API 312 Thread!
- Thread starter fallout
- Start date
Help Support GroupDIY Audio Forum:
Bob's additional reply:
One detail I'll mention, I don't know how experienced you are with electronics, but the primaries of your input transformer (if it has multiple primaries) will need to be jumpered to the appropriate ratio. Neither the schematics nor the PCB indicate this.
Bob
One detail I'll mention, I don't know how experienced you are with electronics, but the primaries of your input transformer (if it has multiple primaries) will need to be jumpered to the appropriate ratio. Neither the schematics nor the PCB indicate this.
Bob
My question:
Thanks for the tip. I probably would have figured that out by looking at the transformer documentation (I'm using the EA2622), but you've probably just saved me a lot of head scratching.
I would rate myself as an advanced novice - this is definitely my most complex project to date, and I'm learning a lot everyday getting up to speed on it. That said, I have a couple of questions from studying your schematic.
1) Before reading all of your documentation, I bought and built a JLM AC/DC power supply that has 3 rails instead of the 5 on the power station. The way your switches work - activating relays - do I really need another 15v rail, or is there some kind of workaround? For instance - I plan to run around +- 18v for the gar2520 based on info from forum posts. Can I just jumper the +18 rail to the +15 input for the relay circuit?
2) As stated earlier, my input transformer is an EA2622. I noted on your schematic that there is a Zobel network available for the transformer secondary, but your BOM has R5 & C1 blanked out. I read in this post - http://www.groupdiy.com/index.php?action=printpage;topic=34515.0 14th reply - that the EA2622 should have the Zobel. Do you have any thoughts on this?
I really appreciate the time you've put in to helping make this project available. I'm having a great time getting all of this stuff together, and I'm really learning a ton. Going through the steps of this project is opening all kinds of doors into my understanding of the equipment that makes my craft possible.
Seth
Thanks for the tip. I probably would have figured that out by looking at the transformer documentation (I'm using the EA2622), but you've probably just saved me a lot of head scratching.
I would rate myself as an advanced novice - this is definitely my most complex project to date, and I'm learning a lot everyday getting up to speed on it. That said, I have a couple of questions from studying your schematic.
1) Before reading all of your documentation, I bought and built a JLM AC/DC power supply that has 3 rails instead of the 5 on the power station. The way your switches work - activating relays - do I really need another 15v rail, or is there some kind of workaround? For instance - I plan to run around +- 18v for the gar2520 based on info from forum posts. Can I just jumper the +18 rail to the +15 input for the relay circuit?
2) As stated earlier, my input transformer is an EA2622. I noted on your schematic that there is a Zobel network available for the transformer secondary, but your BOM has R5 & C1 blanked out. I read in this post - http://www.groupdiy.com/index.php?action=printpage;topic=34515.0 14th reply - that the EA2622 should have the Zobel. Do you have any thoughts on this?
I really appreciate the time you've put in to helping make this project available. I'm having a great time getting all of this stuff together, and I'm really learning a ton. Going through the steps of this project is opening all kinds of doors into my understanding of the equipment that makes my craft possible.
Seth
Bob's reply:
The TQ-12V lists an 18V max control voltage. My intuition says that they should run fine from the +18, but I wouldn't take it absolutely for granted running them right at max voltage. You could also throw a resistor in line to drop the voltage across the coil. You can look at the spec sheet which has voltage and current ratings and figure it out from there. I've never run them off of the op amp supply line but again my intuition says it should be fine.
My 312 BOM is built with a Cinemag transformer in mind which requires no zobel compensation. They're not strictly necessary, but go ahead and use the values specified for your model to get the response the transformer was designed for.
Cool to hear you're getting more into electronics! That's what I've been doing for a while and now I'm actually starting to make some income off of all the time I've spent teaching myself (in addition to knowing my equipment much better)!
Bob
The TQ-12V lists an 18V max control voltage. My intuition says that they should run fine from the +18, but I wouldn't take it absolutely for granted running them right at max voltage. You could also throw a resistor in line to drop the voltage across the coil. You can look at the spec sheet which has voltage and current ratings and figure it out from there. I've never run them off of the op amp supply line but again my intuition says it should be fine.
My 312 BOM is built with a Cinemag transformer in mind which requires no zobel compensation. They're not strictly necessary, but go ahead and use the values specified for your model to get the response the transformer was designed for.
