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Old 19th February 2006, 11:25 PM   #11
fab is offline fab  Canada
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Hi all,

For the scope traces I used 10K Hz square wave.

" presume the L-R network mentioned is an L with damping R in parallel, all in serie with the load. "
yes that is correct.

In trying to summarize some comments:
1) should the RC zobel network be adjusted to a specific amp response for even better damping? And the LR too?
2) I still see hi-fi amps schematics without RL filter. Is the R-8 ohms in parallel with 2uF non-realistic so the RL filter is not really necessary in real life, or "exotic" (what charactistics does it have?) cables do not exist anymore?
3)I have already tried different caps load from 1nF to 2UF but I though it was more the phase and gain margin of the amp circuit that had more influence. Do you suggest that the zobel RC filter and the RL filter have an impcat too. For this test, only at about 3.3 nF it gave some oscillation with 10K H z square wave. All other caps values worked with no problem (1nF, 2.2nF, 4.7nF, 10nF, 22nF, 47nF, 100nF, 220nF, 470nF, 1uF, 2uF). Does it mean that all possible values should be tested?
4) Trials and errors listening tests: this is what I want to avoid when searchng info on this forum.

It seems to me that the RC zobel and RL filters are something important for the amp performance so I thought that there were more "basic" accepted rules.
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Old 20th February 2006, 12:36 AM   #12
iggz is offline iggz  Slovenia
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Quote:
Originally posted by fab
Hi all,

For the scope traces I used 10K Hz square wave.

For this test, only at about 3.3 nF it gave some oscillation with 10K H z square wave. All other caps values worked with no problem (1nF, 2.2nF, 4.7nF, 10nF, 22nF, 47nF, 100nF, 220nF, 470nF, 1uF, 2uF). Does it mean that all possible values should be tested?

It seems to me that the RC zobel and RL filters are something important for the amp performance so I thought that there were more "basic" accepted rules.
Maybe it is the capacitor itself? Try another unit, preferably some wima's or other well designed capacitor. You can experiment with different technologies. Ceramic, plastic, mica, electrolyte...

Did you do a frequency sweep with other capacitor values? Try it, maybe you can find something that gives a clue about what is going on.

Zobel, L, always welcome. Good design practice.
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Old 20th February 2006, 01:04 AM   #13
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Just a few further brief thoughts (bed-time here!)

Firstly we can forget about cable capacitance having much of an effect in audio; it is mostly too low unless you are using tens of metres or have some pretty weird cable. But on another thread SY (whose comments can be respected) mentioned that some amplifiers are not happy even with normal cable, as in that they tend to go unstable, I presume above the audio band. I accept he has seen such results, although I would not like to know about such critical designs........

The slight oscillations in the first oscillogram appear to be of the order of 120 KHz and regarding direct effect in audio can be ignored. That graph with the RL to me indicates that the amp saw an increased load at 120 Khz - the reactance of the L - compared to only a load resistance without it. Analysis will indicate exactly what is going on.

The cap in serie with the Zobel R is as said simply to keep it from shunting the load inside the audio band. As such the value is not important but might depend on the amplifier's "demands" at supersonic frequencies. When there is time I will put this on a Spice program and see what comes up, and come back here (a little tied up at present).

Regards.
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Old 20th February 2006, 07:20 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi Iggz,
Quote:
As frequency increases, the open loop gain drops. The output resistance of the amp rises (because of NFB and reduced open loop gain) and moves that pole
I understand most of your reply, but can you explain this moving pole?
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Old 20th February 2006, 07:31 AM   #15
AndrewT is offline AndrewT  Scotland
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Hi Fab,
go back to that 8r//3n3F dummy load. Check for that amplifier oscillation. Probably in the 300kHz to 10MHz range. Not the natural resonance that the L & dummyR produce @ 100kHz to 150kHz.

Quote:
do a frequency sweep with other capacitor values?
Try some other similar value caps and sweep the frequency to see if they cause oscillation as well.
Try to remove it with adjustments to your compensation.

Finally, the testing with dummy loads and // caps is an attempt to simulate the use of different speaker and cable combinations and over a range of frequencies. A speaker changes it's reactance from resistive (at DC) to capacitive then through resistive to inductive and so on as the frequency goes all the way up.
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Old 20th February 2006, 10:01 AM   #16
Bobken is offline Bobken  United Kingdom
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Default Make your mind up time.

