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Old 11th March 2013, 07:11 AM   #1541
sangram is offline sangram  India
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Shorted input means the input pole is to be shorted to ground. If using RCA plugs, a simple plug with a wire connecting the two terminals is good and a good piece of equipment to keep in your junk box.

You should always do the adjustment with the input shorted and ensure your source has zero DC with a directly coupled amplifier like this. Once you adjust it if you see even the slightest DC (or induced currents due to improper interconnects, ground loops, etc) on the input it will simply multiply by the gain of your amp and show up at the output.

You can work out how best to attach the NTC to the FETs. In the ideal case it needs to be able to track the outputs very very closely, I realise that it is tough with two active devices on each channel but it's possible. For example, you can mount it on a bar that joins the two devices and has thermal contact with the screws holding the devices down. This will give you close enough tracking.

Though I have not yet built my version, the PCB I am using keeps the thermistor bang between the two outputs, which are anyway close to each other. Worst case I will do without the thermistor. My problem was never output transistor anyway, but the source and feedback resistors which got uncomfortably warm when the bias was pumped up either by me or by summer.
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Old 11th March 2013, 07:40 AM   #1542
omishra is offline omishra  India
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Quote:
Originally Posted by pdhanwada View Post
what do you mean shorted input.

the ntcs are close to the transistors not attached. does it matter here. because a ntc can be close to the transistor and other can be little farther.

Few mm difference in the ntcs spacing with teh mosfet can make the temperature difference significant.
Shorted input means open input RCA's center conductor shorted to input ground using wire.

Please look at below pic -
Attachment 330523

Here you can see how I made thermistor in place. Thermistor lead are bent to form a loop around MOSFET without shorting it. Note that it makes spring like force to keep it closer to MOSFET body. I already told you about this. There is white thermal paste in between touching bodies to expel any air. This should give good thermal contact and decrease drift.

Last edited by omishra; 11th March 2013 at 07:42 AM.
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Old 11th March 2013, 07:52 AM   #1543
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om,

I just checked after writing my previous post. one of the ntcs is touching the washer. other one is couple of mm off of it. As suggested by you I will use the thermal adhesive to create contact between the ntc and the washer.

Thanks
Pandu
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Old 11th March 2013, 02:48 PM   #1544
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bought down the bias to 300mv. I would want to keep the amp inside the cabinet.

DC offset is now below 20 mv.

Bias and dc offset look like changing everyday. Even with small adjustment of ntc positioning. Will check it again tommorrow.
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Old 11th March 2013, 03:29 PM   #1545
sangram is offline sangram  India
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Pandu

How are you measuring temperatures?

If you are using your hands, note that 55 feels really, really hot the first few times, if you are not used to touching hot amplifiers. It is 18 degrees above your skin temperature. I do remember the hand-over-heatsink test, but some may be more sensitive than others.

If your dissipation is maximum 20 watts per device at 55-60 degrees on the heatsink, there is too little heatsink.
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Old 11th March 2013, 03:56 PM   #1546
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I have heard Class A amps get very hot. This is my first class A amp.

I am able to pass the 10 sec test. I am playing safe here. I can probably go little higher on the bias.
I have B&K 7270 amp with me. That is the reference. It gets too hot. It is a class A/B 7 channel amp. My heat sinks are N9 from
Aluminium heatsink, Heat sink in bangalore, Heatsink stockist, Heatsink dealer, Heatsink supplier, Aluminium LED Heatsinks, Aluminium Reflector Sheets, Aluminium Caul board Sheet 2.5mm, Aluminium Plywood sheets, Aluminium Alloy Rods,plates in 2014, 7. It is 205 mm wide. I use two of them each side with height of the cabinet being 150 mm.

Also checked Diodes and feedback resistors. The diodes are getting may be 40 to 50 centigrade. Feedback resistors little less.

Source resistors are warm.

Last edited by pdhanwada; 11th March 2013 at 03:59 PM.
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Old 11th March 2013, 03:59 PM   #1547
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Post a pic. It might help.
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Old 11th March 2013, 11:04 PM   #1548
Marra is offline Marra  United Kingdom
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I finished off the power supply container today and the amp [V2] plays music. Hopefully I will get chance for a proper listen tomorrow.
The supply consists of a 350VA transformer with dual 25-0-25 secondaries,dual schottky rectifiers and +- 68,000uf per channel with a further +- 47,000uf Rifas per/ch in the amp case.
Looking forward to a good listen.
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Old 11th March 2013, 11:11 PM   #1549
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Congrats
You might need two 350Va. Another member just went dual mono and was a bit surprised at the difference.
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Old 11th March 2013, 11:38 PM   #1550
nrg2009 is offline nrg2009  United States
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6L6, any updates or photos from your build?

I would love to see a f5tv2 build...
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