Cool to hear you're getting more into electronics! That's what I've been doing for a while and now I'm actually starting to make some income off of all the time I've spent teaching myself (in addition to knowing my equipment much better)!
Bob
My progress update:
I hooked up a very simple voltage divider and took about 11v off of the +18 rail and sent it to the +15 pin, and the relays are working with no problems. With that, there are also no loading issues from the PSU. The + voltage at the op amp is completely unaffected.
Seth
I hooked up a very simple voltage divider and took about 11v off of the +18 rail and sent it to the +15 pin, and the relays are working with no problems. With that, there are also no loading issues from the PSU. The + voltage at the op amp is completely unaffected.
Seth
Yes, you will want a Zobel after the EA2622. I use a 10K R and a 220pF C. It's not what my original input channel skiz or the old 312 skiz show, but it is what is most commonly found on the PCB's themselves. Ed has confirmed this. You will be happy with the results.sethmopod said:
Have fun with your build.
Cheers, Jeff
My question:
I finally finished building the first channel of your 312 circuit (just a couple of hours ago), and in my initial tests, I'm very pleased. Thank you for your work in making this project available!
There are a couple of issues I'd like to run by you though.
In your BOM, I'm pretty certain that R17, 18, & 19 are wrong. These switch positions should be the low gain settings and all I got was squealing when I tested them. Should those values be 63k4, 100K, & 160K respectively?
I am repurposing a used rack box for this, so I won't be able to use the xlr connectors the board was designed for, I'll end up with panel-mount connectors and run some wire from those to the pin holes. When I tested this, phantom power didn't work until I put a jumper from xlrf pin 1 to the power supply ground. Did you have a specific reason for leaving pin 1 floating? Was your expectation that the ground would be supplied by the rack chassis when the board is mounted?
I am getting a 60hz hum in the circuit. I'm pretty sure I'm not grounded quite right yet as the unit is just sitting on a wooden table with no chassis to bolt to, but I am wondering if this could be the result of the still-floating ground on pin 1 of the xlrm. Should I run a jumper there too?
Just out of curiosity - the ground trace that connects c12, d1, d2, & the relays never gets tied into the ground plane of the pcb the way the other two ground pins from the psu do. Is there a reason for that?
Thanks again for your time,
Seth
I finally finished building the first channel of your 312 circuit (just a couple of hours ago), and in my initial tests, I'm very pleased. Thank you for your work in making this project available!
There are a couple of issues I'd like to run by you though.
In your BOM, I'm pretty certain that R17, 18, & 19 are wrong. These switch positions should be the low gain settings and all I got was squealing when I tested them. Should those values be 63k4, 100K, & 160K respectively?
I am repurposing a used rack box for this, so I won't be able to use the xlr connectors the board was designed for, I'll end up with panel-mount connectors and run some wire from those to the pin holes. When I tested this, phantom power didn't work until I put a jumper from xlrf pin 1 to the power supply ground. Did you have a specific reason for leaving pin 1 floating? Was your expectation that the ground would be supplied by the rack chassis when the board is mounted?
I am getting a 60hz hum in the circuit. I'm pretty sure I'm not grounded quite right yet as the unit is just sitting on a wooden table with no chassis to bolt to, but I am wondering if this could be the result of the still-floating ground on pin 1 of the xlrm. Should I run a jumper there too?
Just out of curiosity - the ground trace that connects c12, d1, d2, & the relays never gets tied into the ground plane of the pcb the way the other two ground pins from the psu do. Is there a reason for that?
Thanks again for your time,
Seth
Bob's reply:
Squealing. Are you using an output transformer? Try spacing it off the PCB with a couple washers. I can't remember which board revision you have, but the previous version has an input trace running under the lams of the output transformer that caused feedback at higher amp gains.
Given that you stuffed R6 as ~10k or 20k, R17, 18, 19 should be nowhere near the values you mentioned. You'd be getting really near unity gain at those values. Min amp gain with specified feedback values is, I think 6dB. My BOM assumes vertical mounting of the PCB with the gain switch on top. Any other orientation will mean that actual gain settings no longer match up with assumed gain settings (low at 6:30 and high at 5:30).
60 hz hum. The XLRM pin 1 can probably be grounded with no issues, but I left it floating as a telescoping ground to avoid potential ground loops between equipment. This shouldn't be causing humming. My first suspect would be that it's out in the open without a grounded chassis around it.
