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Why Let an Amplifier Sound Good when You can Force it to?
Why Let an Amplifier Sound Good when You can Force it to?
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Old 29th January 2018, 10:11 AM   #41
DF96 is offline DF96  England
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Quote:
Originally Posted by stocktrader200
Unity gain stable ( 100% NF ) is one of the must haves in any amp I listen to.
It alludes to fantastic stability.
Achieving something which is unnecessary (unity gain stability in an amp not used for unity gain) almost always means losing something else, which may be necessary. You might compromise slew rate, noise etc. Overengineering is almost always poor engineering.

Wavewhipper might like to do some reading about common-mode distortion, which can be introduced when the input signal and feedback are not summed at a single point but at two inputs to a single stage (e.g. two bases of a BJT LTP, grid and cathode of a valve stage). Common-mode distortion cannot be removed by feedback. It is something to be considered, but it is usually small - so small that many designers seem to get away with not even being aware of it.
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Old 29th January 2018, 11:12 AM   #42
Eva is offline Eva  Spain
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Quote:
Originally Posted by PRR View Post
Yes, but I have seen very nearly that plan in several old-old transistor amps.

Comparator? 20mV input will swing the output, 60mV will slam it. That's poor for a comparator. In most audio we have signals up near 1V, and yes it will "comparator" all the medium and loud parts to bent square waves. You even find this done in guitar distortion. (Usually one such stage is not enough for that.) This hyper-sensitivity, and *high* THD even before clipping, is the main problem.

It works fairly well as a mike preamp for dictation speech through a dynamic mike; all those $13 mono cassette recorders (and the low-price 3" reel-reel machines before them). Input levels rarely exceeded 1mV, distortion was tolerated, exact gain was moot because all $0.50 mikes and all talkers vary a lot in level.

I believe at least one Moving Coil phono head uses essentially this AC condition. Signal levels are tiny, so THD is tiny. Stage load is 47K so a 4.69K Zout is fine. Source is 10-100r so a 2K-4K Zin is fine. Temps around a phono are usually benign.

I don't follow the gain vs temp complaint. Yes, in a Military -50C to +100C environment the gain will swing like +/-25%. But this is an audio forum, primarily domestic audio, mostly in living rooms. +/-15C, 10C to 40C, is extreme for most of us listeners. I keep my rooms to +/-3C. The gain change is more like 1%, inaudible. (This stage as shown will not self-heat enuff to matter.)

Output impedance can be whatever, as long as it is not too high and you know what to expect. High Zout can sometimes simplify interstage networks. Here Zout is 99% controlled by the resistor tolerance.

Input impedance does go directly as hFE (and ESR only if the cap is crappy; it often was). However if you cascade such stages the low-hFE units suck more against the high Zout stage before it and voltage gain variation is lessened. (It is often better to design such things on current gain rather than the now-customary voltage gain.)

Zin and input overload level can be increased with a simple series resistor. This is an interesting form but tends to hiss.

"Is this feedback?" There's feedback in it, but not so you can design with it, and not so you can get "good" performance in most audio applications.
In a -50C to 100C environment gain of a 2N3904 can swing from 150 to 400, this is +/-63%, +/-4.2dB.

In the old days transistors were expensive (like 10~100 resistors), incidentally object of cult too, but a single transistor was better than nothing. ICs were even more expensive (like 10~100 transistors), and harsh at zero cross in audio circuits (this is the worst place to have a discontinuity!!) Designers were trying to make consumer equipment with whatever was available and affordable, to cover the needs of a society growing in size and complexity. It was 2nd generation equipment. Previous sound recorder approach was registering vibration mechanically in cylinders or discs. Figure out what gain accuracy did that have.

Imagine a recorder without AGC and with +/-25% (or more) gain change depending on weather, and taking one or more minutes to settle upon power up. Now imagine yourself trying to gather evidence to prove a crime with such a recorder (such that the voice of the criminal has to be recognized by a jury), or trying to record something in nature in extreme hot or cold conditions, or noise analysis in a factory to catch a problem. These circuits were abandoned for several reasons. A dedicated operator was required to continuously compensate the flaws of the machine.

In the end price leveled out, so in SMD it costs about the same to place a resistor, a MMDT3904 (dual NPN) or a LM358 (dual cheap op-amp), with part cost being almost negligible for the resistor and comparable to placement cots for the MMDT and the LM358. All values around 10 cents.

