Poor Man's Pultec EQP1-A Build Support Thread

[ruffrecords]

The second set of DCR readings seem about the same as I meaure for the Carnhills. I suspect there is something wrong with your LCR meter.

Cheers

In

yeah i think so too. that aside is there anything you can tell from these tests? am i correct in assuming that we need as low of a resistance as possible for this circuit?

What you can tell is that the resistance of the Carnhills is OK.  It is not necessary for it to be as low as possible. As long as it is small compared to the overall circuit resistance which ti is.

Ceers

Ian

 

[salomonander]

hey Ian,
alright so you think that the carnhills do not give these issues due to higher resistance (compared to the other ones i have tested)?
just trying to get a better idea of what the problem really is before i go inductor hunting. don will make me two inductors the coming week for testing. one using his rm8 core, the other one the ad ef core.
i also looked at sowter but i think they are really too expensive.  their resistance seems to be extremely low though.

 

[kambo]

hey Ian,
alright so you think that the carnhills do not give these issues due to higher resistance (compared to the other ones i have tested)?
just trying to get a better idea of what the problem really is before i go inductor hunting. don will make me two inductors the coming week for testing. one using his rm8 core, the other one the ad ef core.
i also looked at sowter but i think they are really too expensive.  their resistance seems to be extremely low though.

Cinemag make some too...

 

[ruffrecords]

hey Ian,
alright so you think that the carnhills do not give these issues due to higher resistance (compared to the other ones i have tested)?

Correct

just trying to get a better idea of what the problem really is before i go inductor hunting. don will make me two inductors the coming week for testing. one using his rm8 core, the other one the ad ef core.

The problem is due to stray capacitcance inside the inductor caused simply because the winding wires are close to eachother. All real inductors have this property. It forms a parallel parasitic resonant circuit nside the inductor. The frequency it resonates at is called the self resonant frequency. The more turns you have on an inductor the bigger the stray capacitance and the lower the self resonance.

i also looked at sowter but i think they are really too expensive.  their resistance seems to be extremely low though.

And none of the manufacturers quotes self resonance frequency so you have no idea if they would be any better. The only time I have seen self resonance quoted is for the little 100mH  inductors made by TOKO/Murata/Bourns.

Havr you tried the 5mH plus 390pF plus 1K modification I posted?

Cheers

Ian

 

[salomonander]

hey Ian,

alright i see. i assume its down to luck then. the one "noname" inductor i tested was constructed just like the carnhills with multiple taps and did fine. so it has to be possible somehow i guess. i will try dons inductors and report back.
i have not tried you fix since i assume it will only adress the high frequency ringing not the asymmetric bell shapes. which im majorly concerned with honestly.

 

[ruffrecords]

hey Ian,

alright i see. i assume its down to luck then. the one "noname" inductor i tested was constructed just like the carnhills with multiple taps and did fine. so it has to be possible somehow i guess.

Depends. What were the inductance values on the no name inductor? I ask because the VTB9044 used in my REDD EQ does not have this problem but its maximum inductance is only 200mH. The inductors used in the Pultec have up to 10 times the inductance and hence more turns and more capacitcance and a lower self resonance.


Cheers

Ian

 

[salomonander]

hey Ian,

alright i see. i assume its down to luck then. the one "noname" inductor i tested was constructed just like the carnhills with multiple taps and did fine. so it has to be possible somehow i guess.

Depends. What were the inductance values on the no name inductor? I ask because the VTB9044 used in my REDD EQ does not have this problem but its maximum inductance is only 200mH. The inductors used in the Pultec have up to 10 times the inductance and hence more turns and more capacitcance and a lower self resonance.


Cheers

Ian

hey ian,
the inductance values are in the screenshots names i posted. they were 160mh (on the carnhills); 166mh (on one noname) and 220mh (on the carnhill) and 250mh (on the noname). so rather similar values.  when i tested the carnhills they showed this weired resonance no matter which tap i tried.

 

[ruffrecords]

hey ian,
the inductance values are in the screenshots names i posted. they were 160mh (on the carnhills); 166mh (on one noname) and 220mh (on the carnhill) and 250mh (on the noname). so rather similar values.  when i tested the carnhills they showed this weired resonance no matter which tap i tried.

