Poor Man's Pultec EQP1-A Build Support Thread

[skal1]

Hi Ian.

thanks for the help on the Tl072 make up works a treat.

so i would like to exchange those noisy pots for stepped ones , i did read up on your stepped mastering eq and it confuses me , your are using different pot values to the pultec ones .

i used this app, to calculator the 11 steps does it like right for 0.5 db steps

Step 1, Attenuation = 0 dB, Rx = 0 ohms, Ry = 10000 ohms, Resistor = 10000 ohms.
Step 2, Attenuation = .5 dB, Rx = 559 ohms, Ry = 9441 ohms, Resistor = -559 ohms.
Step 3, Attenuation = 1 dB, Rx = 1087 ohms, Ry = 8913 ohms, Resistor = -528 ohms.
Step 4, Attenuation = 1.5 dB, Rx = 1586 ohms, Ry = 8414 ohms, Resistor = -499 ohms.
Step 5, Attenuation = 2 dB, Rx = 2057 ohms, Ry = 7943 ohms, Resistor = -471 ohms.
Step 6, Attenuation = 2.5 dB, Rx = 2501 ohms, Ry = 7499 ohms, Resistor = -444 ohms.
Step 7, Attenuation = 3 dB, Rx = 2921 ohms, Ry = 7079 ohms, Resistor = -420 ohms.
Step 8, Attenuation = 3.5 dB, Rx = 3317 ohms, Ry = 6683 ohms, Resistor = -396 ohms.
Step 9, Attenuation = 4 dB, Rx = 3690 ohms, Ry = 6310 ohms, Resistor = -373 ohms.
Step 10, Attenuation = 4.5 dB, Rx = 4043 ohms, Ry = 5957 ohms, Resistor = -353 ohms.
Step 11, Attenuation = 5 dB, Rx = 4377 ohms, Ry = 5623 ohms, Resistor = -334 ohms.
NOTE:
An input series resistor of 4377 ohms is required to provide the initial 5 dB of attenuation.

cheers

denis

 

[ruffrecords]

@denis

I have been asked this question many times and back in 2016 I wrote a document that describes how to do it and the resistor values needed for the three band Pultec. The values apply equally to the original PMEQP1A, you just don't need the mid section. Document is attached.

Cheers

Ian

 

[skal1]

cheers ian i will take a look at the document , like i thought way over my head , and the potentiometers in the original pultec are different values compared to your PMEQP1A so they will not work in my case ?.. also i would need to have 11 steps at 0.50db ..

could you break it down for , in small bite size pieces ,step by step for the 1k pot for hi cut ..

cheers

Denis

 

[ruffrecords]

cheers ian i will take a look at the document , like i thought way over my head , and the potentiometers in the original pultec are different values compared to your PMEQP1A so they will not work in my case ?.. also i would need to have 11 steps at 0.50db ..

could you break it down for , in small bite size pieces ,step by step for the 1k pot for hi cut ..

cheers

Denis

OK, I had not realized you are using the original Pultec circuit. In that case simply scale the resistor values in my document by the ratio of the the Pultec ot value divided by my pot value.

However, this will not give you 0.5dB steps because the EQP1A has about 20dB of cut and boost. Do you really want to throw half of that away?

Cheers

Ian

 

[skal1]

Hi Ian , i am would like to use it for mastering my music don't really need that amount of boost or cut so i could live 6 steps at 1/2 a db and 5 steps at 1db..

 

[ruffrecords]

Hi Ian , i am would like to use it for mastering my music don't really need that amount of boost or cut so i could live 6 steps at 1/2 a db and 5 steps at 1db..

OK, understood. It is not at all easy to work out what the resistor values should be. Perhaps the best approach is to simulate the circuit and manually vary the resistors to get the response you want at each switch position.

Cheers

Ian

 

[skal1]

ok makes sense ,what i will do is use a multi turn pot instead of the pot in circuit and turn the pot until i get 1/2 a db of gain for step one then write the resistance down for that step..

one more ? the Tl072 make up amp how do i set it up ..

cheers

Denis

 

[ruffrecords]

ok makes sense ,what i will do is use a multi turn pot instead of the pot in circuit and turn the pot until i get 1/2 a db of gain for step one then write the resistance down for that step..

Sounds like an excellent plan

one more ? the Tl072 make up amp how do i set it up ..

cheers

Denis

Set the EQ controls flat and/or set the Pultec EQ to bypass mode. Feed in a 1KHz signal at 0dBu . Adjust the TL072 gain so the output is 0dBu. And that's all there is to it.

Cheers

Ian

 

[skal1]

is 0dBu equal to 0.775 volts rms , and how do i work out the rms voltage of 1/2 db

cheers
denis

 

[ruffrecords]

is 0dBu equal to 0.775 volts rms , and how do i work out the rms voltage of 1/2 db

cheers
denis

You can do this on your calculator.

dB are defined as 20*log(k) where k is the voltage ratio. What we want is to input the dB and get the ratio k.

On most scientific calculators this can be done as SHIFT LOG dB/20.

So start by pressing SHIFT followed by LOG and your display will usually show 10^(

Then enter the dB you want followed by DIVIDE 20 then close bracket ) then EQUALS

Note you will generally be attenuating so your dB value will be negative

As an example om my calculator if I do this for -1dB I get a ratio of 0.89125

Multiply this by 0.775V and you get 0.6907

It might be easier to set the initial voltage to 1 volt, then the output voltage you need is the same as the ratio you worked out on your calculator.

Hope that helps.

Cheers

Ian

 

[skal1]

cheers Ian your a star

Denis