[silent:arts]

What is the T-Pad marked as on the schem?
R1 (-A / -B / -C)


Thanks for confirming, I thought so but thinking ain`t knowing.  8)

Hi Guys

I am stumped. Which is a first on this build (as I have built many). I'm trying to repair a build that someone else had a go at, but made a complete hash of in every possible sense.

The preamp (with one of my opamp builds - his one was shot to bits and was promptly thrown in the bin!) passes audio cleanly:

output attenuator works
phantom works
-10 and phase, mute all work.
25k reading from outer and inner pin of grayhill

but no gain.

Basically the soldering on the build was so bad, that I have had to desolder most of the board.

I can get the schemo out and start testing, but I just wondered if anyone here has experienced the 'no gain, such a pain' problem, and can get me route one to the heart of the matter.

Cheers Matt

jsteiger

Matt, no gain tells me that there is a cold solder joint in the inverting opamp to ground string. I suggest following the schematic and looking at the 200Ω R, the 330µF cap, the string of Grayhill resistors and the switch. If something in there is floating, the gain switch will not function.
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Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #64 on: November 02, 2015, 09:15:56 AM »
Hey Jeff and  all you good people ,
      I built a 312 and 312DI ..Both with the GAr2520 DOA. I thought they were great..but lately something is a miss...I noticed the sound to be quite thin after recording drum overheads. That sparked me to do some testing. Talking into and using an array of mics and comparing to a UA 610 pre and my console pres, I found they do not have much gain and lacking any sort of low end to the sound. Also to get the required gain , it  brought up a good bit of noise too .. As I was turning knobs and testing the buttons, this is what I noticed ..
 
 A) The output pot all the way to the left ( towards infinity symbol) passed sound just as loud as being all the way to the right but warmer ..as you turn it to 9 o'clock ( from far left ) the sound disappears and as you continue to turn it to the right the sound comes in on a gradual slope ( as one might expect) 

B) If you push in the  polarity switch, the output knob acts in reverse. And I tended to like the sound better with it pushed in.

C) As I stated earier, The gain seems really low. The design boasts 60dbs in gain , which is comparable to the UA, but no where close to the gain I get from the UA or my console pres for that matter...I need to turn the gain almost all the way up to get a usable signal. Introducing a lot of noise too... And this is vocal testing with a Blue Bottle Rocket, an Octova ribbon ( which i couldn't even get the gain needed ) , and a Seinnheiser MD421 ...

The weird thing is that both pres , the 312 and 312DI ,  behave the same. I have them mounted in a Radial Engineering Powerstrip. I could of swore they operated much better than this when I built them months ago. 
     Has anybody else had these issues? Do the pres behave this way by design ? Do you think I could have an issue with power from the 500 series rack? Or is something wrong with the pres? 

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #65 on: November 02, 2015, 10:16:28 AM »
It is very strange to me that this could happen with both preamps at the same time. This is far from being normal. I would first rule out any potential problems with the opamps. Also make sure the opamps are seated correctly.
http://capi-gear.com/catalog/DOA_Install.php

The next thing I would do is check and reflow for bad solder joints. The symptoms you describe to me sound like bad solder joints.
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Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #66 on: November 02, 2015, 11:45:00 AM »
Hey Jeff,
    Tis really strange.  I will get to trouble shooting this evening.

 Just to be clear though,  the output knob " should not"  behave that way?

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #67 on: November 02, 2015, 06:57:17 PM »
Just to be clear though,  the output knob " should not"  behave that way?
No, definitely not.
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Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #68 on: November 03, 2015, 12:44:43 PM »
Hi Jeff,
    So I went thru and took a magnifying glass to all soldier joints and even reflowed the soldier joints. I checked for the proper positions of all components . All good . I did the testing at the end of the VP26 build instructions and came up with 9.5ohms between  C and O . It states there should be over 200k.. I did see in an earlier post from 2012 where you stated it should be about 8.5ohms so I guess this is fine? I won't be able to test it tonight due to destroying the gain pot trying to remove it to isolate where O was shorting to ground..( before I found the 2012 post)..ugh..

  I also went thru the Opamp in the same way..all looks good..

Well I need to jump on your site and order a gain pot..I am also going to order a ready built opamp just in case..

