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Old 8th June 2009, 06:43 AM   #341
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Quote:
Originally posted by AKSA
Gaetan,

Certainly sounds plausible, but in the absence of measurement, I can't verify.

Insofar as this is detective work the thrill of the chase is good fun, isn't it?

Ciao,

Hugh

Hello Hugh

Unfortunately, not very much peoples are interested to do deep researchs and tests in that subject, so I am limited to do speculations base on my knowledges of psycoacoustic and electronics.

But as usual I am currious and I always like to do some Sherlock detective work.

How did you find out about the fact that very low levels of H5, H7, H9 can subliminally cause problems ?

Bye

Gaetan
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Old 8th June 2009, 06:51 AM   #342
AKSA is offline AKSA  Australia
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Gaetan,

Simple deduction.

If very low THD still causes irritation, and we know from psychoacoustics that high order is nasty, then it's a logical conclusion.

Tubes in open loop do not do this (listener fatigue); as soon as negative feedback is applied, the problem starts to creep in.

There are holes in these arguments, but I have not figured out what they might be.....

Hugh
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Old 8th June 2009, 06:59 AM   #343
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Hello Hugh

Is it worth to do a thread about that subject ?

(maby it would go nowhere)

I don't want to hyjack the TGM Amplifier thread.

It would be interesting to know how Nelson Pass see this psycoacoustic and high frequencies H distortions subject.

Bye

Gaetan
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Old 8th June 2009, 07:20 AM   #344
AKSA is offline AKSA  Australia
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Gaetan,

Yes, why not?

However, I doubt Nelson will comment on this, he might, but it's speculation.....

What about some of the scientists and engineers here? Do they have a POV on this?

Hugh
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Old 8th June 2009, 07:58 AM   #345
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Hello

If some guys want to discuss more about all of this subject, maby the moderator can transfer all the concerned posts in a separated new thread.

For those who like to know more about psycoacoustic and ears sound localisation, here is two link;


http://www.aip.org/pt/nov99/locsound.html


http://www.answers.com/topic/sound-localization


Bye

Gaetan
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Old 10th June 2009, 05:10 PM   #346
Bigun is offline Bigun  Canada
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Everyone,

Again, a thank you for your help and patience to lift me up the ladder, so that I could make an amplifier and pointing me in the direction for making it a bit better. It would not have happened without this Forum.

I think interest in TGM1 must naturally subside and I don't plan to do any further changes to TGM1, so I have decided not to continue with this thread on a regular basis. I started with AKSA as the inspiration for this amplifier and with some help via private communications from 'AKSA" I have been able to achieve some wonderful sounds. My opinion is that there is much to be gained by having the option of an amplifier that was not designed simply for the lowest possible distortion. There is room for both the 'purist' and 'artisticic' approach to defining the goal of good amplifier design.


My goal was not to develop an amplifier for others, so I won't be promoting it as a DIY project, there are better documented amplifier projects on this Forum with similar topologies (+ various web sites), e.g. the Carlos DX amplifier series [Destroyer x Amplifier...Dx amp...my amplifier] and the Back To The Future amplifier championed by OS [RCA 1972 Basic amplifier MODS].


TGM2 will have a new thread. [TGM2 amplifier]

====================
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Old 9th February 2010, 07:11 PM   #347
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Hey Bigun...

I didn't catch where anyone answered your question about the "emitter degeneration" resistors in the LTP... They are a form of local negative feedback and they increase linearity since the error voltage is applied across only the BE junctions (which are exponential in their current-vs-voltage characteristic) without them. (I am a proponent if it wasn't obvious). It doesn't take much error to send one transistor into saturation and the other into cutoff, for which the rather large miller cap in the VA stage is a band-aid to keep the amp from oscillating when the inputs are subject to transient error. The equilibrium voltage drop across those resistors widens the error (dynamic) range substantially (because that voltage will withstand doubling without exceeding the linear range), and increases the frequency onset of slewing (when the entire amp is going as fast as it possibly can to restore order, rather than patiently following the waveform).

