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Old 21st September 2016, 05:44 PM   #1
indr is offline indr  India
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Default Building blocks for simple Class A

Any soul devoted for simple elegance would crave for to build something meaningful over this irresistible topology. This one was darling of valve days, can it be replicated with solid state? Could be great to see some inputs.
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Old 21st September 2016, 05:54 PM   #2
Mooly is offline Mooly  United Kingdom
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Of course it can be replicated with SS but you would need a high impedance speaker (typically 800 ohms). It was really a cost cutting exercise used in table radios of the period.
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Old 21st September 2016, 05:55 PM   #3
bwaslo is offline bwaslo  United States
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The problem with that design is that it requires a heavy DC bias current (an ampere or more if you want more than a few watts) continually through the speaker, and most speakers won't "like" that.... voice coil will be pushed far out of the gap, and will likely make smoke.

The tube amps that used similar technology used a transformer between the tube and the speaker, which keeps DC current out of the speaker.
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Old 21st September 2016, 06:07 PM   #4
xrk971 is offline xrk971  United States
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I have been meaning to build this simple topology. I think it could be easily made all SS with a swap from triode to JFET on the input.

Click the image to open in full size.

Above is from Simple Class A hybrid MOSFET follower thread.
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Old 22nd September 2016, 05:42 PM   #5
indr is offline indr  India
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Quote:
Originally Posted by Mooly View Post
Of course it can be replicated with SS but you would need a high impedance speaker (typically 800 ohms). It was really a cost cutting exercise used in table radios of the period.
You're great! Always with some meaningful direction. That led me here UK Vintage Radio Repair and Restoration - Philips High Impedance Speakers
I can remember seeing one in the second hand market. Trying to find one. Can I ask for some SS schematic replicating the design in the above link?
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Old 22nd September 2016, 05:52 PM   #6
Mooly is offline Mooly  United Kingdom
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Thanks but I'm afraid I've no circuits to hand.

I would imagine using high voltage FET's would be the way to go if you really want to do this. Power FET's rated at 400 volt+ are common and inexpensive, even lateral FET's are available in 200 volt rating. The first diagram is AC coupling the speaker which might be a better alternative than passing DC through the voice coil.

It would be an interesting design exercise... one for you to simulate
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Old 23rd September 2016, 09:03 AM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by xrk971
I have been meaning to build this simple topology. I think it could be easily made all SS with a swap from triode to JFET on the input.

--

Above is from Simple Class A hybrid MOSFET follower thread.
Yes that circuit is simple: simply wrong - unless you want an effects box.

There is a choice to be made in audio: do you want "simple elegance" or do you want high quality sound reproduction? Only in other universes with different laws of physics can you have both at the same time.
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Old 23rd September 2016, 02:55 PM   #8
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by DF96 View Post
Yes that circuit is simple: simply wrong - unless you want an effects box.

There is a choice to be made in audio: do you want "simple elegance" or do you want high quality sound reproduction? Only in other universes with different laws of physics can you have both at the same time.
I am not so sure you can say all simply elegant circuits don't have high quality reproduction. I just built this based on F5 topology. Complementary JFET input and complementary MOSFET output in class A. Sounds fanstastic and I believe simulates well too.

http://www.diyaudio.com/forums/pass-...5-headamp.html

Click the image to open in full size.

There's not much to this circuit as you can see here - veroboard build in about an hour:
Click the image to open in full size.

Last edited by xrk971; 23rd September 2016 at 02:58 PM.
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Old 23rd September 2016, 05:30 PM   #9
indr is offline indr  India
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Quote:
Originally Posted by DF96 View Post
Yes that circuit is simple: simply wrong - unless you want an effects box.

There is a choice to be made in audio: do you want "simple elegance" or do you want high quality sound reproduction? Only in other universes with different laws of physics can you have both at the same time.
'Simple Elegance' means just what it means, for all and for ever.
Chosen and blessed ones don't make choices, they just enjoy!
Put this in Google Image search, ' Самый простой усилитель на кт315 ' , I bet you'll forget what's called choice. And all of them are from this universe alone, maybe from a historically sad land!
And high quality sound reproduction? Seems as alien as the aliens themselves! Some say, it exists, some say, not.
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Old 23rd September 2016, 05:32 PM   #10
indr is offline indr  India
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Quote:
Originally Posted by xrk971 View Post
I am not so sure you can say all simply elegant circuits don't have high quality reproduction. I just built this based on F5 topology. Complementary JFET input and complementary MOSFET output in class A. Sounds fanstastic and I believe simulates well too.

http://www.diyaudio.com/forums/pass-...5-headamp.html

Click the image to open in full size.

There's not much to this circuit as you can see here - veroboard build in about an hour:
Click the image to open in full size.
Thanks, indeed a great design.
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