Pultec EQ - High Shelf possible?
« on: April 27, 2020, 08:31:18 PM »
I'm working on a unit which has a stereo pultec and I'm attempting to match it's freq and response to a Bax which I prefer. 

The Low boost seems possible to get close if I can find the right caps...

The High Boost is more difficult due to the peak filter...I was wondering if it might be possible to tweak the circuit and either make the peak very broad (more than stock 0 Q) or turn it into shelving like the Lows...?


squarewave

Re: Pultec EQ - High Shelf possible?
« Reply #1 on: April 27, 2020, 10:18:39 PM »
Not really. The high boost bandwidth control is very simple on the EQP-1. It just dampens the resonant circuit. You could use 10K instead of 2K2. That would broaden the boost quite a bit. But it would also reduce the boost to about 6 dB instead of the usual 12.

The EQP1 is a rather complex circuit actually. I'm surprised it's so popular. You really have to have the curves memorized to use it basically. If you want a simple BAX, you should just make one. It would be way simpler and easier to understand.

john12ax7

Re: Pultec EQ - High Shelf possible?
« Reply #2 on: April 27, 2020, 10:22:30 PM »
Get rid of the inductor and you will then have a high frequency shelving boost.

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #3 on: April 28, 2020, 05:21:49 AM »
agree.

And then put in a handful of mid cuts on the high-cut-frequency dial, and you have a really usable eq

/Jakob E.
..note to self: don't let Harman run your company..

ruffrecords

Re: Pultec EQ - High Shelf possible?
« Reply #4 on: April 28, 2020, 05:32:53 AM »
As others have said, leave out the inductor and you get shelving response. This is what I did in my very first DIY project, the poor man's pultec eqp1. Check out the DIY tab of my web site for circuit details. There is also several threads here on the same topic.

http://www.customtubeconsoles.com/diy

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Pultec EQ - High Shelf possible?
« Reply #5 on: April 28, 2020, 08:14:51 AM »
As others have said, leave out the inductor and you get shelving response. This is what I did in my very first DIY project, the poor man's pultec eqp1. Check out the DIY tab of my web site for circuit details. There is also several threads here on the same topic.

http://www.customtubeconsoles.com/diy

Cheers

Ian

Hi All thank you very much for the suggestions, great news that omitting the Inductor can have this effect

Ian - I am looking at your 'Poor Mans EQP1' - great project! I notice your frequency charts - it appears you are calculating the freq based upon the top of the eq slope as soon as it is flat - is that correct?

One thing I never saw anyone comment about with the Pultec - the strange frequencies from the perspective of the mixer. For instance - the 20Hz Pultec boost sounds absurdly low as it is labelled, when in fact it can be matched with a standard analogue style shelving boost around 250Hz. In fact the 60, 30 and 20Hz boost I find incredibly 'boxy' in use without using the bass attenuation.  At which point you are also scooping the low mids. I appreciate that probably due to the broad shelving response 30Hz is somehow technically correct, but it seems a bit counterintuitive from the mix engineers perspective to have such broad and higher Bass boosts which are labelled very low. Apparently people do use it on the mixbus but currently it is not to my tastes...

Ian your frequencies and the way they are presented make a lot more sense to me. 
« Last Edit: April 28, 2020, 08:24:29 AM by SWAN808 »

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #6 on: April 28, 2020, 08:28:06 AM »
..there's no real consensus as of how to mark frequencies of shelving eq's..

/Jakob E.
..note to self: don't let Harman run your company..

Re: Pultec EQ - High Shelf possible?
« Reply #7 on: April 28, 2020, 03:25:56 PM »
..there's no real consensus as of how to mark frequencies of shelving eq's..

/Jakob E.

thats good because I wasn't planning on labelling at 10Hz if I end up technically using that 'Pultec' frequency...but I thought it was at the flat point of the shelf at max boost?

john12ax7

Re: Pultec EQ - High Shelf possible?
« Reply #8 on: April 28, 2020, 04:37:18 PM »
thats good because I wasn't planning on labelling at 10Hz if I end up technically using that 'Pultec' frequency...but I thought it was at the flat point of the shelf at max boost?

What spec do you use to determine when it becomes flat? Even that is open to interpretation.  What a lot of people do is define it at "X" dB down when at max boost.  But there is no consistency in what people use for X.

