Poor Man's Pultec EQP1-A Build Support Thread

[skeeler87]

Ok. Somehow I imagined I could run the eq into a passive DI, then into the VP`s. Maybe you have a link to a schematic of some sorts? Don`t know what to look for  8)

Yes, you are quite right, you can use a DI. Once again, a high input impedance is preferable so and active type would be best. However, if you happen to already have a passive DI then it is certainly worth a try. The only problem with a passive DI is noise. The Pultec EQ already has an insertion loss of 20dB. A passive DI will have at least as much insertion loss again so you are looking at a total gain make up of over 40dB. Most mic pres will achieve a noise level below -80dBu at 40dB gain so in theory noise should be OK. The other thing with a passive DI is that the input impedance is not as high as the EQ would like. The effect is a reduction in the range of the bass boost control and an increase in the range of the bass cut control.

The simplest form of gain make up amplifier for my passive EQs is one based on a FET input op amp like the TL072. Attached are a couple of schematics for such a gain make up amp.  One is a split supply version and the other is a single supply version. I will also add the schematic to the documentation for the EQs.

Cheers

Ian

Is there any benefit from using a split supply version over a single supply version or vice versa?
Sorry for the extremely elementary question!

 

[ruffrecords]

Is there any benefit from using a split supply version over a single supply version or vice versa?
Sorry for the extremely elementary question!

None whatsoever; I just put them both in because I did not know what supplies people would have available. Also, following some help from others here on groupDIY, I have updated the schematic to include modern FET op amps that can drive a 600 ohm load or a transformer. Revised schematic attached,

Cheers

Ian

 

[kambo]

hey Ian,
been reading your tech documents and EQ documents!
hats off  8)
its almost a  free personal online tutor

before, i was doing some calc and experimentations etc... it was just so confusing; one thing u make better,
next u make the other parameter worst!

after reading and studying ur docs, i feel much better

thank you for the hard work and time you are putting in to DIY community, (also many others !)

BTW:
i understood why ur eq was called poormans EQ, after i added some  inductors to sowter chart 
OMG, almost 350 GBP for set of two mid eq inductors!

 

[ruffrecords]

Sowter inductors are rather expensive but they do have the advantage of being fully screened. If you use unscreened Carnhill inductors from http://www.audiomaintenance.com/your inductor costs will drop dramatically.

Cheers

Ian

 

[G-Sun]

finally getting my PM projects finished now I have the inductors.

Here is my mic pre/DI with Ian's Poor Man's Pultec EQP1-A

Now this is a very cool layout and build.
Thanks for sharing pics.

How does it work for you?

I don't quite understand the psu-part.
I see only a single rail coming in. ?

 

[fragletrollet]

Can the 3-band pultec fit in a 500-series rack? I guess it would be a double width frontpanel to fit all the switches and pots, but does the pcb fit? I was thinking one could use one of these:

http://www.jlmaudio.com/shop/inx5-pcb.html

for the i/o & makeup gain?

 

[ruffrecords]

Can the 3-band pultec fit in a 500-series rack? I guess it would be a double width frontpanel to fit all the switches and pots, but does the pcb fit? I was thinking one could use one of these:

http://www.jlmaudio.com/shop/inx5-pcb.html

for the i/o & makeup gain?

The original 3 band Pultec that uses Grayhill switches has a PCB that is 123mm tall and 73mm deep and mounts at right angles to the fron panel. As long as it is not too tall for a 500 series module then I guess it should fit. The new Lolrlin version (MK2) uses a PCB that is 150mm tall and 50mm wide and mounts parallel to the front panel. I think the 150mm height makes it too big for a 500 series module.

Cheers

Ian

 

[fragletrollet]

Cool, thanks for the input Ian. I`ll measure up some of my 500-series modules.

Not totally sure I got all the differences between the two right, but here goes:

PoorMansPultec: 2 pcb`s + mid extension pcb. Or only one pcb? Confused by the pics on your site, as the wiring diagram shows 3 pcb`s while there are pictures of a pcb fitting all switches (Lo, MID & High)  on one pcb.  Made for Lorlin switches.

3-band Pultec: Lo, Mid and High band all on one pcb, made for grayhills.

Both projects are available to build with inductor for High-band, both need external makeup-gain etc.

...am I getting it right?

 

[fragletrollet]

Also the mid-section is 1x boost / 1x cut, not 2x boost & (1x) cut as on the meq-5...?

