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Old 5th March 2013, 01:31 PM   #1051
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Yes. rade95 spoke of them getting hot. He is using alternative Jfets and they may be hot because he has a v grade type device, meaning high Idss of 12mA+. If so, they are pushing dissipation levels.
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Old 6th March 2013, 02:40 AM   #1052
rade95 is offline rade95  United States
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Hey guys, buzzforb is right, my JFETs were taking way too much current so rather then re-designing the source resistor network, today I met for lunch with my friend Jim (6L6). He brought a pair of almost perfectly matched K170/J74, so today was my day of fun indeed. Once I put those JFETs in the circuit, adjusting the BIAS was a breeze. Started from 0 ohm, but I could have started from 3-4 turns on P1/P2 each. They started 'catching' at about 5-6 turns. At about 7-8 turns I adjusted BIAS to about 0.5V and the R3/R4 voltage drop was about 4.2V, like on the original schematic. DC offset was about 35-40mV, which I think is great. Since I don't have the large heath sinks mounted yet (see pics) I didn't want to push BIAS over 1.2A yet, but I will once the amp is built. Then I did a quick audio test with CJ PV14 as a pre-amp (tube), before the MOSFETS got too hot and the amp sounded outstanding. Next will be to do some measurements now that I have 2 good working modules. Many thanks to my friend Jim who provided me with the JFETs. Like someone said before, if you have a good matched set of JFETs, adjusting this amp is a snap. I'll post some pictures once I figure out how to attach them. thanks Jim, thanks buzzforb
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File Type: jpg _my_F5_ch2.jpg (150.0 KB, 415 views)
File Type: jpg _my_F5_ch1_heathsink.jpg (151.4 KB, 409 views)
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Last edited by rade95; 6th March 2013 at 02:43 AM.
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Old 6th March 2013, 04:39 AM   #1053
6L6 is offline 6L6  United States
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That's fantastic news! I'm very glad it's working properly.

Yes, next you need to attach your Fets to your heatsink so you can listen - You will love it.
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Old 8th March 2013, 03:14 AM   #1054
rade95 is offline rade95  United States
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Default F5 first proper audio test

Today I did the first proper stereo audio test with my newly built F5 stock modules. The amp modules were mounted on a proper 300mm x 300mm heatsink and left to warm up for 1 hour. The heatsink was warm but not too hot after an hour. At first I thought that the amps lack on bass, but when I played Pink Floyd Money and Wish You Were Here, it became apparent that these amps do sound fantastic, crisp and with no coloration of any kind. I am really impressed. I am still to play some classical music and my fav FLAC digital files. Now my dilemma is should I just stick with these stock F5s and finish building them, or opt for a balanced version which will put out more power. I seem to need a bit more power at least with CJ PV14 as a pre and my 12" Altec woofers and Goodmans enclosures, but I should try them with X2.5 and Aleph P and also with UGS pre, which I am yet to finish the case for. Stay tuned for my build thread.
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File Type: jpg _my F5 first stereo audio test.jpg (167.2 KB, 345 views)
File Type: jpg _my F5 first stereo audio test2.jpg (153.0 KB, 328 views)
File Type: jpg _my F5 first stereo audio test3.jpg (147.2 KB, 325 views)
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Old 8th March 2013, 03:22 AM   #1055
6L6 is offline 6L6  United States
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Great!!

How big is your transformer? Are you at full bias?

You do have the heatsink for a Balanced F5...
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Old 8th March 2013, 01:14 PM   #1056
rade95 is offline rade95  United States
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Quote:
Originally Posted by 6L6 View Post
Great!!

How big is your transformer? Are you at full bias?

You do have the heatsink for a Balanced F5...
The toroid is 350VA (18+18VAC), if I decide to go balanced I will have 2nd PS and the toroid. The rectifier diodes I am using were only warm after 4-5 hours of use and I don't think they need a heathsink. My BIAS is at 1A right now. I think I read the recommended BIAS for a single MOSFET pair is 1.3A per side, but what is the max you can get out of this design? I would like to max it out because I have enough heathsinks to do that. The reason I want to try maxing out the BIAS is to see if there is any sound difference with BIAS at the max.
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Old 8th March 2013, 01:22 PM   #1057
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i would not go over around 35W dissipation pr fet. With 23V rails, that comes down to:
35W/23V=1.52A bias. Your sinks is from heatsinkUSA With 0.33"(9mm) base plate?
the sinks are effective in a radius of 10x the base thickness. rest of the sink will be pretty much useless.
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Old 8th March 2013, 06:09 PM   #1058
rade95 is offline rade95  United States
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Quote:
Originally Posted by AudioSan View Post
i would not go over around 35W dissipation pr fet. With 23V rails, that comes down to:
35W/23V=1.52A bias. Your sinks is from heatsinkUSA With 0.33"(9mm) base plate?
the sinks are effective in a radius of 10x the base thickness. rest of the sink will be pretty much useless.
Acquired heathsinks from a local parts store and I think they came from a local RF factory, they were designed for RF amps. The base is about 1/2" thick, and the heathsink was evenly warm but not burning hot after 4-5 hours of use. Thanks for the max dissipation value of 35W per side. Using that value I can calculate my max output voltage swing: 35W / 8 ohm ~ 2A, so 2 x 8 = 16V. Is this correct (simplistic) way to calculate the voltage swing for max dissipation?
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Old 9th March 2013, 07:46 AM   #1059
AndrewT is offline AndrewT  Scotland
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no.
he stated <=35W per device, not per side.
A ClassA Push Pull amplifier can deliver ClassA output current up to 2times the output stage bias current.
If the bias is set to 1.3A, then maximum current in ClassA is 2.6Apk.
Voltage swing across a 8r0 load would be 2.6*8=20.8Vpk
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Old 11th March 2013, 05:40 PM   #1060
rade95 is offline rade95  United States
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Guys, just to let you know that I am looking into ways to push F5 into class AB without much circuit change, with less power, heath and BIAS current. Has anyone thought of bumping up PS to +-40-45V and adding another BJT driver stage and reducing FET BIAS to ~150mA, or something along those lines? Spent most of the weekend in bed sick, reading about class AB and I like it a lot. There is tons of material on the web and in diyaudio, but has anyone tried to convert F5 to class AB, without heavy lifting?
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