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Old 27th March 2012, 12:32 AM   #4001
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Originally Posted by VladimirK View Post
Coming back to the measurements vs sound, I could state that nothing is clear with such terms as "voltage amplifier" and "output impedance". None of these predetermines the bass signature (articulation) at a given system. Also, rated power of an amp helps not a lot for getting clear and articulated bass.
It is quite easy to observe, that 20W class D is better in bass than 200W class AB, or definite versions of 20W class A produce better bass and wonderful overall sound compared to various 100W...200W class AB amps.
These facts are hard to believe and to explain. At present, I see only one possible correlation: current NFB is much more preferrable than voltage NFB, resistive NFB divider with 40mA passing through it is much more stable against speaker back EMF than NFB resistive divider passing few microamps through it.
I believe it comes down the fact that an amplifier & loudspeaker have to be considered as a system. In today's world, or at least not that long ago, a significant majority of speakers assummed a voltage source amplifier & hence tended to work best with any random amp that was more or less a voltage source/low output impedance. ie a chiken & egg thing.

But as diyers we are not constrained by many of the marketing considerations that typical retail oriented manufactures are constrained by.

Having a variable transimpedance amplifer here, and a wide range of speakers -- many of them built without the "gonna be driven with a voltage source" paradym, it is very clear that for every loudspeaker there is an appropriate output impedance, many of mine tend to like highish output impedances.

dave
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Old 27th March 2012, 12:37 AM   #4002
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Originally Posted by dvv View Post
Since 1966, I have learnt a thing or two about audio amps. I have a small collection of them, cosnsisting of Marantz (2 integrateds and one pre/power amp combo, made in 1978, all overhauled), Harman/Kardon (2 integrateds, one overhauled, other in the works), Sansui (1 from 1984) and 1 Karan Acoustics (from 2003), so it's safe to say I can verify most views I have. All the more so since I also have three pairs of very different speakers at home - my own (exceptionally easy load to drive, B&M Acoustivs 1041 Minitor), my wife's (JBL Ti600, floorstanding 3 way, reasonable load) and my son's (AR92, overhauled, changed suspensions, 2.5 way), which is a hard load to drive.

And lastly, a good friend of 30 years owns a pair of Apogee speakers, which are an evil load to drive.
A very small sample and (i guess -- counldn't find any info on the B&M, do you have an impedance curve?) all stuff that only plays well in your corner. Since 1966 i have had, or had the opportunity to play with 100s of different amplifiers & 100s of different loudspeakers.

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Old 27th March 2012, 02:00 AM   #4003
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Parts shipped from DK today. Give the mail a few days and a few days to get them in. To keep entertained, I am putting in the schematic for the RB-951 and RA 840 to see if there is any trend. Rotel was nice enough to leave the values off a few parts in the schematic, so I have to open it up to see. It has a few things in the schematic I don't quite understand yet. I will study before I ask.
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Old 27th March 2012, 03:57 AM   #4004
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Dave,

Quote:
Originally Posted by planet10 View Post
In today's world, or at least not that long ago, a significant majority of speakers assummed a voltage source amplifier & hence tended to work best with any random amp that was more or less a voltage source/low output impedance. ie a chiken & egg thing.
I experimented with current drive and motional feedback back in the 80's and this was hardly the beginning.

The fundamental problems of "Voltage drive" to speakers have been documented for a long time, however as with so many other issues in audio (useless measurements like THD, (T)IMD, Dumping Factor etc) the audio engineering community by and large has elected to follow the course often ascribed to the majestic Struthio camelus; that is to burry it's collective head in the sand and hope those beastly anderdenker (differentthinkers) will eventually go away, a hope in which they have generally so far been confirmed.

Ciao T
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Old 27th March 2012, 04:15 AM   #4005
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Originally Posted by tvrgeek View Post
Parts shipped from DK today. Give the mail a few days and a few days to get them in. To keep entertained, I am putting in the schematic for the RB-951 and RA 840 to see if there is any trend. Rotel was nice enough to leave the values off a few parts in the schematic, so I have to open it up to see. It has a few things in the schematic I don't quite understand yet. I will study before I ask.
schematic?
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Old 27th March 2012, 05:11 AM   #4006
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Originally Posted by ThorstenL View Post
I experimented with current drive and motional feedback back in the 80's and this was hardly the beginning.
Some old posts you made on the subject, and a long talk with an old WE engineer that started me thinking about current drive.

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Old 27th March 2012, 05:19 AM   #4007
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I did not experiment with current drive, but theoretically it should work well if enclosure is stiff and well damped. I experimented with feedback by acceleration. It worked extremely well, but I neglected rumble filter and damaged the driver. Later I experimented with negative impedance, it works almost as good as feedback by acceleration, however in terms of frequency response only. If to go for lower distortions I would go for current drive, but I have too much of other work to do now...
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Old 27th March 2012, 05:21 AM   #4008
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Quote:
Originally Posted by planet10 View Post
In today's world, or at least not that long ago, a significant majority of speakers assummed a voltage source amplifier & hence tended to work best with any random amp that was more or less a voltage source/low output impedance.
This point of view is a common one at present, but it is oversimplified, as usually.
I have got numerous confirmations, that output impedance alone does not describe amp-speaker interaction, there are also differences in how exactly amp damps out the speaker back emf, there is a kind of transient reaction of an amp on the back emf applied.
Given two different amps (on with current NFB and another with voltage NFB, for instance) have the same output impedance, with the same speakers, they produce quite a different bass.
And if one amp, having 0,5 Ohms output impedance, produces more articulated bass than another amp with 0,05 Ohms output impedance, this does not mean that speaker crossovers were optimized for 0,5 Ohms. Most probably, this means that the first amp has more beneficial transient reaction on back emf.

Last edited by VladimirK; 27th March 2012 at 05:26 AM.
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Old 27th March 2012, 05:42 AM   #4009
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Originally Posted by VladimirK View Post
that output impedance alone does not describe amp-speaker interaction, there are also differences in how exactly amp damps out the speaker back emf, there is a kind of transient reaction of an amp on the back emf applied.
Of course it is more complex than that. How much the backEMF screws up the amps performance is another factor, and i think what you are trying to desctibe.

dave
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Old 27th March 2012, 07:41 AM   #4010
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