23 volts of "output bias"? Where are you measuring this 23 v, at the speaker output? I assumed this since you say "zero volts across the output resistors?"
If its at speaker post, sounds like mosfet not turning on? At least you have one working channel to compare to the other.
Russellc
If its at speaker post, sounds like mosfet not turning on? At least you have one working channel to compare to the other.
Russellc
Thanks for the quick response. Right, 23V at the speaker posts, 0v across the output resistors. And the board isn’t getting warm. What would make the mosfets not turn on, any thoughts?
When this happened to mine, I had inadvertently misplaced a decimal point, so I just needed to turn up the bias! Not sure why yours isnt biasing up, someone will be along that will no doubt.
Russellc
Russellc
Hmmm....what voltages do you measure across D1 (the zener) and R8? Are you using 1K fixed resistor for R8?
What voltage do you measure across R8? Please verify that Q2 is a ZTX550 and that it has been installed correctly.
Thanks Dennis, and thanks to 6L6 for some serious help trouble shooting this, it looks like a dead ZTX550 may be the issue. I'm going to replace and will report back later this week. I can't say thank you enough to those of you who make the time to respond and help on these boards.
Hi- over at the AJ Illustrated guide thread there's very helpful advice on assembling the Modushop enclosure which I happened to start (I meant to post it here- but... user error ;-).
Which I think would be uber-helpful for the guide:
Aleph J illustrated build guide
Which I think would be uber-helpful for the guide:
Aleph J illustrated build guide
Have you seen this?
https://www.diyaudio.com/forums/parts/245959-illustrated-guide-4u-deluxe-chassis.html#post3704810
Very similar for 5U.
Russellc
https://www.diyaudio.com/forums/parts/245959-illustrated-guide-4u-deluxe-chassis.html#post3704810
Very similar for 5U.
Russellc
There are a TON of missing photos from that guide... casualties of Photobucket, sadly.
I need to go dig for the originals and upload them here.
I need to go dig for the originals and upload them here.
Both Please! With an explanation of best practice for handling both. I've seen some wired together without a switch, others with. Opinions abound and while there may be no clear "best" it would be great to at least have a summary of how to wire and amp for selectable RCA and XLR inputs.
I'm a mechanical engineer, and DIY audio is new for me - I learn a lot from both experienced folks and newbies on this forum. Keep up the good work!
Mike
Chiming in, as always, with another thought. 😀
I've already shared re: what I'd do if I were you re: XLR / RCA for your builds and possibly how you could incorporate that into the guide.
I'd personally AVOID trying to add a switch into the equation. Your guide is incredible, IMO, and it is for NOOBS. As people progress, they can add a switch if they like. It could be a fun next step, and there is enough information in other threads for someone to do it successfully. My opinion would be that if you're to add XLRs along with (not instead of) RCAs to your guide that you do it similar to the commercial model and recommend an external wire jumper. There are enough threads around chasing hum issues and other niggling things due to input wiring and other considerations. Adding complexity to a noob guide is unnecessary for a (potentially small) group of people that want special additions.
BTW - Mike... I am not implying that you're pro or con with "selectable" or using a switch. Just offering an opinion to the OPs.
I've already shared re: what I'd do if I were you re: XLR / RCA for your builds and possibly how you could incorporate that into the guide.
I'd personally AVOID trying to add a switch into the equation. Your guide is incredible, IMO, and it is for NOOBS. As people progress, they can add a switch if they like. It could be a fun next step, and there is enough information in other threads for someone to do it successfully. My opinion would be that if you're to add XLRs along with (not instead of) RCAs to your guide that you do it similar to the commercial model and recommend an external wire jumper. There are enough threads around chasing hum issues and other niggling things due to input wiring and other considerations. Adding complexity to a noob guide is unnecessary for a (potentially small) group of people that want special additions.
BTW - Mike... I am not implying that you're pro or con with "selectable" or using a switch. Just offering an opinion to the OPs.
Well, last night I traced my dead channel to a bad jfet. Luckily I had another pair of matched LSJ74s on hand because I’d ordered them incorrectly from the store originally. Took out the original JFETs and put the new pair in and she fired right up. I was able to bias to .4V and damn near .000 on the offset. Got a test run in last night and sounds great. Dead quiet at idle through my Zu speakers which claim 101db. I mean dead quiet, no hiss or him or anything. No pops or noises at turn on/turn off. Letting it cook today and will check all the numbers then button it up.
I used XLR connections only, and built this with a full dual mono power supply and I used the store soft start board. I used Antek AS-4200 400VA transformers. I used the 5U chassis and it was pretty tight with two power supplies; I had to mount the toroids vertically with brackets. In retrospect...I might have splurged for two 4U mono blocks and had more room to work. Do the dual supplies sound better? Don’t know! I also used all blue LEDs, recommend this as it gives a nice blue glow; I don’t think I’d use red/green. Glad I went with one color. Also did not drill out the front panel for LEDs, which is ok but I might eventually want to do something on the front as it’s a big expanse of bare aluminum.
This was my second build, after an ACA. I made it a little harder on myself by going with the dual power supply but honestly that didn’t add too much complexity. No way I would have been able to even attempt this without this community and resource. Thanks to all of you for your time. And thanks again to 6L6 for his support.
I used XLR connections only, and built this with a full dual mono power supply and I used the store soft start board. I used Antek AS-4200 400VA transformers. I used the 5U chassis and it was pretty tight with two power supplies; I had to mount the toroids vertically with brackets. In retrospect...I might have splurged for two 4U mono blocks and had more room to work. Do the dual supplies sound better? Don’t know! I also used all blue LEDs, recommend this as it gives a nice blue glow; I don’t think I’d use red/green. Glad I went with one color. Also did not drill out the front panel for LEDs, which is ok but I might eventually want to do something on the front as it’s a big expanse of bare aluminum.
This was my second build, after an ACA. I made it a little harder on myself by going with the dual power supply but honestly that didn’t add too much complexity. No way I would have been able to even attempt this without this community and resource. Thanks to all of you for your time. And thanks again to 6L6 for his support.
Attachments
Usually this amp has a very gentle turn off thump, but isnt distacting in the least. Sort of a characteristic of AlephJ turn off.
Russellc
Russellc
JMcacII - Beautifully done! Congratulations.
FWIW, I particularly love the symmetry. The visual of the chassis ground grabs me for some reason. I dig it.
FWIW, I particularly love the symmetry. The visual of the chassis ground grabs me for some reason. I dig it.