EQP-1S5 500 series PULTEC ---- HELP/BUILD thread

[mitsos]

Hi again Joe,

thanks for the quick reply.  I modified the build document to state that opamps with high DC offset might require circuit modifications, that was what I assumed was the issue in sonic02's unit.  Thanks again for clarifying. 

For anyone interested in using a JLM99V (or other opamp with high DC offset) this mod won't be pretty, but can be done.  The PCB does have a spot for a feedback cap so that's the easy part, but there is no spot for the 100uF, that would be the ugly part.  You would have to put the cap in series with the trimpot (which is in the spot where the 1K47 is in the schematic Joe posted). You can choose to put it before or after, as Joe said, but I think convention is to put it after, as in the drawing.

Thanks again Joe.


@Vernois .. I took the doc down for editing. Will be up in a minute.

 

[okgb]

Thanks for taking the time to post , quality guy I'd say
honestly makes me want to check out more from your site , thank you

Although,  this version is authentic , so it works ,  mod if update or other opamps desired
Mine is working fine using Whistle rock 2520

 

[Winetree]

Both caps would be best fitted so all opamp types can run stable in the circuit with better headroom including the 2520. In all EQP1A3 Solid States that we have refurbed at JLM have added a 120pF feedback cap as recommended in old API 2520 notes as it improves the wideband THD measurements. Most other opamps will be fine with 47pF like our 99v. See circuit mod below.

Please clarify
Sounds like the mod should be done with all opamps including the 2520.
Is really necessary using GAR 2520 or 1731. If so any pictures showing the mod?

 

[sonic02]

If anyone is game to take on the mods  on ;D can you post some pics for the rest of us.

 

[mitsos]

Both caps would be best fitted so all opamp types can run stable in the circuit

Winetree, Joe is not saying here that the caps must be fitted for ANY opamp to work. What he is saying is to fit them for ALL opamps to work. 

The 100uF cap is not necessary if using a 2520 or other opamp with little to no DC offset, but may be considered proper form (FWIW, neither caps were used in the originals). 

For those of you who have built 312 preamps, you'll remember that there is a cap to ground in the gain circuit, same spot as the 100uF in Joe's drawing. This creates a HPF to block DC from causing scratchiness when turning the pot.  In the case of a fixed gain amp like in the EQ there is no pot to get scratchy so that reason is moot in this case. Another reason to use it is if your opamp inherently has high DC offset, like the JLM99V (there may be others, but I am only aware of the JLM as of now), in this case the cap must be used or you run out of headroom real quick.

The feedback is also optional. It creates a LPF with the feedback resistor (20K5), so for 47pf you have a LPF at about 165KHz, and with 100pf at 77 KHz.  (values in between will give you frequencies in betwen those).  Limiting the high end can help some opamps be more stable but, again, the 2520 does not exhibit any problems, so I would only use this if you experience problems with your opamp. 

That said, we have personally tested new and vintage 2520s and a 2520 remake based on the old huntinton's, as well as the 5534. We have also heard back about Scott Lieber's Red Dots and  Mike Fatom's 2520 working properly and I am pretty sure Gar2520's will work flawlessly (I'm sitting on two PCBs I need to build up...), but anyone who has used them please feel free to post.

In fact, if people want to PM me which opamps they are (successfully) using along with any that maybe didn't work, I will add that to the help file and/or the second post (which is still waiting to be filled!).

Hope that explains a bit. 

As for modding the circuit, there are a few ways to do it, none are ideal as the cap would remain sort of flying in the air, but it is possible. For a 100uF cap try panasonic NHG, they are very small.  You have to lift the bottom leg of the trimmer and isloate it from it's pad to ground. connect the + lead of the cap to the now free lead of the trimmer, and the - lead of the cap to the top lead of R5 (the side going to ground). 

 

[Biasrocks]

I'm using a pair of Mike's ML2520's here.

http://www.whistlerockaudio.com/cart/index.php?main_page=product_info&products_id=87

Regards,
Mark

 

[dandeurloo]

Red Dots are working!

 

[JLM Audio]

Both caps would be best fitted so all opamp types can run stable in the circuit

Winetree, Joe is not saying here that the caps must be fitted for ANY opamp to work. What he is saying is to fit them for ALL opamps to work. 