Thanks!!

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #69 on: November 09, 2015, 09:23:07 AM »
Soooo..update...I went thru the 312  replaced gain pot , resoldered, ran tests, said prayer and all is good except for C8 which I had backwards which maybe is why it was noisy.. Installed in rack and still had the same issue...with both the 312 an 312DI..SO I started looking else where..I had recently wired up an extensive patch bay network. Before I had been using mic cables and crawling behind the gear to make connections. So I disconnected it from the patch bay and tested with mic cables..THEY WORK RIGHT AGAIN!! So thru further troubleshooting, and this is where I become a dumb ass , I discover I had yet to connect the sleeve and ring on the TRS connections going into my "unbalanced" line ins on the console. Thus I was not properly converting the out going balanced signal from the pres to an unbalanced signal for the line ins..Ugh...Well..at least I know the pres are working and kick ass. Moral of the story ; don't forget about proper cable connections, and patch bays always will be a menace..At least my knees and back are getting a respite ..Thanks Jeff for the help and making great DIY kits.
 


Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #70 on: January 08, 2016, 06:55:23 PM »
Hey Jeff,

Just received two 312 (w/Litz) kits today! I noticed on the BOM for them though, that R2 is marked with an asterisk which stated

 "optional loading resistor, not typically found on a vintage 312 card"

Please excuse my ignorance, but what does this mean? I'm assuming I do not have to solder in this resistor if I do not want to? How would this affect the sound/performance, and is it ideal to leave it out or solder it in?

Thanks


***EDIT***
So I just finished one of the kits and I simply left the resistor at R2 out. I went to measure the resistance at the sockets for the DOA and I want to confirm some measurements:

-V and +V read "Overload" (pass)

-V and C read "Overload" (pass)

+V and C read "Overload" (pass)

C and O read 9.5 Ohms. I read in an earlier comment in this thread it should be about 8.4 ohms? So does this pass?

+V and O read "Overload" (pass)

-V and O read "Overload" (pass)

SO assuming C and O reading is okay, I should be fine going to testing with power?
Just want to confirm so I dont end up cooking anything!

Thanks again
« Last Edit: January 09, 2016, 02:59:05 AM by Houstnwehavuhoh »

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #71 on: January 09, 2016, 04:37:48 PM »
..... R2 is marked with an asterisk which stated "optional loading resistor, not typically found on a vintage 312 card"

Please excuse my ignorance, but what does this mean? I'm assuming I do not have to solder in this resistor if I do not want to? How would this affect the sound/performance, and is it ideal to leave it out or solder it in?
Installing R2 with lower the input impedance of the preamp to around 1k. Leaving it out will yield appx a 1.2k input Z. The pre will sound a little more "open" without R2.

Quote
C and O read 9.5 Ohms. I read in an earlier comment in this thread it should be about 8.4 ohms? So does this pass?
This is perfectly fine. Possibly just a slight difference in DMM tolerances.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #72 on: January 09, 2016, 08:45:16 PM »
..... R2 is marked with an asterisk which stated "optional loading resistor, not typically found on a vintage 312 card"

Please excuse my ignorance, but what does this mean? I'm assuming I do not have to solder in this resistor if I do not want to? How would this affect the sound/performance, and is it ideal to leave it out or solder it in?
Installing R2 with lower the input impedance of the preamp to around 1k. Leaving it out will yield appx a 1.2k input Z. The pre will sound a little more "open" without R2.

Quote
C and O read 9.5 Ohms. I read in an earlier comment in this thread it should be about 8.4 ohms? So does this pass?
This is perfectly fine. Possibly just a slight difference in DMM tolerances.

Thank you !

Blackdawg

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #73 on: January 11, 2016, 11:12:19 PM »
Jeff,

Im very excited to have two of the VP312 kits! I got them for Christmas from my Dad. I requested them after hearing them at my School, Webster University.

Im about half way through my first one. About to do the power transformer. I've just been using a combination of the VP2x guide on your site as well as the BOM on the site to hash out what componets go where. But I'm not sure what the RG slots are for? I am out of componets so I am not totally sure what they are for.

