Poor Man's Pultec EQP1-A Build Support Thread

[Humner]

Sorry and Thanks Ian!!!!
Just one question
only 3 resistor? no caps?
Thanks!!!!

I created a DOA opamp gain stage and used Ian's recommended resistor vales(which will get you the correct input impedance + gain needed for PMPEQP1) + API's data sheets to help with capacitor values.

here is a copy of API's datasheets for 2520 - look at the line amplifier notes.

https://static.gearslutz.com/board/imgext.php?u=http%3A%2F%2Fwww.danalexanderaudio.com%2FApiInfo%2Fapi2520_02.jpg&h=219268859c89ebb165b98902767ad0c7

I will let someone confirm, but I think with a 10k feedback resistor, you will want to be around the 270pF value for a capacitor across the feedback resistor. This gives you a corner frequency of 59khz to help with potential oscillation issues.

Is this correct?

 

[ruffrecords]

My diagram was intended to show the resistor values you need to get an appropriate input impedance and gain range for the the  amplifier. You do need to also take into account the specific requirements for dc isolation and stability of the device you choose.  In general you should not need an input coupling capacitor. A capacitor inseries with the 2K2 pot is probably a good idea as it keeps bias current out of the pot and also ensures the op amp dc gain remains at unity so offset voltages are not amplified. I think 59KHz is a little on the low side for a 3dB point as this could lead to significant phase shift at 20KHz. I would try 100pF.

Cheers

Ian

 

[theaterguy]

Hi boost with RC or LC?
How do they compare soundwise? Now it seems that VTB9042 is going to be the most expensive item in my  eq. 

 

[ruffrecords]

Hi boost with RC or LC?
How do they compare soundwise? Now it seems that VTB9042 is going to be the most expensive item in my  eq.

The RC high boost is a shelving type. This means just about all frequencies above a certain value are boosted more or less equally. Useful for adding overall clarity to the top end.The LC is a bell shape type which means a set band of frequencies only are boosted. This is handy if you need to pull a particular instrument out of the mix. I am sure you can find a free plug in somewhere that will allow you to compare the sound of the two. Only your ears can tell you which one you need/prefer.

Cheers

Ian

 

[theaterguy]

Thanks, it's very clear now!

 

[ilfungo]

My diagram was intended to show the resistor values you need to get an appropriate input impedance and gain range for the the  amplifier. You do need to also take into account the specific requirements for dc isolation and stability of the device you choose.  In general you should not need an input coupling capacitor. A capacitor inseries with the 2K2 pot is probably a good idea as it keeps bias current out of the pot and also ensures the op amp dc gain remains at unity so offset voltages are not amplified. I think 59KHz is a little on the low side for a 3dB point as this could lead to significant phase shift at 20KHz. I would try 100pF.

Cheers

Ian

Thanks to all!!!
Now my unit it's working Thanks!!!

Just one info about a 2k2 series capacitor...
Little help for the right value?
THANKS!!!

 

[ruffrecords]

Just one info about a 2k2 series capacitor...
Little help for the right value?
THANKS!!!

Just calculate it for a subsonic -3dB point. For example 2K2 and 47uF has a -3dB point close to 1.5Hz.

Cheers

Ian

 

[ilfungo]

Thanks Ian
cap negative  to 2k2 pot?
Sorry for a stupid question...

 

[Humner]

Hi boost with RC or LC?
How do they compare soundwise? Now it seems that VTB9042 is going to be the most expensive item in my  eq.

The RC high boost is a shelving type. This means just about all frequencies above a certain value are boosted more or less equally. Useful for adding overall clarity to the top end.The LC is a bell shape type which means a set band of frequencies only are boosted. This is handy if you need to pull a particular instrument out of the mix. I am sure you can find a free plug in somewhere that will allow you to compare the sound of the two. Only your ears can tell you which one you need/prefer.

Cheers

Ian

i was going to ask the same question but also had another question.

I've been using this calculator - http://www.calctool.org/CALC/eng/electronics/RLC_circuit

Very helpful for determining center frequency values - however I'm getting stuck on converting the results to Q.

They give the FWHP - which as I understand is the width between the half way points(halfway on each side of the bell curve). However, we actually want the value from the -3db point on each side instead so we can convert this to the Q value - is this correct?

So my question is, is there a simple(or at least a way) to convert the FWHP value they provide to Q?

Secondly - have I opened up a can of worms I shouldn't have opened?  ;D

 

[ruffrecords]

It is very easy to calculate Q. It is just wL/R where w is the frequency in radians (2,pi,f), L is the inductance and R is the resistance.

The hard part is determining the value of R. Most passive EQs use a series resonant circuit. This means R is the sum of all series resistances in the series resonant circuit. These include the resistance of the inductor, the resistance of the Q pot if there is one and the resistance of the boost/cut control itself. You also have to consider what you mean by Q. If you set a resonant EQ circuit to give 2dB of boost, how would you measure the -3dB points? For this reason it is common to specify the Q at maximum boost or cut.

Cheers

ian

 

[unklepablo]

Hi can anyone help me with the pad switch wiring for the new mk2 boards (with mid)? I see in Dave's wiring diagram some of the pots (and from T1 on the high board) are wired to the dpdt switch,  yet my board has no T1-4 connections, instead just 'in' and 'out'  as the pots are wired direct to there own 3 pin connectors on the board. Everything else is in order (pcb's complete, pots wired, power wired and ready to test) but just the wiring diagram is not making much sense as it for separated hi and low board which required manual wiring of the pots which mine does not seem to need. If I had the separated hi and low boards it would make perfect sense to wire it as per Dave's hook up diagram. If I wanted to I could wire the same way as Dave but using my mk2 boards but wont that defeat the object of the updated board with nice 3 pin connectors for the pots?

