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Review and Measurements of Meier Corda Jazz Amp

solderdude

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#81
Specs say: Maximum output 15V / 300 mA
The BUF364 output devices are certainly capable of that.

15V peak in 300 Ohm = 0.37W
In 32 Ohm one can expect 1.4W based on 300mA peak currents.

No idea why output measures that low.
 

MRC01

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#82
...
No idea why output measures that low.
The only reason I can imagine is because the active ground is an inverted differential channel that doubles the output, and wasn't connected when Amir measured it.
Just speculation, but it fits the observations here.
 

solderdude

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#83
The active ground would be just a buffered ground so the output would be 0V but not directly from the ground plane but via a BUF634.
Otherwise it would be 'balanced' out and not called 'active ground'.
Follwing the PCB traces on the bottom side seem to suggest the 'active ground' does not carry any signals.

It is strange that there would be signal on it when it is called ground.
 

flipflop

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#84
but srsly 0.2db will never be heard
That's at one gain setting. It might be higher at others.
what the actual frequency reproduction channel deviation on their headphones! is like (were talking 0.x to multiple dB), right?
Two wrongs don't make a right.
output impedance is 4Ohm and i would not call 4Ohm"high" at all... thats perfectly ok for 32Ohm headphones id say?
It could shift the frequency response by up to 1 full dB with 32 ohms headphones. Definitely audible and completely unacceptable for an amp.
 

MRC01

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#85
The active ground would be just a buffered ground so the output would be 0V but not directly from the ground plane but via a BUF634.
Otherwise it would be 'balanced' out and not called 'active ground'.
Follwing the PCB traces on the bottom side seem to suggest the 'active ground' does not carry any signals.
It is strange that there would be signal on it when it is called ground.
The way Meier's "active balanced" actually works:
The ground signal carries 1/4 L + 1/4 R, inverted.
L carries L + ground
R carries R + ground
So if you have a 1 kHz sin wave (or any other signal) that is equal in both channels, the output looks like differential balanced.
Amir confirmed this in an earlier post in this thread. So if you don't connect that ground, you lose 6 dB of output.
 

solderdude

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#86
http://www.meier-audio.homepage.t-online.de/grounds.htm

Indeed not just buffered ground it seems

still..
Specs say: Maximum output 15V / 300 mA
The BUF364 output devices are certainly capable of that.

15V peak in 300 Ohm = 0.37W
In 32 Ohm one can expect 1.4W based on 300mA peak currents.

If it were half the voltage it would measure 0.18W in 300 Ohm not 30mW
 
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MRC01

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#87
Yeah, it's still below spec according to Meier's site. But we're fairly confident that Amir's connector left the ground signal floating, not differenced from + signal. That would have halved the voltage, so for a conservative estimate, take the power Amir actually measured & multiply by 4. That's where the 240 mW / 1.2W I suggested earlier came from.
 
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#88
but srsly 0.2db will never be heard
--> That's at one gain setting. It might be higher at others.
uhhh yeah... and it "might" be not aswell? were discussing whats measured, and not whats not measured?
or is there an indicator for you to make you believe it might be higher, that i dont know or dont understand? (no offense, serious question)



what the actual frequency reproduction channel deviation on their headphones! is like (were talking 0.x to multiple dB), right?
--> Two wrongs don't make a right.
i never said that... im talking about what is relevant, and what is not. i even did some "fine-tuning" (fixing) on headphones myself to even out channel deviations... but if a specification of any kind on any thing is irrelevant for a specific purpose, i dont take it as a "failure" or "wrong", i take it as a given fact that is, well, irrelevant... and since im going to use my audio equipment myself and not that bat that lives in the tree next to the house, its not an relevant issue for me. better is better, no question. question is, how i benefit or not benefit from the better... but then again, im not an audio scientist, im an audio equipment user.
 

