Review and Measurements of Meier Corda Jazz Amp

[Deleted member 2944]

Of course a ground must be connected for power to flow. The Jazz amp carries an inverted signal on its ground channel, like a balanced amp. So even though the Jazz has an unbalanced 1/4" headphone output, it must be measured as you would a balanced output. So the question is, which ground did the AP analyzer use when taking the measurements? I think Amir is looking into this.

Nonsense. All that's required is a difference of potential for power to flow. (A ground is not required.)
In the context of this particular headphone amplifier the output connection should be noted as "ground" and not an actual ground. Too much fast/loose language being employed here.

I don't think there's anything for Amir to look into. I believe the AP can measure balanced output schemes in a floating-mode, so his testing setup seems perfectly valid.

Dave.

 

[Soniclife]

Nonsense. All that's required is a difference of potential for power to flow. (A ground is not required.)
In the context of this particular headphone amplifier the output connection should be noted as "ground" and not an actual ground. Too much fast/loose language being employed here.

That fits my limited understanding of how this should work, thanks. I agree, common might be a better term than ground for a headphone socket.

 

[amirm]

I agree, common might be a better term than ground for a headphone socket.

That is the term I attempted to use earlier for that reason.

 

[invaderzim]

A very valid point. But isn't objective measurements what separates this particular forum from many other forums? My impression is that the members here come to discuss the objective performance of a product in addition to the subjective performance. Objective performance also says something about engineering skill, and is a proof of what is achievable within certain budgets. Other aspects matters as well, of course, like availability, warranty, support and robustness. All else being equal, I will still pick the unit that measures better. Paying the same for something that measures worse just doesn't make sense to me.

Granted this is a rare case of an item that doesn't test well at the same levels as other equipment but I'd still say retest it at a usable level.

I'll admit I have very little knowledge about any of the testing or setup for testing but since it was mentioned that most comparable amps perform best at full volume it would seem the stats could be swayed towards amps that might have great specs at levels you'd never use while having less stellar specs at usable levels. In that case wouldn't the one you'd really want be the one that tested better in the conditions you'd be using it? To me it is a bit like testing all cars on a racetrack and dismissing those that don't do well even though they'd be great for sub 150mph speeds and far better as a daily driver than the supercars. Similar to formula 1 cars that have to be going fast to have any grip. At lower speeds you'll slide off the road. I don't want that as my commuter.

 

[amirm]

I'll admit I have very little knowledge about any of the testing or setup for testing but since it was mentioned that most comparable amps perform best at full volume it would seem the stats could be swayed towards amps that might have great specs at levels you'd never use while having less stellar specs at usable levels.

For intermodulation test, that is what I did. At max volume results were horrible so I backed the volume off to get the measurement you saw:

Even after doing so, performance was not great.

 

[MRC01]

The ground of the signal generator should not be connected to the ground of the measuring input.
When these are electrically connected you short the 'active ground' and at the same time don't measure the correct output as the real output signal is between the 'active ground' and output.
The output opposite the input ground thus contains the amplified signal + inverse of the 'active ground' signal, which b.t.w. is combined with the ...

Yes, that was exactly what I thought when I first saw the low output figures.

If I understand Meier's active ground page, and my simple algebra is right, when the signal is the same on L and R channels, the Jazz output should look the same as a balanced output.

For example, consider input signal of a 1 kHz sin wave in both channels:

Ground carries 1/4 L + 1/4 R, inverted, which is 1/2 the voltage (inverted) since L and R are the same.
L output carries L + Ground, which is 1/2 the L voltage since G is 1/2 the signal inverted.
Likewise for the R channel.
Thus the output at either L or R alone looks like 1/2 the voltage, the ground is the same signal inverted. They're equal but inverted, so when you difference it the total signal doubles.

PS: just look at post # 56: https://www.audiosciencereview.com/...ents-of-meier-corda-jazz-amp.7197/post-164810
That looks like a balanced differential output: the - channel carries a signal that is the mirror image of the + channel.

Of course, listening to stereo music, the L and R won't be the same, in which case the output + and - won't be mirror images of each other like they always are with true balanced. But in the special case when L and R are the same, it looks like it should be.

