Hello everybody
I did search for P270 bias but couldn't find the exact figure, numbers seem to differ. I finally got to measure mine. It seems to be awfully high. I get 108 mV across a 0.22 ohm bias resistor, which gives bias current of 490 mA! Am I correct to assume that it should be around 120 - 150 mA? Should I mention that amp after about 10 minutes is too hot to touch?
Thx
I did search for P270 bias but couldn't find the exact figure, numbers seem to differ. I finally got to measure mine. It seems to be awfully high. I get 108 mV across a 0.22 ohm bias resistor, which gives bias current of 490 mA! Am I correct to assume that it should be around 120 - 150 mA? Should I mention that amp after about 10 minutes is too hot to touch?
Thx
If I remember this amp has a mosFET output stage.
There is no limit to mosFET bias current for better audio performance.
It's quite different from an EF BJT output stage which requires either an optimally biased ClassAB or a full ClassA for good performance.
MosFETs just get better as the bias is increased towards full ClassA
Borbely told us to bias all mosFET output stages to at least 500mA, irrespective of how many pairs were installed.
The limit is heat.
How hot can you tolerate and still be reliable with the size of heatsinks provided?
There is no limit to mosFET bias current for better audio performance.
It's quite different from an EF BJT output stage which requires either an optimally biased ClassAB or a full ClassA for good performance.
MosFETs just get better as the bias is increased towards full ClassA
Borbely told us to bias all mosFET output stages to at least 500mA, irrespective of how many pairs were installed.
The limit is heat.
How hot can you tolerate and still be reliable with the size of heatsinks provided?
Output devices
Hello Andrew and thank you for your reply. Output devices are K226 / J82. I have to check what they are...
Hello Andrew and thank you for your reply. Output devices are K226 / J82. I have to check what they are...
Keep it cool.
If the outside is too hot to touch, then how hot is it inside?
Reliability would be my concern.
If the outside is too hot to touch, then how hot is it inside?
Reliability would be my concern.
I understand from comments by others, that this and similar models like the P270-2, P370, run with a constant and quite high bias. Consequently, they will run hot but not as hot as a full class A amplifier of the same power rating.
Generally, 60C is considered too hot to touch for more about 5 seconds but I would actually measure the top edge of the heatsink, on the thick inner section to be certain. Any exposed surface that reaches or exceeds 60C is probably too hot for general home use but I have to say that plenty of class A and glass tube fanatics have amps that are roasting hot and they like it that way
There seems to be plenty of ventilation for the inside of the amplifier adjacent to the heatsinks though, so it may not be the kind of problem that need fixing.
If you don't have any suitable kind of thermometer, get a cheap wire thermocouple lead and adaptor for your multimeter or a complete meter with digital readout type for not much more than $10. You can even use a quality meat thermometer or such from the kitchen drawer, if you can maintain proper thermal contact with the metal surface.
Some old comments on heat and other issues here: Musical Fidelity P270-2 output devices?
Generally, 60C is considered too hot to touch for more about 5 seconds but I would actually measure the top edge of the heatsink, on the thick inner section to be certain. Any exposed surface that reaches or exceeds 60C is probably too hot for general home use but I have to say that plenty of class A and glass tube fanatics have amps that are roasting hot and they like it that way
There seems to be plenty of ventilation for the inside of the amplifier adjacent to the heatsinks though, so it may not be the kind of problem that need fixing.If you don't have any suitable kind of thermometer, get a cheap wire thermocouple lead and adaptor for your multimeter or a complete meter with digital readout type for not much more than $10. You can even use a quality meat thermometer or such from the kitchen drawer, if you can maintain proper thermal contact with the metal surface.
Some old comments on heat and other issues here: Musical Fidelity P270-2 output devices?
Hi Guys
Im new to the forum, some very interesting threads thanks to all for the information. I have a P270.2 which I have been using for many years. I have recently replaced the power supply caps with 105C, 63V types fantastic improvement.
