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Old 10th July 2003, 02:24 AM   #1
owdeo is offline owdeo  Australia
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Default Amplifier topology subjective effects

Hi everyone,

The thread entitled "What do you think of this schematic" posted by PMM began with the schematic of an Aussie kit amp published in the 80s by AEM magazine. Unfortunately the thread got way off track and didn't explore what I think would be a very interesting subject - hence this thread.

In the designer's words, this amplifier was designed to provide the very best subjective performance, while offering superb objective performance. Hence it was titled the "Ultra-Fidelity" amplifier, project AEM6000 - commonly referred to as the "6000".

The author stated that he believes that the use of fully symmetrical stages in the input of an amp is inappropriate and that an asymmetric stage offers superior subjective and objective performance for this stage. However for the next stage (voltage amplifier), he believes the opposite: that symmetrical VAS stages are superior. This is why the 6000 was designed as such, and I've not seen any other amp with this topology. I think this amp is really exceptionally good sounding, and so does PMM.

The generic "Lin configuration" that Self discusses in his "Distortion in Power Amps" series always sounds similar to me - I have heard good and bad amps that use this topology, but they always have to my ears a rather forward midrange, bright but very detailed treble and hard but not extended bass. Based on Hugh Dean's comments about his AKSA amps, it is a matter of tweaking this circuit and getting the fine details correct to make it sound great - I don't dispute this and haven't heard these amps, but I have built many variations of this topogy and to me it always has similar sound characterisitcs. This is a fully asymmetrical topology.

Then you have fully symmetrical topologies such as the Leach amp, which to my ears sound more correct and civilised, but also less interesting than the Self topology (more laid back mids I suspect are the main reason for this).

The 6000 amplifier seems to offer the best of both worlds subjectively (and objectively according to the designer, David Tillbrook), and I wonder whether it is due to the mixture of symmetric and asymmetric stages.

Does anyone have any experience/comments on this subject?
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Old 11th July 2003, 07:37 AM   #2
AKSA is offline AKSA  Australia
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Hi Owdeo,

I guess I can't add much to this argument. I still hold that the devil is in the details, particularly the stability issues.

I also believe there is something to be gained through close control of the open loop gain. Feedback factor is definitely important; too much gives too much damping which kills decay and thus affects vitality. Too little makes it loose, like a bass amp in the bottom end, and allows a little too much of the voltage amp non-linearities through. It is definitely a balancing act, the management of compromise. In this regard an audio amp is like most other machines.......

I have done very little with fully complementary designs, so can't comment too much. By and large I feel their complexity is not justified by improved sound, and complementary voltage amplifiers seem to me to be inappropriate because the OLGs will not be the same. This mandates a lot of emitter degeneration on each voltage amplifier, which greatly reduces the Zout of the VAS. This is very important since the VAS must cope with wildly varying impedances at the input to the output stage as it cycles from positive to negative half cycles through the crossover point.

Cheers,

Hugh
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Old 11th July 2003, 05:11 PM   #3
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Hugh hasn't designed any(?) symmetrical amps but I have _only_ designed symmetrical and the reason why is simply I like the looks of symmetry. I gather also that distortion is "kinder" and more symmetrical.

In contradiction to Mr Dean, I have no problems in using many parts. I'm pleased with my results. That's enough for me.
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Old 11th July 2003, 05:22 PM   #4
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Quote:
Originally posted by AKSA
This is very important since the VAS must cope with wildly varying impedances at the input to the output stage as it cycles from positive to negative half cycles through the crossover point.
This is not really a problem, just design a high impedance emitter follower (or source follower).

With 10-15 mA in the VAS stage it's easy to interface it with a power output stage.
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Old 12th July 2003, 05:46 PM   #5
Pabo is offline Pabo  Sweden
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Per Anders

Quote:
I have _only_ designed symmetrical and the reason why is simply I like the looks of symmetry. I gather also that distortion is "kinder" and more symmetrical.
I don't quite understand what you mean by "kinder". A symmetrically designed amplifier does reduce second harmonics but instead it introduces more of third harmonics. This is because of the exponential nature of the transfer functions for both MOSFETs and bipolar devices.

Third harmonic is known to sound poorly compared to a similar amount of second harmonic. You can easily see it with your eyes if you sum two waves in pspice. The third harmonic affect the appearance of signal much more than the second harmonic.
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Old 13th July 2003, 04:02 PM   #6
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To be honest, this "kinder" thing is more rumour that facts for me. Something I have heard but I feel that a symmetrical amp is good. When we talk distortion, it's rather low in both cases, not a problem really.

BTW: Hej Pabo, are you here also?
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Old 13th July 2003, 04:46 PM   #7
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Hugh and I have had a little discussion about his AKSA amp. The amp may be a little bit "retro" but Hugh have choosed this (with good justifications) and I haven't heard it so I won't dismiss it unheard.

This amp is rather slow and some people claim that an amp should be rather fast with fairly high slew rate. I belong to the high slew rate wing, surprised?

This could be a really burning topic, is it?

Note: Hugh wants his amp to be slow by purpose, his design choice.

BTW: AKSA, does it mean something?
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Old 13th July 2003, 05:07 PM   #8
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Peranders,
Why do you favour high slew designs? Do you agree that 5V/us is about all you'll find in an exact speaker signal at normal room listening levels? Why do you think higher slew than this is superior?
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Old 13th July 2003, 05:38 PM   #9
Pabo is offline Pabo  Sweden
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traderbam

I have actually read in an article by Robert R.Cordell ("Another view of TIM" I think) that a normalised slew rate of 0,02V/us/V is sufficient in order to cover all music examples. This figure was determined after measuring the FFT of a large amount of LPs if I remember correctly. In a 100W/8ohm amp this would mean that about 0,8V/us would be enough.

LPs probably contain much less information at high frequencies than CDs and especially DVDs or SACDs so triple the figure and end up at 0,06V/us/V which will mean that a 200W/8ohm amp will need 3,6V/us.

It would be interesting to analyze a SACD with a FFT
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Old 13th July 2003, 05:43 PM   #10
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Quote:
Originally posted by traderbam
Peranders,
Why do you favour high slew designs? Do you agree that 5V/us is about all you'll find in an exact speaker signal at normal room listening levels? Why do you think higher slew than this is superior?
BAM
(kindly give us all a break and change that picture)
Slew rate is depending of output power. The amp should be far away from slew rate limiting at all audio levels. Why do we need 192 kHz sampling and and slow amps? My theory is that the speaker should be the worst link in the audio chain.

To be honest: not fun to make slow amps! It's a lot more challenging to design fast ones. Have you followed the CFB thread?

I don't think that many designers, DIY'ers really have made a calculation of which figures different parameters should have, for example 0.05% dist, 35 kHz BW etc. I think we want something better regardless of needs.

How many think that MP3 is alright? How many think that CD is OK? How many think that CD isn't any good and want SACD, DVD etc?

The avatar, you have a vivid imagination! Most living stones are green, brown or beige but Lithops optica is the only one which is purple. I have no intention to change it at the moment, not tired of it yet.
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