The 100uF cap is not necessary if using a 2520 or other opamp with little to no DC offset, but may be considered proper form (FWIW, neither caps were used in the originals). 

For those of you who have built 312 preamps, you'll remember that there is a cap to ground in the gain circuit, same spot as the 100uF in Joe's drawing. This creates a HPF to block DC from causing scratchiness when turning the pot.  In the case of a fixed gain amp like in the EQ there is no pot to get scratchy so that reason is moot in this case. Another reason to use it is if your opamp inherently has high DC offset, like the JLM99V (there may be others, but I am only aware of the JLM as of now), in this case the cap must be used or you run out of headroom real quick.

The feedback is also optional. It creates a LPF with the feedback resistor (20K5), so for 47pf you have a LPF at about 165KHz, and with 100pf at 77 KHz.  (values in between will give you frequencies in betwen those).  Limiting the high end can help some opamps be more stable but, again, the 2520 does not exhibit any problems, so I would only use this if you experience problems with your opamp. 

That said, we have personally tested new and vintage 2520s and a 2520 remake based on the old huntinton's, as well as the 5534. We have also heard back about Scott Lieber's Red Dots and  Mike Fatom's 2520 working properly and I am pretty sure Gar2520's will work flawlessly (I'm sitting on two PCBs I need to build up...), but anyone who has used them please feel free to post.

In fact, if people want to PM me which opamps they are (successfully) using along with any that maybe didn't work, I will add that to the help file and/or the second post (which is still waiting to be filled!).

Hope that explains a bit. 

As for modding the circuit, there are a few ways to do it, none are ideal as the cap would remain sort of flying in the air, but it is possible. For a 100uF cap try panasonic NHG, they are very small.  You have to lift the bottom leg of the trimmer and isolate it from it's pad to ground. connect the + lead of the cap to the now free lead of the trimmer, and the - lead of the cap to the top lead of R5 (the side going to ground).

Yes all of above is correct. Don't Panic.

The 100uF would be a good add on if a new PCB revision was done in the future only but with 2520 and 1731 or any opamp with diff pair input the offset depending on the match of the diff input pair and input current mismatch so should be no worse than a couple of hundred millivolts offset on its output pin so loss of headroom is minimal.

The thing I would add is the CF cap as the unit may function fine but random radio/mobile/digital pickup without the feedback cap will happen and make its way into the audio output as at least short bursts of higher %THD or at worst audible noise. A 2520 without the CF cap at the low gain used in the EQ will be flat response out to several MegHz. Remember there wasn't any mobile phones, bluetooth, WiFi around when the original EQP1A3 was designed.

The 99v does need the 100uF due to it non diff pair input and low overall gain (which is why is has the huge low end warm and silky highs of a single ended tube circuit instead of the tight normal opamp sound. ;-)

 

[mitsos]

The 99v does need the 100uF due to it non diff pair input and low overall gain (which is why is has the huge low end warm and silky highs of a single ended tube circuit instead of the tight normal opamp sound. ;-)

You had me up until the GS marketing shpeal!  ;D

seriously, thanks for confirming.

FWIW, offset on a 2520 can be measured on one, maybe two hands, in rare cases, feet too. Unless you are running a DI straight into it, like the Bo Hansen DI mod for the 312 preamp, then you'll see about 100mV.  That's the most I've ever measured.  I always match my input pair when building them, maybe that helps.

 

[GregNey]

I've got the GAR2520's in both of mine and they sound great to my ears. I have not done any formal measurements.

 

[mitsos]

Great, thanks for confirming that.  I think that's it for DIY2520s then?  I should try to get my melcors built, but I'm pretty sure those will work if the 2520s do.

So far, the only thing to look out for in choosing an opamp is high DC offset, which should work with the mods Joe Malone described above for the JLM99V.
Here is a somewhat easy way to install the cap, not the only way of course. I have not tested it, but I will in the next couple of weeks:

Install feedback cap (something between 47 - 100 pF, check with opamp mfg).

Remove the trimpot if already installed. Put a piece of tape over the pad going to ground, and bend the trimmer leg that was installed there towards the front of the PCB.
Install trimmer using the top two legs, and connect the now free leg to the positive lead of the cap. The negative lead goes to the top of R5. 

If you use a small cap (Panasonic NHG, for example) you may be able to ziptie it or otherwise secure it to the trimmer for stability. 