The environmental conditions for using single transistor stages are no longer met. What remains? Cult to something that original designers were not giving cult to? The appearance? Cult to appearance can govern human tastes when environmental conditions are not opposing, but this is a side effect of seduction. Humans not under seduction are not vulnerable to cult to appearance.
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Old 29th January 2018, 04:05 PM   #43
Wavewhipper is offline Wavewhipper  United States
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I remember looking at an LM723 series pass power supply of a few hundred watts and figured I could make it function as an audio power amp by tapping in at a Vref input.
It would have been a Class A amp and who knows what bandwidth as it was designed.

If a DC audio amp were modified for bipolar adjustable power supply duty, the power rating would need a good bit of de-rating. They are meant for pulse duty.

Last edited by Wavewhipper; 29th January 2018 at 04:10 PM.
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Old 6th March 2018, 03:48 PM   #44
Wavewhipper is offline Wavewhipper  United States
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I recently forced an amp with unbalanced output transistors, I forced it to sound good with negative feedback. The amp didn't tell me what components to use, I showed that amp that it will accept the components I made it with, forced it to work, and I like that.
Yes I'm beginning to like that very much.
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Old 7th March 2018, 11:20 AM   #45
suzyj is offline suzyj  Australia
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Ample negative feedback really is the only way to go. Go hard or go home. If you're not hitting 120dB OLG, you're really not trying hard enough.
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Old 8th March 2018, 09:52 AM   #46
Eva is offline Eva  Spain
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There are limits imposed by physics on how much OLG can be obtained. Each gain device introduces phase shift and propagation delay.

The following rule of thumb applies: OLG*UGF/NGD=nearly_constant
OLG=open-loop gain
UGF=unity gain frequency
NGD=number of gain devices, cost
So cost is roughly proportional to gain bandwidth product (cost ~=GBW=OLG*UGF).

In practice it is more convenient to consider how much GBW can be obtained from a fixed number of stages (considering other constraints), as a design problem with wider scope is difficult to close.

However, when the distortion originates from self disturbance within the loop, such as capacitive or inductive coupling, or summing node non-linearity, increased OLG does not reduce THD (not proportionally):
- The magnitude of the error (drive) signal becomes reduced by the same amount of dB the open loop gain is increased.
- The magnitude of disturbance signal, relative to error signal, increases by same amount of dB (for 1st stage, progressively less for intermediate stages).
- Thus, past some point, X more dB of open loop gain result in: X more dB of open loop distortion, attenuated by X more dB of NFB gain.

So additionally: there is an optimum range in the number of stages, more stages would not allow to reduce THD arbitrarily, as non-NFB-correctable sources of THD become dominant.
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Last edited by Eva; 8th March 2018 at 10:07 AM.
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Old 8th March 2018, 10:12 AM   #47
mchambin is offline mchambin  France
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Can you give clear exemples with proofs of non correctables by NFB ?
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Old 8th March 2018, 10:41 AM   #48
jcx is offline jcx  United States
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things not measred by the feedback - typical positive gain diff input amp feedback network dosen't measure diff Q input base current which may vary from Early effect, or parasitic Ccb whic also Vcm dependent
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Old 8th March 2018, 10:57 AM   #49
Eva is offline Eva  Spain
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The optimum number of stages for a class AB audio amplifier is between 4 and 6. Less than 4 stages: more HF THD. More than 6 stages: no improvement in HF THD.

THD sources not correctable by NFB:
- In class AB amplifiers: Distorted inductive coupling from: halves of output current waveform flowing through different inductive paths. Inductive coupling from distorted base current waveform. Inductive coupling from base current switching peaks at current waveform zero cross.
- In BJT transistor stages with miller capacitor: Logarithmic nature of Vbe=log(Ic).

The field where this knowledge is obtained is multi-channel class D, when the analog audio guru learns RF.

Attached picture shows a 50ppm THD @ 50W class D minimalist balanced modulator with single integrator, "a piece of analog audio electronics". NFB at 20khz is around 38dB. Open loop THD is below 1% typically. The limiting factor is: about 20dB stronger magnetic fields than in class AB, permanent magnetic fields at any volume, square wave and periodic glitches in nature.

But there is a technique to bury the THD of inter-channel disturbance under the THD of the own channel, and under noise floor at low volume (about 120dB-A). The SO-8 is balanced comparator. The SO-14 is quad op-amp.
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Last edited by Eva; 8th March 2018 at 11:21 AM.
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Old 8th March 2018, 01:20 PM   #50
Telstar is offline Telstar  Italy
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What really matters is low noise (120dbA is NOT sufficient, the target should be 144 unweighted) not ppm-figures THD.
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