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One Sympatico channel running hotter, higher output
Rank: Member
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Joined: 2/10/2016(UTC) Posts: 4 
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Hi Russ and Brian Building a complete TP package with Buff 2 and IVY sending balanced signal into Sympatico boards. Everything works great first time apart from one sympatico channel is louder and running hotter. The sound is very good, no distortion but the hot channel is almost twice as loud. I hoped you may have some ideas. As the boards are very plug and play with no calibration I'm at a loss unless one LM4780 is a bit dodgy. Boards are to standard BOM with R10 = 2K. All connections are crimped and I thought of re-doing them with solder. All Ground connections are star grounded. Power transformer is 25-0-25 each side with 26.5VAC at the power input terminals on both boards. Some measurements of cathode and anode from VREF 1-4 on each board, both boards have very symmetrical readings: Cool board: REF1 C =+14.37V REF4 C =+35.30V REF2 A =-12.37V REF3 A =-32.81V Hot board: REF1 C =+14.0V REF4 C =+35.34V REF2 A =-12.38V REF3 A =-32.85V I was surprised to see 35V at REF 4. Below are some pics, in the first pic the hot channel is on the right. Please let me know of any other measurements that would be useful.  DSC_0809.JPG (254kb) downloaded 22 time(s). DSC_0812.JPG (146kb) downloaded 14 time(s). DSC_0810.JPG (183kb) downloaded 13 time(s). DSC_0807.JPG (163kb) downloaded 11 time(s). DSC_0808.JPG (177kb) downloaded 13 time(s).
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Rank: Administration
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Joined: 10/24/2006(UTC)
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Location: Massachusetts, USA
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I see you are not using the included K-10 thermal pad for the LM478s. How are you isolating the heatsinks (electrically)? With no isolation, the will be connected to the V- task (-35vdc). This can lead to shorts or shocks, either of which can cause damage.
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Rank: Member
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Joined: 2/10/2016(UTC) Posts: 4
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These were bare boards not kits sorry Brian. I have Kapton tape handy.
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Rank: Administration
Groups: Administration, Customer
Joined: 10/24/2006(UTC)
Posts: 2,868
Location: Massachusetts, USA
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Ah, that makes sense.
First, how are you measuring temperature?
Are you measuring the chip or heatsink? On the cold side, is the chip as cool as the heatsink? What I am getting it is whether it is the chip-to-heatsink interface.
Another thing to double-check is the soldering of R10, to make sure joints are good on the cold side, as that resistor increases gain.
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Rank: Member
Groups: Member
Joined: 2/10/2016(UTC) Posts: 4
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Thanks for this Brian, the process of elimination will start with the chip heat sink interface. I will remount and re-check soldering.
Temp measurement was with my hand on the heat sink only. I think I touched the cool chip itself and wasn't too alarmed but won't power up again until the insulation is sorted.
Do you think lack of thermal interface would cause the cool channel to operate at such lower output level?
It felt like the hot channel was out only due to the position of the volumite which seemed low considering the volume of that channel.
I'll get back to you Brian cheers.
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Rank: Member
Groups: Member
Joined: 2/10/2016(UTC) Posts: 4
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Right, fixed it but it's a bit of a hack.
After some fiddling around I determined that the erroneous channel is actually the cool or lower output side.
I now have R10 = 562R on the low channel and R10= open (stock gain) on the high channel. Both channels now sound better balanced to my ears. Very good sound to be fair, the quality of sound from the previously quiet channel seems to be actually better than the other now, so some tweaking of R10 values might be worth it. I may consider installing pots across R10 and really dial the balance in.
Strange business I know, Everything else is fine. All soldering, resistor values and placement and now thermal insulation is fully sorted so it may only leave the LM4780 chip itself?
Both chips are running cooler since the improved thermal bond. I ended up buying a sheet of Red Keratherm type material from Farnell.
Cheers for the help anyway Brian, trust me to build a board that doesn't need calibration and then have to calibrate it...
Everything else is bang on, all the kit is exceptional quality and worked first time. The Buffalo 2 and Amanero is a stunning combo. Very pleased.
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Rank: Administration
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Glad to hear your report. :) Sympatico is definitely one of my favorite modules I have designed.
Cheers!
Russ
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One Sympatico channel running hotter, higher output
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