Patrick from Davis
Well-known member
That's what the file describing the circuit on that board looked like, but I can convince myself that it just can't be that easy.
Patrick
Patrick
[BUILD] CAPI VP312DI~500/51x Series~Preamp + Direct Inject~Official Support Thread
- Thread starter jsteiger
- Start date
Help Support GroupDIY Audio Forum:
Patrick from Davis
Well-known member
Despite my best attempts to try and break this thing in every possible way, and some I believe were considered impossible until recently, everything works. I just had to take the Sub DI board with the 1/4" jack off and reattach it, add a jumper so the 48 V would work, and viola, everything is functioning as it should. I am very happy with the FET module on it for bass sounds, it is very nice. Now to get some decent stuff down with a mic through this pre, other than, "La la la, this thing on?" Thanks again Jeff for providing such wonderful toys.
Patrick
Patrick
Jeff, finished the VP312DI. The preamp gain pot makes a lot of loud noises when turned. It's not the stepped gain. This is when I try it with a phantom powered microphone. The Output attenuator does not make noise, but increases/decreases a hum. What should I look for in troubleshooting?
As always, rule out the opamp. Next go thru and touch up any bad or iffy looking solder joints. The biggest trouble folks have with this preamp build are the relay boards.
Jeff, Ok. After knocking around on my main pcb with my multi-meter, I found that the diode at CR6 is not giving me any reading. Whether this is the problem or not, it's obviously wrong - correct? The other similar diode CR5 is giving me a reading.
OK. I'm getting a reading from the CR6 diode now. However, hum is still there. No audio. Bad noise when turning the Preamp Gain and pressing the mute and phase reverse buttons. No noise when pressing the pad button. Luckily, I bought an extra op-amp. I'll try building another one. I can't imagine being more careful than I was on the first op-amp, but I'll try. Thanks for you help.
I hooked up the 312DI with the test jig. The hum was the same. Have not built the new op-amp. However, after a few minutes, the Cap C10 blew. Pop! I was hoping that I installed backwards or something and that would be my problem, but I installed C10 correctly. I think it's something with the Input Gain pot. It's super noisy. Anyway, I'll get a new cap for C10 from Mouser and try to build the new op-amp. Should I just buy an op-amp that I know has been tested?
The only way that cap will blow is if there is DC offset from the opamp. There is a major problem with the opamp. Is it seated properly? http://classicapi.com/catalog/DOA_Install.php
Trying to test a fresh preamp build with only a fresh opamp build can be a tail-chasing experience. I never recommend it.
Trying to test a fresh preamp build with only a fresh opamp build can be a tail-chasing experience. I never recommend it.
Patrick from Davis
Well-known member
As sort of an off shoot of what Jeff recommends, I'd get a hybrid op amp (or two) to have around for testing. They are much easier to build than the discreet ones. Once you confirm that they work, you can use them for your builds to hold as many constants as possible when debugging. They are also pretty cheap at less than $10 a piece unassembled. I bought two for exactly this purpose, and they really have been quite helpful in narrowing down my errors.
Patrick
Patrick
I had read that about fresh pre amp builds needing a solid op-amp. I'm really, really careful, but I still make mistakes. I'm going to buy a working, tested op-amp right now. I'm obsessed with getting this thing going.
Patrick, thanks for the advice. Much, much appreciated....I'll look into that too.
Jeff, on another note, your test jig kit is genius. I did however, have a lot of problems with the Omron female receptacle - it's really cheap plastic and very easy to break when applying enough pressure to puncture the ribbon cable. I broke one, ordered one that seemed a little stonger, then broke the other one but somehow made that work. Is there another option?
Patrick, thanks for the advice. Much, much appreciated....I'll look into that too.
Jeff, on another note, your test jig kit is genius. I did however, have a lot of problems with the Omron female receptacle - it's really cheap plastic and very easy to break when applying enough pressure to puncture the ribbon cable. I broke one, ordered one that seemed a little stonger, then broke the other one but somehow made that work. Is there another option?
