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Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
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Old 26th November 2019, 02:27 AM   #431
jameshillj is offline jameshillj  Australia
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A'noon Joe,

It's pretty self evident, especially for explanations on sites like yours - and then there's the area of acoustical transforms, compliance, acoustical impedance, etc

Another quite complex 'thing' would be the output transformers on valve amps ...
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Old 26th November 2019, 02:33 AM   #432
rayma is offline rayma  United States
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Quote:
Originally Posted by Joe Rasmussen View Post
Interesting... not a single comment or disagreement on that point.
It is critical, but current measurements are necessary to see this. Most don't use current probes, and are unaware.
For example, you can run into audible current limiting much earlier than would be expected, on either music
or asymmetrical test waveforms.

Last edited by rayma; 26th November 2019 at 02:36 AM.
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Old 26th November 2019, 03:00 AM   #433
Bigun is offline Bigun  Canada
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Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
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Originally Posted by Joe Rasmussen View Post
Interesting... not a single comment or disagreement on that point.
Well, one of the challenges could be-that the amplifiers people are none too familiar with speakers and the speaker people are none too familiar with the amplifiers so it’s not easy mixing it altogether like that.

The late Graham Maynard looked at the behaviour of his JLH inspired amplifier differently. He was concerned about the impact of a non-resistive speaker on the output stage of the amplifier. He investigated it and designed his amplifier accordingly.

And for some folk, amplifier technology is already fine and great speakers are available too. The challenge is what to do with the room interaction.

For me, it’s mostly academic interest, the Elephant In The Room is actually the difficulty of finding good recording quality. I got ‘ruined’ by a couple of Chesky CDs years ago and have yet to recover.
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Last edited by Bigun; 26th November 2019 at 03:04 AM.
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Old 27th November 2019, 11:15 AM   #434
Joe Rasmussen is offline Joe Rasmussen  Australia
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Originally Posted by planet10 View Post
Well Joe, perhaps because it is althou a cartridge/record/RIAA is also a complex acousta/mechanical interface.

dave
Yes indeed.
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Old 27th November 2019, 11:38 AM   #435
Joe Rasmussen is offline Joe Rasmussen  Australia
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Originally Posted by rayma View Post
It is critical, but current measurements are necessary to see this. Most don't use current probes, and are unaware.
For example, you can run into audible current limiting much earlier than would be expected, on either music
or asymmetrical test waveforms.

And 'current measurements' is the key and right now looking at a series of such measurements that will show results that will be surprising: That amplifiers can have two different distortion profiles at the same time, one on the voltage side and another on the current side - and the current side will have more distortion than on the voltage side. The first two measurements have already been done, not just by me but also by somebody else. Take a typical 2KHz sinewave into a single driver, put that on Channel 1 on your scope. Now fit a 0.1R current sense resistor on the return side, a small voltage that represents the current. Now put that small signal through a low distortion amplifier with a gain around 60. Since the DC resistance of the voice coil is 6R, you have now the Vre of the driver. Multiply the Vre by the current of the amplifier and you will have the dissipation of the voice coil. Only this part of the impedance dissipates heat, the rest of the impedance above 6R does not. I have been attacked for calling this part of the imepdace the "back-EMF impedance" and yet I stand by it. Now put Vre on Channel 2 of the scope. It's amplitude will be less that the voltage of the amplifier, but typically delayed by 30 degrees with most drivers.

Since we now have the voltage sinewave of the amplifier and we also have the current sinewave of the driver, and that there is a difference in time, the question must be asked: WHICH sinewave are we listening to? We cannot be hearing two sinewaves at the same time and we are not. The answer is simple, we are listening to the current, as represented by the Vre of the driver.

What if I was to tell you that you will measure distortion on the Vre (putting it through an analyser) that you won't measure on the voltage side of the amplifier? Well, I am here to tell you that there is, And since we have admitted that we are listening on what we see on Channel 2 is the sinewave we hear, then were are listeining to that distortion.