OK, understood. Does the no name have any other higher inductance taps like the Carnhill or does it just have the 166 and 250mH ones?

Cheers

Ian

 

[salomonander]

hey ian,
the inductance values are in the screenshots names i posted. they were 160mh (on the carnhills); 166mh (on one noname) and 220mh (on the carnhill) and 250mh (on the noname). so rather similar values.  when i tested the carnhills they showed this weired resonance no matter which tap i tried.

OK, understood. Does the no name have any other higher inductance taps like the Carnhill or does it just have the 166 and 250mH ones?

Cheers

Ian

250 is the highest.

 

[ruffrecords]

hey ian,
the inductance values are in the screenshots names i posted. they were 160mh (on the carnhills); 166mh (on one noname) and 220mh (on the carnhill) and 250mh (on the noname). so rather similar values.  when i tested the carnhills they showed this weired resonance no matter which tap i tried.

OK, understood. Does the no name have any other higher inductance taps like the Carnhill or does it just have the 166 and 250mH ones?

Cheers

Ian

250 is the highest.

OK, so that is the difference. The Carnhills have extra windings which add to the stray capacitance. Other Carnhill inductors like VTB9044, which has a maximum inductance of 200mH, does not have the problem. So we conclude that as some point above 200mH, the stray capacitcance in an inductor means the self resonant frequency drops so low that ir can affect the EQ response. What I am not sure of is how much far over 200mH we can go.

Cheers

ian

 

[salomonander]

hey ian,
the inductance values are in the screenshots names i posted. they were 160mh (on the carnhills); 166mh (on one noname) and 220mh (on the carnhill) and 250mh (on the noname). so rather similar values.  when i tested the carnhills they showed this weired resonance no matter which tap i tried.

OK, understood. Does the no name have any other higher inductance taps like the Carnhill or does it just have the 166 and 250mH ones?

Cheers

Ian

250 is the highest.

OK, so that is the difference. The Carnhills have extra windings which add to the stray capacitance. Other Carnhill inductors like VTB9044, which has a maximum inductance of 200mH, does not have the problem. So we conclude that as some point above 200mH, the stray capacitcance in an inductor means the self resonant frequency drops so low that ir can affect the EQ response. What I am not sure of is how much far over 200mH we can go.

Cheers

ian

makes sense. we'll know for sure once i get dons inductors.  ill post my findings as soon as they arrived.

 

[ilfungo]

Hi Ian
I'm building your 3 Band pultec but with Peter custom inductors ( finally rrived)
From your PDF

"To account for QMAX (470 ohms) and the average resistance of the VTB9042
inductor (40 ohms), you can subtract 510 ohms from the final 10K resistor making it
9490 ohms. This can be made from 9K1 resistor in series with a 390 ohm resistor."

It's ok also with this inductors?
THANKS!!!

 

[ruffrecords]

Hi Ian
I'm building your 3 Band pultec but with Peter custom inductors ( finally rrived)
From your PDF

"To account for QMAX (470 ohms) and the average resistance of the VTB9042
inductor (40 ohms), you can subtract 510 ohms from the final 10K resistor making it
9490 ohms. This can be made from 9K1 resistor in series with a 390 ohm resistor."

It's ok also with this inductors?
THANKS!!!

That pdf refers to the untested swtiched pot modification. That said, yes that should work also for theose inductors with those values.

Cheers

Ian

 

[ilfungo]

OK
THANKS!

 

[salomonander]

hey,
bad news. dons inductors measure just like the carnhills. i assume we either ignore the issues or ian has to design this around different inductor values. cheers

 

[ruffrecords]

hey,
bad news. dons inductors measure just like the carnhills. i assume we either ignore the issues or ian has to design this around different inductor values. cheers

Which unfortunatey just shows  it is a property of that kind of mulit-tapped inductor. What surprises me is that nobody has noticed this before but I guess that is because people mostly just measure up to 20KHz. Most of the Helios clones use the VTB9042 inductor so I guess they must have this feature but just don'rt realise it. Even some classic Neve's use inductors of the same basic construction. Maybe its just part of what gives those vintage designs their 'sound'.