Hi fab,

This appears to be a situation where you wish to 'have your cake and eat it' and this rarely works out in real life.

You started off by saying "what is the best way to get the best most natural *sound* " etc., but now you say you wish to "avoid trials & errors listening tests" for this!

Regrettably, all amps vary with regard to their stability issues and how speaker cables and speakers themselves will affect this area. Therefore, there simply is no 'one size fits all' answer here.

Many posters have tried to explain the reasons for these output network components (including myself) but unless you have incipient instability, which shouldn't be the case anyway, I doubt that you will ever *hear* any real differences here. This, of course, assumes that the components don't actually roll-off the signal anywhere near to the audio range, which they shouldn't.

Otherwise, having carried out numerous *listening tests* on similar areas, myself, in my experience you will *hear* far greater effects from the type of components used than with any minor variations in the square-wave overshoot at say 10kHz, or above.

For good reason (Luminaries like) D.Self, B. Duncan, and JLH all state that the output inductor should be air cored (and subjectively I found this to be true) and two of the above who also comment on the sonic effects, agree that these effects (which are always to give a ripple on the square wave) are of no consequence, sonically.

JLH." ... a ripple on the square wave/reactive load test waveform, which is an inevitable effect of any steep cut, low pass filter", and "provided there are not more than 3 wiggles on the square wave, this is sonically benign"..

B.D. "... which may have to be accepted as one of those compromises that measures bad but sounds fine".

In my own experiments, I achieved better results with regard to overall stability and 'clean' square waves (initially without any output inductor//damping resistor) through varying the compensation caps. Then, where a *suitable* damped output inductor has been added later, I could hear no difference to the sound, at all.

However, experiments with different components (of the same nominal values) being used for the cap and resistor of the output Zobel, showed marked changes to the overall sound. To show how seriously I take this, I have ended up using $30 worth of components here for this Zobel alone, in a stereo amp.

Such expenditure nearly breaks my heart, but every alternative component I tried out had some different (and more adverse) effect on the outcome, and my present best choice here is to use MIT RTX (multiple-section) polystyrene & tin-foil caps, together with a Caddock resistor.

Varying the values of these (within reason) has little (if any) sonic effect, but changing to an alternative type/make of component alters the sound by an unintuitive degree, but which I cannot ignore.
Running for a short while without any Zobels, sounded even better still, but I am not prepared to take a chance on this on a long-term basis.


No amount of theorising, simulation, nor measurements (which I have also done!) explain the full 'sonic' picture here, and that is why the very best designs have had hours of development-time spent on them, painstakingly listening to these kind of effects, and consequently also explains (not necessarily justifies!) to some extent, their often apparently inordinately high costs.

You can see from posts on this Forum just how much trouble designers like Jonathan Carr, John Curl, Hugh Dean, Peter Daniel, and many others will go to to achieve the best possible *sound* from their designs, on top of the already technically well thought-out circuits they use.

So, as I said in post #8, if you really do wish to end up with the best *sonic* results, you have no choice but to carry out these trials for yourself, however inconvenient/unattractive this may be, and no-one else can tell you exactly what is best to make it easy for you. If, however, you wish to maximise the results from purely a measurement point of view, that is a different matter altogether.

Regards,
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Old 20th February 2006, 10:13 AM   #17
iggz is offline iggz  Slovenia
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Quote:
Originally posted by AndrewT
Hi Iggz,


I understand most of your reply, but can you explain this moving pole?
This holds true for voltage feedback voltage amplifiers, wich is 99% of all amplifiers made.


I don't know any equation by hart, but roughly the uotput impendance of the amplifier is:

Zout=Z0/(1+beta*A)

Z0 is the output impedance withouth feedback
beta is the feedback factor
A is the open loop gain, which is frequency dependant

assume a beta of say 0.01
a A of 100000 at DC
and a Z0 of 10 Ohm, purely resistive

Zout=0.00999 Ohm at DC.

