Poor Man's Pultec EQP1-A Build Support Thread

[spaceludwig]

OK, so initial breadboarding done with resistors and pots to be added shortly (no pun intended...)

Had a question regarding the Mid EQ: Is there a readily available formula or calculator sheet available like Ian has for the Hi/Lo sections? 

[edited for spelling]

 

[spaceludwig]

Hmm, my picture is not posting. Are files hosted by Dropbox forbidden?

 

[ruffrecords]

OK, so initial breadboarding done with resistors and pots to be added shortly (no pun intended...)

Had a question regarding the Mid EQ: Is there a readily available formula or calculator sheet available like Ian has for the Hi/Lo sections? 

[edited for spelling]

Unfortunately there is no ready reckoner for the mid band. The reason is that the same value inductor is used for several frequencies. What that means is that the Q is different for each frequency that uses the same inductor. If we had the luxury of a unique inductance value for every frequency then we could keep the Q constant in which case there would be a simple ready reckoner you could use for frequency.

However, I can give you some general guidance that will allow you to work out the inductor value at a given frequency for a given Q.

The basic equation is Q = 2*PI*F*L / R

where F is the frequency and R is the characteristic resistance of the EQ circuit. For the mid boost, R is about 8500 ohms.
You can re-arrange this to give you L:

L = Q*R / 2*PI*F and since we know R and PI this simplifies to:

L = Q*1353 / F

So if we want a Q of 1 at 3KHz we get L = 1353 / 3000 = 0.45H

So pick a value of Q (1 is not a bad start) and use the formula to calculate L for the required frequency.

Note that high values of Q ae not a good idea at low freqeuencies. For example, the Q of the 200Hz boost in my design is about 0.2.

Hope that helps.

Cheers

Ian

 

[spaceludwig]

Thanks so much for the info, Ian. I am still researching if/how a gyrator could be used for the mid band section. Am i right in interpreting the circuit as having the cut inductor grounded via the 1k resistor and floating in the boost circuit?

Unfortunately there is no ready reckoner for the mid band. The reason is that the same value inductor is used for several frequencies. What that means is that the Q is different for each frequency that uses the same inductor. If we had the luxury of a unique inductance value for every frequency then we could keep the Q constant in which case there would be a simple ready reckoner you could use for frequency.

However, I can give you some general guidance that will allow you to work out the inductor value at a given frequency for a given Q.

The basic equation is Q = 2*PI*F*L / R

where F is the frequency and R is the characteristic resistance of the EQ circuit. For the mid boost, R is about 8500 ohms.
You can re-arrange this to give you L:

L = Q*R / 2*PI*F and since we know R and PI this simplifies to:

L = Q*1353 / F

So if we want a Q of 1 at 3KHz we get L = 1353 / 3000 = 0.45H

So pick a value of Q (1 is not a bad start) and use the formula to calculate L for the required frequency.

Note that high values of Q ae not a good idea at low freqeuencies. For example, the Q of the 200Hz boost in my design is about 0.2.

Hope that helps.

Cheers

Ian

 

[ruffrecords]

Thanks so much for the info, Ian. I am still researching if/how a gyrator could be used for the mid band section. Am i right in interpreting the circuit as having the cut inductor grounded via the 1k resistor and floating in the boost circuit?

In the cut position the inductor is connected to ground via a 1K resistor. In boost it is connected to the input via a 4K7 resistor. In the off position it is floating.

Cheers

Ian

 

[audiosway]

Go to the DIY tab of my web site:

https://www.customtubeconsoles.com/diy

Scroll down and click on the PMEQP1A folder. All the info you need should be in there.

Cheers

Ian

Your site is down Ian. I clicked the link and it doesn't resolve to the site.

 

[ruffrecords]

Your site is down Ian. I clicked the link and it doesn't resolve to the site.

