Symetrical schematics are alike plague in Brazil, do you like them?

[PMA]

Originally Posted by Bigun

Or put it another way, it has been claimed that distortion harmonics are significantly less objectionable if they follow a clear decreasing trend from 2nd to 3rd and so on. Would the higher order harmonics be as audible in this case.

Hi,

you mean something like this, for sure:

[PMA]

I was forced to work with the LM3886. I still have a large set of measurements. This stuff is a real crap, both sonically and in parameters. Good just for beginners.

[Elvee]

Originally Posted by PMA

Dear Ms. Eva,

please try also higher harmonics (in a controlled test).

I would add my results:
1% of 2nd H is at the edge of audibility
1% of 3rd H is distinguishable and audible
1% of 4th and higher H is horribly audible
0.1% of typical classB crossover distortion is easily audible.

Best regards,

I am not sure I agree with these figures.

Let us make something absolutely clear first:

I do not belong to the golden ear/subjectivist clan, and I would classify myself as a typical tin-eared guy.

And anyway, I never use listening tests to assess anything seriously.

But, a number years ago, I was setting up a low distortion FM discriminator.
A 1KHz signal was used for the test and I used a distortion-meter to control the result. I wanted to find the optimum setting for a double tuned quadrature demodulator, which was capable of under 0.1% THD.
There was also a monitoring amplifier/ speaker connected at the output, and to my amazement, I noticed I could sort of "detect" very small amounts of distortion on the pure sinewave.

As it seemed suspicious, I set up a crude "blind test": I disconnected the distortion meter, and made the adjustment only by ear a number of times, then checked the actual figure with the meter.
I consistently got results in the 0.1% region.
The figure in itself has nothing particularly striking: a nasty crossover artefact can be heard easily at levels significantly lower than that, but the effect of a misadjustment of the second coil of such a demodulator is very soft, and only increases the level of even harmonics, mainly the second.

Now, this test is only indicative, and there are a lot of arguments that could be used to dismiss it: a pure 1KHz sinewave is unrealistic, I was "looking for" and expecting something, there was a sort of dynamic feedback between the hearing and the movement of the hand adjusting the screwdriver, but anyway, I would recommend anyone to try it: it shows that even harmonics are not necessarily pleasant, undetectable or benign.

PS

I don't like the sound of single ended amplifiers, at least the ones that display significant amounts of objective distortion.

[godfrey]

Originally Posted by Eva

Excuse me, 125dB at 10Khz? 225dB at DC? Are you sure? It seems quite far from practical. Where is so much gain coming from and how it is made stable?

That would be the LM3886-based composite amplifier he claims to have designed and built. Apparently it has 100dB of global feedback at 10kHz and is unconditionally stable into any load, with no output inductor.

A remarkable achievement indeed, especially as he claims to have designed it eight years ago, but didn't know what group delay was until last month. See here.

[wg_ski]

Originally Posted by godfrey

That would be the LM3886-based composite amplifier he claims to have designed and built. [/URL]

And it's not a gainclone, nor for beginners. I've seen LM1875's tucked inside the feedback of precision op amps 20 years ago. Distortion just went away.

I don't know about 100dB of global feedback at 10k, though. It was probably less.

[wahab]

Originally Posted by Eva

The 0.001 ohm output impedance figure at 20khz is unrealistic. That would be a damping factor of 8000 at 20khz. .

A "little" mistake by an order of magnitude.
But still 8000 below 2khz, the freqency of roll off.

[sumaudioguy]

Originally Posted by Eva

Excuse me, 125dB at 10Khz? 225dB at DC? Are you sure? It seems quite far from practical. Where is so much gain coming from and how it is made stable?

Yes that is it. Now 8 years back. A practical composite amplifier. Have 100 in the field which have been turned on since install and have given no trouble of any kind. I do see a few changes to enhance performance now though. Excellent sonics none the less.

But this is off topic of the symmetric circuit amplifier. The schematic is for an "early" high symmetry amp from the 70's. Highly symmetric and yet today still so very popular.

Not everyone uses the present popular electronic models and formula for calculation of filters and an amplifier is simply a filter with gain. Just because I do not happen to use those does not mean I do not know what I am doing thank you. I am a physicist and not an engineer so as usual for us physics guys- we tend to look at things differently. Physics is the direct study of nature and not electronics in particular.

As anyone can see there is not much to this composite amp with servo. Checked the gain calculation and the LP is set to 106kHz for compensation so the gain claimed should be very close to correct via direct measurement.

What I wonder about is why it is so important to say that I am wrong or that I do not know. I see we disagree but that does not automatically mean I am wrong and someone else is correct. Please all, it used to be the earth was flat and square and most of the learned ones agreed. This is how I see some here with their flat earths. Allow us the chance to learn and review from one another without these items which prick the ears of the moderators. I have sworn off that behavior and hope I never trip again.

[wahab]

Three symetrical differentials + vas in serial..

The accuphase P300 did use two, back in the 70s,
but each one with its own negative feedback loop,
and no global loop.

Since a single such stage can provide some 80dB open
loop gain without too much effort, ,i guess that the 225db
gain at DC is quite realistic...

[godfrey]

Originally Posted by sumaudioguy

The schematic is for an "early" high symmetry amp from the 70's.

The schematic you show is completely unrelated to the thing in the photo, which is what you claim to have designed and are attempting to sell in the commercial sector of this forum.

Originally Posted by sumaudioguy

As anyone can see there is not much to this composite amp with servo.

As anyone can see, the schematic you provided is not a "composite amp with servo".

Regarding the amp that you claim to have designed and built, you have made it clear in various threads that you are not prepared to show the schematic, you are not prepared to share any technical details as to how it works, you don't have any measurements or test results , and you can't even offer any meaningful specifications.

Is there any point in mentioning it here, other than to bolster your claims of expertise or perhaps promote a sale?

[sumaudioguy]

The highly symmetric amp is the thread subject so this early version schematic, 1974, belongs here and could be of interest to some.

For LM3886 composite amp specs please see the thread:

So just how "good" can a chip amp be ?

where that information belongs. I mention this chip amp here because of the high gain brought up earlier and this composite amp does have very high gain so is related to the subject of discussion of that post.