Linsley Hood's all-mosfet power amp - diyAudio
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Old 31st May 2010, 02:28 AM   #1
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Default Linsley Hood's all-mosfet power amp

I'm trying to make it sound right, made my own PCB (checked, no mistakes), but I'm using dual 50V supply (actually, 53). The matter is I can't bias it, from both trimpots, still have distortion. I'm using IRF640-9640 as output (couldn't find K1058 or J162, yet), IRF530-9530 as drivers and BSS92 in differential stage, but I don't think this could be the problem.

Is the one that appeared on Electronics Magazine on March, 1989, I think.
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Old 31st May 2010, 06:10 AM   #2
Nrik is offline Nrik  Denmark
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Can you link to a copy of the schematics?
Notice that the IRFs need something like 3.2-3.8volt to open, where the K1058/J162 needs less than 1v, so maybe you just needs a higher voltage-drop in the bias circuit.
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Old 31st May 2010, 06:21 AM   #3
Nrik is offline Nrik  Denmark
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Is it this one?
Click the image to open in full size.

The outputs are in the feedback loop, so even though they wouldn't be open (pure class B), you would still get decent sound. So when you say that you have "audible distortion", it must be the VAS not being open.
Please check that there is some current (22mA) flowing through TR7 - simply measure the voltage over R12. It should be something like 0.22V. If not, then increase R5 to 8,2K - 9.1k (or use a 22k trimmer to determin the value) .

When you get to the output transistors : The voltage over the trimmer R20 needs to be over 7V before the outputs open up, so while on the testboard -you could just increase the value of the trimmer.
HOWEVER....IRFs have thermal runaway (wich the K1058/j162 dont have) and therefore you need to replace the trimmer wth a proper bias circuit ( one extra resistor and one extra transistor)
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Old 31st May 2010, 06:54 AM   #4
Nrik is offline Nrik  Denmark
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Slideman82

Make these modifications - that should work.
Attached Images
File Type: jpg EWW3-89 modified to IRF Mosfets.jpg (128.6 KB, 910 views)
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Last edited by Nrik; 31st May 2010 at 06:58 AM.
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Old 31st May 2010, 07:39 AM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by slideman82 View Post
I'm trying to make it sound right, made my own PCB (checked, no mistakes), but I'm using dual 50V supply (actually, 53). The matter is I can't bias it, from both trimpots, still have distortion. I'm using IRF640-9640 as output (couldn't find K1058 or J162, yet), IRF530-9530 as drivers and BSS92 in differential stage, but I don't think this could be the problem.

Is the one that appeared on Electronics Magazine on March, 1989, I think.
The whole point of the amp is that it uses Lateral FET's

Try Nrik's bias generator (transistor should be on main heatsink to track outputs) if you must use HEXFET's.

The correct devices (later type numbers) should be readily available.
Magnatec. ALFET Lateral MOSFETs

Also 2sk1058 and 2sj162 are fine.
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Old 31st May 2010, 10:18 AM   #6
AndrewT is offline AndrewT  Scotland
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exicon & renesas are two other Lateral FETs that can be used.

The JLH is designed to use Laterals Not Verticals.

If you want to use Verticals then find a circuit designed for these devices.
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Old 31st May 2010, 10:32 AM   #7
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Default Interesting catalogue

Quote:
Originally Posted by Mooly View Post
The correct devices (later type numbers) should be readily available.
Magnatec. ALFET Lateral MOSFETs
Thanks, Mooly. Interesting catalogue here http://www.semelab.com/pdf/acc.pdf.
Max.
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Old 1st June 2010, 12:57 AM   #8
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Thanks a lot for all replies! I'll try all these mods! I know it was made for lateral mosfets, but I made some others and these vertical devices worked right! I know about thermal runaway, I'll use a proper heatsink and probably a cooler fan! Even with lateral ones.

Yup, that was the schematic!
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Old 1st June 2010, 01:01 AM   #9
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Quote:
Originally Posted by Nrik View Post
Can you link to a copy of the schematics?
Notice that the IRFs need something like 3.2-3.8volt to open, where the K1058/J162 needs less than 1v, so maybe you just needs a higher voltage-drop in the bias circuit.
I have had problems getting the bias high enouh on amps with MOSFETs too.

The IRFP240 needs about 3.5 volts to turn on but the IRFP250 needs a bit more.

My first amp was a JLH amp and was 50 watts, brings back some happy memories of my first years in electronics.
It worked well for many years.
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Old 1st June 2010, 05:48 PM   #10
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I used vertical mosfets in my guitar amp, using Flavio Dellepiane's 60W Mosfet Power Amp, and it hasn't a voltage multiplier, I even set up quiescent current so high that after 2 hours of playing (and pushed it really hard!) anybody can't almost touch the heatsink! IRF640-9640 work really well in that circuit.
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