Poor Man's Pultec EQP1-A Build Support Thread

[ruffrecords]

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!

It is 4K7 just as in the orignal 3bandpultec. When I first did the calculations I mistakenly thought you could use the same set of resistors for boost and cut but it was an error in my spreadsheet. Unfortunately this means you need separate sets of resistors for boost and cut  so you need to use a 2 pole switch.

Cheers

Ian

 

[ilfungo]

Hi Ian
thanks for your help!
I'm working on make up stage with PPA Doa
Could work fine like this?
Thanks!

 

[ruffrecords]

Hi Ian
thanks for your help!
I'm working on make up stage with PPA Doa
Could work fine like this?
Thanks!

That is basically right. If the DOA has split supplies then you should not need the 47uF cap but otherwise it looks OK.

Cheers

ian

 

[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

I have done a lot of work on this trying to find a workable solution. I think I have one at least for the Hi Boost. The current design uses the VTB9042 inductoe which has taps up to 1 Henry. The designonly uses the taps up to 0.6H. We can replace this inductor with four 100mH inductors. The top five frequenies only need 100mH, 200mH or 300mH so the capacitor values stay the same as the original. Only the 3K uses the 0.6H winding which will now be 400mH. The required capacitor to resonate at 3KHz is  7.1nF which you can make form a 6.8nF and a 330pF. Just fit these to C7 and C7A. The inductors could be fitted directly to the PCB. As this is just a boost, we a re less concerned about the series resistance of the inductors. So we can use axial leaded types. I recommend theEPCOS B82144A2107J type avialabke from Farnell part number 518300. You need to fit one between the S  pin and the one immediately to its left  (unlabelled). Then fit one between the unlabelled pin and the pin labelled B. Then one between the B pin and the D pin and lastly one between the D pin and the E pin. These inductors have a dc resistance of  420 ohms so we now longer need the Qmax resistor. The Q range will be corret for the 16K and 10K frequencies but the max will be slightly lower for the lower frequencies because of the added resistance of the extra inductors. If you want to avaoid this you need to go to radial leaded inductors (like the Toko) which have a sereis resistance of only 80 ohms and they will be more awkward to retrofit.

For the mid, the problem is more difficult because the lowest frequency is 200Hz and uses a 2H inductor. Even if we used 120mH inductors we would need 16 of them to reach 2H so this is not practical. I suggest we use 8 of them. With this many, the series resistance of the axial types is so high that the when switched to cut there would be no cut at all. With 8 of the Tokos the resistance ia about 640 ohms which is not too far away from the 470 series resistor normally fitted  with the Carnhills. This will give use 960mH total so the Q of the frequencies between 200Hz and 700Hz will be halved but I think this is no bad thing at low frequencies. Fitting 8 into the space of the existing inductor should just about be possible but it wll need an add-on PCB. My next task is to work out the new capacitor values and design the add-on PCB.

Cheers

Ian

 

[ruffrecords]

I have now worked out the values for the mid.

F=200Hz, L=960mH, C=680nF, Fact=197
F=300Hz, L=960mH, C=270nF, Fact=313
F=500Hz, L=960mH, C=100nF, Fact=514
F=700Hz, L=960mH, C=47nF+6n8, Fact=700
F=1KHz, L=960mH, C=27nF, Fact=988
F=1K5Hz, L=960mH, C=12nF, Fact=1482
F=2KHz, L=480mH, C=12nF+1nF, Fact=2014
F=3KHz, L=480mH, C=4n7+1n2, Fact=2990
F=4KHz, L=240mH, C=6n8, Fact=3939
F=5KHz, L=240mH, C=4n3, Fact=4954
F=7KHz, L=120mH, C=4n3, Fact=7006

Where:

F is the desired frequency,
L is the inductance
C is the actual capacitance
Fact is the calculated actual resonant frequency.

When I get back from my holiday I will look into designing an adaptor PCB.

Cheeers

ian

 

[salomonander]

hey Ian,
you are the man! thanks so much for not giving up on this. i hope you have a great vacation.

 

[salomonander]

hey Ian,
maybe you can elaborate on the output transformer when using a 2520 opamp. im not sure what to use. the origina pultec u. is used a 250:600 for the output. will we also need something in the 1:2 range, or will a 1:1 be fine as well?

 

[ruffrecords]

hey Ian,
maybe you can elaborate on the output transformer when using a 2520 opamp. im not sure what to use. the origina pultec u. is used a 250:600 for the output. will we also need something in the 1:2 range, or will a 1:1 be fine as well?

I am no expert on the 2520 (tubes are my thing) but if a 1:2 is normally used then a 1:1 will be fine. You will just have 6dB less headroom.

