Pultec EQP-1A3 solid-state clone LF boost question

[fazeka]

Hi all,

I have built a clone of a solid-state Pultec EQP-1A inspired by Chris Preston's document:

http://vintagewindings.com/tech%20swag/Pultec%20EQP-1A3%20DIY.pdf

I used original Triad HS-56 transformers in and out, along with Chris's custom inductor.

I spent a lot of time analyzing the wiring layouts in his document, his wiring diagram (http://vintagewindings.com/tech%20swag/Vintage%20Windings%20EQ-1A%20Wiring%20Guide.pdf) and rainton's tube layout (filter section) (http://www.mediafire.com/file/2b8py0igkenkh7l/Pultec_EQP-1A__layout_rev1_31.pdf) and planned my approach of the build.

I finally got it wired up and right out of the gate I have it working. Well, sort of...

Everything works as expected except it seems that the 20Hz boost is difficult to detect. Not sure if it's because I'm feeding the input of the EQ with iPhone audio (I know, I know but my home studio is in various states from my patch bay being rewired or gear stuck in storage  ). Or if it's because of my monitors (pair of JBL 4311, don't think so?). I double-checked my wiring, seems OK. The other LF frequencies are fine. Just 20Hz boost...

Any ideas?

Thanks,
Chris

 

[john12ax7]

Can you do a frequency test to confirm? With something like REW or ARTA software,  or a scope?

Possible the cap that selects the 20 Hz is not connected,  miswired,  or  less likely the wrong value.

 

[ruffrecords]

iphone should be fine as a low impedance source and I would not suspect the JBLs.

More likely a dry joint or wrong value component.

Cheers

Ian

 

[fazeka]

Can you do a frequency test to confirm? With something like REW or ARTA software,  or a scope?

Possible the cap that selects the 20 Hz is not connected,  miswired,  or  less likely the wrong value.

Hi,

Thanks for your reply.

I'll admit, I haven't done a frequency test. Nor do I know exactly how to execute one? I have a Tek 2465.  Is it just a matter of 'scoping the EQ's output, with an oscillator on the input?

Thanks,
Chris

 

[fazeka]

iphone should be fine as a low impedance source and I would not suspect the JBLs.

That's what I was thinking, Ian, as the other frequencies are working as expected. Couldn't be too sure, 'though, thus the question. 

More likely a dry joint or wrong value component.

OK, maybe I'll try reflowing the joints. Wrong value component? I hope not, the 1.5uF and 56nF caps (330nF and 12nF on the poles, respectively) are sealed in the filter can... 

Otherwise, I'm hoping it's my wiring... triple check, coming up!

 

[fazeka]

Hmmm... I triple-checked everything again. Wiring, connections. Disconnected the filter and checked each with my capacitance meter. OK. Triple-checked them to the right terminals of the switch. Even reflowed the solder joints just to be safe. Tried it out again. 20Hz hard to hear boost. 30Hz, I can hear some. 60Hz and 100Hz are apparent. Wonder if the 2213 woofers in my 4311s just can't get down into 20-30Hz? They are the original cones, 40+ years old... 

 

[Murdock]

If you don't hear a change when fully boosting 20Hz something is wrong with the wiring or parts. I'm quite sure that the full boost affects frequencies beyond 100Hz...
Maybe the switch is faulty. Have you checked continuity from common terminal to the 20Hz terminal?

 

[ruffrecords]

Hmmm... I triple-checked everything again. Wiring, connections. Disconnected the filter and checked each with my capacitance meter. OK. Triple-checked them to the right terminals of the switch. Even reflowed the solder joints just to be safe. Tried it out again. 20Hz hard to hear boost. 30Hz, I can hear some. 60Hz and 100Hz are apparent. Wonder if the 2213 woofers in my 4311s just can't get down into 20-30Hz? They are the original cones, 40+ years old... 

Try using headphones.

Cheers

Ian

 

[fazeka]

Maybe the switch is faulty. Have you checked continuity from common terminal to the 20Hz terminal?

Yes, also to the other connections (e.g., LF boost pot, etc.), that's why I'm scratching my head here... 

I'll try looking again...

 

[kosi]

As far as I know, Chris does NOT seal his filter cans. He puts some foam in them !
Have you tried to open it ?