Let say that the A at 10kHz is 1000, a change of 40 dB, others parameters unchanged:

Zout=0.909 Ohm

Asume that we have a 1uF capacitor at the output. Zout and the capacitor compose a impedance divider, this is where we take our feedback. Pole at DC is:

POLE_dc=1/(2*pi*Zout*C)= 15.9 MHz

The same pole at A=1000 (f=10kHz), Zout is now 0.909 Ohm:

POLE_10kHz=same equation, different Zout= 175 kHz

When the amplifier is excited with 10kHz the pole moves/is located at 175kHz. The same pole is located at 15.9MHz. Both of the values are located away of the 20kHz region in this case. However this is not necesarily so at other frequencyes and other Zout/C combinations. How this affecta the amplifier, is it good or bad, is in the domain of the stability analysis. In other words, it is higly dependant on the amplifier itself (and the components the amplifier is composed of + the topology of the circuit and even layout).
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Old 20th February 2006, 10:19 AM   #18
Eva is offline Eva  Spain
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In my experience, an amplifier may be oscillating like hell, distorting badly, cross-conducting like if it were class-A and starting to smoke, while most people will still tell you that it sounds right. In other words, listening is definitely not the way to optimize the behaviour of an amplifier above the audio band. Analyzing such behaviour with any other tool than an oscilloscope is to me an absolute waste of time.

Concerning air cored output inductors, they will obviously produce a stray magnetic field proportional to the loudspeaker current and to its frequency, that will obviously cause some degree of feedback over the small signal circuits and signal wires. So these inductors should be placed away from the amplifier circuit itself, in the output binding posts for example. A feet or so (30cm) is already a good distance.
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Old 20th February 2006, 11:50 AM   #19
Bobken is offline Bobken  United Kingdom
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Hi Eva,

Regrettably, in your anxiety to disagree (a trait I have seen before in some other posts) you have not read (or perhaps understood) what I said here.

The original query (this was clarified subsequently by fab, and as I specifically reiterated in my previous post) was about the *sonic* differences (or preferences i.e. "best most natural sound") of the two different square-wave forms which had been shown.

To take your comments:

"oscillating like hell, cross conducting, starting to smoke" etc.

Where on earth does this come into the discussion?

I certainly have gone to a lot of trouble to point out that obtaining stabilty is important in the first place, and, together with several other posters, have suggested some ways and means of achieving this. Also, because it is fundamental to the 'stability' issue, in this last post I even suggested it was best to look at the compensation caps first, as no-one (IIRC) had yet mentioned this.

"Listening is definitely not the way to optimise the behaviour of an amplifier above the audio band".

Of course it is not, and no-one has suggested that it was!

What I have said (supported by several references) is that any such sonic differences are unlikely to be heard when conducting *listening tests*. Therefore, they are a red-herring as far as the sound goes in this instance, and no choice is likely to be possible here, based on these wave-forms.

I have looked many times, and for many weeks at such 'scope waveforms, and as I have also said before, there is aurally no difference (IME) when I have carried out these tests.

I simply went on to point out in what I hoped was a helpful comment to someone who had asked about the *sonic* preference here, what *does* most certainly make a noticeable difference to the sound in my experience.

If and when you achieve the experience of some of the people I mentioned before, you will also appreciate that careful choice of components in any audio circuit is also important, and can make a good amp into an excellent one, even if if this not intuitive to you now.

Measurements alone are simply not sufficient to tell the entire story here, whether you accept this or not.

I don't wish to start any retalliatory action here, but your comments on output inductors and crosstalk etc. , do need some clarification if one is to believe Doug. Self (who is far from being a 'subjectivist'!) and my own ears.

In his book "Audio Power Amplifier Design Handbook" he devotes 6 pages to this subject, so it is not nearly as simple and straightforward as you imply. Perhaps this is as well, since many (most?) amps will probably not have an enclosure sufficiently large to allow for your suggested 30cm.

Doug Self has carried out many trials here, and with output inductors which are orientated in the same way (the worst case scenario, similar almost to a transformer, and which he considers to be "the least favourable combination") he measured -126dB total crosstalk at 1kHz, with merely 110mm spacing between them. He sensibly comments with some confidence that "this is not going to be a problem".

I realise that you also include other parts of the circuitry in your comments (at low-level) but cannot accept that they would be any more liable to 'pickup' here than a similar coil tested in a 1:1 'semi-transformer' configuration.

How did you arrive at your recommended and possibly almost unattainable 30cm, which you say is already a good distance?

Regards,
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Old 20th February 2006, 12:12 PM   #20
Eva is offline Eva  Spain
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I arrived at that number by measuring (or no longer being able to measure) the maximum voltage induced in a 10cm diameter loop of wire at 10Khz (that as you have mentioned, is orientation dependent). You can't realiably measure the crosstalk in the own amplifier because the feedback effects are taking place inside it and there may be positive or negative feedback, or a mixture of them over various stages. I was also able to measure induction with improper wiring, but that's another story.
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