If you measure the Jazz + output against Earth ground, ignoring the Jazz - output, you'll get half the signal voltage.
If you latch the Jazz - output to Earth ground, you'll short the ground channel and possible damage the amp.
If you measure the Jazz + output against its - output, leaving the - output floating (not attached to Earth ground), then you will measure the full output voltage.

 

[MRC01]

This has me re-wondering about my question in post # 57: if the AP analyzer left the - output floating and differenced it against the + output, why did this amp measure the same as unbalanced vs balanced?
I would imagine the unbalanced input differences the + output against Earth ground, ignoring the - output. If the analyzer did that, it would have measured 1/2 the output voltage. Looks like the AP analyzer doesn't do that.

 

[MRC01]

Nonsense. All that's required is a difference of potential for power to flow. (A ground is not required.)

You can have a difference in potential without a ground. But you can only measure it relative to something else. And power won't flow until you connect it to something else. That something else is ground; at least, relatively speaking. That ground may not be Earth ground, so it can be less confusing to call it "common".

 

[Deleted member 2944]

That something else is ground; at least, relatively speaking. That ground may not be Earth ground, so it can be less confusing to call it "common".

Huh? My goodness.
A load can have a connection to ground or "ground" or "common" or whatever....but it doesn't necessarily have to. In fact, a load can have no ground connection (relative or otherwise) at all. In fact, that's exactly what we have here with the Meier amp.

Your understanding of this leaves me puzzled.

Dave.

 

[MRC01]

You can't measure a voltage at a single point. A voltage is a potential difference between 2 points.

Suppose you give the Jazz input a 1 kHz tone equal in both L and R and measure its output. Your AP analyzer has 2 probes to measure it labeled + and -. The AP analyzes the difference between them. Let's assume the AP keeps both floating; it does not latch the - to earth ground.

If you connect the AP's + to the Jazz +, and the AP - to earth ground (ignoring the Jazz - output), you will measure voltage V and power P. You're differencing the Jazz + signal against Earth ground, ignoring the signal on the Jazz - output.

If you connect the AP's + to the Jazz +, and the AP - to the Jazz -, you will measure voltage 2V and power 4P. You're differencing the + signal against the - signal, which is a mirror image, so it doubles the voltage and quadruples the power.

With normal unbalanced amplifiers, their - output doesn't carry a signal, so there would be no difference between these 2 ways of measuring.

 

[amirm]

The input thus is single ended and the output is also single ended but the output 'ground' is not and must not be connected to the input ground.
Indeed one way of measuring would be to use the + and - input of the AP but the ground pin should not be connected to the input ground as the output signal is not truly balanced.

The AP's inputs and outputs are completely floating and isolated by default. Not only is the input "common" is insulated from the same on the output, each input and output is floating between left and right.

 

[MRC01]

Good, that's exactly how it should be. That implies the AP doesn't really know or care whether the thing you're measuring is balanced or unbalanced. It simply sees the difference between + and -, just like a speaker or headphone would.

You mentioned earlier that the AP can latch the - to ground. I would assume this is not normally done, except in rare cases when you're getting grounding problems like hum or DC offsets.

 

[amirm]

You mentioned earlier that the AP can latch the - to ground. I would assume this is not normally done, except in rare cases when you're getting grounding problems like hum or DC offsets.

In my old AP, this was a relay that you could control in software. With my new/current APx555, I literally have to connect a jumper wire from ground lug to whatever I want grounded. When I see high mains interference in dashboard, I will play with optionally grounding either input or output (where it cannot do any damage) and show the best case setting.

 

[MRC01]

It sounds like the mystery is solved. The original AP measurements were differential and correct (the Jazz "ground" channel wasn't ignored).

If the Jazz output power really is that low, I must be listening at quieter levels than I thought. With the LCD-2 I'm on low gain up to about 12:00 max. With the HD-600 and its lower voltage sensitivity I use high gain at lower volume settings. That's with an input that does 2Vrms at digital full scale. By comparison, the JDS Element on low gain with the LCD-2 the volume gets pretty loud at 9:00 with most sources.

 

[solderdude]

The max output power seems to be dependent on the volpot setting.
The higher the vol setting is and/or the higher the input voltage is the higher the max. output power will be.

I cannot estimate what the influence of the 'digital' volpot circuit does at higher gains (volpot settings) and output voltages though.