OK, maybe I'll try reflowing the joints. Wrong value component? I hope not, the 1.5uF and 56nF caps (330nF and 12nF on the poles, respectively) are sealed in the filter can... 

 

[gyraf]

The Pultec works by forming a general attenuator, then "not attenuating" in particular areas for achieving boosts (and attenuating even more for forming cuts).

The Pultec bass boost works by inserting a capacitor in the general attenuator's path to ground and partly bypassing this capacitor with the "low boost" potentiometer.

Pot shorted= no boost = full attenuation. Pot open= path-to-ground goes through capacitor thus not attenuating below that caps corner freq.

That is, 1.5uF shorted, not open.

This means that for the lowest step to be no-function, there has to be a short from that step to ground when the pot is at full-boost.

Jakob E.

 

[abbey road d enfer]

Hmmm... I triple-checked everything again. Wiring, connections. Disconnected the filter and checked each with my capacitance meter. OK. Triple-checked them to the right terminals of the switch. Even reflowed the solder joints just to be safe. Tried it out again. 20Hz hard to hear boost. 30Hz, I can hear some. 60Hz and 100Hz are apparent. Wonder if the 2213 woofers in my 4311s just can't get down into 20-30Hz? They are the original cones, 40+ years old... 

The actual cut-off frequency of 4311 is about 45Hz, below which the level falls at a rate of 24dB/octave, so at 20Hz, teh response is about 30dB down. I have much better speakers than 4311's, and yet cannot hear anything significant at 20Hz, except distortion at 40 and 60Hz when hitting hard.
Many subwoofers even have poor response at 20Hz. The elusive 20Hz LF response is mostly a lab thing.

 

[Winetree]

At 17hz my Tannoy M-1000s just make a clicking or pulse sound.
It turns into a steady sine wave at about 22hz.
Those JBLs don't go that low.

 

[fazeka]

As far as I know, Chris does NOT seal his filter cans. He puts some foam in them !
Have you tried to open it ?

No, I don’t need to. I used a capacitance meter to measure. It’s wired up correctly. 

 

[fazeka]

The actual cut-off frequency of 4311 is about 45Hz, below which the level falls at a rate of 24dB/octave, so at 20Hz, teh response is about 30dB down. I have much better speakers than 4311's, and yet cannot hear anything significant at 20Hz, except distortion at 40 and 60Hz when hitting hard.
Many subwoofers even have poor response at 20Hz. The elusive 20Hz LF response is mostly a lab thing.

Response would be about 30dB down at 20Hz. But not nearly as down, at say 50Hz, right?

Here's what's confusing to me. Looking at the frequency response curve for the Pultec at 20Hz:

it seems that I should hear *something* when boosting 20Hz "full up"? Say, 50Hz at 6dB? I mean, I can detect when I boost at 30Hz and I can *definitely* hear when I boost at 60Hz.

Otherwise, I guess I am struggling with the point of having a setting for 20Hz? I mean, lots of monitors "back in the day" when the Pultec was king didn't perform as good as the 4311 anyway (?)...

 

[abbey road d enfer]

Otherwise, I guess I am struggling with the point of having a setting for 20Hz?

It comes from how shelving filters are defined. They are not defined by their turning point, they are defined by their maximum boost/cut. Indeed a 20Hz (or 20kHz) shelving filter will be used and perceived for its limited action in the audible range, the audibility being the result of the limited BW of both the speakers and the ears.

 

[fazeka]

Thanks Jakob for the explanation.

However, I get no short when EQ is engaged at 20Hz with full boost.

The Pultec works by forming a general attenuator, then "not attenuating" in particular areas for achieving boosts (and attenuating even more for forming cuts).

The Pultec bass boost works by inserting a capacitor in the general attenuator's path to ground and partly bypassing this capacitor with the "low boost" potentiometer.

Pot shorted= no boost = full attenuation. Pot open= path-to-ground goes through capacitor thus not attenuating below that caps corner freq.

That is, 1.5uF shorted, not open.

This means that for the lowest step to be no-function, there has to be a short from that step to ground when the pot is at full-boost.

Jakob E.

 

[abbey road d enfer]

Thanks Jakob for the explanation.

However, I get no short when EQ is engaged at 20Hz with full boost.

Can you post a frequency response plot? That would cut short to any speculation.

 

[fazeka]

Not readily, but I'll work on it.

Can you post a frequency response plot? That would cut short to any speculation.