[BUILD] CAPI LC53A~500 Series~Love Child EQ Kit~Official Support Thread

[bdunard]

Can I attach my common ground anywhere on the chassis for ground...or does it need to me on a common from one of the removed opamps?

And I'm assuming I should leave the DTO5 inplace?

 

[bdunard]

Ok with everything in place except the Boost/Cut PCB I attached a ground to the chassis and ran a 500Hz signal in to the unit at 1.23 V.

My readings are:

1.14 V  --  1st reading: negative side of C2 on the Main PCB.
1.14 V   --  2nd reading: output pin of A1, DF on CB PCB
1.135 V  --  3rd reading: output pin of A2, DF on CB PCB
.777 V    --  4th reading: output pin of A4, DTO5 on CB PCB
.781 V   --  5th reading: output pin of A3, DTO5 on CB PCB
.782 V  --  6th reading: neg side of C12 on the Main PCB

 

[jsteiger]

You should be looking for the THD that you said was present. Check for it at the mentioned test points to see where it begins.

 

[bdunard]

Just wanted to post an update to my LC53A issue.

Turns out the Eq was fine the whole time.  I was testing it in a rack that has both xlr and db25 connections.  I was using xlr to tt cables just to quickly patch into the channel  on the rack  and test.  Meanwhile the db25 connectors were patched into a normalled patch on the patch bay which allowed it to feedback into itself while using the xlr connections at the same time. 

 

[fancytimestudio]

Hey everyone!
New to post here but I've been reading this forum a lot. I assembled a Love Child and I'm having an issue with it. I was hearing some sort of distortion so I re-did the CMR calibration and that didn't seem to fix the issue. Anyway, I have a couple of screen shots here. The top audio file is the "clean" 400Hz sine wave. The middle audio file is the sine wave returned from the EQ. Lastly, I lined up the two audio files and flipped the phase so I could see the difference. That's what the bottom audio file is, the difference. I looked for bridged solder points and didn't find any. I also checked to make sure my diodes were all attached with the correct polarity. I would love some help with this. Pretty sure this isn't how this EQ is supposed to sound. Any ideas?

 

[fancytimestudio]

Here is a zoomed screen shot.

 

[jsteiger]

Is this audible THD? A % of THD+N would be a more helpful measurement IMO. I don't have a completed unit here in front of me but don't recall it being much over .05% on avg. Maybe in the .1% @ 20Hz with a +4dBu input sine.

Distortion will most likely be coming from an active stage like an opamp or discrete follower. I would suggest measuring THD at all of the test points and see where it starts. Make sure all of the amps are fully seated. http://classicapi.com/catalog/DOA_Install.php

 

[Studio Mollan]

Hi,
I've built 18 of these and use them every day in my api console. Super nice eq that was well worth the effort of putting them together. (Took quite a while..)
I recently modified a diy neve 1073 eq for a wider Q midrange filter and now Id like to try out a mod on the lc53. How about getting rid of the hi-pass and instead use that switch as a wide Q (wider would be more appropriate). Could you point me in the right direction here Jeff? I need to find the resistor in series with the mid band inductor and increase the value right?
Since I have hi-pass filters in the console I could live without it on the eq.

/
Emil

 

[jsteiger]

This would take some doing. All of the inductance values would need to be changed as well as caps and then resistors. With this EQ, I don't think you could easily toggle between wide and narrow Q's. It would almost need to be one or the other.

 

[Studio Mollan]

This would take some doing. All of the inductance values would need to be changed as well as caps and then resistors. With this EQ, I don't think you could easily toggle between wide and narrow Q's. It would almost need to be one or the other.

Hi jeff and thanks for your reply.
On the neve 1084 the hi-q switch switches the resistor directly after the inductor. Higher value gives wider q. On the 1073 I managed to do this pretty easily but the lc53 is not designed the same way then?
/
Emil

 

[jsteiger]

No Emil, its a different topology. Only switch the R would mostly change the amount of cut or boost.

 

[fancytimestudio]

Thanks for replying Jeff. Yes, this is an audible distortion more noticeable at lower levels. During CMMR calibration using the extension jigs, I was able to get the 400 Hz sine wave down close to -80dB (lowest I was able to meter).  I checked all of my solder points on the DOA sockets (they were fine) and tried two different sets of tested opamps with the same result. I forgot to mention that this distortion occurs whether or not the EQ is engaged ("in" button not pushed in). Here is a screen shot of what is returning from the EQ with the 400 Hz sine at -10dB generated from Pro Tool's signal generator.

 

[jsteiger]

The in/out is not a hard bypass. All it does is lift or connect the LCR filters to ground. The common mode adjust is simply a null for the receiver amp. It has nothing to do with THD. I would suggest making up a cable or probe so you can "follow" the signal thru the circuit and see where the distortion starts. That will help us narrow down the cause. Use the test points PDF that was posted pretty close to after 'Chung's build pics.

 

[fancytimestudio]

1st reading: negative side of C2 on the Main PCB. -clean
2nd reading: output pin of A1, DF on CB PCB -clean
3rd reading: output pin of A2, DF on CB PCB -clean
4th reading: output pin of A4, DTO5 on CB PCB -clean
5th reading: output pin of A3, DTO5 on CB PCB -real dirty
6th reading: neg side of C12 on the Main PCB -real dirty

I popped out the A3 and checked for cold solder joints and popped it back in and it's the same so there must be an issue with that DTO5

 

[jsteiger]

Yes indeed. Check all of the resistors and transistors for proper location. Something must be wrong.

 

[fancytimestudio]

Thanks Jeff. Everything looks good on the layout. Transistors and resistors are in their proper places. I went back over all of the solder points one more time just to be sure there wasn't a cold joint and I'm still getting that same distortion. 

 

[jsteiger]

OK, swap the position of the DTO5's A3 and A4. Then see if the distortion follows the amp or stays after A3.

 

[fancytimestudio]

Did that already and it follows the amp.

 

[jsteiger]

Can you send a close pic of the amp? Top and bot please

 

[fancytimestudio]

Here's the Bottom.