Sigh. Would you settle on 2k?
I went for LME49710 and 1k feedback, the LME will do 50mA's.
Original definition: an amp where each output device conducts 360 degrees of the signal cycle.
Which begs the question: an amp where the output devices conduct less than 360 degrees, but do not turn off due to some residual bias, is that a genuine class A amp?
jan
In my opinion it is, as long as there are no 'sharp' edges in the curves (see LA Cube-Law amplifier).
But it is not according to the definition (cube law is, as both polarities conduct 360 degrees of the signal).
Another way to look at it is to look at the supply currents. Class A, including cube law, has supply currents that resemble the signal. 'Fake' class A, like class AB with a non-zero switch-off current have supply currents that look more like class B 'pulses'.
This is important as a large part of the sound quality of class A comes from the fact that the supply currents don't have the high level of harmonics that can cause audible degradation in non-genuine class A.
Jan
A reminder:
Class A: both polarities conduct for 360 degrees of the signal
Class B: each polarity conducts for 180 degrees of the signal
Class C: each polarity conducts for less than 180 degrees of the signal
Class AB: each polarity conducts for more than 180 but less than 360 degrees of the signal
Jan
Class A: both polarities conduct for 360 degrees of the signal
Class B: each polarity conducts for 180 degrees of the signal
Class C: each polarity conducts for less than 180 degrees of the signal
Class AB: each polarity conducts for more than 180 but less than 360 degrees of the signal
Jan
Bonsai, it has little to do with output impedance (that's easy to make low enough), it's distortion when tested with a load that is irrelevant to audio. John is exactly correct in that respect. I have two preamps in house at the moment, both with about a 150R source impedance (dominated by a series resistor at the output). One of them won't even break a sweat at a 600R load, the other will show a bass rolloff and increased distortion. So what? There are no 600R loads in a home. Why design for something that isn't actually an intended use?
If there is a need to drive 600R for some pathological reason and you want to use an IC, there are plenty of them which will do so.
If there is a need to drive 600R for some pathological reason and you want to use an IC, there are plenty of them which will do so.
I am trying to understand why there's a problem designing something that can drive 600 ohms and loading it with say 10 k?
I don't think I am suggesting you need low output Z - just that a line stage should not break into a sweat driving a 600 a Ohm load. Anyway, you almost always have to insert a 50 ohm resistor in series with the output to isolate Cload.
For interconnects, a bit of current is not a bad thing - very hi Z are more prone to noise pickup and Richard pointed out.
I don't think I am suggesting you need low output Z - just that a line stage should not break into a sweat driving a 600 a Ohm load. Anyway, you almost always have to insert a 50 ohm resistor in series with the output to isolate Cload.
For interconnects, a bit of current is not a bad thing - very hi Z are more prone to noise pickup and Richard pointed out.
You can certainly do so. But since intended loads are between 1 and 3 orders of magnitude higher, why bother? There's good reasons to keep the source impedance low, but as I and others have pointed out, that's not equivalent to capability of driving unrealistic loads.
A 100K terminating R for the source located at the input of another amp (eg power amp) actually has that Z only at low freqs. The shunt affects of the input circuit's C and cable C causes the drop as freq increases. What affect is there on the feedback of the driving amp with such varying load Z (phase shift). [assume a VFA using GNFB and neglecting any non-linear C's affect].
If only the terminating Z was lowered, the C's all the same, the affect of the phase shift would be reduced.... and maybe over-all system distortion reduced some as well? And, esp. when the source Z is high as well.
Best to have very low source Z and lower load Z. Approx 10K gives a flat Z vs freq with many amps. 1K Ohm (600?) is even better from a system view and lowest interfacing distortion. This is still within the realm of many IC amps to drive fairly well. It is only what level of distortion increase with lower Z load is to be allowed/tolerated.
THx-RNMarsh
If only the terminating Z was lowered, the C's all the same, the affect of the phase shift would be reduced.... and maybe over-all system distortion reduced some as well? And, esp. when the source Z is high as well.
Best to have very low source Z and lower load Z. Approx 10K gives a flat Z vs freq with many amps. 1K Ohm (600?) is even better from a system view and lowest interfacing distortion. This is still within the realm of many IC amps to drive fairly well. It is only what level of distortion increase with lower Z load is to be allowed/tolerated.
THx-RNMarsh
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If you're more concerned with a magazine than users, sure, why not? There's also no reason not to design the unit to function at 500 degrees or under high gamma flux or on a shake table. You never know.
For the fashion market, I do note that the first couple of Stereophile "Class A" preamps I looked at either weren't measured at 600 ohms or showed increased distortion at unrealistically low loads. That didn't seem to inhibit them from being declared "best of the best."
If it is your living, you would be out of your mind to not take into consideration how the relevant magazines rate products. This should only be frowned upon if it were to detract from the user value, but this is not the case here. Rather the opposite, because you serve both users and misusers by building in some redundancy.
To design an amplifier to function on a shake table is actually a good idea.
Good news: this means that all my Circlophone, Synclophone and Symclophone are now upgraded to class A, while I designed and described them as sliding class AB
Like I said, having significantly increased distortion under an unrealistic load did not inhibit the first couple of Stereophile Class A preamps that I looked at from getting their "gold medal." Here's an example of a very good one http://www.stereophile.com/content/...ogy-sl1-renaissance-preamplifier-measurements that shows significant bass rolloff and increased distortion at low loads.
As an aside, although I think that Stereophile generally does a very good job of measuring, I don't see why they do an irrelevant measurement like this on preamps but don't do a highly relevant PowerCube-type measurement on power amps.
What's the real value of such "award"? I am really doubtful. And I am doubtful about anything that is unable to drive 600 ohm, or even worse, it has problems with 8k and is "excellent" only into 100k. You such rigorous sometimes but admit technically mediocre circuits. What a controversy.
And anyway, what's the big deal about designing line stages capable of driving anything even way below 600 Ohms? What with today's availability of very fast medium power transistors, at very reasonable prices even in retail?
It's not as if they have to run awfully hot in pure class A mode, they can be adjusted to reasonable limts, but still retain the current capability in case of need. It can't hurt, unless one is a bean counter.
This is why I used 50W devices on a headphone amp. I know a few people who still swear by their WW2 Beyer headphones, rated at 8 Ohms. Just about all op amps will choke on that load, all the more so since such ancient cans are much less efficient than the modern models.
Everybody will be fine if they own Beyer 600 Ohm cans, but what happens if they own modern Sennheiser and other units, rated at 32 Ohms?
Or for some exotic reason simply have to drive a 50 Ohm device?
That's the cross each and every designer has to bear, completely in the dark regarding what someone somewhere might attach to their device. Ditto for my speakers, as well; while they have exemplary traits regarding myself, using them for testing at large would be ridiculous as none other known to me is as well behaved.
It's not as if they have to run awfully hot in pure class A mode, they can be adjusted to reasonable limts, but still retain the current capability in case of need. It can't hurt, unless one is a bean counter.
This is why I used 50W devices on a headphone amp. I know a few people who still swear by their WW2 Beyer headphones, rated at 8 Ohms. Just about all op amps will choke on that load, all the more so since such ancient cans are much less efficient than the modern models.
Everybody will be fine if they own Beyer 600 Ohm cans, but what happens if they own modern Sennheiser and other units, rated at 32 Ohms?
Or for some exotic reason simply have to drive a 50 Ohm device?
That's the cross each and every designer has to bear, completely in the dark regarding what someone somewhere might attach to their device. Ditto for my speakers, as well; while they have exemplary traits regarding myself, using them for testing at large would be ridiculous as none other known to me is as well behaved.
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