XLRF pin 1 is routed to chassis ground via the PCB mount XLR I specify. I'll make a note for the next rev. to not require this specific part to make that ground connection. What's more standard is to route pin 1 of input to chassis ground. You might try that, though I don't suspect it will matter in your case.
Relay ground return was left on a separate pin just in the case of switching transients showing up as voltage fluctuations on the ground plane. In actual use it's probably a non issue but I did it anyways. Just star ground the two ground busses at your power supply or chassis input.
Bob
Squealing. Are you using an output transformer? Try spacing it off the PCB with a couple washers. I can't remember which board revision you have, but the previous version has an input trace running under the lams of the output transformer that caused feedback at higher amp gains.
Given that you stuffed R6 as ~10k or 20k, R17, 18, 19 should be nowhere near the values you mentioned. You'd be getting really near unity gain at those values. Min amp gain with specified feedback values is, I think 6dB. My BOM assumes vertical mounting of the PCB with the gain switch on top. Any other orientation will mean that actual gain settings no longer match up with assumed gain settings (low at 6:30 and high at 5:30).
60 hz hum. The XLRM pin 1 can probably be grounded with no issues, but I left it floating as a telescoping ground to avoid potential ground loops between equipment. This shouldn't be causing humming. My first suspect would be that it's out in the open without a grounded chassis around it.
XLRF pin 1 is routed to chassis ground via the PCB mount XLR I specify. I'll make a note for the next rev. to not require this specific part to make that ground connection. What's more standard is to route pin 1 of input to chassis ground. You might try that, though I don't suspect it will matter in your case.
Relay ground return was left on a separate pin just in the case of switching transients showing up as voltage fluctuations on the ground plane. In actual use it's probably a non issue but I did it anyways. Just star ground the two ground busses at your power supply or chassis input.
Bob
My progress update:
I realized that about the vertical vs horizontal mounting last night as I was falling asleep. I felt like smacking my head and shouting "dooh." When I turned it on expecting the lowest gain settings, I got the highest instead. I've been out all day today, but this evening I'll retest. I think the squealing was probably just feedback as my initial tests were though a guitar amp with a mic input that I dragged back to my work space. I'll be mounting these horizontally, so on the second channel, I'll rearrange the resistor bank to account for that. Once I figure that out, I'll probably pull the resistors from this first channel and rearrange them for the expected results.
If it still squeals at high gains even after testing without having open air speakers, then I'll look at the output transformer (Ed Anderson's 2623). I have board revision 2.3.
I realized that about the vertical vs horizontal mounting last night as I was falling asleep. I felt like smacking my head and shouting "dooh." When I turned it on expecting the lowest gain settings, I got the highest instead. I've been out all day today, but this evening I'll retest. I think the squealing was probably just feedback as my initial tests were though a guitar amp with a mic input that I dragged back to my work space. I'll be mounting these horizontally, so on the second channel, I'll rearrange the resistor bank to account for that. Once I figure that out, I'll probably pull the resistors from this first channel and rearrange them for the expected results.
If it still squeals at high gains even after testing without having open air speakers, then I'll look at the output transformer (Ed Anderson's 2623). I have board revision 2.3.
My further status update:
I tested listening through headphones, and it was fine at all gain settings. It's just that the resistors are a little out of order compared to where I put the stop pin on the Grayhill switch.
I was trying a couple of things to see if I could get rid of the hum, though, and I managed to fry a resistor on the psu! So now I'm off to see if I can figure that one out.
One of my main goals here was to learn more about electronics, so I'm kind of enjoying some of these stumbling blocks as it's making me stretch and figure stuff out. In that spirit, I've realized that much of this correspondence probably should have been on the prodigy-pro API 312 forum thread. Would you mind you mind if I posted sections of this there so others can benefit?
Seth
-----This post concludes the archive of my private correspondence with Bob Schwenkler. I would like to thank him once again for his help with this project. As I encounter further issues, I will post them here so all can benefit.
I tested listening through headphones, and it was fine at all gain settings. It's just that the resistors are a little out of order compared to where I put the stop pin on the Grayhill switch.
I was trying a couple of things to see if I could get rid of the hum, though, and I managed to fry a resistor on the psu! So now I'm off to see if I can figure that one out.
One of my main goals here was to learn more about electronics, so I'm kind of enjoying some of these stumbling blocks as it's making me stretch and figure stuff out. In that spirit, I've realized that much of this correspondence probably should have been on the prodigy-pro API 312 forum thread. Would you mind you mind if I posted sections of this there so others can benefit?