I'm wiring 2 mk2 boards up in 1 case for a stereo pair. Will this make any difference? Ive got 2 dpdt switches 1 for each channel.

Hope someone can help.

Thanks

Paul

 

[ruffrecords]

Dave's hook up diagram is intended for the original design. The MK2 aimed to simply construction by containing a lot of the wiring on the PCB and having standard pin outs for the pots and switch. You need to read the MK2 assembly guide which you can get here:

https://drive.google.com/file/d/0B_n67A1hN3qtbXNYWHc0STBlZGM/view?usp=sharing

It is also in the MK2 folder in the PMEQP1A folder of the DIY tab of my web site.

Cheers

IAn

 

[unklepablo]

Thanks Ian, I have read the guide over and over and I can see just looking at the board/components everything is built in for the switches. I dont see any mention of switch wiring in your guide. Ive never wired a pad switch before.  Am I meant to wire 1 side of the dpdt switch to the in/out of the pots and the other to the in/out of the pad? it seems to me that would bypass the gain stage. Im just learning here and feel so confused. So sorry to ask these stupid questions but I need to understand.

Im trying to wire 2x of your mk2 board to 2x your tube gain stages. This is the part Im finding most difficult.

Cheers
Paul

 

[unklepablo]

Maybe I should just follow the in/out for the eq on the 2 wire digram? Would that work ok?

https://drive.google.com/folderview?id=0B_n67A1hN3qtfkNFTjdqVUtDeUM0YlNzWHZUQ1BKUHpPTkt4TnB3aV8xREtIVmo4Z0xPVlU&usp=drive_web

Thanks again
Paul

 

[ruffrecords]

Maybe I should just follow the in/out for the eq on the 2 wire digram? Would that work ok?

https://drive.google.com/folderview?id=0B_n67A1hN3qtfkNFTjdqVUtDeUM0YlNzWHZUQ1BKUHpPTkt4TnB3aV8xREtIVmo4Z0xPVlU&usp=drive_web

Thanks again
Paul

Apologies, I had not noticed you were talking about the EQ bypass switch. Yes, you wire that up like any passive EQ as shown in the 2-wire diagram.

Cheers

ian

 

[ruffrecords]

They give the FWHP - which as I understand is the width between the half way points(halfway on each side of the bell curve). However, we actually want the value from the -3db point on each side instead so we can convert this to the Q value - is this correct?

So my question is, is there a simple(or at least a way) to convert the FWHP value they provide to Q?

I just realise the FWHP stand for full width half power which is the -3dB points. So just divide the centre frequency by FWHP to get Q.

Cheers

Ian

 

[unklepablo]

Maybe I should just follow the in/out for the eq on the 2 wire digram? Would that work ok?

https://drive.google.com/folderview?id=0B_n67A1hN3qtfkNFTjdqVUtDeUM0YlNzWHZUQ1BKUHpPTkt4TnB3aV8xREtIVmo4Z0xPVlU&usp=drive_web

Thanks again
Paul

Apologies, I had not noticed you were talking about the EQ bypass switch. Yes, you wire that up like any passive EQ as shown in the 2-wire diagram.

Cheers

ian

Thanks Ian at I think i'm on the right path, its making sense now.  Just to clarify on the 2 wire digram the eq board shows in and out being 1 wire with a 0v line ran from the transformer to the eq board and finally the output switch. I have 2 wires each side of my mk2 board for i/o. Do I simply measure the voltage to find out which pin is 0v and link them together at the switch? Also do I omit the 12k resistor on the input with my version being unbalanced (may add transformers for the next eq build, but keeping costs low for this one).

Ill get some pics up when I have the front panel sorted.

Thanks for all the help.

Paul

 

[ruffrecords]

Thanks Ian at I think i'm on the right path, its making sense now.  Just to clarify on the 2 wire digram the eq board shows in and out being 1 wire with a 0v line ran from the transformer to the eq board and finally the output switch. I have 2 wires each side of my mk2 board for i/o. Do I simply measure the voltage to find out which pin is 0v and link them together at the switch? Also do I omit the 12k resistor on the input with my version being unbalanced (may add transformers for the next eq build, but keeping costs low for this one).

Ill get some pics up when I have the front panel sorted.

Thanks for all the help.

Paul

Just take the 0V from the EQ input connector to the input. Use the 0V connection of the output for the screen of the output cable shown on the 2-wire. The 0V pin on both connectors is the one with the square pad which is the bottom one in both cases. You can omit the 12K resistor at the input.

Cheers

Ian

 

[unklepablo]

I have drawn and attached my own version of the 2 wire hook up diagram based on the power transformer I am using and the MK2 boards. Its is for unbalanced operation. I will be using this to power 2 gain stages so I'll be duplicating many of the power connections from one toroid/psu but decided just to keep it simple and show the one in the diagram. I hope it can help someone else in the future.  Re-drawing it myself really helped me to understand how and why it works.

Fingers crossed it might actually work.

Cheers
Paul

 

[ruffrecords]

Looks OK to me ap[art from the output socket is wired incorrectly. Right now the Out goes direct to chassis.

Cheers

Ian