In my own designs, similarly to as outlined by W.M.Leach (GA Tech EE professor with very good and stable audio amplifier designs) in his designs, I use hundreds of ohms for LTP emitter degeneration to reduce gain and feedback, and thus distortion, and increase dynamic range by allowing the transistors to both still be in their active regimes even with substantial error. The more you can keep active devices from turning off and/or entering saturation (which can be thought of as "sticky"; it causes lagging distortion because both conditions are sort of momentary commitments for a transistor) in real-world transient conditions, the better.

A design example for me is to have a 2.5 mA bias current in the LTP, with 300 ohm resistors on the emitters. with the 50/50 split at zero error, the drops sit at 0.375 volts, and the response is theoretically linear up to +/-0.75 volts of error. Whether you actually forward the benefit of that linearity to your VA stage is another matter, but you definitely will not have it at your disposal with without any emitter degeneration.

Consider that your (any) amplifier takes a finite time to source or sink current and get the relatively massive speakers moving and return the confirming measurement to the input, so if your audio signal source of choice supplies a step change beyond this voltage reach that is quicker than that finite time, the LTP has its proverbial "needle pinned" until feedback from the output confirms that the universe has returned to sufficient balance, several tens of microseconds later. This is the "transient intermodulation distortion" that some erroneously believe to be mythical.

Without those resistors (and indeed with extremely small values), the error voltage required to produce this ugly condition is quite infinitesimal. The design without them must rely on interstage feedback in the form of bandwidth-consuming capacitors to reduce this problem, causing the amp to simply not bother responding to fast signals rather than chase them hither-and-yon across the voltage rails.

The 'penalty' of degeneration is to reduce open-loop gain. I consider this a minimal trade-off since the gain gets more linear and feedback correction (along with harmonic distortion) is greatly diminished as a consequence.

Sorry about the long-windedness here, but I feel somewhat passionate about this and I see no reason perfectly good people should be making bandwidth compromises for the sake of infinite gain. The greater the open-loop gain, the greater the finesse required in global feedback. The "automotive analogy" would be like having cruise control that could only either give your intakes full throttle or kill the engine entirely until your speed is properly controlled, rather than just applying little bits of effective correction on a dynamic basis.

Note that current mode feedback (like your headphone amplifier design) circumvents this problem in its own way that I am not prepared to explain, and is preferred by some designers.
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Old 10th February 2010, 03:00 AM   #348
Bigun is offline Bigun  Canada
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Howdy,

Since 'finishing' (it's not really finished) the TGM project I have become a lot more enthusiastic about the application of local feedback via emitter degeneration and my current projects will use this to various degrees.

I haven't given much thought to the question of a limit to the dynamic range of the differential pair at the input. As you say, when there is little phase shift of the feedback relative to the input the input differential will not attempt to generate a large error signal and all is well. I can see that a large phase shift due to a nasty load for example, will compromise the accuracy of the error amplifier with unfortunate consequences. If I'm not mistaken about the work of Graham Maynard, he invented the GEM amplifier to address the issue of the active load messing with the amplifier.
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Old 5th February 2012, 10:45 PM   #349
Bigun is offline Bigun  Canada
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Well, my chasis and power supply were needed for another project [TGM5 - all-BJT Simple Symmetric Amplifier ] so now my poor TGM1 amplifier is sitting idle. Since it was my first ever amplifier (kind of my first ever electronic circuit for that matter) that I made at home I feel it's a shame not to find a new box.

I have found a new box. But now I need to build a new power supply. This time around I'm going to use all Nichicon Gold capacitors, matching those that I used on the amplifier it'self. A new pcb is in the design stage...

And maybe a couple of improvements will be made to my TGM1 modules - nothing major (no topology changes).
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File Type: jpg psu.jpg (91.3 KB, 336 views)
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Old 7th February 2012, 02:33 AM   #350
Bigun is offline Bigun  Canada
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Progress made - pcb patterned, etched and drilled. The unusual shape is to match up with the new box, itself recycled from an industrial application it has already some mounting holes etc. A recycled trafo from a Yamaha Tuner installed already.
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File Type: jpg TGM1 new box2.jpg (240.7 KB, 257 views)
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