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Old 8th February 2006, 08:47 AM   #1
Mad_K is offline Mad_K  Norway
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Default OTA - One Transistor Amplifier

I while back I posted a amplifier design with only one transistor. I named it SEWA (Seven Watt Amplifier). It seems most people interested in the design actually wanted LESS power (power dissipation I guess). While the SEWA design has evolved into something different (coming later), I have renamed this design OTA - One Transistor Amplifier. Attached is the revised schematic. Enjoy! And please let me/us know if you decide to try it
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File Type: pdf ota3,5r0.sch.pdf (50.9 KB, 3648 views)
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Old 8th February 2006, 08:49 AM   #2
Mad_K is offline Mad_K  Norway
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Default OTA PSU

Attached is the schematic for my preferred PSU arrangement.
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File Type: pdf sewa+ota3,5 psu.pdf (65.7 KB, 1800 views)
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Old 8th February 2006, 09:00 AM   #3
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Default Re: OTA - One Transistor Amplifier

Quote:
Originally posted by Mad_K
I while back I posted a amplifier design with only one transistor. I named it SEWA (Seven Watt Amplifier). It seems most people interested in the design actually wanted LESS power (power dissipation I guess). While the SEWA design has evolved into something different (coming later), I have renamed this design OTA - One Transistor Amplifier. Attached is the revised schematic. Enjoy! And please let me/us know if you decide to try it

Growing up with OTA meaning Operational Transconductance Amplifier, you fooled me! I also grew up with designs like yours, known as a Source Follower. But what's in a name, as long as you succeed in confusing everyone!

Nice schematic, BTW. Why don't you bias the source at the supply midpoint 11.5V to maximise output power?

Jan Didden
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Old 8th February 2006, 09:06 AM   #4
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...and next generation will be " 1 / 2 " transistor.... or " transistorless " ?
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Old 8th February 2006, 09:14 AM   #5
Mad_K is offline Mad_K  Norway
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Jan, if you do that you get unsymmetrical clipping (negative). The resistor needs enough voltage headroom across it to be able to deliver the required outputcurrent. So when the amp swings - 7,5 volts you have 5,5 volts left over the 5R resistor to deliver 1,1A of current to the load.

Upupa, -yes, that would be nice
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Old 8th February 2006, 09:17 AM   #6
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Default Re: Re: OTA - One Transistor Amplifier

Quote:
Originally posted by janneman
Nice schematic, BTW. Why don't you bias the source at the supply midpoint 11.5V to maximise output power?
For a push-pull or CCS class A, midpoint is most effective.

But
when using a single end with power resistor
it is different.

Say 8 Ohm resistor and 8 Ohm load.
At negative swing, the load will be in series with power resistor.

A theorectical ( not counting drop across transistor )
most effective class A single end
will be if 2/3 (66.66 % ) of voltage supply is across power resistor.
This is if LOAD=Resistor (for example 8 Ohm and 8 Ohm)


The circuit by Mad_K
is actually more effective than if using output at midpoint.
He uses output at 57.77% ( 13 Volt of 22.5 Volt )
I often use output at something like 60%.

Why often used 50%, is to give the transistor some better space to work in.
This will lower efficiency, but possibly reduce distortion.

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Old 8th February 2006, 09:29 AM   #7
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Hi Mad_K,
I have built the " power follower 99" ( Andrea Ciuffoli design) and
have one observation to do.
After a long experimentations ( two years) of listening tests I have found this amp sounds wonderful on the mid and high but sounds also
without any type of punch / slamm on the bass. Big delusion.
This is not a problem of power, I am sure . I listen at low levels with "easy" speakers ( in mid-field).
I have compared it with a aleph30 and the result is very similar to my ears. Monotriodes also.
Compared with a X-250 the difference is total, another world! One of the best bass I know.
Again I repeat I listen a very low levels.
Is it a sort of "genetic trade/ carachter" of the SE (only output stage
I suppose)?

All the comments are appreciate
Cheers,
Inertial
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Old 8th February 2006, 09:44 AM   #8
Mad_K is offline Mad_K  Norway
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Inertial, yes your observations are correct. SE excels in mid and treble region, while PP usually has it' strength in the lower regions. The bass caracter of a Common Drain SE amp is way better than a Common Source, so with this design I feel you get the best of both worlds (within the power range). The "power follower 99" is a nice design, and my SEWA design has evolved in that direction.
A good example of this taken to extremes are the Passlabs Rushmore speakers which (from my memory) has A Zen on top, Aleph in the middle and X amp on the bottom.
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Old 8th February 2006, 10:51 AM   #9
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Oh thank you Mads,
THEN I am not crazy!!!
Now I understand zero of electronics but what is the physics explanation of this "behaviour"?

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Old 8th February 2006, 11:01 AM   #10
Mad_K is offline Mad_K  Norway
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I'm shure others can explain it better, but some clues are:

SE: 2. harmonic distortion dominates. often low damping factor designs. limited output current.

PP: 3.harmonic distortion dominates, higher damping factor designs, usually lots of output current avaliable.
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