flipflop

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#89
uhhh yeah... and it "might" be not aswell? were discussing whats measured, and not whats not measured?
or is there an indicator for you to make you believe it might be higher, that i dont know or dont understand? (no offense, serious question)
The presence of channel mismatch shows that it's got analogue volume control, which can be quite bad at a low volume setting.
But you're right, it might not be any worse than what is shown, hence why I chose the word 'suspicious'.
i never said that... im talking about what is relevant, and what is not. i even did some "fine-tuning" (fixing) on headphones myself to even out channel deviations... but if a specification of any kind on any thing is irrelevant for a specific purpose, i dont take it as a "failure" or "wrong", i take it as a given fact that is, well, irrelevant...
0.2 dB of mismatch is irrelevant. So is 0.5 dB in most cases. As you pointed out yourself, headphones tend to have worse channel matching than amps, and if you got a headphone with 0.5-1 dB of mismatch plus 0.5 dB of typical analogue volume control mismatch, it starts adding up and pushes it into bothersome territory. This is of course not an issue limited to the Corda Rock.

I don't think we're in major disagreement on anything in this discussion. Let's leave it :)
 

trl

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#90
frequency response might be "bad" as for measurements, but 0.5dB drop @15k max will not be audible aswell... [...]
I assume you are referring to the measurements for Corda Rock, right? Because Jazz has an almost flat FR. Instead, Rock has a crappy FR with a rolloff of about 0.5dB @15KHz and about 1dB @20KHz, per Tyll's measurements. It's not complicated to design a LowPass Filter to make a perfectly flat FR, I don't see why they don't do it?!? I doubt that a carefully designed amplifier will start to oscillate if bandwidth increases from 30 KHz to let's say 50 KHz or 100 KHz.

[...]if you do a blind test and proof to hear a difference between a filter according this FQR and no filter on an actual recording, id hire you for a studio[...]
Make me an offer, then. :)

But I'm sure you can do a simple test by yourself, just need to find an amp and identify the low-pass capacitor, then simply double the capacitance in real time (if possible, directly on the PCB). You can use low volume 12-14 KHz sinewaves, but you can also use bright headphones and a song with enough trebles.

Trust my ears and my "instincts": an amp that has a freq. deviation higher than +/-0.5dB within 20-20000 Hz is not correctly designed. It simply doesn't follows human ears hearing pattern!

Have a look how Pavel Macura designed his headphones amplifier:
- http://pmacura.cz/dispre_en.html (bandwidth up to 3 MHz and not oscillating!)
- http://pmacura.cz/dispre2_en.htm (Freq range (-3dB) 0.1 Hz – 220 kHz)
- http://pmacura.cz/dispre2-jfet_en.htm (Freq range (-3dB) 0.1 Hz – 220 kHz for potentiometer set at 100%)
Just have a look how the square wave signals look like @ 100KHz. This proves that this amplifier will be compatible with several headphones and different headphones cables, no matter their internal resistance and impedance. So why not designing amps correctly just to eliminate few more variables from the eqution?
 

trl

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#91

amirm

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#92
Taken from here: http://www.meier-audio.homepage.t-online.de/grounds.htm

The concept is very nice, indeed, and I assume the 4 x BUF634 from the mainboard are used per the picture above (2 for R/L and 2 for G).
However, perhaps the increased THD could be caused by this way of driving the audio signals.
That would naturally be the case. Distortion quadruples for each channel since all four output buffers are used to produce each channel. Noise would also quadruple. All of this is meant to solve a non-problem. This may be the reason that when this thing clips, it clips super hard since all four opamps are overdriven at once.
 

KSTR

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#94
The idea behind the Meier scheme is that a common-mode voltage is added to the output so that its total far-field voltage potential against "world" is zero which is alleged to lower "dielectric distortion" and "stored energy". For this to work (if that distortion is actually present, that is) it must be assumed that PE (or the input GND from the feeding device, depending on the GND scheme of the amp core) and "world" have the same potential. But that is not typically the case, so I would think close to nothing is gained in practice because the few volts of audio just don't matter on top of the dominating real-world potential difference that is present anyway.
 
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solderdude

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#95
What's weird is that the output stage seems to clip at 3V in high gain and 0.5V in low gain mode.
Regardless of load impedance.

