It’s supposed to, yes. But I’ve never had good results with the R156 method.
the bias trimmer adjusts the operating voltage
AML 52F50
- Thread starter Bonsaimaster
- Start date
Help Support GroupDIY Audio Forum:
the bias trimmer adjusts the operating voltage
The Bias trimmer is the one that adjusts the measurement across R156 - or is supposed to. Mine seems to be barely changing although am getting together a better system for calibrating so will try again when ready.
I couldn't get my meter working on in or out when i tried but not sure if that's due to not having a correct stating point. Am currently building an oscillator pcb to help get a a guaranteed 0dBu signal so will get back to calibrating as soon as it done - just waiting on some components to arrive in the post.
Let me know if you get it working!
Yes will reply here with all of my results. Still waiting on some parts arriving for a little oscillator PCB that i'm building thanks to ruffrecords on here but as soon as I get somewhere I'll report back on here. I've not had chance to try anything else since my previous efforts as been waiting to get my calibration set up straight before continuing. I'm also waiting on a PSU that is away for repair that I can use to measure the current on the bench outside of my 500 rack.
Update: I found a short between AGND and -16V rail. The short was seen across U1, C61 and R157. I removed U1 and is still shorted when out of the PCB between GND and Input pin. The short across C61 is now gone but still seen on R157 so next i'll remove R157 from the PCB and see what that tells me. Unclear at the moment if U1 was the cause or something else shorted which damaged U1 so before replacing the 7808 i'll investigate further possible short locations. I'll report back again when I have more info and in the mean time purchase a replacement 7808.
Removed R157 from the PCB and is still showing as a short out of the PCB and the PCB is now free of the short so maybe I had a duff resistor that stuffed the 7808. Need some replacements now before I can continue. Will report back when I have new info.
Did some more investigation and realised that 10 ohm will read as a short on my DMM in beep mode. R157 is measuring 10ohms perfectly so refitted. I removed R156 to measure out of circuit and found it to be measuring 1M so its gone faulty. It is a 1 ohm resistor so have fitted the correct part. I'm wondering now if I somehow slipped with my probes and shorted something and fried R156 - don't remember but was looking at my DMM while adjusting the trimmer so could easily have slipped without knowing. Wires on either end of R156 to clip probes to from now on so advise anyone else doing this to do the same. Held up again now as purchasing replacement 7808 and 1ohm resistor.
Calibration PCB is calibrated to give 775mV at 1KHz so good to go with that as soon as the replacement parts arrive.
I'll update again as soon as I can.
Calibration PCB is calibrated to give 775mV at 1KHz so good to go with that as soon as the replacement parts arrive.
I'll update again as soon as I can.
I still think 500 series kits should have more consistent calibration methods. While most of us have ribbon cables, the double wides can be problematic….
glad you’re getting results. My stereo pair are rocking so hard. I’m really enjoying them. Using them on drum bus, vocals, bass and just dig a live piano session and they ripppped. Really cool compressor.
glad you’re getting results. My stereo pair are rocking so hard. I’m really enjoying them. Using them on drum bus, vocals, bass and just dig a live piano session and they ripppped. Really cool compressor.
Yeah the calibration side of things did surprise me a bit as didn't see anywhere about calibration until downloading the build instructions once I'd purchased. Although it wouldn't have stopped me buying a kit to build I'd have definitely got the calibration stuff together before building as it's frustrating having spent the money on the kit plus the extender cards and not being able to use them right away. Especially as you can buy a very good compressor off the shelf for the price of the kit and extender cards such as an Elysia mpressor 500, ISG Audio Photon, TK Audio TKomp, etc.... I know you're getting a more expensive and unique unit once you're done with a 52F50 build but it's definitely not for everyone with the options available. I did mine to learn and I'm definitely doing that. Cool thing is the the next builds I do I'll be ready for the calibration as planning some 1073/1066 line pre's and EQ's when I can afford the next kits. Hopefully more news soon as my replacement parts are due to arrive tomorrow.
Any progress on your calibration?
Not just yet but am collecting my power supply from repair tomorrow so will be able to carry on with it this weekend so will report back.
Replaced U1 and R156 and have been able to set the bias to measure 130mV across R156. Anyone thinking of building one of these I advise tacking wires to either end of R156 and using clip probes to measure to avoid any risk of shorting while looking away at the trimmer. I must have slipped with a probe when doing mine first of all leading to a blown U1 and R156.
Paused there with calibrating as waiting now to borrow a scope from work to measure the output correctly. Will report back again when I get there.
Paused there with calibrating as waiting now to borrow a scope from work to measure the output correctly. Will report back again when I get there.
Continuity check between each pin is all you need to do which can be done in circuit. An easy check to see if that might be the problem. If you suspect the same fault as mine check the value of R156 as mine had gone faulty which I attribute to accidental shorting while looking away at the trimmer.
Tried running audio now that the bias has been set up correctly and everything appears to be functioning perfectly as far as all controls affect the sound. I still have no adjustment on the gain which possibly wont make much difference as won't be using in stereo ever but also still no adjustment on the meter which is annoying. I don't want to put into the rack and start using until the meter works so more to do.
If you don't have a short between any pins on U1 that's good as there shouldn't be. If you measure U1 for voltage do you get anything on the output pin?
