Have you built a Drip EQP-1A?

[AusTex64]

I built two, using Sowter input, output and interstage transformers and the Vintage Windings Pultec inductor (www.vintagewindings.com). I measured the frequency centers of the EQ and discovered that on 10K, 12K and 16K, when boost is at full and bandwidth at sharp(narrow), they are not right. 10K measures about 8.5K, 10K measures about 9K and 16K measures about 13K. Before I buy different inductors, I would really appreciate someone measuring their unit built with all Sowter transformers and inductors, as that is what Drip recommends. I need to confirm that the EQ actually performs as it should before acting further. I assume Greg at Drip did this, but I've asked and not received an answer yet.

Thank you in advance for any help on this!

 

[AusTex64]

No one? C'mon, help a brother out. Just feed it white noise, look at a spectrum analyzer plugin, put the boost to full and bandwidth to narrow, and step through the high frequencies. Just need to know if the centers match the front panel - 10K, 12K and 16K. Pretty please? Best case, it will line up. Worst case, you'll find the same problem I have. Thanks!

 

[gato]

I haven't built a Drip Eqp1a. I assume, however, that the filter section is more or less the same as the original. It sounds like your inductances are consistently high.  Let's say your capacitors are 20% tolerance so the 12k cap is 6.6nF, then your inductor would be around 44mH. Often inductors have a 10% +/- tolerance, but that is more like a 25% difference. Since caps are cheaper I would just select for lower value caps throughout the filter section rather than switching inductors.

 

[AusTex64]

Gato, thanks for your reply. I understand the Drip Pultec filter section is supposed to be identical to the original. I'm going to go through it tonight and confirm the cap values. At this point I suspect the inductor is the problem. Another member of the forum pointed the problem out to me with the Vintage Windings inductor in his PTP build, asking me to check mine to find out if I saw the problem. I did and found the issue I've outlined here.

In addition, I also measure a gradual 10dB rise from 20kHz to 50kHz when boost is at full. With the filter section bypassed the line amp is amazingly flat out to 50kHz. That indicates a ringing inductor to me.

I still would like to confirm that the Drip Pultec, when built with all Sowter transformers and inductor, performs as it should.

 

[AusTex64]

Found this plot of the Cartec clone. These curves look almost identical to what I am measuring on my Drip Pultecs. So is this what real Pultec HF boost curves look like?

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

 

[gato]

Even if the inductances are high you can compensate for that with the caps. With pretty wide tolerances I imagine that the originals either sorted for correct parts (probably by changing cap values) or were simply off by whatever the tolerances allowed for. In terms of the schematic the math shows that the frequencies are correct for the values of the components.

 

[AusTex64]

Gato,

Once I manage to get rid of the 20K to 50K rise in HF, I'm going to tune the filters with different caps to get the freq center closer to spec.

The HF rise is proving very difficult to get rid of. It only happens with HF EQ boost. Recovery amp is flat to 50K with no EQ engaged. I changed the Zobel values on the interstage transformer to get rid of the rise, but when the boost is dialed out or EQ is bypassed, everything over 10K drops drastically. I have some different inductors on the way to try, hoping it's a ringing inductor. But I've been advised that's not possible, so don't really know what to do next.

 

[emrr]

Here's another to study. 

http://groupdiy.com/index.php?topic=48328.msg703839#msg703839

Note the link to MEQ responses in a link there. 

Seems to me you have to tailor caps to inductor, no multi-tap inductor is going to drop in to a particular set of caps and line up.  I doubt real Pultecs ever matched the panel markings either. 

 

[AusTex64]

Thanks Doug. Those plots are very interesting. Looks like I'll be buying a selection of caps and dialing it in.

The HF rise over 20K is the problem to solve now. Just can't figure out where it's coming from.