bdinnev

I will get probing soon and see what the results are. Is going to take me a little bit to make sense of the schematic and how it refers to the physical board but that is what learning is all about.... On the learning topic - not sure where the feedback resistors are. The boards are fully populated except for the 2 x surface mount resistors on the underside. Here is a picture -

OK, a bit more testing. The previous tests I did were all with the SS jumpers in place. I have taken them off and jumped all the ZF jumpers and get some different results. In ZF mode, on the board that was reading -2.5v the DC offset comes down considerably - 145mv and -81mv @ ~150mA bias. On the other board that was measuring -500mv it still measures the same in ZF mode, however I need to adjust the 5k trim-pots a fair amount to get back to ~150mA bias - I think I saw a post a few pages back with a similar situation so will go back and read the thread again!

All of the ones in both boards at present are the same (all purchased in one batch from Mouser), but I will try swapping them all out for the ones MLA mentioned above and see if it makes a difference - I am ordering all new PNP/NPN transistors (I think I ordered the batch I have at the height of COVID when stock was hard to come by and although everything else was sourced from Mouser, those may have come from somewhere less reputable) and some new resistors / capacitors to bump up the GRLV's to 30v so tacking on some LED's to the order won't make much of a difference in the scheme of things.

I have noticed that the LED's on the -ve side of the board don't glow as bright as those on the +ve side of the board - is that normal or has anyone else noticed this in correlation with an issue? I will try changing the LED's and see what happens.

That does not appear to be the same switch - the C&K is rated at 5A, not 6A, and the writing for the ON / ON is orientated differently. I have been able to find plenty of 5A rated switches, but no 6A.

Can anyone help identify who makes this toggle switch? I need to source a replacement as this one is broken but am struggling to find a 6A / 3A 125/250vac model

I have visually checked all the components on the boards and tested what I could with a meter and all seems OK - all resistors are in the correct location and of the correct values (as per the boards), all the caps seem to be in the correct orientation, all the PNP's and all the NPN's in the correct location, diodes etc, and the same across both boards. All the transistors are in the correct locations (15030 v 15031) and there does not smell or appear to have been any magic smoke come from anything. Is there anywhere on the board I can probe for a voltage / resistance to try and narrow down the issue?

I am reading -500mv on one board with the 10k pot trimmed to the max (or min depending on how you look at it) , the other board is reading around -2.5v! Any ideas where to look?

OK, will go across 1 resistor, makes the math easy Hopefully last question - what dc offset voltage should I be aiming for? My research leans towards "as close to 0+- as possible" as any voltage whilst "idle" simple gets turned into heat / noise on the transducers??

When I was getting all the parts some time ago, someone who has built one recommended I get 21v transformers, I actually ended up with 24v transformers. My understanding is that the output of the GRLV's will / should be lower than the input voltage so 20v seemed about right? Circa 150mA bias is what I have seen throughout this thread as well so seems to be on the money, my only question is is that when measuring across 1 of the 2 1ohm series resistors, or across both (mine read 2.3 ohm when measured across both on both the +ve and the -ve circuits). My measurements at present show a DC voltage of approx 150mv on both +ve and -ve after a quick tweak on the 5ohm pots, however the voltages are not stable, they are consistently rising (they are not oscillating, just increasing), the rising does slow down after a few minutes, but they do continue to go up - caused by temp increase across the circuit? If the measurement is across 1 or the 2 series resistors, then 150mv will be circa 150mA, but if I need to measure across both resistors in series, then I need to bump up the bias by double - a considerable increase.

Thanks Pars - I dialed in the GRLV's many moons ago, they are both running at 19.995v +- .002 on all outputs. When you say "across the 1ohm resistors" - is that across 1 of them, or across the pair on each circuit? I have the protector board ready to wire in, that will happen as part of the casing process, just want to get the CFA3 boards sorted ATM, one step at a time Thanks, Ben

Hi All, As with most of us, "life" caused some delays with my CFA3 build and I am just getting back around to trying to finish it off. I have 2 x GRLV's and 2 x CFA3 boards ready to go, just need to get initial setup done and then will case it all up. I have got the general idea of initial setup steps but is anyone able to assist with a step by step guide for a CFA3 - I.E. what do you do when powering up the first time? I understand I need to pull the op-amps out, power a board up and adjust trim-pots for bias, but can anyone help with where I should be measuring and what I should be aiming for, what steps should be taken and what safety measurements should be taken to ensure everything is functioning correctly? Thanks! Ben

Thanks audiostar. Yes, the transistors on the GRLV's will be mounted on the heatsinks on the side of the case - I temporarily soldered them into the boards just so I could do some quick tests on the GRLV's to make sure there was not any unwanted smoke etc but will remove the transistors and then re-mount them onto some aluminum angle which will then be mounted to the case heatsinks and solder them back into the boards. I have a headphone protector board ready to go as well as a pair of the 1st gen digital attenuators and a ss / zf switcher board. I am going to be running 3 transformers in the PSU - 2 x 24v 100va for the GRLV's and 1 x 15v 50va a to run the headphone protector, DA's, DA controller and ss / zf switcher along with 2 linear power circuits for 12v and 5v. I know it is a bit of overkill but I have space for it so figured why not. I need to go back and read the main thread for the CFA's to wrap my head around adjustment as it has been a while since I started this project. I will hopefully have the case for it in the next couple of weeks and once it is all cased up will get around to adjustments etc.

Finally received the last of the components for the CFA boards today from DigiKey - Mouser who I usually use were out of a couple of items and whilst DigiKey had the values I needed, they didnt have them in RN60 so ended up going for the slightly more expensive CMF series resistors - was only for a few values so was not a big deal and am happy to have everything in now. Some quick soldering and the CFA3 boards are finished (except for the output transistors - they will go in when I have the cases and heat-sinking sorted out) - has been a long time between start and finish but I can feel the end of this build getting near. PS, managed to find a free way to host images - Google Firebase - so can now insert images without having to worry about chewing up quota

I think the MJL* versions are the same except for different leg / pin sizes and slightly different packaging. Electrically they are the same as the MJW* (I think from memory)