Poor Man's Pultec EQP1-A Build Support Thread

spaceludwig

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

Image00001.jpg


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]
 

ruffrecords

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

Image00001.jpg


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

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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?

ruffrecords said:
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

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spaceludwig said:
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
 

craig

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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

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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
 

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ruffrecords

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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.
 
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