Pultec filter section question

[NewYorkDave]

Yeah, I realized on the drive home tonight that my quick guess was wrong. It occurred to me that the "low boost" pot in the MEQ forms part of the series resistance that helps the mid-cut circuit to do its job. Like I said earlier, it was a quick guess and not a careful analysis :wink:

As you discovered, moving the mid-cut section to the end of the chain takes care of the problem since the treble boost and bass cut pots and the 510-ohm and 1K resistors are providing plenty of series resistance to form the upper part of the midrange voltage divider. The overall insertion loss must be increased by the shunting of the mid-cut pot across the EQP filter section output, but hey, just add more gain to taste :guinness:

 

[NewYorkDave]

By the way, here's a conceptual outline of how you could add mid-boost as well. ("Q" controls omitted for clarity).

 

[CJ]

I think we can just parallel a new boost section with different freqs right along side the trebele boost. I think this is the way manly does it. Works here:

Here is a curve showing boosting at two different freqs:

This thing is turning into a mini massive!
:guinness:
Bite off as much as you want to build.

 

[JPrisus]

are you guys familiar at all with the concept of varying the 'drive' to the inductors? I've been told this can be a very musical form of coloration, might be worth looking into. I'm planning to research inductor construction a LOT for this box.

Would be nice to add a stepped output level for the SRPP gain stage as well... but let's take care of the filter section first!


This is gonna be a monster, i'm very excited! :grin:

 

[Guest]

I drew out your circuit in my SPICE simu8lator and can't seem to get a flat response. With pots set to middle I get a 10dB cut at 150Hz, a couple if 1.5dB humps between 1kHz and 6kHz and a 3dB cut at 8kHz.

I could have made an entry error or my program may not be doing something right. I'll double check. I am interested in doing a project like this.

 

[JPrisus]

Thomas, i don't think you'd be looking for flat response with the pots at mid-position, since each pot is either boost or cut specifically. I'm a novice and could be wrong or just misunderstanding your post, this is all very enw to me.

is it full resistance or zero resistance of a pot that would essentially take that section 'out' of the circuit, i.e. flat?

PS - CJ, i'd love to talk on the phone about this circuit and the simulator program, maybe some evening this week? Thanks for the offer, very much appreciated!

 

[Guest]

JPrisus, you know, I didn't think of it like that. I am so used to thinking of boost AND cut that I completely ignored the obvious. Thanks.

 

[ChuckD]

Does anyone have the ability to do an Edit job to the Gyraf passive EQ section PCB so that some of us can give this a go?

Could be a viable new revision to the Gyraf Pultec don't you think?
I would my self but I cannot edit Gerber file format and would want to be able to out source the build to Gustav.

I think a new name for the design should be offered as well.

Anyone for the PCB idea?

-ChuckD

 

[JPrisus]

ChuckD, i'm gonna go P2P for the filter section and use Gustav's PCBs for the gain stage and power supply. Seems to make the most sense, but if there's enough interest for a full PCB, we'll have to make some serious decisions. I know exactly what I want out of this box but it may be different than what everyone else is into. I always liked the sounds of Pultec EQPs but there are some things it doesn't need and other things it lacks IMHO, so this would be like my version of a 'more perfect' Pultec LMAO :thumb: :sam:

 

[JPrisus]

anyone have the HPF schematic from that Pultec filter box? Can't find it anywhere but i don't remember the exact name of that unit. Basically i'm lookin for a variable HPF with Hz choices of 30, 60, and 120 or thereabouts. Would be nice if we could implement this first in the EQP schematic, as it will sonically affect what proceeds it. As i'm getting more and more familiar with the simulator, hopefully with CJ's invaluable help, we can get this planned out this week.

 

[CJ]

Point to point makes sense, if you have a boatload of different brand caps and want to plug and play for the best tone.

OK, we are just getting started. How many freq bands do you want in each section? This will be determined by what you use for rotary switches, 5 band, 11 band, etc.

Boy, lots of simulation if 11 bands everywhere!
Tremaine has some useful tables for component values.
Are you happy with stock 16 db cut/boost, or do you want to be able to screw up the music with 24 db?
Remember, the more makeup required, the more noise.
The circuit will always have interaction between sections even with all the post turned down.
To make room for the added features, we could fix the bandwidth of the treble boost, 1k seems about right for the Q resistor.
Do you want stock Pultec freq ranges, or would a Neve like eq suit you better, ie 220, 440, 880 hertz? Supposedly more musical.

 

[alk509]

[quote author="cjenrick"]Boy, lots of simulation if 11 bands everywhere!