What causes this distortion? The multitude of imperfections of the driver, that distorts the "back-EMF impedance" and hence corrupts the current of the amplifier. And it can be backed up with measurements.
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Last edited by Joe Rasmussen; 27th November 2019 at 12:00 PM.
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Old 27th November 2019, 11:53 AM   #436
Joe Rasmussen is offline Joe Rasmussen  Australia
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Quote:
Originally Posted by Bigun View Post
He was concerned about the impact of a non-resistive speaker on the output stage of the amplifier. He investigated it and designed his amplifier accordingly.
Absolutely right and doing the same here, based on the legacy work of the late Allen Wright a new amplifier is being developed over the last three years, with the help of another friend of Allen's, Brian Gurr in Auckland, New Zealand.

This amplfier has effectively negative gain (-24dB) in the output stage, this makes it highly immune to current corruption caused by the "back-EMF impedance" of the speaker, that which makes the driver non-resistive.

Quote:
Originally Posted by Bigun View Post
And for some folk, amplifier technology is already fine and great speakers are available too. The challenge is what to do with the room interaction.
Got no beef with you about room problems, but can't agree about the speakers and amplifiers.

If you get amplifiers that cope with back-EMF issues (or current issues, as it is the same thing), you get an extraordinary soundspace, openness and clarity and the sound seemingly coming away from the confines of the speaker box. Also, EQ the current of the speaker and once again you get the same sort of thing. It seems that current management at the interface and you get an audible clarity that seems so less Hi-Fi sounding, textures of instruments so much more realistic in space. Sadly, not the sound that you hear in Hi-Fi shops and shows.

But agree 100% about the room, but to me, that is one of three to get right.
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Old 27th November 2019, 12:24 PM   #437
Joe Rasmussen is offline Joe Rasmussen  Australia
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Quote:
Originally Posted by jameshillj View Post
A'noon Joe,

It's pretty self evident, especially for explanations on sites like yours - and then there's the area of acoustical transforms, compliance, acoustical impedance, etc

Another quite complex 'thing' would be the output transformers on valve amps ...
Hi James

Down in Tasmania for a break, so late this evening catching up...

Quote:
Originally Posted by jameshillj View Post
and then there's the area of acoustical transforms, compliance, acoustical impedance, etc
Absolutely, but these show up as imperfections and modulating the "back-EMF impedance" of the driver and hence as actual current distortion of the amplifier. This is measurable, that is what makes it so interesting, but work in progress...

Quote:
Originally Posted by jameshillj View Post
Another quite complex 'thing' would be the output transformers on valve amps ...
They may actually prove a good thing. We think of an output transformer as a step-down device, but it is as much a step-up device when it comes to current. But in reverse they are step-down devices when it comes to current that is caused by the back-EMF of the speaker. Now try get the output tubes as close to unity gain as possible, and voila! This is not just theory, we got working amplifiers doing just that. Being 'local' I think you will be hearing more about that.

There is now a new world-wide interest in this topic, that saying "follow the money" means "follow the current" will get us a much better understanding of what is happening.
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Old 29th November 2019, 12:56 PM   #438
Ken Newton is online now Ken Newton  United States
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Originally Posted by Joe Rasmussen View Post
Hi James

Down in Tasmania for a break, so late this evening catching up...
Hi, Joe,

We haven't corresponded in some time, so I just thought I would drop in here to say hello. I hope all is well with you and your family.

I see from your Custom Analogue website that you are still developing unique approaches to obtaining good sound. I check the site from time-to-time to see what new ideas you are currently about. I've always appreciated your rejection of rigidly doctrinaire thinking, which, as you well know, entails enduring slings-and-arrows from the self-righteous, who, with minds closed, reflexively defend orthodoxy.
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Old 30th November 2019, 08:57 AM   #439
Max Headroom is offline Max Headroom  Australia
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Recent topic on other threads Bob Cordell's Amp Thread and JC's BT Thread.
Cyril_Bateman/Bateman_Speaker_Amp_Interaction.pdf
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Old 2nd December 2019, 09:37 PM   #440
esl 63 is offline esl 63
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JOE Update your picture link on page One.
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