Pultec EQP-1A Turnover Frequencies

I have been analysing the turnover frequencies used in the Pultec EQP-1A and also the attenuation/boost at each set frequency. I looked at the frequency at which the boost/cut was 3dB and the actual attenuation at the set frequency. For the low cut we have:



The turnover frequency is an amazing 13.5 times the set frequency at which the cut is 17 to 18dB.

For the low boost we have:


The turnover frequency is about 6 times the set frequency at which the boost is 12 to 13dB.

And for the high cut we have:


The turnover frequency is about 7.5 times the set frequency at which the cut is 16dB.

The low boost and high cut seem to have the sort of turnover to set frequency ratio you would expect from a passive EQ but I am surprised by how high this ratio is for the low boost.

Thoughts?

Cheers

Ian

Is there a reason why they give the frequencies such misleading names? Is it just that back then they didn't have the standard '3dB down at X frequency' we have today?

Is it perhaps that the labelled frequency is where the shelf bottoms out?

I found some interesting Pultec frequency response graphs from UA Audio that may be relevant:

http://www.uaudio.com/webzine/2004/july/text/content4.html

According to those plots, the 100 hz cut is within about 3dB of its maximum at about... 100Hz. Could that be the standard they were going by?

I've added more questions than answers here, sorry!

Hi Mike,

Thanks for the reply and especially for the link. I think you are right that their criteria for labelling the frequencies is not the 3dB point but more like where you get within 3dB of the maximum attenuation.

The graph of low cut and boost combined in the link you provided I found especially interesting and I think it explains why the turnover frequencies for boost and cut are so different. Notice from my tables that the low cut turnover is at a much higher frequency than the low boost - in fact they are more than an octave apart. So the low cut is bound to start acting first and if you set the boost level greater than the cut level you get the famous Pultec bump. If the boost and cut turnovers were identical this effect would not be produced. Was that inspired design, pure luck or a design oversight?? -I don't know.

Cheers

Ian

MikeClev said:

Is there a reason why they give the frequencies such misleading names? Is it just that back then they didn't have the standard '3dB down at X frequency' we have today?

Click to expand...

There's nothing like a "standard 3dB down" in equalizers. Shelving EQ's have always been characterised by the frequency where the shelf bottoms out or tops up +/- something. A typical baxendall type EQ quoted at 15dB boost/cut @100Hz and 10kHz actually has its turnover (3dB) frequencies at roughly 500Hz and 2kHz, will go asymptotically to +/-18dB (at 50Hz and 20kHz).

Ian,
It's a little surprising.
I don't have an EQP1, but I have sim'd it and I see that the bass boost and cut are reasonably symmetrical for the 100Hz position, but not for the 30Hz position.
The EQP1 operates, as I'm sure you know on the basis of  the interaction of two RC circuits in series. The RC values are such that the base signal is attenuated  by 22dB. One RC determines the boost turnover, the other the cut turnover. The correct ratio for reciprocity is 1:12, which is verified for the higher position, with 10nF and 128nF, but for the lower, the ratio is much higher 36.2nF:1uF, about 1:27. I guess we'll never know why...
I don't think the EQP1A is much different in this respect.

Here's the Altec 9061/Langevin 251, looking at the 40 Hz and 100 Hz boosts. 

Thanks for that Doug. Very similar to the Pultec with the turnover frequencies in the 250 to 1000Hz region.

Cheers

Ian