Poor Man's Pultec EQP1-A Build Support Thread

pacorro said:

ah, ok, I saw your freq response graphs and are similar to mine, so it's the natural response of the Eq and not a problem in my device.
Sorry 

Click to expand...

In a simple passive EQ like this one, the steepest rate of cut/boost is 6dB per octave which is the same as 20dB per decade. This means if you want 20dB of boost or cut at 10KHz then you have to start boosting/cutting close to a decade below which is why it starts at around 5KHz. The same arguments apply to the low boost cut which is why the 20Hz curt starts at around 150Hz. If you look on line you wll find curves for the original Pultec EQ and they are very similar.

Regarding the log pot for the bass cut, again this is the same as used by the Pultec. However, if you load the output of the EQ with anything less than about 470K you do get more cut at the 100% setting so the last 25% does seem rather crowded. The bass cut always flattens out eventually so it is not a true high pass filter. I think you are seeing the combination of the bass cut and the roll off already present in your circuit.

Cheers

Ian

mylesgm said:

do you mid sharing the faceplate and where you got it made?

Click to expand...

Sure.
I've got it from Frank, at http://www.frontpanels.de
You can make your own design with a software called frontpanelexpress.
If you want mine, just give me an email and I'll send it to you.

thanks Ian.

By the way... Do you think that my rolloff in 20hz is because the output impedance of my interface?
I'm using an Rme Multiface 2, wich is 100 ohm.
Is possible that entering to the Eq at this impedance can causes this rolloff?
Thanks for your patience

pacorro said:

thanks Ian.

By the way... Do you think that my rolloff in 20hz is because the output impedance of my interface?
I'm using an Rme Multiface 2, wich is 100 ohm.
Is possible that entering to the Eq at this impedance can causes this rolloff?
Thanks for your patience

Click to expand...

An output impedance of 100 ohms should be fine. With all the controls set to off, the EQ looks like a load of over 47K so a 100 ohms output should not have any roll off. It might be worth doing a loop  test without the EQ to check the response of the system before the EQ is added.

Cheers

Ian

I've found it.
The roll off is coming from the DI box.  :-[
I'm working now to add an opamp make-up.
Thanks, Ian

pacorro said:

I've found it.
The roll off is coming from the DI box.  :-[
I'm working now to add an opamp make-up.
Thanks, Ian

Click to expand...

That's interesting. The Behringer specification says it has an incredibly flat response.

If you are going to add an op amp make up it is probably worth checking with some of the other builders on here who have already done that. I am sure they have circuits/PCBs etc you can use.

Cheers

Ian

Cheers

Ian

I finished assembling my eq and it fired up with no black smoke! It passes signal and sounds clean. Dual mids and high bands boost nicely at apparent frequency settings. Low doesn't boost at all ??? NONE of the cuts seem to do anything either I've got something connected wrong. I will retrace the connections tomorrow and see if I can find something stupid. Ive used all Lorlin step switches.
What and where should the tube makeup gain trimmers be calibrated at?
Almost there!

duantro said:

What and where should the tube makeup gain trimmers be calibrated at?
Almost there!

Click to expand...

With the EQ in but all the controls off, adjust the VR1 pot on the tube gain make  up PCB for overall unity gain. So just feed in a 0dBu signal for example and adjust the pot for 0dBu out.

Next switch the EQ out and adjust the VR2 pot for unity gain. All this pot does is set the bypass attenuation to be the same as the EQ attenuation.

Cheers

Ian

Thanks Ian, that adjustment makes perfect sense.
Looking back through the thread, I think I may have connected my caps 1-6 boost values, and 7-12 cut values. I will have to confirm when I get back to my unit to look.
I tried to remember your comment on the first page, but think that since the capacitor calculations chart has boost listed first, I may have went ahead and did it with boost first. I suppose I will just need to swap the t1 to t3 and t2 to t4. I'll report back....

duantro said:

Thanks Ian, that adjustment makes perfect sense.
Looking back through the thread, I think I may have connected my caps 1-6 boost values, and 7-12 cut values. I will have to confirm when I get back to my unit to look.
I tried to remember your comment on the first page, but think that since the capacitor calculations chart has boost listed first, I may have went ahead and did it with boost first. I suppose I will just need to swap the t1 to t3 and t2 to t4. I'll report back....

