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Old 20th August 2014, 03:09 PM   #21
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Nice build Only one concern. What is your bias goal?
This will be around 85W @8ohm amp (voltagewise). If you want it to stay in class A all the way, you wil need 1.15A pr device with 2 pairs. With +/-40V rails, that comes to 46W dissipation pr device. and a total of 184W pr ch. that might make the amp a little to hot. and the Lifetime of the outputs a little to short.
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Old 20th August 2014, 05:02 PM   #22
cviller is offline cviller  Denmark
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Quote:
Originally Posted by toxic ingestion View Post
Hi. I don't understand exactly what you mean by your statement that you mount "them upside down (with mosfets in the bottom)." Might you explain a bit or provide a quick photo or sketch?

Thanks.
I think Zen Mod said it pretty well. So, if you have mosfets on one side of the board and mount the board parallel to the heatsink, you can choose to turn around the board to have the mosfets below or above the board. Below is better.
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Old 20th August 2014, 05:16 PM   #23
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Quote:
Originally Posted by cviller View Post
I think Zen Mod said it pretty well. So, if you have mosfets on one side of the board and mount the board parallel to the heatsink, you can choose to turn around the board to have the mosfets below or above the board. Below is better.
Yes, for sure. I see that the photos posted thus far were not clear in exactly which position on the sinks the boards and MOSFETs were mounted using spacers. I will post additional photos later today. I could have simply mounted the boards horizontally on the sink at a higher level, supporting them from below. But as pointed out by ZM, I wanted to keep them on the lower third of the sink, which is why I abandoned the horizontal position.
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Old 20th August 2014, 05:33 PM   #24
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Quote:
Originally Posted by AudioSan View Post
Nice build Only one concern. What is your bias goal?
This will be around 85W @8ohm amp (voltagewise). If you want it to stay in class A all the way, you wil need 1.15A pr device with 2 pairs. With +/-40V rails, that comes to 46W dissipation pr device. and a total of 184W pr ch. that might make the amp a little to hot. and the Lifetime of the outputs a little to short.
Hi. My intent has always been to bias to tolerable temperature. One of the reasons for building this was I and 6L6 were curious as to how high a bias could be achieved using the 5U sinks without a fan. That is, how high in life, rather than by calculations. I was shooting for 0.8 to 1 A without a fan, but hoping for more with crossed fingers. I'm not particularly concerned about leaving class A during peaks based on the very tiny bump on the distortion curve during the switch, especially with the large cap banks for each channel. (I just learned in the last few days of NP's article on leaving class A.) From my very limited understanding, classical music CDs are not as compressed and have a wider dB range (don't know that I am wording it correctly). Thus I wanted something to handle a moderately increased output and was counting on the caps handling the peaks. But I anticipate my comments may raise a rather vigorous discussion and corrections of my (mis)conceptions.

As the posts proceed, I'll post results of numerous measurements of distortion, clipping thresholds, waveforms, heat sink and device temperatures, etc. But you, indeed, are perceptive to recognize the heat issue. Another thing to be considered is that the electrostatics are supposed to average about 6 ohms. I don't think you will be surprised with the outcome. Please be patient with me as I work on getting these posts together in the evenings. More to come.
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Old 20th August 2014, 09:58 PM   #25
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After soldering MOSFETs in place, I mounted thermistors. The board provides for mounting on both of the interior two MOSFETs (one N channel and one P channel), and allows two location options for each device. I used the holes closest to the edge of the board. In the first photo, I have simply soldered a single lead on the MOSFET and then placed the thermistor in place for the photo. Thermistors were pulled through and then bent over to lie on top of the MOSFETs.

I had mounted spade connectors on the amplifier boards for connections pertaining to power in and speakers out. For signal in, I used shielded twisted pair, with the ground connected to negative side at one end of the cable. While I wanted to use some very small spade connectors (and tabs) for signal in on the boards, I decided to solder the leads directly to the board to minimize the chance to pick up extraneous noise, though I donít know this was essential. I then mounted the boards and MOSFETs, using mica and thermal paste on the transistors (had some arctic silver Ė not conductive). I confirmed lack of conductivity between any lead of a MOSFET and a non-anodized area of the the heat sink using an ohm meter. The M3 taps in the sinks worked perfectly.
Attached Images
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File Type: jpg F5#20.jpg (384.6 KB, 174 views)
File Type: jpg F5#18.jpg (553.0 KB, 169 views)
File Type: jpg F5#21.jpg (309.0 KB, 161 views)
File Type: jpg F5#22.jpg (499.0 KB, 160 views)
File Type: jpg F5#23.jpg (376.0 KB, 73 views)
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Old 21st August 2014, 12:31 AM   #26
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I meant to ask everyone whether they also would have soldered the shielded twisted pair input cable directly to the board or whether I would have done fine with a small spade and tab connector, instead. I have always soldered it directly to the board previously. Still, being able to easily disconnect input at the board could be convenient during repair/troubleshooting.

I had 2.79 mm wide male and female connectors I could have used. Digi-key A247TR-ND and WM18259-ND. Photos attached.

Thanks.
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File Type: jpg F5#25.jpg (19.0 KB, 13 views)
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Old 21st August 2014, 12:38 AM   #27
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I get mildly concerned about the JFETs becoming hot when the board is mounted in proximity to a hot heatsink. So I'd prefer the board at 90 deg to the heatsink, putting the input JFETs at a greater distance to the heat and in cooler air (hopefully). It might be a good idea to put heatsinks on the JFETs that are connected to each other, for better thermal tracking and cooler temps... JFETs move bias point nicely with temperature, iirc.

Of course, mounting off soldered connections only is going to create breaks in the solder eventually. So nobody reading should do that.

Nice clean looking build.

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I'd never use push on terminals for this application, others might, not me.
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Old 21st August 2014, 12:59 AM   #28
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Quote:
Originally Posted by bear View Post
I get mildly concerned about the JFETs becoming hot when the board is mounted in proximity to a hot heatsink. So I'd prefer the board at 90 deg to the heatsink, putting the input JFETs at a greater distance to the heat and in cooler air (hopefully).
Interesting you mention that, Bear. I had wondered about the same thing. I'll take some temperature readings on the JFETs and on the sink and let you know how they compare. Thanks for bringing this up. As I recall, you have a set of these boards. Have you done anything with them yet?
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Old 21st August 2014, 02:05 AM   #29
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oh yeah, they are doing well collecting excess dust particles so far.

not happy about this either. not at all.

living vicariously through DiyAudio these days...

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Old 21st August 2014, 02:35 AM   #30
sangram is offline sangram  India
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Arctic silver is not conductive, but it is capacitative. Results for builders have been unpredictable in earlier builds, don't recall which one. I would exercise caution. The body/drain is connected to the output and the heatsink to ground, so you might have to deal with some additional stray capacitance on the output.
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