Seth
-----This post concludes the archive of my private correspondence with Bob Schwenkler. I would like to thank him once again for his help with this project. As I encounter further issues, I will post them here so all can benefit.
The PSU is fixed. I had accidentally shorted the heatsink from one of the voltage regulators to ground. The JLM website even states specifically that this will burn up the 10-ohm resistor.
There is still noise. It turns out it was more than just 60hz though.
There was a buzz that came on from just plugging the unit in (even with power off). When I connected the screw on the psu labeled "case" to the ground from the mains, that disappeared completely.
When the unit is powered on with no mic plugged in, there is horrible buzz & hum. When a mic is plugged in, the buzz disappears, but there is still a significant hum. I've tried grounding various points of the circuit to the mains ground, but nothing I've tried so far seems to affect it.
Any hints as to common places to look for hum would be appreciated.
Seth
There is still noise. It turns out it was more than just 60hz though.
There was a buzz that came on from just plugging the unit in (even with power off). When I connected the screw on the psu labeled "case" to the ground from the mains, that disappeared completely.
When the unit is powered on with no mic plugged in, there is horrible buzz & hum. When a mic is plugged in, the buzz disappears, but there is still a significant hum. I've tried grounding various points of the circuit to the mains ground, but nothing I've tried so far seems to affect it.
Any hints as to common places to look for hum would be appreciated.
Seth
I've been trying anything I can think of to affect this hum/buzz, but I can't seem to make it better or worse ???
The only thing that seems to affect is that when I switch the power off, I get 3-5 seconds of beautiful, clean sound while the caps in the power supply discharge. Does this mean that the problem is likely in the power supply?
I'm using a JLM AC/DC
Seth
The only thing that seems to affect is that when I switch the power off, I get 3-5 seconds of beautiful, clean sound while the caps in the power supply discharge. Does this mean that the problem is likely in the power supply?
I'm using a JLM AC/DC
Seth
Definitely the power supply. I had an extra power supply that I had salvaged from a piece of outdated, broken gear. When I hooked it up the whole thing was pristine.
I need the JLM AC/DC to work though - this other psu only goes to +/-16v and has no +48....
I apologize for being clueless on this one, but at this point, I have no idea where to start for troubleshooting this any farther. Any tips or possible first suspects would be appreciated. If there are certain bits of data that I need to start with, hints toward that would be great too.
Seth
I need the JLM AC/DC to work though - this other psu only goes to +/-16v and has no +48....
I apologize for being clueless on this one, but at this point, I have no idea where to start for troubleshooting this any farther. Any tips or possible first suspects would be appreciated. If there are certain bits of data that I need to start with, hints toward that would be great too.
Seth
Found the noise. It is completely an issue of power transformer placement. Moving the wires that connect the psu to the preamp and rotating the transformer around a bit eliminated all of the noises completely.
I still have a couple of questions though:
I was talking to a friend who suggested that I use "shielded wire" to connect the psu to the pre. Does such a thing exist? Does it matter much what kind of wire I use - solid core, stranded, etc?
He also suggested that I wrap all of the wires in aluminum foil and ground the foil to the chassis. Does this sound right? I've never seen that in a picture of anyone's finished gear or in anything I've ever taken apart.
When there is no mic plugged in, the interference is much stronger - I'm guessing that the xfrmr is creating a much stonger magnetic field while not under load (does that sound right?). When I moved the xfrmr around the wires in this state, it sounded just like a light saber! Do you think that's how they did it in the movies!?
Seth
I still have a couple of questions though:
I was talking to a friend who suggested that I use "shielded wire" to connect the psu to the pre. Does such a thing exist? Does it matter much what kind of wire I use - solid core, stranded, etc?
He also suggested that I wrap all of the wires in aluminum foil and ground the foil to the chassis. Does this sound right? I've never seen that in a picture of anyone's finished gear or in anything I've ever taken apart.
When there is no mic plugged in, the interference is much stronger - I'm guessing that the xfrmr is creating a much stonger magnetic field while not under load (does that sound right?). When I moved the xfrmr around the wires in this state, it sounded just like a light saber! Do you think that's how they did it in the movies!?
Seth
mitsos
Well-known member
- Joined
- May 4, 2007
- Messages
- 2,886
An unloaded preamp will buzz/whatever. Stick 150R between pins 2 and 3 of the input XLR (mimicking a mic load) and the noise should disappear.
