plus, more then anything, I like the idea of having outboard eqs because I just work better with them. For the actual projects were recall is important, I will either leave the channels set the same way (hence wanting 16, my ghetto form of recall  ;D) and just patch into some other unused ones. If it becomes a problem in the future, I will just go back to doing everything in the box, no big deal. More then anything, I just like being able to eq projects in 10-15 minutes. I spend FAR too much time fiddling with stuff in the box. Anything super surgical will be done in the box however.  Okay, Ill stop rambling now.

I have seen this thread
https://groupdiy.com/index.php?topic=67084.0
Did he do some schematic modifications minus adding a band? I am thinking of being an unbalanced in/out version of the neteq, with only shelving low/high filters. That would be pretty simple, and if it sounds as good as everyone says, pretty damn hard to beat!

The  Barry Porter Net EQ was a result of an early online contest for the best SVF  design. It’s a winner. 

Thanks for that! This design was created long before I was around here, so thats great to know.

 

[JohnRoberts]

Maybe you will have some thoughts on this John....I have been contemplating what makes different state variable filter designs sound different. Part of me thinks that manufacturers may have slightly different potentiometer tapers, thus the control ranges on some eqs may "feel" better. I am also wondering if part of it comes down to ringing/pcb layout. I wonder if certain opamp types are more unstable being used as integrators. Any other ideas, outside of well, the circuits being different on paper? I know these could be completely off base, just trying to think slightly outside of the box. 8) ;D

In theory with full parameter control of center frequency and Q, they "should" sound the same, but as with everything the details matter.

#1- The SVF describes the BP filter topology but how that gets added to or subtracted from the full band audio signal makes a difference in control range and performance.  The simple add or subtract at unity can deliver infinite cut, but only +6dB of boost, but with very low noise gain (quiet EQs are nice). The symmetrical boost/cut topology involves higher noise gain for large amounts of boost/cut but delivers more conventional control range.

#2- Q/Bandwidth adjustment. There are a number of ways to vary the Q in a SVF and I've seen most of them over the years. One of the slickest that is only available using DPOTs for the frequency control is to stagger or spread the poles apart for wider bandwidth, pretty much impossible to do with conventional pots. The Q can also be varied by the bandpass gain (I've seen this done with a second dual pot section used to scrub of the extra unwanted gain). Q can also be varied by R to ground increasing the noise gain of the first section (HF out).

#3- a subtle third concern is headroom... Different SVF implementations can use other than unity gain for different HP/BP/LP sections. The BP will always be scaled for unity, but a HP or LP with more than unity gain will clip before that BP section. How this matters in use, depends on how much extra gain it has, and the signals it sees. An unused HP or LP section clipping should not affect the BP output, but it always does so this can be a source of unexpected spurious distortion.

#4- frequency control. Besides the DPOTs I mentioned, I have seen this done a number of different ways.  The name "potentiometer" reflects their strength as voltage (potential) dividers. They are not called dual rheostats as you would expect for variable resistances. Pots are best at delivering ratios of the input signals they see. The bulk resistance of pots can be 20% so their absolute value is not great, and adding end limit resistor to set their LF range will not be very repeatable. The most accurate way I have found to implement these to vary resistance is to connect the bottom of the pot to ground so when set for LF it is out of the circuit, then bypass with a large value resistor to set LF. This way you get 5% (or 1%) accuracy for both HF and LF endpoints, with decent tracking in between. IIRC I cap coupled the pots but DC coupled the LF resistance. The caps need to be large to prevent some odd LF instabilities.

#5- tricks, most popular implementations vary the Q with a variable resistance from either the + or - input of the HF output section (ignoring that one oddball that varied Q with BP gain), working again a fixed resistance in the other input. I found by spanning across both the + and - input with a pot using a resistor from wiper to ground delivers a wider range of Q adjustment with lower noise gain. For a trick that is perhaps too tricky for general use, in one parametric design I did, I spanned the Q pot with one end adding the BP feedback R to vary the BP gain, and the other end going to ground  like typical Q pots. This gave me the unique intentional interaction between boost/cut (actually BP gain) and Q. This EQ was designed for Hifi use on wideband material, so intentionally dropped the amount of boost/cut as bandwidth got wider, and increased it for narrow Q. This gave a nice first order loudness correction when EQing full range source material.

JR

PS: The parametric kit I published back in the 70s used the trick boost/cut interaction, but not all the several tricks I mentioned that I picked up in later designs. In fact my professional EQ designs did not have any interaction between Q and boost/cut as you would expect for studio use on individual instruments.

 

[abbey road d enfer]

It would be nice if you could slug potentiometers to get the reverse log law you need in this application (I know you cannot). 

Yes you can! Look at how it's done on the dbx905. Use of Lin pots gives much better repeatability unit-to-unit. I've never looked back.

 

[JohnRoberts]

Yes you can! Look at how it's done on the dbx905. Use of Lin pots gives much better repeatability unit-to-unit. I've never looked back.

As described in my #4 above. 

Resistor tolerance accuracy at both endpoints, mid pot is still subject to poor (20%) bulk resistance of typical potentiometers.

JR

 

[abbey road d enfer]

As described in my #4 above. 

  Yes.

  Resistor tolerance accuracy at both endpoints, mid pot is still subject to poor (20%) bulk resistance of typical potentiometers.

Absolute value is not as significant as it is in rheostat mode. And Lin pots are pretty good at tracking percentage.