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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 11th December 2011, 07:34 PM   #18981
Wavebourn is online now Wavebourn  United States
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John Curl's Blowtorch preamplifier part II
Quote:
Originally Posted by PMA View Post
Anatoliy, you might be surprised. No need of testing 2W SE tube amps only or overload protection function. Try it and you might be surprised.
So, what's the point, in such case, in frequency response curve of the amp loaded on some kind of dummy load that represents some standardized model of some standard speaker?

To show that the amp has very low output impedance? Is it good, or is it bad? What about amps that were made especially with high dynamic resistance, so called Current Drive Amps? Do they sound worse than amps with negative output resistance?
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Old 11th December 2011, 07:57 PM   #18982
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Quote:
Originally Posted by Wavebourn View Post
So, what's the point, in such case, in frequency response curve of the amp loaded on some kind of dummy load that represents some standardized model of some standard speaker?

To show that the amp has very low output impedance? Is it good, or is it bad? What about amps that were made especially with high dynamic resistance, so called Current Drive Amps? Do they sound worse than amps with negative output resistance?
When you get into specialized topography such as current output amplifiers then you then get very loudspeaker design specific results. (But you knew that.) But most of the amplifiers I have looked at have quite interesting differences in distortion results both at low level and at clipping when run into a real loudspeaker load.

These days I use a dual 18" subwoofer for the load because it was leftover from an arena system. (They only had enough space for 30 of them not the 32 the system was designed to use.)

Even at very low levels where I would not expect any real strain from the load differences show up, there often is a difference. Now is it due to back EMF into a feedback loop, or just due to the V/I phase shift, or some other cause is an interesting consideration and points to what may be weaknesses in the design.

As you may be aware I am starting to look at noise modulation of circuitry and that may be an issue with different loads.

Now at high levels collapsing inductors in loads and other issues pop up. Although most of your work assumes that the load crossover inductors don't saturate, the behavior during saturation makes a very big difference in the perceived sound quality of many pro amplifiers.

This is much less of a problem in amplifiers just used for reproduction of recorded music with purpose built loudspeakers. But unless you are controlling all the users of your products you may get complaints or even a bad reputation among some users.
 
Old 11th December 2011, 08:03 PM   #18983
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You have a good point, Wavebourn. If anyone wants to check, my JC-1, because of its unique output stage, heavy bias, use of negative feedback, and NO output coil has one of lowest output impedances over audio frequency in the industry. Still, it does not mean that much. Other factors count more, and I use added .3-.5 ohms in series with my amp to drive my WATT loudspeakers. I prefer it that way.
John A, I meant to challenge the inference that we were buying our reviews through placing ads in either 'TAS' or 'Stereophile' . Some people just imply things that have no justification.
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Old 11th December 2011, 08:07 PM   #18984
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John Curl's Blowtorch preamplifier part II
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But most of the amplifiers I have looked at have quite interesting differences in distortion results both at low level and at clipping when run into a real loudspeaker load.
Yes, the keyword is distortion results. I totally agree here. But the point was about frequency response curve. If I want to check frequency response curve of my amp most revealing would be the curve taken on plain resistive load, even 1/4'Th of nominal load resistance. If I want to see frequency response curve with loudspeakers I would add measurement microphone into the equation, but frequency response of the amp loaded on some kind of dummy load is meaningless.

If to compare amps by frequency response curve taken as function of output voltage to input voltage when loaded on reactive load, of course Bob's amp will look always better than Nelson's amp in such reviews because they have different output resistances by design.
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Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
 
Old 11th December 2011, 08:25 PM   #18985
Wavebourn is online now Wavebourn  United States
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John Curl's Blowtorch preamplifier part II
Quote:
Originally Posted by john curl View Post
You have a good point, Wavebourn. If anyone wants to check, my JC-1, because of its unique output stage, heavy bias, use of negative feedback, and NO output coil has one of lowest output impedances over audio frequency in the industry. Still, it does not mean that much. Other factors count more, and I use added .3-.5 ohms in series with my amp to drive my WATT loudspeakers. I prefer it that way.
That means, you need to add a switch: "For Stereophile Review / For Listening Preference".
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Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.

Last edited by Wavebourn; 11th December 2011 at 08:28 PM.
 
Old 11th December 2011, 08:28 PM   #18986
StereoEditor is offline StereoEditor  United States
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Quote:
Originally Posted by Wavebourn View Post
So, what's the point, in such case, in frequency response curve of the amp loaded on some kind of dummy load that represents some standardized model of some standard speaker?
It gives an easily grasped picture of how and if the amplifier's output impedance changes with frequency and whether the inevitable response changes with a typical speaker will be large enough to be audible or not. You can see the circuit diagram and the complex impedance of my standard simulated speaker at Real-Life Measurements Page 2 | Stereophile.com .

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Old 11th December 2011, 08:29 PM   #18987
simon7000 is offline simon7000  United States
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Never mind, I read the actual reviews not the quick quotes version of them!

In pro use flat frequency response is the goal of many, of course that is not the goal for really successful systems. (I will not reveal that secret!!!!)
 
Old 11th December 2011, 08:30 PM   #18988
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Quote:
Originally Posted by Nelson Pass View Post
Peter Moncrief said that the role of the reviewer is to help the reader
differentiate between products. There is no such thing as a "best"
amplifier, only ones which best fit a particular customer's needs.

Amen to that thought, Nelson, with the proviso that there are some designs that would never fit any customer's needs!

John Atkinson
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Old 11th December 2011, 08:43 PM   #18989
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John Curl's Blowtorch preamplifier part II
Quote:
Originally Posted by StereoEditor View Post
It gives an easily grasped picture of how and if the amplifier's output impedance changes with frequency and whether the inevitable response changes with a typical speaker will be large enough to be audible or not. You can see the circuit diagram and the complex impedance of my standard simulated speaker at Real-Life Measurements Page 2 | Stereophile.com .
John;
I respectfully disagree with you here.
If the amp has output impedance that changes with frequency it is much more useful to compare result with straight line than with some kind of standardized curve that also depends on output resistance of the amp, even if it is absolutely frequency independent. Also, dips and peaks on impedance curve of loudspeakers are not directly translated into dips and peaks on sound pressure curve.

During one BAF in San Francisco there were 2 listening rooms, one in front of another. In both rooms were installed speakers and amplifiers. In one room were flat panels, like doors, with 1 full-range driver in each, and amps that drove them had relatively high outpt resistances.
In another room speakers were made by professional carpenters, they were complex dipoles with crossovers, amplifiers had very low output resistance.

In your tests amps in room 1 would look much worse than amps in room 2. But music in both rooms sounded nice. I personally prefered reproduction in room number one, because closing eyes I could hear real instruments in the room, but in the room 2 I always heard nice sounding system.
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Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.

Last edited by Wavebourn; 11th December 2011 at 08:45 PM.
 
Old 11th December 2011, 08:43 PM   #18990
wayne is offline wayne  United States
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John Curl's Blowtorch preamplifier part II
I like Johns measurements consistency is important. One of our techs was testing an Int150 and I couldn't remember a curve when my machine was backing up. Well a quick look at Johns review measurements and all was well.
As an old prof said measure! Measure! Measure! You had to bang the table with a red nut driver while saying it.
 

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