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-   -   Thoughts about First Watt F1: am I right? (http://www.diyaudio.com/forums/pass-labs/67336-watt-f1-am.html)

oehlrich 5th November 2005 04:20 PM

Thoughts about First Watt F1: am I right?
 
Thoughts about First Watt F1: am I right?

Recently I read a lot about the F1 from First Watt and was impressed by the idea to supply current instead of voltage to the speaker. The advantage is clear. Current is proportional to the movement of the speaker so delivering current from the amp would give quite a better result than delivering voltage. Also parasitic effects like resistance and inductance of wires and the voice coil will be of no importance any more. So far the theoretical basics.

As we all know it is impossible to realize ideal voltage or current sources. Both of them have internal resistance. Each real world voltage source delivers - as we all know only a limited amount of current. Also real world current sources have a limitation in output voltage.

From theory we also know that a real world voltage source can be composed of an ideal voltage source with a resistor in series (Rs) and a real world current source can be composed of an ideal current source with a resistor (Rp) in parallel. We also know from theory that a real world voltage source with its resistance Rs could be substituted 1:1 by a real world current source with Rp = Rs.

Now back to the F1. It is real world current source with Rp = 80Ohm. Using a speaker with 16Ohm this leads to a damping factor of 0.2. As I read the F1 with a fullrange speaker leads to a very good result for the mid and high region and a very undamped bass.
My system is an active F.A.S.T. (Usher 6-channel poweramp, AER MD 1B as open baffle 16Ohm) http://freerider.dyndns.org/anlage/i...L-Prototyp.jpg x-overed at 100Hz so bass should be no problem. But I have no F1!

No problem! Accordingly to the theory above a voltage source (e.g. conventional amplifier) with a series resistance Rs of 80Ohm should give the same result. Interesting isnt it? All you need to do is increase the amplification of the fullrange accordingly to the loss of power over Rs. For 16 Ohm speakers the output voltage has to be increased by factor ????.

Those 80Ohm seemed to be a lot so I started with two 15Ohm power resistors I bought today. Before installing them I made a frequency measurement. Then I increased the amplification by 6db inserted the resistors and measured again. With the resistors installed I got nearly the same frequency response but above 3kHz the response increases linear up to +4db at 20kHz. Interesting isnt it? Inserting Rs leads to better mid and hight performance!!! With the Rs of 15Ohm installed the damping factor is decreased down to 1.0 lower than most tube amps.

Then I sat down and listened while switching to and forth between the two configurations. The improvement was really tremendous. I heard things I never heard before. Listening the Friedmann SACD the strokes to the strings of its guitar were astonishing real, much more live than before. Also the room got more width and depth. Female voices got much smoother. I gut the impression a curtain was removed playing over the serial resistance. I had the impression the full range really played freed and much more open.

Now the electronic store is closed so I could not test the 80Ohm alternative :-(

The first test was really encouraging. So may this be the final solution to improve mid and high quality. Just take a big conventional amp (you need power to compensate for the voltage loss over the 80Ohm resistor) and a series resistor to reduce damping factor and therefore provide current instead of voltage to the chassis.

What did you think about my thoughts? Am I right?

Charly

Orgonom 5th November 2005 05:56 PM

Think about it!
 
As you change the output res. your speaker will gain more level
in the mids or highs because your impedancy is rising (inductivity from the voice coil).
When you read nelsons papers, you will see that you could adjust
is with a R-C parallel after the "output Resistor" in your case 15 or
80 Ohms.
Hope this helps.
Andreas

acaudio 5th November 2005 07:21 PM

Charly,

F1 is the absolutely best amplifier for fullrange speakers.
I listended my F1 clone by AES in Kassel ( http://www.aes-kassel.de/breitband.htm ) with Oris horns and AER speakers.
I listened Fostex FE206E too . It was a short decision, Im owner of a pair Fostex ! (The AER are expensive...) I built the Fostex in my old enclosures and I listened the whole night! I heard things the I have never heard bevore. Crazy! Have a look here: http://www.diyaudio.com/forums/showt...536#post758536

Man, you should build F1 and you have with the AERs the best system which you had.

Best Regards
Adam

mastertech 6th November 2005 12:12 AM

oehlrich-
The advantage is clear. Current is proportional to the movement of the speaker so delivering current from the amp would give quite a better result than delivering voltage.

Tell me what advantage? in detail please!

SY 6th November 2005 12:47 AM

That's a pretty inefficient way to do things. Do you have the ability to modify your amplifier's feedback to make it act more like a current source?

chrisr 6th November 2005 01:19 AM

i have a question isn't it POWER we're delivering to the speaker, i'm trying for figure out what the advantage is, sounds pretty inefficient.

SY 6th November 2005 02:55 AM

You can make it as efficient as you like. Using a resistor is a nice let's-see experiment, but not the way an amp should be designed.

The Mills/Hawksford papers available at Nelson Pass's site detail the potential advantages of current drive.

oehlrich 6th November 2005 05:58 AM

Quote:

Originally posted by SY
You can make it as efficient as you like. Using a resistor is a nice let's-see experiment, but not the way an amp should be designed.
Current amps have big advantages right! The way I tried it is ineffcient true. But I have a power amp with more than 100 Watts so what?
The question for me is: Is a current amp with 80Ohm the same as a voltage amp with 80Ohm? Theory says yes. If you say no where is the difference?

oehlrich 6th November 2005 06:31 AM

Quote:

That's a pretty inefficient way to do things.
I made some calculations. Take a voltage amp delivereing 100W in 8Ohm. Such an amp outputs aprx 29V. Using a resistor Rs of 47Ohm and speakers with 16 Ohm you will loose aprx. 10W over Rs and have aprx. 5W at the speaker. If 5W are enough this approach seems not to be that inefficient. In this case damping factor will be 0,34!
My point is: There is no true voltage and no true current amp on this planet. Each real amp is anywhere between. High damping factor meens the amp is more at the voltage side, low damping factor means the amp is more at the current side.
This also means tube amps (low damping) are more on the current side than on the voltage side. Despite destortions may this be a reason why they sound better especially at fullrange speakers?

SY 6th November 2005 11:51 AM

Oehlrich, what I'm trying to say is that you can retain all of the amplifier's power by changing the way that feedback is taken so that it presents an 80 ohm (or whatever) output impedance without losing most of the power through a series resistor.

Certainly to the extent that highish source impedance will cause frequency response variations, that is an explanation of the different sound of some tube amps. Better tube amp designs have a low enough source impedance that this is much less of an issue and the explanation of sonic differences must lie elsewhere.

In my experiments, I found it quite easy to turn a pentode amp into a current source merely by removing the feedback loop, then knocking down the gain of the driver stage.


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