Pultec EQ and (bad) frequency response ???

[matthias]

hi,

while I was running some tests with my pultec eqs, I found out,
that there's a dip of 1db around 5khz in the frequency response

pultec=white
channel without pultec=green

that's the schematic I used:

I ommited the 1k resistor at the output (thanks Chris, for pointing that out)


after a lot of tests, checking for shorts and measuring of components,
there's something unclear for me...

the hicut pot hast to be 1k, to let the audio pass uncuttet
the resistance is exactly 1k in my circuit, but if I measure it, I get 910R
so there is something in parallel... I don't know if that's normal...

I traced it back and it's the 10k resistor, that is in parallel...

but I don't know if that's an error and responsible for thar problem??

beside that small problem, everything is fine and works like it should...


I hope you have an idea of what could be wrong ...

thanks,
mat


____________________________________________________________



for testing I didn't use a transformer at the input or output, just an unblanced insert cable... for makeup I use the channel amp of my soundcraft ghost mixer.

@Chris: I also tried it with the 1k resistor, but that acted in a deeper cut and something like a hishelf at 20khz ?? no cut in the bass section... strange...

 

[CJ]

1k and 10k in parallel is about 910.

I think that the Pultec will never run completely flat if it is keyed in. That's why they have the bypass switch. As soon as you turn a pot to eq something, that 1 db dip won't mean much.

 

[matthias]

I was really wondering, because without that dip, everthing is very, very close...

do you think every pultec eq has the same kind of freq response??

If I'd try to simulate the circuit ... do you think it will be the same??


mat

 

[cayocosta]

I ran some sweeps on mine after it was completed and it was flat. Also the boost and cut response was accurate.

I used the same filter schematic, but with the original tube circuit and sowter transformers.

 

[CJ]

I had pretty good luck with the simulator.

Are you seeing this 1 db dip after going thru the line amp, or are you completely by passing the unit?

Ohm out your pots in the off position to make sure they really read zero ohms. I have seen pots before that don't go all the way to zero when there turned off.

 

[matthias]

thanks for the help....!!!

I just tested my second pultec and the result is exactly the same...

I tested the amp of my mixer at lower amplification...
but it doesn't affect the result...

I didn't use pots, but switches, so 0ohm is really 0 ohm


what software do you use to simulate ?? p-spice ??

thanks,
mat

 

[CJ]

I used this free stuff:

SIMetrix:

http://www.catena.uk.com/

You can use this as a guide to your simulator schematic. Just eliminate all the extra filters:

http://vacuumbrain.com/The_Lab/TA/Pultec/eqp_reactance.jpg

 

[cayocosta]

Matthias,

Here's my layout of the circuit including the filter:

http://home.earthlink.net/~cayocosta/diy_files/eqp_1a/eqp-1a_layout_final.jpg

 

[matthias]

I GOT IT !!!!

one of my channels of my ghost mixer caused the small dip in the
frequency range... the channel amp itself is ok...

final result: my pultec is ok...!!

I made some other tests this day...

here's a frequency response with transformer and without transformer...

I think the oep A262A7E fits very well here... don't you think??

cheers,
mat

 

[Michael Krusch]

Once I had the same problem. Software was drawing a problem but there was'nt a problem. :grin: