Pultec EQP-1A stock inductor values and HF response curves

[AusTex64]

Does anyone know the coil resistances of the stock Pultec EQP-1A inductor they could share with me?  Gyraf?  CJ?

Also, does anyone have measured HF boost curves with bandwidth at narrow for a real Pultec? I am starting to wonder if even the real thing is dead on.

I built two Drip Pultecs using the Vintage Windings inductors. The frequency centers at 10K, 12K and 16K are low, and the unit exhibits a gradual 10dB rise from 20kHz to 50kHz. With no filters engaged the line amp is flat out to 50kHz! I may have figured out why. Drip suggests using Sowter transformers and inductor.

Here are the specs for the Sowter inductor:
http://www.sowter.co.uk/eqinductors.php

And here are the specs for the Vintage Windings inductor: http://vintagewindings.com/products/VintageWindings%20EQ-1A.html

Note the resistances listed for the VW inductor are twice that of the Sowter.  I don't have a firm understanding of the implications of this. Kinda going nuts trying to figure this out, and don't want to change inductors unless I have to - cost, time, hassle.

Any help will be greatly appreciated!

 

[rainton]

Here's a screenshot from that part of the original filter schematics of the EQP-1A

As far as I know Chris with vintage windings follows the original specifications with his inductors - even uses the same core material as the original inductors.

But I also noticed the frequencies were a little off when building my p2p pultec EQP-1A.

I ended up  measuring the frequency response while playing with different caps and cap values until I had a close to specs response and the sweetest sound

I didn't experience the 10db rise above 20k though

What output transformer did you use?

 

[ruffrecords]

Here's a screenshot from that part of the original filter schematics of the EQP-1A

As far as I know Chris with vintage windings follows the original specifications with his inductors - even uses the same core material as the original inductors.

But I also noticed the frequencies were a little off when building my p2p pultec EQP-1A.

How close do you expect them to be? The calculated 10KHz frequency is 9675Hz from the schematic you posted.  With 5% tolerance components the measured centre frequency could range from 9191Hz to  10158Hz -basically anywhere between 9KHz and 10KHz.

Cheers

Ian

 

[AusTex64]

I used Sowter input, output and interstage transformers in my build.

If I understand correctly, the impedance of the coil should only affect the Q of the filter, not the frequency centers. So the Sowter inductor with lower impedances shound have a higher Q than the Vintage Windings inductor.

I found this thread, which taught me a lot about this inductor. http://groupdiy.com/index.php?topic=25482.0

Still can't account for the rise over 20K though. Where else could it be coming from but the inductor? Seems there is not much else in the HF filter circuit that would cause it.

 

[AusTex64]

Here is a HF filter plot for the Cartec EQP-1A. This is very close to what I'm seeing, which leads me to believe maybe this is really what the HF curves look like. So far I have not been able to find any measured curves for an original EQP-1A. I've seen the ones in the original catalog, but those look like what they calculated them to be.

http://dt7v1i9vyp3mf.cloudfront.net/styles/news_large/s3/imagelibrary/c/cartec_eqp1a_04_HFboostbroadnarrow.jpg?uGV_VGAZE24BV0vjZ5EINeC2J2W4F_Pt=&itok=pmqfp4vQ

 

[JW]

Just came across the Vintage Windings page mentioed earlier: http://vintagewindings.com/products/VintageWindings%20EQ-1A.html

Hmmmmm. Just bought two Carnhill inductors with exactly the inductances that he states are wrong.

I guess I'll be stacking caps. Is that what you ended up doing? Just adjusting caps until you get close enough?  Anybody have a source for some good sounding polystyrene caps?

 

[ruffrecords]

Just came across the Vintage Windings page mentioed earlier: http://vintagewindings.com/products/VintageWindings%20EQ-1A.html

Hmmmmm. Just bought two Carnhill inductors with exactly the inductances that he states are wrong.

I guess I'll be stacking caps. Is that what you ended up doing? Just adjusting caps until you get close enough?  Anybody have a source for some good sounding polystyrene caps?

Everything he says is true but not necessarily correct or important. The Q is determined by the total circuit resistance which includes the inductor resistance. However, the inductor resistance is less than 10% of the the total circuit resistance so whether it is 30, 40, or 50 ohms makes very little difference to the Q. The circuit resistance includes the pots whose values have a 10% or even 20% tolerance. These therefore have a much greater effect on the achieved Q than does the inductor resistance.

Even if the inductors are dead on the calculated value for the given capacitance, the capacitors themselves have a tolerance so your frequency will always be out unless you take the time to measure and select capacitors that are dead on.

Cheers

Ian