Use heatshrink tubing to ensure against shorts.


Again, this mod is only necessary for opamps with high DC offset, not the 2520s that the original circuit used.

 

[kilmister]

Hi all!

Just finished my pair of EQP1S5's.
Nice straightforward build and really good sounding eq's.
At the moment I'm using Mitsos 5534 boards and waiting to get my gar2520's.

Huge thanks for Mitsos for great work!



-Paavo

 

[mitsos]

Hi all!

Just finished my pair of EQP1S5's.
Nice straightforward build and really good sounding eq's.
At the moment I'm using Mitsos 5534 boards and waiting to get my gar2520's.

Huge thanks for Mitsos for great work!



-Paavo

Nice work!  Enjoy the EQs!

 

[Axelerator]

almost ready with first unit here, two questions left so far :

i suggest i have to solder the headers after mounting filter board and bending them to fit ?
- i tried to  get my trimpot in middle position, but i can screw it in any direction as far as i want and will find no end position ? tried much more than 20 turns so is is alright ?
  would like to verify this before slodering the pcb to pcb headers !
thanks
axel

 

[kante1603]

- i tried to  get my trimpot in middle position, but i can screw it in any direction as far as i want and will find no end position ? tried much more than 20 turns so is is alright ?
  would like to verify this before slodering the pcb to pcb headers !
thanks
axel

Hi Axel,

Measure directly on the pot contacts from whiper (normally the middle pin) to both ends for resistance.When end position is reached one of those connections will show zero ohms.The other connection will show the full pot resistance when out of circuit.In circuit the resistance might differ depending on the surrounding parts,but an end position always shows zero on one side.

Cheers,

Udo.

 

[Axelerator]

just measured in the two opamp sockets while screwing the trimpot and i could fade between 104.6 and 99.5 k  so thats 5k difference as that is the value of the pot so
i screwed it to 102.3 for start . hope this makes sense.
stillnot know if i should solder the pcb headers after bend and fix ..
greets axel

 

[mitsos]

Hi Axel,

Almost there!  You should be fine, you might want to try to measure the pot value between the opamp -IN and ground. Should give you more or less the correct value. 

And as for the headers, yes, they need to be soldered once the top (filter) PCB is fitted.

good luck!
cheers!

 

[Axelerator]

..4 fully working EQ´s here !! ;D 8)
this together with the 511x rack and dual psu supply is my first really working DIY sofar..
went together like a charm many thanks
just tried reddots and let the sound of my machinedrum through it and you can make the beat sound very fatt quick and nice
when you give it some serious boost at 70 hz while cutting the same freq it sounds like having some  compression-like fx
while sounding very warm, mids are wonderfull opening up and doing some high cutoff is always nice on digital synth and
drums!
now up to the lovechilds,and then to the helios eqs which i had to stop because farnell finished my account
always prepaid them but they dont want any nonprof. any more..
Having all in a full kit is a major time and money safer 
greets
axel

 

[mitsos]

..4 fully working EQ´s here !! ;D 8)
this together with the 511x rack and dual psu supply is my first really working DIY sofar..
went together like a charm many thanks

NICE! Congrats!

just tried reddots and let the sound of my machinedrum through it and you can make the beat sound very fatt quick and nice
when you give it some serious boost at 70 hz while cutting the same freq it sounds like having some  compression-like fx
while sounding very warm, mids are wonderfull opening up and doing some high cutoff is always nice on digital synth and
drums!

simultaneous low boost/cut is very nice. It's like natural compression or nice analog saturation, while the notch in the low mids (from using the low controls simultaneously) keeps it from getting muddy.    With both the low tricks and high boost you can make things totally stand out in a mix.  And the high boost on vocals is beautiful, (if I may say so myself ).

Having all in a full kit is a major time and money safer 

The idea was to save everyone time/money, but I really did it so people would finish these quickly (and not have them sitting in a drawer for 2 years). 

 

[Dr Gris]

Got two of these wonerful EQ' s coming my way.

Do you think the APP992 opamp http://www.groupdiy.com/index.php?topic=33576.0 would work in this?
By reading the spec I can' t tell about DC offset... I' m too much newbie, too old and sometimes I smell bad ;D
I' m thinking I want a neutral sounding opamp in these. I' ve got GAR 2050' s and 1731' s to try as well.


Kind regards
Magnus