pultec lo cut problem

i'm hoping someone can help me. i've point to point wired two different pultec peq type boxes over the last year, both with the same problem. the slighest turn of the lo cut kills the the signal. i did not use a tube amp stage for the eq's, instead i used an api opamp for one of them and for the other a neve ba283 pre circuit. i used both of the schematics on the gyraf site, (one of each). any and all help is greatly appreciated.

joe

Is the signal gone completely? or just the low end?

Peace,
Al.

the signal is not gone completely but it is quite drastic. it is also more than just the low end. when i increase the lo boost the level recovers.

joe

Get a highlighter pen and go like this:



Your friends will be amazed that you have turned into a troubleshooting genius!
:razz:

If this does not work, I will send you my Pultec. :thumb:

i've done that and i'm pretty sure i'm ok. i am suspicious of the impedance at the input of the amp stage. i think the neve circuit input is 120k. i've done some simulations that i can post the results of when i figure out how to upload an image. :?

joe

[quote author="monumental"]i've done that and i'm pretty sure i'm ok. i am suspicious of the impedance at the input of the amp stage. i think the neve circuit input is 120k. i've done some simulations that i can post the results of when i figure out how to upload an image. :?

joe[/quote]

Joe,
I'd love to see ehat your simulation shows. I don't think that loading is the issue, the primary of the original interstage transformer used in the pultec was only 2.5k.

Geoff Tanner, former neve designer, said this:

http://www.auroraaudio.net/dcforum/DCForumID1/55.html


>Hi, Geoff,
>
>Can you tell me what the input impedance is of pin L on a
>BA283 card? I'm wanting to use a 283AM for gain make-up for
>a Pultec-type passive eq I'm DIY'ing.

Hi! I'm not in the office at the moment but, from memory, believe that it's up around 68Kohm... it's certainly high enough to bridge any circuit you hang it on!

>Ideally, the output of the passive filter section would
>prefer to see about 10kohm. As I currently have it, I'm
>going straight from the filter circuitry into pin L. If pin
>L is not close to this, what would be the best way to
>implement this output stage? Would I then want to use a
>31267 as a step-down interstage transformer and use a
>fully-populated 283AV?

No, I don't think that you need all that extra complication... just whack the signal straight into it and adjust the input level and/or gain to suit. An "un-dabbled" 283 output has 11dB gain with another 4dB in the output transformer.

I'm still working on adding images to my posts but I think this will work for now. The schematic I used for simulation can be found here:

http://groupdiy.twin-x.com/displayimage.php?pid=67&fullsize=1

and the output graph with the lo cut (VR4) set at 0%, 10%, 20%, 30%, 50% and 100% is here:

http://groupdiy.twin-x.com/displayimage.php?pid=68&fullsize=1


I think the lo cut freq should be 20hz with a 100n cap (C1).

Joe