 

[ruffrecords]

There are three implementations of my Pultec design:
1. Original poor man's consisting of separate PCBs for hi, mid and lo uising Lorlin switches. The hi boost is RC only
2. Grayhill 3 band Pultec with everything on a 124mm by 73mm PCB. The hi boost is RC or LC (as in the original)
3. The MK2 poor man's, using Lorlins but with all three bands on a 150mm by 50mm PCB. The hi boost is RC or LC (as in the original )

The MK2 is mean to replace the poor man's three boards with an easier to build version.

In all three versions the mid is a single set of frequencies and a switch selects between cut or boost.

I guess I ought to create a single document that explains/clarifies the three.

Cheers

Ian

 

[3nity]

After 3 years of having this completed i just tested....today..
using an 325 output like...

Now questions:
Bypassed EQ signal is just too loud..what was guys your solution^
DO i need the pad IAn sugested here: http://www.ianbell.ukfsn.org/pics/EQSystem.png

Now for calibration ..wheres the info?

 

[ruffrecords]

DO i need the pad IAn sugested here: http://www.ianbell.ukfsn.org/pics/EQSystem.png

Now for calibration ..wheres the info?

Yes, that is exactly what you need. The Pultec EQ has an insertion loss of about 20dB. The pad is designed to have the same loss when in circuit so the level stays the same. To calibrate set the EQ in and all the controls flat. Feed in a 1KHz signal and measure the output level. Switch the pad in (EQ out) and adjust the preset pot for the same output level.

That's all there is to it.

Cheers

Ian

 

[ilfungo]

Is there any benefit from using a split supply version over a single supply version or vice versa?
Sorry for the extremely elementary question!

None whatsoever; I just put them both in because I did not know what supplies people would have available. Also, following some help from others here on groupDIY, I have updated the schematic to include modern FET op amps that can drive a 600 ohm load or a transformer. Revised schematic attached,

Cheers

Ian

Hi Ian
have you the schematics ( Fet makeup for driving 600 ohm transformer) whit better resolution?
I can't read the components value...
Thanks!!!

 

[ruffrecords]

Hi Ian
have you the schematics ( Fet makeup for driving 600 ohm transformer) whit better resolution?
I can't read the components value...
Thanks!!!

Crikey, they are unreadable! I am surprised nobody mentioned it before. Here is a pdf version which should be much better.

Cheers

Ian

 

[ilfungo]

Thanks!!!

 

[jonasmaks]

Hello,
Has anyone measured Ian's Pultec frequency responses beyond 20 kHz? I've just built 3 Band Pultec inductive version (EQWithGainMakeUp) and found that HiBoost starts to ring above 26kHz and MidBoost at 16-17kHz . What is the cause of such behaviour (damping?), is it possible to avoid it?
Measurements were done @96kHz rate with RME Fireface400 Mic input, 4dBu Line Out, Eq input unbalanced, loaded with 100 K resistor, output via Ian's TL072 buffer, loaded with 470 K resistor.
G-Pultec HiBoost @2K&16K with 100Hz LoCut LoBoost added for comparison.

 

[ruffrecords]

Interesting. I have not seen that before but I have not made extensive measurements above 20KHz. What setting was the Q control for the high boost? Which inductors are you using?

Cheers

Ian

 

[jonasmaks]

Q control was at maximum - here is a new plot with max and min Q values. I'm using Carnhill inductors - VTB9042 for HiBoost and VTB9050 for MidBoost. I've measured inductance before fitting them into PCB - all looked like OK. I've built 2 channels (for stereo unit) - one is fitted with tube gain stage already (VTB9071/ VTB2291) - same  response.

 

[ilfungo]

Hi
I'm building the same 3 band Pultec  with inductors.
I want to use stepped pot instead regular pot.
How I can realize 1 db for step (lorlin switch 1 x 12)?
Suggestions?
Thanks!


Hi jonasmaks
have you some wiring plot?
Thanks!

 

[ruffrecords]

Q control was at maximum - here is a new plot with max and min Q values. I'm using Carnhill inductors - VTB9042 for HiBoost and VTB9050 for MidBoost. I've measured inductance before fitting them into PCB - all looked like OK. I've built 2 channels (for stereo unit) - one is fitted with tube gain stage already (VTB9071/ VTB2291) - same  response.

Thanks for that. it appears the Q control has no effect on the spurious response. I was thinking along the lines of stray capacitance inside the inductor causing a secondary resonance but that would have reduced as the Q control resistance increased. Your graphs show this is not the case. Is the 26K bump there with the controls flat?

Cheers

ian

Cheers

Ian