You should be able to make this work fine without crazy amounts of shielding. The only place I'd use shielded cable is for the audio in/out but IIRC, bob's boards have the xlr's mounted on the PCB, so that is moot. For the power, ground, etc, cables just use normal hookup wire.
It's also important to check for continuity between all panels of your case, as that is the unit's main shield.
You should be able to make this work fine without crazy amounts of shielding. The only place I'd use shielded cable is for the audio in/out but IIRC, bob's boards have the xlr's mounted on the PCB, so that is moot. For the power, ground, etc, cables just use normal hookup wire.
It's also important to check for continuity between all panels of your case, as that is the unit's main shield.
falcorpilot
New member
- Joined
- May 1, 2009
- Messages
- 3
All,
I am a newbie.
I finally have my ACCESS 312 Passing audio. I used cinemag trannies etc.
Issues Noted: Distorts at High Gain (I bought some 2520's from a guy on this forum. They look great.)
When I touch the connected Microphone or anything that should be grounded I get a strange slow "whoop" sound. (Do I need to ground the XLR jacks?)
Any ideas?
I am a newbie.
I finally have my ACCESS 312 Passing audio. I used cinemag trannies etc.
Issues Noted: Distorts at High Gain (I bought some 2520's from a guy on this forum. They look great.)
When I touch the connected Microphone or anything that should be grounded I get a strange slow "whoop" sound. (Do I need to ground the XLR jacks?)
Any ideas?
martthie_08
Well-known member
Well yes, Pin 1 of the XLR input should be grounded in some way. As I do not know the Access PCB, are you running a shielded cable from the XLR to the main PCB? If yes, does the shield connect to the PCB ground or is it a telescoping shield?
I would like to ask a question as well, not knowing if this has been discussed here before: I am currently building a 312 style circuit based on Mitsos boards and it has provision for a DC coupling cap between the opamp output and the output transformer.
On my past 312 style builds I have omitted this cap, like on the original API schematic. So is it a matter of how much DC is "on" the audio signal and how much DC the specific output transformer can "take" or is it generally considered not good circuit design to omit the coupling cap?
thanks, Marten
edit: Jeff if you read this, any idea when you will get the EA 2623-1 output transformer back in stock? Or do you happen to have a pair left over somewhere, can be used, don't matter...
Hi Marten,martthie_08 said:
The coupling cap really depends on the particular DOA you are using and the DC offset it induces...like you said.
For my VP312DI, I have a 3 pin header so you can have the coupling cap or completely bypass it. You are right, the original 312 card had no coupling cap after the DOA. If you measure the DC offset it is very minimal. Now, if you have a straight into the 2520 DI circuit, you will notice more DC offset. DOA depending, I have measured over 1V. The original 2520's and Scott's SL-2520 are the lowest I have seen. Both of Gary's DOA's are only slightly more. I would say to just keep the cap in and measure. If it is only a few mA, take the cap out if you want.
I was expecting a large shipment before Christmas. They are late which has been the norm. Ed tells me now that I will get them just after the 1st of the year.
Happy new Year!
Jeff
martthie_08
Well-known member
great, this will be nice as then eventually I'll have 8 channels of nearly identical preamps only with different outputs: two Profile 4804, two EA2503, two EA2623 and two THAT chip based. Will be interesting to hear the differences...
Thanks for the info on the opamps generating different DC offsets, I will mostly be using Gary's 2520, so I'll see how that works out. Does the DC offset depend on opamp gain in any (linear) way?
Is there any limit as to how much DC say a 2503 can take before showing "side-effects"? The output transformer should act as a "free" high-pass filter anyways, at say 10-20 Hz :
sorry I have a problem ....
I just finished 2 x 312 API (bobschwenkler) and I used:
input - API 2622
out - API 2503
Gar 2520
I have not yet installed in the case but I have a strange problem:
it is as if there was a low cut ....
What could be the problem?
the colors are correct on the PCB than API 2503?
Thanks
I just finished 2 x 312 API (bobschwenkler) and I used:
input - API 2622
out - API 2503
Gar 2520
I have not yet installed in the case but I have a strange problem:
it is as if there was a low cut ....
What could be the problem?
the colors are correct on the PCB than API 2503?
Thanks
Latest posts
-
-
Calculating capacitors after bridge rectifier and before regulator.- Latest: JohnRoberts
-
-
-
-