I built the other GAR 2520, VERY carefully. I used a magnifying headset and triple checked my work. I got the new C10 cap in. Pluged in the VP 312DI. It's making the same loud clicking noise as it did before, so I turned off the power as quickly as I could. I tried it with the test jig....same thing.
There's not a lot of info about the DC Block/Cap coupled output. I installed it like in Chungers thread, but I think that he was building a 51X....not sure. Anyway, I need help. I really don't think it's the op-amp. What else could it be? I made the relay boards as carefully as possible.
There's not a lot of info about the DC Block/Cap coupled output. I installed it like in Chungers thread, but I think that he was building a 51X....not sure. Anyway, I need help. I really don't think it's the op-amp. What else could it be? I made the relay boards as carefully as possible.
There is info on the purpose of the coupling cap earlier in this thread as well as on the PCB page at my site. With an API preamp, in mic mode, you will see typically around 80mV max of DC offset around the opamp. With the VP312DI (or any DI that uses the post 2622 method) you will see 1V or more of DC offset. This can be a little less or a lot more depending on the opamp. This is true for the 51x or VPR version of this build. That is enough DC to damage the 2503 output transformer. So we have the 470uF blocking cap. That cap is rated at 16V so 1 or 2 V will be no problem. Slamming it with significant DC from a bad or improperly installed opamp and the cap goes boom. If this cap explodes, it is a sign that something major is wrong with the opamp.
No offense Reverend but "I really don't think it's the op-amp" is not the same thing as "I have tested this opamp in another known working circuit and it is fine". Maybe a red dot would be a good choice at this point? Scott tests them all before shipping so the likelihood of getting a bad one from the start is basically null.
Also, folks have had weird problems getting these going most always related to the relay boards. I would also suggest reading thru this thread from the beginning.
No offense Reverend but "I really don't think it's the op-amp" is not the same thing as "I have tested this opamp in another known working circuit and it is fine". Maybe a red dot would be a good choice at this point? Scott tests them all before shipping so the likelihood of getting a bad one from the start is basically null.
Also, folks have had weird problems getting these going most always related to the relay boards. I would also suggest reading thru this thread from the beginning.
I've ordered a pre-built and tested op-amp from someone else. I wanted to purchase one from CAPI, but I couldn't find them on the website. I'll just wait until the pre-built and tested one gets here and see what happens. Thanks.jsteiger said:
This is backwards from what it read the other day. I was wondered if one of them was dead (I know, not likely). So I pulled CR6 and re-soldered (I know it's not a good idea). After I reinstalled it, it was giving me a reading, but then, CR5 now isn't giving me a reading.Reverend said:
Here's an issue I had. In the file I was emailed, in the pdf of the Relay layout, the layout is not like the relay that I received. The pdf has R1 in the top left corner. The relay I received has Q1 in the top left corner. Where the diodes go in the relay, there are not circles to show polarity, there is the diode symbol. I guess that I could have made a mistake on the relay since I've never really seen the layout for my relays. Both are the same for PK1 and PK2....each says P-1 Rev. A 2. It may be hard to check the relays without the right layout.
I updated the layout for the relay board quite a while ago when I changed board houses. All of the part #'s and values remain the same for each and every part. I changed the diode silkscreen to match what I have been using on some of my other small footprint boards. The silkscreen arrow points to the pad that the black band (cathode) goes to. Its pretty standard fare.
Besides wrong facing diodes, the main problem on the relay boards is folks not identifying the 4V3 zener and just thinking all the diodes are the same. They are NOT the same.
Besides wrong facing diodes, the main problem on the relay boards is folks not identifying the 4V3 zener and just thinking all the diodes are the same. They are NOT the same.
No matter as the part values and numbers are the same for both versions of the PCB. The circuit did not change.Reverend said:
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