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Amplifier topology subjective effects
Amplifier topology subjective effects
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Old 13th July 2003, 07:15 PM   #11
Pabo is offline Pabo  Sweden
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peranders

Designing a 300V/us amplifier is like designing a 2000W amplifier,

You will feel pretty confident that the speed is enough in the first case as well as the power in the second case.

As long as those goals does not affect the sound quality I agree that it is more fun to design fast and powerful amplifiers but in most cases those goals will affect the sound quality.

Take for example the fastest amps you can find and you will see that they use current feed back. A current fed back amplifier normally have poor PSRR compared to a voltage fed back amplifier. Sure, you can put a linear regulator on the power rails to the input stage but the second harmonic is also much higher than with a differential stage.

Some might say that the distorsion is low enough anyway but in that case you can say the same about the speed.
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Old 13th July 2003, 09:29 PM   #12
peranders is offline peranders  Sweden
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Amplifier topology subjective effects
Patrik, you sound like a boring engineer but you are right. "More" isn't allways better. You have somewhere a technical optimum.

You are rather new here so you have maybe missed, putting CD's in the freezer, peeling of covers of caps, using 17 um laminate for multi ampere amps and 10 mil /17 um for signal traces, etc, etc, etc? (The last one is a voodoo tweak from a known member here. )

The world is crazy
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Old 14th July 2003, 02:10 AM   #13
jcarr is offline jcarr  United States
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I have a voltage feedback power amp with a slew rate of about 200v/us (the slew rate of the voltage gain section is about twice this value). This is a proof-of-concept prototype - neither the schematic nor the board layout have been optimized, so I expect that with more development effort the performance can be improved.

Given the amount of RF noise that is omnipresent in today's world, an audio amplifier has to deal with much higher bandwidth energy than the music signals themselves.

IME, amplifiers work best when: 1. there is a low-pass filter at the input. 2. the physical size of the circuitry is small, so that the amplifier acts like a smaller and therefore less sensitive antenna for RF. 3. The amplifier has enough bandwidth and slewrate that any HF energy that gets through 1 and 2 can be treated just like any other signal. I find that when the above measures are not done, the amplifier is more prone to IM distortion, and the results of that are sum-and-difference signals that are not related directly to the musical signal at all. When this happens, you may be able to hear the distortion as a separate mask over the music signal, or the amplifier may simply sound nasty.

I have noticed that frequently amplifier manufacturers (including myself) tend to get into trouble when they first demonstrate their prototypes in a different country. I believe that this is because the RF band is allocated differently according to local laws (not to mention hotel rooms with cable transmissions running through the walls on poorly-shielded cable).

hth, jonathan carr
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Old 14th July 2003, 04:04 AM   #14
djk is offline djk
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"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. "

So deliberately choosing a 'slow' cap for the bootstrap current source makes the OLG the same for positive and negative going signals?

Right.
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Old 14th July 2003, 06:12 AM   #15
Pabo is offline Pabo  Sweden
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jcarr

Not quite sure I understand. The best way to make sure that HF-disturbances aren't interfering with the audio signal would be to limit the bandwidth of the gainstage, wouldn't it?

peranders

I don't know if you have seen these wonderful little resistors called MINIMELF? They are packaged in a cylindrical case and they sound great because of their low noise and high bandwidth. You can buy them i Farnell and they don't cost more than regular thickfilm resistors. A bit harder to solder though
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Old 14th July 2003, 06:27 AM   #16
peranders is offline peranders  Sweden
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Amplifier topology subjective effects
Minimelf is cool especially when you put them in via holes! I have never used them myself, haven't had any need for this extremely tiny resistor.

I wonder why no DIY'er use them? They use anything else which is weird of some kind.
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Old 14th July 2003, 06:58 AM   #17
phase_accurate is offline phase_accurate
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Pabo wrote
Quote:
The best way to make sure that HF-disturbances aren't interfering with the audio signal would be to limit the bandwidth of the gainstage, wouldn't it?
Regarding RF I fully agree with Jonathan Carr:
It is definitely better when your amp is able to handle the RF at it's input properly rather than trying to get araound the problem by making any amp stages sluggish.
Since a passive lowpass is always more linear (given the right choice of components and layout) and always better able to handle RF signals than any active circuitry, the input filter is a good aproach for me.
I don't remember ever having built or owned any power amp without one !!!!!

Who ever heard a broadcasting station through his audio equipment knows what the the effect of improper handling of RF signals means (rectification/demodulation within active stages). Even if you dont' hear them distinctively doesn't mean that they are not there and don't impair the sound.

Regards

Charles
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Old 14th July 2003, 09:19 AM   #18
capslock is offline capslock  Europe
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There was an app note by AD on this. A lot of RFI rectification seems to take place in the input transistors, where it produces an error voltage. Running the LTP at higher current makes it less susceptible. High speed bipolar and FETs were less susceptible than low speed, precision devices.

Limiting the VAS bandwidth is probably not a good idea, as it will allow the LTP to go into saturation on a RFI transient. And input filter to throw away the unneeded bandwidth before the active circuit is always a good idea.

Regards,

Eric
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Old 14th July 2003, 10:54 AM   #19
jan.didden is offline jan.didden  Europe
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Quote:
Originally posted by Pabo
Per Anders



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.
Pabo,

I beg to differ. Indeed the second harmonics largely cancel in the symmetrical case, which leaves the 3rd harmonics more "visible". But it does not increase the 3rd harmonics wrt the non-symmetrical case because of the non-symmetry itself.

Jan Didden
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Old 14th July 2003, 03:40 PM   #20
Pabo is offline Pabo  Sweden
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janneman

I state that you are wrong.

Simply draw the transfer functions of a PNP and a NPN and then combine these curves.

You end up with something that looks like a third order function.

The output voltage of a gain stage is equal to the input voltage multiplied with the transfer function so a third order transfer will introduce or increase the third harmonic.

If you don't believe me, simply simulate!
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