The out of band rise is the the problem you need fixing which is due to the top boost circuit. Fortunately this does not need a huge inductance so we could probably substitute a number Toko 100mH inductors or the equivqlent. Five of the frequencies need just three of these. The 6th is qite close to the fifth so we can probably get away with i=one more for this. So I am thinking maybe I can squeeze four Toko 100mH inductors on a small PXB that yo can wire in instead of the Carnhill. What do you think?

Cheers

Ian

 

[salomonander]

hey,
bad news. dons inductors measure just like the carnhills. i assume we either ignore the issues or ian has to design this around different inductor values. cheers

Which unfortunatey just shows  it is a property of that kind of mulit-tapped inductor. What surprises me is that nobody has noticed this before but I guess that is because people mostly just measure up to 20KHz. Most of the Helios clones use the VTB9042 inductor so I guess they must have this feature but just don'rt realise it. Even some classic Neve's use inductors of the same basic construction. Maybe its just part of what gives those vintage designs their 'sound'.

The out of band rise is the the problem you need fixing which is due to the top boost circuit. Fortunately this does not need a huge inductance so we could probably substitute a number Toko 100mH inductors or the equivqlent. Five of the frequencies need just three of these. The 6th is qite close to the fifth so we can probably get away with i=one more for this. So I am thinking maybe I can squeeze four Toko 100mH inductors on a small PXB that yo can wire in instead of the Carnhill. What do you think?

Cheers

Ian

hey Ian,
yes im leaning towards the tokos as well. its most likely the best Methode. if you redesign around another inductor i assume all cap values and resistors - well everything would Change. ill try some single inductors. i guess it doesnt matter which manufacturer as Long as the values are not too high. do you think there will be a big Impact to the Sound of the unit? is it worth trying a few different manufacturers?

i also find it super akward that no one measured this before. the mid band seems to Show the issues well below 20k. remember - i have only tested the inductors outside of the unit. only jonas did the tests in a finished unit. i wonder if there is anyone else with a finished unit who could give  it another test drive. but i assume People who finished it dont lurk around the thread anymore.

 

[salomonander]

PS: alternatively - if we'd know how high we can go in inductance - we could ask don to wind us say 2x 0-300mh inductors in one housing. but we dont know how high we can go. the one i measured was 250mh. so that is most likely still fine...

 

[ruffrecords]

hey Ian,
yes im leaning towards the tokos as well. its most likely the best Methode. if you redesign around another inductor i assume all cap values and resistors - well everything would Change. ill try some single inductors. i guess it doesnt matter which manufacturer as Long as the values are not too high. do you think there will be a big Impact to the Sound of the unit? is it worth trying a few different manufacturers?

Difficult to tell how the sound will be affected. Certainly many of the characteristics of inductor based passive EQs are due to the particular inductors used. Both the Carnhill and the small Toko type inductors use ferrite as the core so at least in that respect they are similar but there are lots of different ferrites so the chance of them being the same is small. You will have noticed that the Carnhills are a lot bigger than the Toko. A bigger core means the ability to handle a bigger signal level so one possible concern about the Toko is whether the core will saturate under normal conditions. The worst case is at full boost when the full signal is right across the inductor. I have ordered some of these type of inductors and I will check them out before designing them in.

The other problem with the Tokos is they have now ceased production and I don't really want to design in an obsolete aherd to find component. However, there do seems to be manufacturers of very similar inductors (Bourns for example) which seem to be readily available. I have ordered a bunch from Mouser to try out.

i also find it super awkward that no one measured this before. the mid band seems to Show the issues well below 20k. remember - i have only tested the inductors outside of the unit. only jonas did the tests in a finished unit. i wonder if there is anyone else with a finished unit who could give  it another test drive. but i assume People who finished it don't lurk around the thread anymore.

Lot's of people have built and are building these EQs but I guess nobody has thought to measure above 20KHz because for the vast majority of people it is not important.  Everyone who has commented on the EQ said they like the way it sounds. As I mentioned before, I only added the mid band after numerous requests and even then I made it clear that the design was at best compromise.

Cheers

Ian

 

[ilfungo]

Hi Ian
sorry for a stupid question:
I've download the " SteppedPotsfor3BandPultecv0.2"
I don't understand if the series Mid boost resistor need to be 4k7 or 300k....
Thanks!