Cheers

ian

 

[ilfungo]

Hi Ian
I'm at the end of 3 Band pultec
I must use 12k resistor on the input transformer as poor man's pultec?
Input transformer Sowter 3575.
Another question is the stepped mid connections:
DPDT on off on Switch
one side go to PCB Boost -off-Cut
other side ???
I don't understand how to select one or other set of resistor..
Thanks!!!

 

[ruffrecords]

Hi Ian
I'm at the end of 3 Band pultec
I must use 12k resistor on the input transformer as poor man's pultec?
Input transformer Sowter 3575.

No, the 12K is not required for the 3 band Pultec.

Another question is the stepped mid connections:
DPDT on off on Switch
one side go to PCB Boost -off-Cut
other side ???

This switch selects the mid band between boost, off and cut.  You can use a SPDT switch with a centre off position. If you use a DPDT type the other half is not used.

Cheers

Ian

 

[ilfungo]

Hi Ian
Thanks but
I'm building with your great help to switch stepped version and in your "SteppedPotsfor3BandPultecv0.2" you say to use a DPDT to select the row of different resistor for cut and boost.
I don't understand how select the different resistors whit the same switch.
Thanks!

 

[salomonander]

Hi Ian
Thanks but
I'm building with your great help to switch stepped version and in your "SteppedPotsfor3BandPultecv0.2" you say to use a DPDT to select the row of different resistor for cut and boost.
I don't understand how select the different resistors whit the same switch.
Thanks!

if you have a doube switch it has two poles. so you can use each one individually.

 

[ruffrecords]

Hi Ian
Thanks but
I'm building with your great help to switch stepped version and in your "SteppedPotsfor3BandPultecv0.2" you say to use a DPDT to select the row of different resistor for cut and boost.
I don't understand how select the different resistors whit the same switch.
Thanks!

OK, now I understand! I have just reviewed the documentation and I think there is a more simple way to achieve what is needed. However, the original method is as follows:

The documentation says:

K is connected to 1&2
L is connected to 3

For the mid boost/cut you have a 2 pole 12 way switch instead of a single pole oneWhat you actually need to do is:

K is connected to 1&2 of both poles of the 12 way switch
L is connected to the wiper of the second half of the DPDT switch
The boost connection of the second half of the DPDT switch goes to the boost pole pin 3
The cut connection of the second half of the DPDT switch goes to the cut pole pin 3.

This is to accommodate the different resistor networks needed for boost and cut.

That is basically it.

However, as I mentioned above, there is a simpler way to do it using the original SPST switch. Simply wire the boost pole in series with the 1K resistor and the cut pole in series with the 4K7 resistor and short K and L. If you wanted you could even use seprate 1 pole 12 way switches, one for mid boost and one for mid cut which means all three bands have seprate boost and cut controls.

Cheers

Ian

 

[ilfungo]

THANKS IAN!!!
My 3 Band Pultec is Alive and it sound wonderfully!!!
I have only one problem:
when mid boost is on it add 1db of boost
when mid cut is on it cut 1 db cut
Tomorrow I will do some tests and after in Studio for more impressions.
I will post some Images and some info about the combo:
Ian 3 band Pultec
PPA Make up gain and DOA
Begis custom Toroidal
THANKS!!!

 

[ruffrecords]

I am really pleased you got it working. I don't understand why you get the 1dB mid boost/cut. If the  stepped pots are wired correctly then the boost/cut circuit should be completely disconnected at the 0dB setting. However, you are the first person to build this so we are breaking new ground.

It would be nice if you could email me some pics so I can send them to Pierre to include in his gallery of projects of my designs.

Cheers

Ian

 

[ilfungo]

Thanks Ian
Could be just When the inductor is in circuit also without resistor?
Thanks!

 

[ruffrecords]

Thanks Ian
Could be just When the inductor is in circuit also without resistor?
Thanks!

There would have to be some stray capacitance to complete the circuit but that would likely be very small which would alter the resonant frequency. Jus the inductor hanging off the main circuit but connected nowhere else should produce no effect at all. Does the 1dB bump change in frequency as you alter the mid frequency control?

Cheers

Ian

 

[ilfungo]

Yes on all mid frequencies.

 

[ruffrecords]

Yes on all mid frequencies.

What happens if you disconnect the wipers of the new  mid level switch?

Cheers

Ian

 

[ilfungo]

if I disconnect the wiper there is no boost or cut at all.
But I think it's one of my stupid error:
I've seen that i've started with resistor from R1 instead of open.
But I think it's not a big  problem because if If Mid come in it's for cut or boost something.
Ian Sorry for my stupid error.