I've read in this thread you generally email a build guide for the 312 after one purchases the kit. Could I get a copy of that just so I know I'm doing everything right? I'm very certain i have all the resistors and capacitors in the right spots. But couldn't hurt to double check. I think I am missing some washers too for the Power Transformer. I just have lock washers.

My other question is the op amp. My Dad didn't get my any so I am going to get some. My question is, are there any big differences? I am a student so the cheaper ones are tempting, but I also know you get what you pay for generally especially in audio gear. So basicially are the VF600s you sell worth it? Or should I go with an SL2520 or a gar or ML, JLM, John Hardy? There are so many. How different are they?

Thank you!

Monte
« Last Edit: January 11, 2016, 11:23:35 PM by Blackdawg »

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #74 on: January 12, 2016, 10:37:17 AM »
Monte, the RG positions are for the resistors required if you are building a stepped gain module with a Grayhill switch. I assume you have a Bourns pot for variable gain so no VP-Gainswitch R's were shipped.

All of the support docs are published on my site. You will find a hardware BOM that shows how to install the output transformer (its not a "power" transformer) with the split lockwashers that were provided. http://capi-gear.com/catalog/support_docs.php

As for the opamp, it depends on what you want to do. If this is your first DIY build, I would suggest a prebuilt opamp so you know its working, in case there is trouble at startup. I would go with the tried and true so a red dot or a gar2520. The 990 is not the best match for the EA2622 transformer so I never recommend using them in a 312 style circuit.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Blackdawg

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #75 on: January 12, 2016, 02:00:32 PM »
Monte, the RG positions are for the resistors required if you are building a stepped gain module with a Grayhill switch. I assume you have a Bourns pot for variable gain so no VP-Gainswitch R's were shipped.

All of the support docs are published on my site. You will find a hardware BOM that shows how to install the output transformer (its not a "power" transformer) with the split lockwashers that were provided. http://capi-gear.com/catalog/support_docs.php

As for the opamp, it depends on what you want to do. If this is your first DIY build, I would suggest a prebuilt opamp so you know its working, in case there is trouble at startup. I would go with the tried and true so a red dot or a gar2520. The 990 is not the best match for the EA2622 transformer so I never recommend using them in a 312 style circuit.

Correct! No stepped Gain pots. Good to know! Thank you.

ah output transformer, thank you again. It seems very straight forward with the nice labeling of the PCB, just wasn't totally sure if the lock washers should go in between the transformer and the PCB. Thank you for confirming!

I was going to go with a prebuilt one for this pair of pres, as you said to just simply things. Its not my first build, but its my first mic pres. Im still curious about the VF600, is that a modified 2520 or a 990? Im assuming 990. what, if any, are the sonic differences between the gar and SL?

Thank you for your quick reply! Very nice to beable to ask the maker himself questions! Love the kits. Really dig the new face plates too.

Monte

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #76 on: January 12, 2016, 02:12:01 PM »
The VF600 is Steve Firlotte's (Inward Connections and Tree Audio) amp block. Its his own design.

The red dot and gar2520 are pretty similar. Both are nearly identical in sonics to mid '70's Huntington's and Melville's. Some folks prefer the red and some the gar. Neither is a bad choice.
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CAPI is not affiliated with, endorsed by, or sponsored by API.

Blackdawg

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #77 on: January 12, 2016, 03:03:53 PM »
The VF600 is Steve Firlotte's (Inward Connections and Tree Audio) amp block. Its his own design.

The red dot and gar2520 are pretty similar. Both are nearly identical in sonics to mid '70's Huntington's and Melville's. Some folks prefer the red and some the gar. Neither is a bad choice.

Cool thank you again! My Schools has the Sl2520, so I think I will opt for the Gar2520 and compare the two for myself! Maybe try a VF600 at a later time or another build.

Thanks again Jeff!

Monte

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #78 on: March 01, 2016, 11:58:02 PM »
Still a bit curious as to why this cap blew. Was it because those two resistors seem soldered in the wrong spot? Is it as easy as swapping them and grabbing a new kit? Any help is appreciated!
 http://imgur.com/a/sMehZ

jsteiger

Re: [Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread
« Reply #79 on: March 02, 2016, 10:15:26 AM »
I would just replace the 120µF cap. Should be no biggie. Not sure why it failed.
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