Yes, there is a problem I hope to resolve in a couple of days. In the meantime you can access it here:

https://sites.google.com/site/customtubeconsoles/home

Cheers

Ian

 

[audiosway]

Yes, there is a problem I hope to resolve in a couple of days. In the meantime you can access it here:

https://sites.google.com/site/customtubeconsoles/home

Cheers

Ian

ok cool. Just wanted to make sure you knew about it.

 

[ruffrecords]

ok cool. Just wanted to make sure you knew about it.

Thanks for that. I started a thread about it in the Brewery. Hope to have it fixed soon.

Cheers

Ian

 

[ruffrecords]

Site is back up now.

Cheers

Ian

 

[craig]

Hello Ian
After re reading the whole thread and searching the info on your site I still need to ask.
With PMEQP1A V1 with mid board and using 9050, which inductor taps connect to which caps/switch positions in relation to Mid schematic. And is the winding start connected to the boost cut switch pole.
Thank you

 

[ruffrecords]

Hello Ian
After re reading the whole thread and searching the info on your site I still need to ask.
With PMEQP1A V1 with mid board and using 9050, which inductor taps connect to which caps/switch positions in relation to Mid schematic. And is the winding start connected to the boost cut switch pole.
Thank you

The poor man's mid circuit is basically identical in all versions. If you look in the PMEQP1A folder and then the MK2 folder you will find a schematic where all the cap values and the pin letters of the inductor are shown. I have attached it to this post

 

[craig]

Thank you, of course I didn't think to look in the MK2 folder.

 

[ruffrecords]

Thank you, of course I didn't think to look in the MK2 folder.

The PMEQP1A was my very first project on GDIY so the documentation was not the best. Then I added the mid band, then designed the 3U3Band version and lastly the MK2 by which time the documentation was hopefully somewhat better.

 

[skal1]

Hi Ian,

just a quick ? about TL072 circuits for the PMEQP1A and all my other passive EQs:



can i use this to work with the original pultec filter circuit , without any changes to your TL072 circuit.

Regards

skal

The PMEQP1A was my very first project on GDIY so the documentation was not the best. Then I added the mid band, then designed the 3U3Band version and lastly the MK2 by which time the documentation was hopefully somewhat better.

Hi Ian

i now have the bits to make the TL072 make up for the original pultec filter , on your schemic you have a R3 470k on the noninverting i/p would i need to change this to a different value,

I am using the 1246 differential line receiver to drive the filter , and i will load the o/p with the 620 ohms to the i/p of the filter the o/p from filter will pass to the TL072 make up Amp then it passes its signal on to the differential 1646 driver .

Would i have to change these components hanging off the o/p of the TL072 ..

cheers

Denis

 

[ruffrecords]

Hi Ian

i now have the bits to make the TL072 make up for the original pultec filter , on your schemic you have a R3 470k on the noninverting i/p would i need to change this to a different value,

No, 470K is fine for all my passive EQ designs

I am using the 1246 differential line receiver to drive the filter , and i will load the o/p with the 620 ohms to the i/p of the filter the o/p from filter will pass to the TL072 make up Amp then it passes its signal on to the differential 1646 driver .

There is no need to load the driver with 620 ohms; just connect it straight to the input of the EQ. EQ out to TL072 and then straight to 1646 is fine.

Would i have to change these components hanging off the o/p of the TL072 ..

Keep the 47uF coupling cap and the 100K to ground. You can probably dispense with the 47R series resistor on the output because the 1646 likes to be driven by a very low impedance

cheers

Denis

Cheers

Ian

 

[skal1]

cheers , just testing the TL072 , 1k sine wave looks good can get 26vpp from 1vpp I/P that should be enough make up ,i believe..

ive used RF[100K]. AND 10K TRIM POT. looks stable to me .

regards

Denis

 

[skal1]

can i use a bipolar cap for c1?

 

[ruffrecords]

can i use a bipolar cap for c1?

Yes you can. 100uF will be fine but you are using bipolar supplies you can usually just direct couple the TL072 output to the 1646 input.

Cheers

Ian

 

[skal1]

cheers Ian