The LCD2 does not need a lot of volts. It already puts out 110dB at 1V so at 3V this is 119dB !
That's enough power to reach average SPL of 95dBA which is loud.

 

[MRC01]

Yeah, the LCD-2 has about 6dB higher voltage sensitivity than the HD-600 (half the voltage gives the same output level), but their efficiency is about the same due to the impedance difference.
It seems strange for the Jazz to have a beefy output stage whose potential power is never used because it's voltage limited upstream.

 

[solderdude]

Yeah, the LCD-2 has about 6dB higher voltage sensitivity than the HD-600 (half the voltage gives the same output level), but their efficiency is about the same due to the impedance difference.
It seems strange for the Jazz to have a beefy output stage whose potential power is never used because it's voltage limited upstream.

In the case of low output R impedance the only important efficiency number is dB/V not the dB/mW.
The reason for this is that an amp supplies an output voltage, not an output power.
Power is the resultant of voltage and impedance and as impedance is a variable it makes more sense to use dB/V.

On the same volpot setting (output voltage) 2 headphones with the exact same dB/mW rating can sound VERY different in actual SPL depending on their impedance.
So the same efficiency number really says little to nothing about how loud it plays from say a mobile phone.

When 2 headphones have the same dB/V efficiency they will play about equally loud. They probably won't play exactly as loud though due to differences in tonal balance and the way/frequency at which the efficiency number is obtained.
But dB/V is the number that should be used when determining how much difference there will be in loudness between them at the same volpot setting.

Unfortunately, in headphonedom, some quote an SPL number without any meaning as the number could be at 500Hz, 1kHz a certain noise spectrum, @ 1mW, @1V, at maximum continuous power rating.

Agreed that there seems to be unused potential in the output stage. It could be a 'design' philosophy from Jan that the output stage should never come anywhere near clipping level. Could also be that he did not find it was necessary to have huge output powers available that would never be used.
Given the fact that most opamps he uses perform best at higher power supply rails and is of the opinion that opamps must be 'pulled' into class-A makes me believe that he believes that subjective evaluations and theories are more important than technical limits.

The Soul amp seems like an interesting choice for headphone aficionados that want to faff around in the digital DSP realm with preselected options.

Given that the specs Jan provides seem to be incorrect and belonged to another amp says to me that he does not find it an important spec.
As most amps seem to use the BUF634 as output stages and work at similar voltage rails the output specs that are written down all seem to give the specs of the BUF output capabilities (15V / 300mA).
No output powers in any impedance are given.

I do think that the spec 15V - 300mA is all that is important IF the amp really reaches those values and one can calculate all available output powers in any impedance with these 2 parameters but when the possible potential output voltage is never reached then quoting the voltage value of 15V is nonsensical. Jan admitted the specs were not from the Jazz but did not correct it on his website.. says something about the importance of output power capabilities I reckon.

 

[MRC01]

Subjectively, the Jazz does sound different from other good solid state headphone amps, like the JDS Element. Overall it seems slightly richer and smoother in the mids and treble, similar to a good tube amp but the difference is more subtle than that, making the Element just a bit leaner sounding in comparison.

If that subjective observation is reliable, it should be possible to qualify and quantify it with measurements. But I don't see anything obvious here to explain it. Distortion and noise are low enough in both, and frequency response is flat enough, differences should not be audible. Perhaps there is some other measurement(s) that could explain it.

 

[Veri]

If that subjective observation is reliable, it should be possible to qualify and quantify it with measurements. But I don't see anything obvious here to explain it. Distortion and noise are low enough in both, and frequency response is flat enough, differences should not be audible. Perhaps there is some other measurement(s) that could explain it.

If you listen to music on a rather low volume the specs are pretty good. On normal/high volume you are nearing distortion thresholds and it could sound worse. I would find it difficult that it'd sound "better" though, nothing in the measurements really lends to that.

 

[MRC01]

Sometimes a little extra THD, especially 2nd harmonic, can make it sound subjectively richer or sweeter, and possibly mask less euphonic higher order or IM distortion. More generally, distortion or noise having different frequency distribution can sound different. But that means the distortion or noise has to be at audible levels, perhaps at least 0.3% in the midrange, and neither of these amps has that much.
The audible differences are subtle, and it's hard to reliably characterize differences near the thresholds of hearing.