Click to expand...

It's an easy mistake to make. A few posts down from the top I posted a schematic for how to connect up the boards if you make this mistake. I have a strong suspicion that most people build it this way anyway.

Cheers

Ian

So... it looks like I have my cut caps installed first, on T1 and T2, and boost caps installed second, on the T3 and T4 sides. I did have my low boost pots (stepped switches) wired on the wrong side. However, I'm still not getting any of the cuts or low boosts controls working after correcting this.

I have modified and attached Dave's wiring diagram to SHOW HOW MINE IS CONNECTED. please let me know if you can see any errors.

1. I don't have the red wire (on Dave's modified drawing that is attached) from the low boost to the "G" on the mid boards. I didn't see this connection in your schematics. Is this needed? If so, after all the shuffling of the labeling of the mid boards, where should it attach to? to the screen pad ("S")?
2. I would only need 1 set of the 4k7 and 1k resistors (like in the drawing as well) for both mid connections, correct?
3. Is the 12k (in the drawing) that is between the white and yellow wire of the input transformer needed? I don't see it in any of the schematics.
4. The necessary jumper on the low boards (I still can't find this documented anywhere, except the question on post #4, pg, 1by EJ Whyte) is this supposed to be a jumper from 1 leg of r3 to 1 leg of r2?

I have ChrioN's Inductors hooked up as following:
Hi Mids 9042: Ground Clip to "S"
                      tap 0 to "ind"
                      tap 1 to "D"
                      tap 2 to "E"
                      tap 2 to "F"
                      tap 3 to "G"
                      tap 4 to "H"
Lo Mids 9047: Ground Clip to "S"
                      tap 0 to "ind"
                      tap 1 to "E"
                      tap 2 to "F"
                      tap 2 to "G"
                      tap 3 to "H"

Thanks for any insight!
                   
Edit: revised image mid board labeling and related questions

@Duantro,

Swa[[ing T1 theu T4 as you have done should do the trick but you will need to swap the values of R1 and R3 on this board as well. On the lo PCB you need to have shorting links in place of R1 and R2. You also need to fit R3 = 56K and a shorting link where the little vertical dotted line is at the bottom right hand corner of the PCB.

Don't forget on both PCBs to put a shorting link in place of L1.

Hope that helps.

Cheers

Ian

Then, If my low boost caps are C1-C6 I should have R3 as 56K or R1?

I'm building another one of this and my low cut pot acts strange, it reduces the overall volume and the cut goes really up, tuned to 20Hz, at max cut I have almost no sound and filtered up to 1khz, could it be related? or it's just a wiring mistake? I've checked it and looks fine, but I'll recheck it

dirtyhanfri said:

Then, If my low boost caps are C1-C6 I should have R3 as 56K or R1?

Click to expand...

It is always R3. R3 connects between the wipers of the two halves of the switch so uit does not matter which is boost and which is cut.

I'm building another one of this and my low cut pot acts strange, it reduces the overall volume and the cut goes really up, tuned to 20Hz, at max cut I have almost no sound and filtered up to 1khz, could it be related? or it's just a wiring mistake? I've checked it and looks fine, but I'll recheck it

Click to expand...

Two possibilities. The first is that you are not feeding the EQ output into a high impedance input. Feeding into too low an impedance makes a big increase in lo cut.

If that aspect is OK then it is most likely a wiring error.

Cheers

Ian

ruffrecords said:

Two possibilities. The first is that you are not feeding the EQ output into a high impedance input. Feeding into too low an impedance makes a big increase in lo cut.

If that aspect is OK then it is most likely a wiring error.

Cheers

Ian

Click to expand...