Was listening yesterday and the amp blew the fuse on the RHS, LHS all OK. Fuse looks to have vaporised.
Have done a visual check of the FETs nothing obvious burnt. I have the original Hitachi J162 in there.
Am I correct in assuming its likely to be the FETs time to change them ? Any tips for things to look for would be great.
Is there a good source for equivalent types / replacements ?
Thanks in advance
Andrew
Im new to the forum, some very interesting threads thanks to all for the information. I have a P270.2 which I have been using for many years. I have recently replaced the power supply caps with 105C, 63V types fantastic improvement.
Was listening yesterday and the amp blew the fuse on the RHS, LHS all OK. Fuse looks to have vaporised.
Have done a visual check of the FETs nothing obvious burnt. I have the original Hitachi J162 in there.
Am I correct in assuming its likely to be the FETs time to change them ? Any tips for things to look for would be great.
Is there a good source for equivalent types / replacements ?
Thanks in advance
Andrew
Further investigation showed the bridge rectifier on the RHS had gone, replaced with RS part number 687 5803, 50A, 1000V the original bolt fits fine.
Hope of use to someone.
Hope of use to someone.
You blew the bridge rectifier which probably had current and voltage ratings of several times the amplifier's maximum requirement. I would inspect to make certain that there is no sign of other failures among the replacement caps and wiring where your work may have left some soldering debris or such that shorted the power supply. It is much more assuring to have a reason why such a meltdown would occur.
2SJ162 is the PMOS complement of NMOS type 2SK1058 lateral Mosfet from Renesas in Japan. Check that there isn't more than one type there. These are one of a kind nowadays and hard to get even in the US but Ampslab supplies them online: Signal and Power Transistors @ AmpsLab
Otherwise, Profusion PLC in the UK can supply Exicon branded equivalents, reasonably priced. Lateral MOSFETs | Profusion
2SJ162 is the PMOS complement of NMOS type 2SK1058 lateral Mosfet from Renesas in Japan. Check that there isn't more than one type there. These are one of a kind nowadays and hard to get even in the US but Ampslab supplies them online: Signal and Power Transistors @ AmpsLab
Otherwise, Profusion PLC in the UK can supply Exicon branded equivalents, reasonably priced. Lateral MOSFETs | Profusion
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Hi Ian,
Many thanks for the information very useful indeed.
Symptoms - for some considerable time the unit had experienced quite a hum from what I thought was mains borne DC or bad AC supply. So thought I would take a look...
took off the lid and found that one of the capacitors on the RHS had its sleeve partly decomposed the plastic lid had fallen off. I found that the caps were 50V and the supply rails measured around 55V. Decided to change all 8.
This went well, all OK, new caps fitted same value but 63V and 105C types. much better used it for around a week before the RHS fuse went mid listen. During this time to my disappointment the hum was still there.
So was looking around for my stupidity short somewhere no luck. Pulled out the whole amp RHS seems OK found one dry joint but all fine put back together. Isolated the bridge and found that it was the culprit.
New bridge, no hum all appears OK, sounds great 🙂
Thanks for your help, forum full of great info thanks to all. Will report back if I see smoke
Many thanks for the information very useful indeed.
Symptoms - for some considerable time the unit had experienced quite a hum from what I thought was mains borne DC or bad AC supply. So thought I would take a look...
took off the lid and found that one of the capacitors on the RHS had its sleeve partly decomposed the plastic lid had fallen off. I found that the caps were 50V and the supply rails measured around 55V. Decided to change all 8.
This went well, all OK, new caps fitted same value but 63V and 105C types. much better used it for around a week before the RHS fuse went mid listen. During this time to my disappointment the hum was still there.
So was looking around for my stupidity short somewhere no luck. Pulled out the whole amp RHS seems OK found one dry joint but all fine put back together. Isolated the bridge and found that it was the culprit.
New bridge, no hum all appears OK, sounds great 🙂
Thanks for your help, forum full of great info thanks to all. Will report back if I see smoke
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