[...]

Do you want stock Pultec freq ranges, or would a Neve like eq suit you better, ie 220, 440, 880 hertz? Supposedly more musical.[/quote]

Actually, using 11-position switches lends itself to doing a 1/3-octave type dealy with 30 frequencies: 25, 31, 40, 50, 62, 80, 100, ............., 10000, 13000, 16000 and 20000 Hz) with each switch (low, mid, high) doing 10 frequencies.

May be completly stupid, but I'm a newbie, so I'm allowed.

Peace,
Al.

 

[JPrisus]

let's see... that Neve idea is nice for the mids, and 16dB should be more than enough... Off the top of my head, this is kinda what seems right to me:


HPF: 30, 60, 120
Low boost: 40, 60, 80, 100, 120, 140
Low-mid cut: 220, 330, 390, 470, 560, 680, 820
Hi-mid cut/(boost?): 1.2k, 1.8k, 2.2k, 2.7k, 3.3k, 3.9k, 4.7k, 5.6k
HF boost: 10k, 12k, 16k

How do those sound? I'm thinkin that with the right inductor values, one could play around with these Hz options and get things right where they want them. I'm definitely planning to experiment a lot before I close the box. Getting the inductors right seems important though.

Any ideas for a midrange inductor source? Might go with the Sowter for the HF.

 

[Joechris]

I guess every body has to cook their own, but this is my humble opinions:
Leave the low cut, low boost and high boost as it is in the pultek.
It works very well. Hi cut could even have a lower frq. to make it a bit more brutal. With the luxery of a mid cut I would like to start at 170 Hz, then walk the neve route up to 1K or so, then make a jump to 4,4 kHz as it can soften up symbals and such. Mid boost could be the 1073 route. Mabye add a hi shelf in parralell with the hi boost, or a switch to bypass the inductor in hiboost. The frq. will change then, but mabye still usefull. Sounds like a dream to me...
who wants to bring out their calculator :grin:

jo

 

[CJ]

an excel spread sheet program would sure save a lot of hassle.
you can calculate the stock reactance values form the original and then just use that as a guide for other combinations.
interesting that Tremaine recomends 150 volt caps in passive filters.
i guess you can actually generate more voltage than you input due to cap and inductor being 180 out of phase.
the voltages add up together.
threr are a bunch of other options for filters such as wave filters, zero insertion loss filters, balanced filters, but I think the stock Pultec setup is fine.
I wissh someone made a variable inductor that was good for audio, kind of cool to have an infinite pot instead of a selector switch.

 

[JPrisus]

Jo, u feel the original low cut would be better left alone than replacing with a dedicated HPF? I'm not against it, just tryin to figure out which is more useful/practical to have.

Starting the mid cut a little lower is cool, like 170 as you said. Having the mid-cut split into two bands is important to me, as i'm often wanting to make gentle cuts in both areas simultaneously. It's also pretty important to cover that 1k-5k area in-depth, since lots of problems occur in there if you're not careful.

By leaving the Hi boost alone, i'm assuming you mean including 3k, 4k, 5k, and 6k in there? I'd kinda prefer to have them in a dedicated hi-mid boost band, like maybe making the hi-mid-cut section boost and cut?

 

[Joechris]

Well, my thoughts was based aroud the layout that sir Cjenrick came up with.
In this design you can not have boost and cut on the same pot, and I dont know if you could have more than one mid cut. The boost works difrently as you can paralell several boost in the front of the circuit. I like it simple..unless you want to turn this into something MASSIVE :grin:
And yes the low cut is great , as it interact with the low boost....but its not a hipass filter..just 6db/octave I think

jo

 

[CJ]

check out the owners manual for the MP.
great reading for ideas on where to go from here:
mp
Also, don't forget you can put two pots on the same shaft if you want.
cj

 

[CJ]

oh boy. a filter program.

anyone have a White Instruments eq schematic?

this looks promising:
http://www.filter-solutions.com/

 

[CJ]

Awesome!
http://www.filter-solutions.com/
this program does everything but wash the dog and take out the trash!!!
And, it is easy to use!
You can enter source and load resistance, slope in db per octave, frequency that you want the filter to work at, high pass, low pass, passband, bessel, butterworth, gausian, whatever, and it spits out a bunch of different circuits with all the values!
then you can plot everything.
, if your into designing filters, grab this program!
:guinness: :guinness: :guinness: :guinness:


cj
jeez! impulse response, zero coef,.reflection coef's,.... amazing.
but only a 20 day trial. might have to buy this bad boy.

transfer function...