Question About Pultec EQP-1A Schematics Differences

[vintage-mike]

I've been researching some information about the Pultec EQP-1A - I noticed that the Gyraf audio schematic has a 1K in series with the LF attenuate control, but the LF frequency selection capacitors only bypass the control itself (and do not include the 1K resistor also). In other schematics, the LF frequency selection capacitors bypass the control and the 1K in series with it. Does anyone know which versions is 'correct'?

It would seem that in the Gyraf (G-Pultec) configuration, this would mean the HF boost Q would be limited by the 2K2 bandwidth pot at low Q settings, and limited by this 1K resistor at high Q. Also providing an essentially constant Q with HF boost frequency selection and better isolation between the controls.

In the alternative configuration, the HF Boost Q would be limited by the 2K2 bandwidth pot at low settings, and by the parallel combination of the 1K and selected LF frequency capacitor at high Q. Resulting in a max HF Boost Q which varies, dependent on the HF frequency selection (getting sharper for higher frequencies) but also dependent on the LF frequency selection. (All other boost / cut controls being set to minimum)

I've seen measured graphs from units that appear to show both characteristics, If my understanding is correct I would assume that the former (G-Pultec) EQ behaviour is desirable because it gives better control isolation, which one would expect in e.g. a more modern EQ, however it seems that other (and some 'official' units do the opposite and have much greater control interaction. So, does anyone know which is 'correct'. Is there an error in the G-Pultec schematic, or is there in fact an error in the real Pultec schematics (that has now become imbued with some 'magic mojo' as a result? )

 

[ruffrecords]

I do not think either is 'correct' they are simply different design decisions. There is a;lways a degree of interaction between bands in a passive EQ like this which is one reason the LF and HF bands are kept so far apart.

Cheers

Ian

 

[dmp]

Does anyone know which versions is 'correct'?

The Gyraf version is derived from the tubetech (see the schematic at the top of the page)
It is not intended as an exact eqp-1 clone.
The Eqp-1 manual has the schematic for the actual Pultec filter section.

 

[vintage-mike]

The Gyraf version is derived from the tubetech (see the schematic at the top of the page)
It is not intended as an exact eqp-1 clone.
The Eqp-1 manual has the schematic for the actual Pultec filter section.

Thanks for all the replies - yes I have the original Pultec schematics too, my problem with that was that its yet another version (which doesn't have the 1K resistor I mentioned at all, and only has five HF bands instead of the seven on all the Pultec EQP-1s I've seen, and only has two inductor taps as opposed to four or even five in other designs) so perhaps its an early incarnation.

I agree all passive EQs will have some control interaction - in this case I was puzzled by the differences I was seeing between published measured specs for two different 'authentic' re-issues, one which has essentially constant Q for the high boost, irrespective of the high boost frequency selected, and another which has noticeably sharper HF boost at the 16kHz setting than at the lower frequency settings e.g. 3kHz, 4kHz etc. (I'm assuming all other controls are set to minimum). The effect being most noticeable for the 'sharp' bandwidth setting.

I'm still a little confused, It seems like every time I find another piece of the puzzle I know less than I did before (I also have some software - VST Plug-in - emulations, all of which behave differently from one another, one that just doesn't work at all and none of which completely match any of the measured hardware performance)

 

[gyraf]

..take care not to interpret any "magic" into any particular implementation: Contrary to the massive marketing(s) of this eq type, there is no magic ingredient that makes or breaks the deal - no matter how much the idea of esoteric parts (often different for each new re-issuer) has been hyped through the years.

The simple answer is that passive eq - boost-eq'ing by removing all things not-boosted, then simply not removing in the boost area - just sounds "right" to our ears, most probably because of absence of frequency-versus-gain-related nonlinear residues (distortion) that "gives away" what you're doing

even with rough-wound iron inductors and a simple opamp gain stage the pultec-type filter sounds right out excellent