Small signal Mosfet AB PA?

slideman82 · 12th February 2010, 02:10 AM

I wanted to build a low wattage class AB PA using BS170 and BS250... could this be possible, using max voltage with bipolar power? Maybe could exeed current rattings... well, just an idea.

Bigun · 12th February 2010, 03:23 PM

I'm not familiar with these devices, can you tell us more about them ?

From my perspective, I don't see why you can't build something along the lines of 'just do it'

satoru · 12th February 2010, 06:04 PM

BS170 is a 60V 0.5A MOSFET. Max power dissipation is 0.3W. May be you can parallel many of them to achieve your goal but you may need more than 100...

Slideman82, DIY power amplifier design starts by deciding how much power to put on what kind of load. From there, you can pick a suitable output device, heatsink, power transformer, etc. Then you can have fun meddling around on the circuit.
I design low voltage amp for my own use since I rarely need more than 2W output on 6 ohm speaker. But even for me, BS170 won't do well. Regardless, if you come up with an extraordinary design with BS170, share with us!

Have fun!

slideman82 · 13th February 2010, 12:49 AM

Quote:

Originally Posted by satoru

BS170 is a 60V 0.5A MOSFET. Max power dissipation is 0.3W. May be you can parallel many of them to achieve your goal but you may need more than 100...

Slideman82, DIY power amplifier design starts by deciding how much power to put on what kind of load. From there, you can pick a suitable output device, heatsink, power transformer, etc. Then you can have fun meddling around on the circuit.
I design low voltage amp for my own use since I rarely need more than 2W output on 6 ohm speaker. But even for me, BS170 won't do well. Regardless, if you come up with an extraordinary design with BS170, share with us!

Have fun!

Great reply!!!!!!!! So, we could do it quasi complementary with BS250's? Load? 8 or 4 ohms...

Unfortunately I don't know how to design a PA yet...

I was thinking a bit, and the idea of using an existing PA with a small power transformer (for example, 12V dual supply and 1 or 2A) could work for me!

Bigun · 13th February 2010, 01:06 AM

Yikes, 300mW maximum dissipation is not a good omen, it really does need dozens of them to get any meaningful power unless you want to design for some very high efficiency speakers. If you have to go buy a transformer etc. it might be worth to consider buying some power transistors. If this is acceptable you could pick out a circuit that is known to work as there are quite a few around (search for Carlos' DX amplifier and look at Nelson Pass DIY)

If you do want to use these devices - how many do you have that you can parallel up?

satoru · 13th February 2010, 06:08 AM

There is a Japanese DIYer who made a 11W amplifier with 2SA1015/2SC1815 (TO-92 300mW dissipation) as output transistors. He literally paralleled 100 each...

The web page is in Japanese but you can see the photos of the amp.

A1015/C1815_100???????????

slideman82 · 16th February 2010, 04:21 PM

Hahahaha 100! Great! But I want it with mosfets! I'm gonna try some nelson pass amp for sure, probably a lower voltage version of the Citation 12... just and idea.

Thanks to all!

Oh, I'd parallel as many as I needed for 2 or 5 W

satoru · 16th February 2010, 04:44 PM

Hmmm. Even for 2 to 5W output power, the output devices have to be packed in case with around 30W to 50W power dissipation and better be mounted on a heatsink. You still need around 100 devices if you stick to BS170. BS170 (and almost all other MOSFETs) has higher saturation voltage compared to BJTs (so they need higher power supply voltage) which in turn results in more power to dissipate.

Bigun · 16th February 2010, 05:45 PM

Well, Nelson is leading the way, he has a 1,000 JFET monster under construction so 100 parts is just an appetizer

AP2 · 16th February 2010, 05:54 PM

Hi,
What is the advantage of using n.100 BS170? on stage A-B?

12th February 2010, 02:10 AM	#1
slideman82 diyAudio Member Join Date: Oct 2007	Small signal Mosfet AB PA? I wanted to build a low wattage class AB PA using BS170 and BS250... could this be possible, using max voltage with bipolar power? Maybe could exeed current rattings... well, just an idea.

12th February 2010, 03:23 PM	#2
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	I'm not familiar with these devices, can you tell us more about them ? From my perspective, I don't see why you can't build something along the lines of 'just do it' __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

13th February 2010, 01:06 AM	#5
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	Yikes, 300mW maximum dissipation is not a good omen, it really does need dozens of them to get any meaningful power unless you want to design for some very high efficiency speakers. If you have to go buy a transformer etc. it might be worth to consider buying some power transistors. If this is acceptable you could pick out a circuit that is known to work as there are quite a few around (search for Carlos' DX amplifier and look at Nelson Pass DIY) If you do want to use these devices - how many do you have that you can parallel up? __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

16th February 2010, 05:45 PM	#9
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	Well, Nelson is leading the way, he has a 1,000 JFET monster under construction so 100 parts is just an appetizer __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

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12th February 2010, 06:04 PM	#3
satoru diyAudio Member Join Date: Jan 2007 Location: Berkeley, California	BS170 is a 60V 0.5A MOSFET. Max power dissipation is 0.3W. May be you can parallel many of them to achieve your goal but you may need more than 100... Slideman82, DIY power amplifier design starts by deciding how much power to put on what kind of load. From there, you can pick a suitable output device, heatsink, power transformer, etc. Then you can have fun meddling around on the circuit. I design low voltage amp for my own use since I rarely need more than 2W output on 6 ohm speaker. But even for me, BS170 won't do well. Regardless, if you come up with an extraordinary design with BS170, share with us! Have fun!

13th February 2010, 06:08 AM	#6
satoru diyAudio Member Join Date: Jan 2007 Location: Berkeley, California	There is a Japanese DIYer who made a 11W amplifier with 2SA1015/2SC1815 (TO-92 300mW dissipation) as output transistors. He literally paralleled 100 each... The web page is in Japanese but you can see the photos of the amp. A1015/C1815_100???????????

16th February 2010, 04:21 PM	#7
slideman82 diyAudio Member Join Date: Oct 2007	Hahahaha 100! Great! But I want it with mosfets! I'm gonna try some nelson pass amp for sure, probably a lower voltage version of the Citation 12... just and idea. Thanks to all! Oh, I'd parallel as many as I needed for 2 or 5 W

16th February 2010, 04:44 PM	#8
satoru diyAudio Member Join Date: Jan 2007 Location: Berkeley, California	Hmmm. Even for 2 to 5W output power, the output devices have to be packed in case with around 30W to 50W power dissipation and better be mounted on a heatsink. You still need around 100 devices if you stick to BS170. BS170 (and almost all other MOSFETs) has higher saturation voltage compared to BJTs (so they need higher power supply voltage) which in turn results in more power to dissipate.

16th February 2010, 05:54 PM	#10
AP2 diyAudio Member Join Date: Dec 2009 Location: Italy	Hi, What is the advantage of using n.100 BS170? on stage A-B?

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