PTP Pultec all frequencies a bit low

I have a PTP Pultec EQP1A that i built a few years ago. I have been using it with great results since.
I don't have any real audio testing software, so most of my testing is done in my DAW using a signal generator and Blue Cats free spectrum analyzer.

Out of curiousity I decided to look at the frequencies a couple of days ago. The low end was difficult to scope with my limited means, but i was able to look at the high end frequencies with decent clarity. What i found was that all of the frequencies were lower then they should be.

I am using the Cinemag pultec style inductor, which has 150, 82, 68, 47, 33, and 27mH. I have married those with the appropriate caps as well. Assuming the reactance charts are right, i should have pretty close to the schematic--3k, 4k, 5k 8k, 10k, 12k, and 18k
All of my frequencies are a bit low--more in the line of  2k, 3k, 4k, 6k, 8k, 10k. and 15k.

Has anyone else noticed this with the cinemag inductor? I'm curious if this is something that has popped up before. I didn't see anything specific to this inductor. The caps are all in spec, and the frequencies are consistent just lower

Interesting!
Ian

im no expert by no means but have you measured the caps with an lcr? when i got mine and randomly tested caps almost all modern ones were too low. only the old styroflex measured the values that were written on them. even good quality caps like modern siemens mkt measured too low. like 20% in lots of cases. as said, im no expert or maybe my lcr is bad....

Lots of threads about Pultec variance.  One within the last 60 days. 

Ive read the threads i think you are referring to. My question was in regard to anyone using the Cinemag inductor, and if they had found similar results. The other threads did not address that.
I'm gonna look at some different cap options and see if i can get closer there

Doing some plug and play with the caps, I am getting my 3k and 4k dead on with .082(.084) , and my 5k with .068(.07)
Got to get back to recording, but will try swapping some other caps in tomorrow for the other high frequencies

For reference, i had a typo with my caps, missed a zero:
3k/4k 8.2nF, 5k 6.8nF, 8k/10k 3.3nF 12k 2.2nF and 18k 1.5nF

my actual center frequencies are:
3k, 4k,4.9k, 8.2k,10k, 12.1k, 15.7k

The caps were relatively accurate. I guess the inductance on the cinemag is skewed high? I don't have a way to measure it.
Definitely happy with the measurements, and glad i did it.
Now to find some caps. These are random spares i had laying around

Doesn't matter who's inductor you use, tolerances insist on individual tuning.  You can't tweak the inductor, so it has to be the caps. 

Yes, agreed. I guess i was surprised at how off the values were. The caps i tested were relatively close to their spec, and didn't account, at least to my math, to the fequencies being so far off. It seems, unless i am missing something, that the inductance was out of spec

Seasonal temp changes will make inductors measure differently too.  Possible all Pultecs swim around. 

Wow! Did not know that..So that would explain why all the old soul music sounded better in the summer?

I've spent about two months closely studying the Pultec EQP-1A circuit, installing and measuring inductors and transformers from Sowter, Vintage Windings, Carnhill, Cinemag, UTC and Triad, messing with Zobel values, etc. Also measured a Pulse Techniques unit, which is as close as I could find to an original. Learned a lot about this seemingly simple circuit.

With regard to the frequency centers on HF boost, the only conclusive way to get to the bottom of this is to measure the inductance of all the taps on the Cinemag inductor to confirm their values, along with all the caps.  I've measured units using the Vintage Windings and Carnhill inductors, along with the Pulse unit. Unfortunately for you I did not check out the Cinemag inductor. At 16K, the freq center was low on all three inductors (VW, Cinemag, and Pulse). I think the stated frequency centers on EQP- 1A are approximate, especially 8K, 10K, 12K and 16K. The ones where they use a single inductor tap with two different caps. What I found odd about your description was that 3K, 4K, and 5K were low. These were dead on with all three units I tested. Hmm.

Having said all that, the freq centers you listed are all quite within the ballpark of what I've seen, and I would say are pretty close to spec. As close as you're gonna get with standard cap values. As noted, tolerance on caps can be -20%, so you could get a bunch of caps and find the ones closest to spec. If you're dead set on the freq centers being spot on, swapping caps as you're doing (as tedious as it is) is the correct path to take.

I have rediscovered why some pieces of gear cost what they do - it takes a LOT of time and work to dial some of this stuff in. DIY looks real good until one sees they have 40 hours into chasing some problems....

Good info! Nice to compare notes with someone who has dived so deep!
I am a former austinite who is often there!
I still find it odd that my cap values are so far off from the schematic. This seems beyond any 20% spec. The caps are running less then half of the original values..
I went ahead and ordered some orange drop 715s for those frequencies.

Hello from Austin!

I measured a Warm Audio Pultec style EQ on Friday that uses the Cinemag inductor. I didn't measure inductance of the taps nor the cap values, but will report several of the freq centers apoeared to be low. But again, not so far out of spec compared to what I've seen with other units.

Here's something to check out on your build. Select 16K, bandwidth sharp (narrow) and boost at full. Does your analyzer show freq response to 50kHz? If so, do you see a gradual HF rise starting around 30kHz extending to 50kHz? I'm betting you will. I saw it on my Drip build and on the Warm Audio unit too. I think the Pultec inductor wants to ring. I have not looked at an EQP-1A inductor yet that did not ring.

What interstage transformer is in your build?

I don't think my frequency analyzer goes up to 50k.
I'll check on what interstage I used tomorrow.
Now I'm curious to know if my getting my frequencies to "spec" is actually pulling them past the real pultec frequencies. I know from usage that there are a couple of sweet spots in those high boosts.
I haven't seen any measured results from old units. What info so you have?

If you're audio interface samples to 96K, you should be able to see out to 48kHz.

I've not had the opportunity to measure an original EQP-1A. Could not locate one here in Austin. Many people feel the reissue Pulse Techniques units are indistinguishable from the originals, and I did measure one of those. The frequency centers were dead on at 3K, 4K, and 5K, and slightly off one way or the other at 8K, 10K, 12K and 16K.

Here's the deal on the HF boost,  even on "sharp" (narrowest bandwidth) the EQ curves are pretty broad. Moving the centers around by 100 to 500 Hz isn't going to make a big difference, IMHO. Now being off by 3K, that's worth chasing. The difference between 15.7K and 16K isn't. Just my opinion.

The Pulse unit also had a 1dB bass bump from 20hZ to about 1K with no EQ in. Interesting. This was measured with a 100K bridging load. I just recently got the proper cabling and terminator to measure I/O at 600 ohms. 600 ohms is harder for any unit to drive and the worse case test condition between 600 ohms and bridging.

Someone came up with 75 ohms as the worst case input Z for a particular EQ setting, I don't know how that was determined. 

Has anyone experimented with tacking a variable L on the end of the L chain or tweaking?  I have feeling it wouldn't trim the whole system appropriately, but I don't know. 

You can find a bunch of my measurements in MEQ500 and Analog Allstars threads. 

emrr said:

Someone came up with 75 ohms as the worst case input Z for a particular EQ setting, I don't know how that was determined. 

Has anyone experimented with tacking a variable L on the end of the L chain or tweaking?  I have feeling it wouldn't trim the whole system appropriately, but I don't know. 

You can find a bunch of my measurements in MEQ500 and Analog Allstars threads.

Click to expand...

Interesting. I will have a look with both 600 and 47 ohms (native output impedance of QA401)  into the Pultec. I do not recall seeing much of a difference before.

I found no measurable change in EQ range comparing 50 ohm source with 600 ohm source.  Affect on source equipment may be another matter.