The pultec labeled frequency is a few dB down at max boost.  And for the lows the cut and boost aren't even the same frequency,  which is also part of its charm.

ruffrecords

Re: Pultec EQ - High Shelf possible?
« Reply #9 on: April 28, 2020, 05:03:33 PM »
As has been mentioned, there is no accepted definition of the 'frequency' of a shelving EQ. If you look at the 100Hz cut curve of the picture below, which is taken directly from the Pultec manual, you will see that at maximum attenuation, 100Hz is 3dB before the flat part of the curve.



All shelving EQs have two 'turnover' frequencies. The first is where the response starts to change and the second is where it starts to flatten out again. As you can see from the Pultec curves, these frequencies are about a decade apart. The frequency where the response starts to change is fairly constant but there is no point labelling this as the frequency of the EQ  because it is far away from the frequencies at which any audible effect occurs. However, the frequency at which the curve begins to flatten out varies a lot with the amount of boost or cut selected, indeed it is by varying this turnover frequency the  the EQ operates.  So which of these many frequencies should you use to label the EQ?  Pultec seems to have chosen to use the frequency where the boost/cut is 3dB from the maximum when the boost/cut is also set to maximum. In my version I decided to do the same.

If you compare the Pultec low shelving and boosting curves with this in mind, you will see that the turnover frequencies for a given labelled EQ frequency differ between boost and cut. This is deliberate because the boost and cut have separate controls. If they both operated at the same turnover frequencies they would effectively cancel each other out. Instead Pultec separated the frequencies with the result that if you apply both boost and cut you get the well known Pultec bump.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Re: Pultec EQ - High Shelf possible?
« Reply #10 on: April 30, 2020, 08:27:04 AM »
I have a 2k linear pot connected to the 47mH inductor for the High Q - is it possible to have a wider Q than stock Pultec High Boost Q? I'm wondering if this could be a middle ground maintaining some peak characteristics...

Ian your Poor mans pultec project as well as the universal EQ are some great resources thank you for making them available

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #11 on: April 30, 2020, 09:49:48 AM »
yes, you can put in a higher value pot to widen the Q - but note that you already get significantly smaller boost range at lower Q's

/Jakob E.
..note to self: don't let Harman run your company..

squarewave

Re: Pultec EQ - High Shelf possible?
« Reply #12 on: April 30, 2020, 11:16:20 AM »
Actually the proper way to lower the Q without messing with the "Q" control (which is really just a crude damping) is to decrease the size of the inductor and increase the size of the cap by the same amount. So for example 56nF with 47mH would give you an Fc of 3KHz. But if you do like 200nF with 15mH you still get 3KHz but with a much wider Q and no change in level.

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #13 on: April 30, 2020, 11:58:34 AM »
Agree. But this would be a very different way than in the original circuit..

/Jakob E.
..note to self: don't let Harman run your company..

Re: Pultec EQ - High Shelf possible?
« Reply #14 on: April 30, 2020, 12:10:35 PM »
yes, you can put in a higher value pot to widen the Q - but note that you already get significantly smaller boost range at lower Q's

/Jakob E.

thanks thats great...the boost range is not a worry seeing that there is ample and for mix bus usage mainly...I can swap out for a 2.2k, 5k, 10k and 22k - even up to 500k...any thoughts on the range or should I just try it...?

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #15 on: April 30, 2020, 01:08:01 PM »
try and see? perhaps plot response for comparison?
..note to self: don't let Harman run your company..

Re: Pultec EQ - High Shelf possible?
« Reply #16 on: April 30, 2020, 01:50:26 PM »
try and see? perhaps plot response for comparison?

ok will do  8)

Re: Pultec EQ - High Shelf possible?
« Reply #17 on: May 21, 2020, 12:59:28 PM »
I tested the 10k pot in place of the 2k on the High Q - I like the broader effect but as predicted above it limits the wide Q boost to about 6db. This is fine for mix bus however when the Q is adjusted the boost can be absolutely massive...I demonstrated it in the video link below (looks like I can't embed in here...)...using the plugindoctor app in hardware mode...in case it is interesting to anyone...

https://youtu.be/Wl4_wG5zwBc

I think next job is to test without inductors-because I'm not sure a Q adjustment is useful anyway....only question is how to work out adjusted cap values...

gyraf

Re: Pultec EQ - High Shelf possible?
« Reply #18 on: May 22, 2020, 04:48:21 AM »
I'm not sure a Q adjustment is useful anyway....only question is how to work out adjusted cap values...

Q control does not make sense in a first-order shelving filter, only works in conjunction with the inductor.

For working out cap sizes, simply try a handful of what you have and select the ones to your liking. Note that doubling the value (in nF) results in halving of frequency, nothing complicated like the LC calculations..

/Jakob E.
..note to self: don't let Harman run your company..


 

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