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
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
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
"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
Doesn't this answer your question(s)?
C
I think you will find that Bob Carver is favoring the 1.5 ohm output resistor.
A very reasonable choice.
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.
http://www.diyaudio.com/forums/pass-labs/204823-f2j-parts-4.html
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
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.
http://www.diyaudio.com/forums/pass-labs/204823-f2j-parts-4.html
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
I'm at this point of experimenting.
My noise making things are:
Cyrus 2 + PSX
P10 EmKens
'Some' resistance cross the Outputs.
Stock - Cyrus could be described as a little bit bright and light, slightly bass shy.
With 'R0' - Shares most of the stock amp traits till it comes to bass, which 'goes' a little deeper..... due to a lessening of damping factor?
Imaging has improved, some of my reference music has regained the really spooky imaging / spatial clues.
Badly recorded music sound bad, excellent recordings sound excellent.
When listening to 'complex' music, there is no sense of the system 'giving it a go', can sound really really tiny.
Can hear that the amp is class B - A/B, it has a slight grain to its reproduction, which you cant hear when 'R0' is not across the outputs.
Q: Would putting R across the outputs of say an F5 work?
Hi Sippy,
I am not sure exactly what you did. It is good that you have some good results. However not that the resistor should be in series with the speaker rather than 'across' the speaker. Unless you are using a current amplifier (effectively only F1 and F2). The Cyrus should be a voltage amp and you need to put the resistor in series..
Oon
I am not sure exactly what you did. It is good that you have some good results. However not that the resistor should be in series with the speaker rather than 'across' the speaker. Unless you are using a current amplifier (effectively only F1 and F2). The Cyrus should be a voltage amp and you need to put the resistor in series..
Oon
Dear Zen mod,
You are using my ground as your cap? The cross talk between our system will be terrible
Oon
Buddha is everywhere ........
even if it's fattest in vacuum
I'm using 2.3 ohms in series with my horn drivers (TAD TD-2002s), which are being driven by a J2. Since the J2 has 0.4 ohms output impedance, the total impedance each driver sees is about 2.7 ohms. I arrived at the 2.3 ohm external resistance experimentally, to tame a slight "sizzle" or metallic sound (hard to describe, really) that the resistor quashed nicely. The end result is pretty darn good sounding, but I still have no firm idea what the resistor is doing electronically. I suppose it must have to do with lowering the damping factor.
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What's the wattage (say, into 8 ohms) of your gainclone, and what value resistor do you plan to try? The most straightforward approach is to pick a wirewound resistor of about the same wattage as the amp output, that way you know you can't burn it up. You can usually get away with a lot lower wattage, but wirewound resistors are pretty cheap. Remember the resistor may get pretty warm in use.
Thanks fpitas. The gainclone is approx 50 watts and i would like to try values from the, apparently famous, value of 1.5ohms to 47ohms to try to shelve the rising response of my fe108es. Am i better off putting two resistors in series to arrive at the value?
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