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Old 9th October 2013, 02:31 AM   #1
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Default Amplifiers with series output resistors.

Hi,

In Nelson Pass article which is the prelude to the F1 amplifier, He mentioned that one way of achieving high impedance output is by adding a resistor in series with your amplifier.


http://www.firstwatt.com/pdf/art_cs_amps.pdf

Excerpt
Most of the examples we will examine do not require true current source amplifiers, only amplifiers of quite high output impedances. Most of these cases will be happy with an output impedance of approximately 47 ohms or so and prefer 47 ohms loaded in parallel with the output of a current source. That being the case, you can build a Thevenin Equivalent of such
a current source by placing a large resistor (here later referred to as R0) in series with theoutput of a high wattage voltage source amplifier and get similar results. I’m not saying it will equal a spiffy First Watt F1 (being Class A and no feedback and all), andyour resistor will run hot. On the other hand, you probably already have such a voltagesource amplifier and some of these speakers are quite cheap, allowing you a taste of theseforbidden pleasures without high expense. I want to emphasize that this does not serve as
any sort of comprehensive guide to designing systems around this approach.



Now 10 years on. I wonder if anybody ever got around to try this and compare this with their F amplifier and how does it sound. I guess I am especially interested in class D since the power economy will really come in at this point.

I will try this myself, when I have some time.

Would love if somebody have tried this, also share their experience.

Oon
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Old 9th October 2013, 03:47 AM   #2
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Oon,

I built an F2 for myself after trying this experiment. I used 24R in series with a small chip amp. I remember being amazed at the direction it took the sonics. Try it!
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Old 9th October 2013, 03:53 AM   #3
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You used a full range Tangband on OB ?

How would it sound with a typical 2 way bookshelf speaker. The effect of damping would theoretically be reduced , but would the improvement in sound work well with an OB rather than a regular enclosure ? Worth checking out.
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Old 9th October 2013, 05:44 AM   #4
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Quote:
Originally Posted by cochleus View Post
Oon,

I built an F2 for myself after trying this experiment. I used 24R in series with a small chip amp. I remember being amazed at the direction it took the sonics. Try it!
I built a F2J myself without the 16 ohm output resistor. The amp sounds fantastic. I would like to make a comparison one day, when I can find the time.

Did you compare your F2J versus the chipamp and what are your findings?

Oon
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Old 9th October 2013, 01:51 PM   #5
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"you probably already have such a voltage-source amplifier and some of these speakers are quite cheap, allowing you a taste of these forbidden pleasures without high expense. I want to emphasize that this does not serve as any sort of comprehensive guide to designing systems around this approach."

Doesn't this answer your question(s)?

C
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Old 9th October 2013, 02:15 PM   #6
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Hi,

You can convert a typical low output impedance voltage source
tranny amplifier to a high output impedance current source by
reconfiguring the feedback loop of the power amplifier.

This will give high impedance with no power losses.

Rod Elliot has an article on how to do it.

rgds, sreten.
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Old 9th October 2013, 11:13 PM   #7
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Quote:
Originally Posted by oon_the_kid View Post
I wonder if anybody ever got around to try this and compare this with their F amplifier and how does it sound. I guess I am especially interested in class D since the power economy will really come in at this point.
I think you will find that Bob Carver is favoring the 1.5 ohm output resistor.

A very reasonable choice.

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Old 16th October 2013, 01:57 PM   #8
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Care to point me in the direction of Bob carver.....

Oon

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Old 16th October 2013, 08:51 PM   #9
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You can google him, but I don't see a link that references that. In any case
he is on record as saying so.



Correction: google:

bob carver 1.5 ohm
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Old 17th October 2013, 05:59 AM   #10
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Thanks Nelson,

It was quite interesting. I think output impedance is one of the biggest factors that affects the nature of the sound.

BTW, finally finished my F2J. I used a 490uF Polyproplene instead of the 15,000uF output .. I am stacking my claim to being your fanboy with the biggest capacitor (physical) in his F amp.

F2J parts.

The interesting thing is the capacitor is also designed for solar inverters... as a DC blocking capacitor.. So that put two solar inverter parts in my amp. Should use a solar panel as the cover...

Oon
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