I'm feeding the EQ out to the Tube make up gain stage, so I think it's ok, and the wiring looks fine (Actually I redid it for one channel), I should be misunderstanding something... I'll check it again, and see how it works right now (If I remember right, the low boost pot doesn't boost as expected and the low cut cuts up to 1khz and acts like a master volume)

EDIT: Cheking everything can't find nothing wrong in the wiring, and as I said, I'm using your tube gain stage, and it's output straight to xlr, maybe that's the issue? I'm not sure, because in bypass everything works fine, but maybe when I turn up the low cut pot the impedance goes up or something? I was thinking in build some simple opamp circuit for input unbalancing and output balancing, based in 5532's, but I have a 600:600 transformer at hand for trying

@dirtyhanfri

OK, if you are using my tube gain make up stage then the input impedance should be OK. Just to be sure can you measure VR1 to make sure it is 470K and also that the 1Meg resistor from its wiper to ground is 1Meg. Does the lo boost work OK?

Note: this is a simple passive EQ with a slope of 20dB per decade. So to get 20dB cut at 20Hz you have to start cutting at around 200Hz. In fact, on the 20Hz setting, the response is about 3dB down at 200Hz. Check out the curves I measured here:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/PMEQP1A/LFresponse.pdf

Cheers

Ian

Yes, I understand the slopes, but the cuts seems to be much wider than expected, the -3db point is around 1200Hz when tuned to 20Hz, it doesn't look fine, actually I was looking your response curves for reference.

Also, boost is not working properly, it doesn't give me more than 3 or 6 dbs.

I reworked the grounding, just forgot to route the pin 1 of xlr's to star ground, now it's done, all the rest remains the same, and now, the low cut pot acts as a master fader, no cut, just lows the volume of the whole signal.

My components looks fine, actually VR1 it's adjusted to it's minium, and anyway I can't get the same level with the eq in than in bypass.

I have jumpers for R1 & R2 in the lo board, R3 is 56K, The inductor and the little dotted line under R3 are jumpered, In Hi boards, I have R1 & R2 with their correct values, no jumper in R3, no jumper in dotted line and L1 jumpered. The wiring scheme, as far as I know looks fine, cap values are ok, the pots value also are ok

My ground (and general wiring) is according to dave's wiring diagram omitting the transformers, but I can't get it working propperly

ruffrecords said:

@Duantro,

Swa[[ing T1 theu T4 as you have done should do the trick but you will need to swap the values of R1 and R3 on this board as well. On the lo PCB you need to have shorting links in place of R1 and R2. You also need to fit R3 = 56K and a shorting link where the little vertical dotted line is at the bottom right hand corner of the PCB.

Don't forget on both PCBs to put a shorting link in place of L1.

Hope that helps.

Cheers
Ian

Click to expand...

I should clarify, I didn't actually swap T1 thru T4 on the boards, just swapped the wires coming off of T1-T4 to go to the correct cut boost pot switches. Thus I'm assuming that I don't need to swap any of the r1,r2, etc.
I didn't have the dotted line jumpered, but still have the same problem ( no Low boost and none of the cuts, except lows work) after applying this jumper.
I have all L1's, and r1 and r2 on the low boards jumpered.
I also have connect the red wire in question, in my updated Dave's drawing, that goes from the Low boost to the ground clips of the inductors of the mid boards and see if that would fix anything, but it didn't solve my problem.
Thanks for the help.

There is clearly something basically wrong here. I would suggest simplifying first. Disconnect the two mid EQ boards by disconnecting the wires from the point labelled 'pot' on each mid board. Then check if the hi and lo EQ works ok or not on its own.

Cheers

Ian

ruffrecords said:

There is clearly something basically wrong here. I would suggest simplifying first. Disconnect the two mid EQ boards by disconnecting the wires from the point labelled 'pot' on each mid board. Then check if the hi and lo EQ works ok or not on its own.

Cheers

Ian

Click to expand...

I tried disconnecting the mid boards as you described, and it still is malfunctioning in the same manner. I've triple checked my wiring to my updated Daves dual mid guide, and everything is as listed. I am using stepped Lorlin switches instead of pots. Man this thing is kicking my butt! I will try again tomorrow.