mwl168

You are doomed... doomed...

That noise you are hearing is the sucking sound of the KG ES amp vacuume...there is no escape...

Is this offered as a kit? The PCB looks to fit the chassis perfectly.

I started running mine at 150mA and experimented with higher bias up to 220mA and settled for 200mA. Also experimented with supply rails from 22VDC to 26VDC. Also tried with and without the servo engaged. Frankly, in the balanced mode, I did not hear a big difference. YMMV. With servo in, the DC offset of each channel and each board is within 1mV according to my DMM. Without the servo, DC offset on each board sits around 60mV and does not change much as the amp warms up, and in balanced mode, the offset is about 30mV and very stable. I did not match the MJF output devices.

I had forgotten about it but this is basically what I did with my GRHV except I used a long M3 metal screw with the shoulder washer.

Actually, I could not tell from the pic if you have the servo engaged or not. You should power the amp up without the servo engaged. Adjust the balance and offset then engage the servo. From what you described above, it sounds like you have the servo engaged?

Does the offset respond to the adjustment of the trim pot?

Yes, you want to use the one in the BOM. The idea is that the tube of the washer needs to go through the transistor tab and extends into the insulator below so the metal screw does not cause a short. Congo 5 just saved you from a disaster that was waiting to happen

7721-10PPSG is the washer part that works well for me and it's brown in color.

If anything, I suggest you wait till you actually loaded the PSU with the Carbon amp before you start "fine tuneing" your rail voltage. I was the same way - bugged by the "uneven" regulated voltage even though it's totally within spec of the parts' tolerance (the LT1021 and the resistors in the error amp). Actually, with 350v secondaries you will likely be able to get the rail voltage higher if you desire as long as the parts you use are rated for it.

Thanks guys. Just ordered the silver XLR socket and let's see how that works. I want to keep Nate happy!

Exactly my thought Nate. Mounting behind the panel will require some recess on the back of the panel. I checked my closet but did not find a CNC router in there. Plus, it will still have the two holes for the screws exposed on the front panel which is not ideal either. It would be great to have XLR jacks made similar to Justin's Stax socket. Never scored high in the art classes I took. The black XLR socket looks fine on my silver SS Dynalo chassis but somehow looks a bit off on this particular chassis. Maybe I should put a black color knob on it...

Thanks Nate. On the hindsight, I wish I had ordered the XLR socket in the silver color. May still give that a try.

Got the XLR socket in. Here is the finished amp. The volume pot is cosmetic only - not wired.

Congrats! I would not worry too much about the bias voltage being a little higher - we are talking about 2% here. Looks like Antek is out of 25va/15v but has 50va/15v in stock. They are $6.5 more and just a bit larger. May be an option?

Nice one JoaMat. Two questions, how is the delay time calculated and would it work with 12V supply? I am thinking about the application with Megatron where we'll have a 12VDC supply for the front end tube filament.

I know congo5 has implemented his delay switch using this module from eBay: http://www.ebay.com/itm/DC-5V-9V-12V-Adjustable-Timing-Delay-Time-Timer-Turn-on-off-Relay-Switch-Module/191949280682?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao%3D1%26asc%3D40130%26meid%3D1b0997a4bcd44fc9a34aabfdf754af52%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D131776109221

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Yes, the 5K pot sets the bias current. Turing it clockwise raises the bias current. I left mine untouched from how they were shipped (my experience is they typically will sit around the middle of its adjustment range) when I first powered up. IIRC, they did not start up at anything crazy although I was only running 22VDC rails at the time. Those MJF1503x parts are 36W devices so as long as you have proper heatsink (it's a must) they should be fine. It's probably advisable to turn a pot a few turns counterclockwise before powering on to be safe. I do like the balanced setup more than I do the initial SE setup. The balanced amp seems even faster and more control. But even at its SE form, this amp was an jaw-dropper to me. By the way, have been tinkering and experimenting and I believe that, everything being equal, the bias current drift is primarily a function of the case temperature of the MJF1503x devices. As their temperature rises, the current drops. The higher the temperature variance the more the current swing. Hope this helps! EDIT: I powered down the CFP amp to measure the 5K pot setting in situ - for 210mA bias on 23VDC rails when warmed up, the pot is set around 1.8K. So I think starting up at 2.5K or higher for the 5K pot is probably a good idea.

The boards you have are the GRHV PSU, different from the KGSSHV PSU. The 600 R resistor and the CPC1117N are there for those that want to implement a timed delay for the HV (for the amps using tube output like Blue Hawaii) You can simply leave them out if you don't need the delay function.

Kevin and Birgir already answered but I am using standard silicon insulators for the GRLV pass transistors. Different story for the GRHV where aluminum oxide insulators and shoulder washes are a must otherwise arcy sparky!

Thanks! The bias current drifts at 22VDC rails sitting in open air as well. I do think that it drifts more with higher supply rails and with all the guts in the chassis (re: higher running temperature). By the way, although this chassis makes for a compact and neat build, I would suggest using a larger chassis unless you like your amp to run rather warm. The heatsinks for the amp boards are adequate (45 C after 5 hours) but I think the pass transistors of the GRLV are putting out lots of heat - case temperature is about 55 C bolted to the bottom plate. It probably has to do with my stupidity - I am using a 28vac trafo so the pass transistors have to drop 12VDC and provide over 1A current. I should probably have opted for a trafo with 24 vac secondaries. (EDIT: I should mention that the Antek AS1228 secondaries (28 vac) sag to about 26.5 vac with this heavy load and with that input the GRLV could not sustain regulation for 28VDC rails as I originally planned. I am not certain how high a regulated output a 24 vac trafo can provide.) This amp rekindles my love for the LCD 2. Thanks Kevin for yet another great amp that seems curiously overlooked by many. And thanks to congo5 for leading me to it and all the help and advices through my build.

Close to finish casing my balanced CFP amp. Waiting for a 4-pin XLR socket. This one runs on 26VDC rails and 220mA bias current. I am posting an unfinished amp because I want to report that I tried OPA551 for the servo OP and it appears to work fine at less than half the cost of OPA445 but only good to 30VDC. something to consider for those that may be in the process of sourcing parts for their builds. The bias current proves tricky to dial in. It drifts downwards as the amp warms up and seem to continue to go lower even after a hour of operation. The range of drifting is significant too. I am observing 70mA and possibly higher.

I tried this before with my universal PSU for Blue Hawaii, Grounded Grid, etc. running on Antek transformers. My empirical experience is that a few tenth of volts is lost (measured at the filament connectors on the amp PCB) from the transformer through the umbilical, internal wiring and the PCB trace. I took out the "gizmo" I rigged together to lower the EL34 filament supply inside the PSU. My suggestion; don't do anything additional until you measure it on a running amp.

Yes, I do like those bridges. This pair is wider than the other ones I used for the SuSy Dynalo so I had to "relocate" the 2 10 R resistors to the reverse side of the PCB. Current draw stated is for a balanced CFP amp - 4 boards.