A BUF634 is always used combined with an opamp.
As the amp seems to work on at least +/- 15V internally (probably even +/- 17V) there would be no reason fo the circuit to clip nor have such high distortion (could be gain stages are noisy opamps)

This is weird. Gain should not affect clipping, just the gain unless something before the output stage is clipping for some reason.
Even if the signal is doubled and later on subtracted again in the driver (just so the cable always 'sees' a balanced voltage for whatever that would solve in a headphone cable that is not build up like a flat cable) then one would expect a higher voltage.

Edit: gain issue is solved... engineering thing by Meier. The input to the output stage clips before the output stage does. Probably a 'theory' thing.
Still 3V = 9Vpp is a very low clipping level when 30Vpp is available in the output stage. Even when considering the way the signal is made up there is plenty of headroom left to accomodate for a higher output voltage.

When looking at the scope and dialed up (no load) to clipping levels what voltages are there ?
The scope shot does not show V/div or values.


Also have a look here: http://mclements.net/Mike/mrc-blog/blog-140615.html
 
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Jan Meier

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#96
Dear friends,

the Jazz is one of my best selling amplifiers. It is loved by many and noone ever complained about its performance. Search the internet and you will find quite a few opinions/reviews.

So why is there such a discrepancy between its popularity and the measurement results as presented over here?

Before I explain you first have to know a little bit about the topology of the amp.

Inside the JAZZ the signal passes three stages. An input amplification stage with volume control, a passive network for the crossfeed and finally the output stage with two (LOW and HIGH) gain settings.

The topology of the input amplification stage is uncommon. It does not have a fixed amplification factor with a volume control (potentiometer) in front, but volume is set by changing the feedback resistance. The advantage is, that the gain-factor is decreased with lower volume settings and lower gain factors imply lower background noise. The resistance is varied by electronic switches that activate/deactivate various resistors.

The measurements present two major issues with the Jazz. First there is a higher than usual mains noise (humm) and secondly the amp starts to clip signal at a level that is much lower than most other amps.

As for the mains noise. The amp has a rather powerfull transformer and the enclosure is very slim. Although a toroidal transformer is used there always is a remaining magnetic field and this induces a signal into the traces of the audio signal. Simple physics.

A possible solution would have been to use a bigger enclosure, with the signal traces further apart from the transformer, or to use an external powersupply. However, I wanted the amp to be compact and for ecological reasons I don't like external supplies that are ON all the time.
A more modern solution would be to implement a switched powersupply, but at the time the Jazz was designed/developed these were not very common yet.

So the Jazz has a clearly higher humm level than e.g. the Corda Classic (which has a much larger enclosure) or the Corda Rock (which is a newer design with switched powersupply).

But is it really a problem?

Sure, compared to the random noise background the 60 Hz component is very high. One should however be aware that the sound pressure of a signal is proportional to both amplitude and frequency! And the frequency is very low.

Yes, the humm of the Jazz can be heard with sensitive headphones at very high volume settings. But if in this situation you would add a normal level input signal your ears would be blown instantaneously.
At normal listening levels humm is not a problem.

It should be noted that, in contrast to more conventional designs, the humm signal at the output decreases with lower volume settings. This is a result of the topology of the amp in which volume is set by changing feedback. Actually this can well be seen by the measurement results presented here.

At maximum volume the Jazz has one of the worsest SNR-ratios. But at a 50 mV output level the amp is 20 dB better than all other amps in the graph! It's a pity that the Amirm did not comment on this.

As for the clipping/distortion, why is this amp starting to do so at an output level that is much lower than would be expected by the supply voltages and the amplification elements used?

Well, the electronic switches used at the volume control do have a much lower supply voltage than the amplification elements. As soon as the output signal of the first amplification stages surpasses a certain threshold the signal will start to distort. In High gain, this distortion will start at around 3.5Vrms.
With a 2Vrms input signal this distortion starts at around 2 o'clock.
With a 200mVrms input signal (portable player) distortion will not be seen at all.

But again, is this really a problem?

Even with an ineffizient headphone like the HD800 (300 Ohm, 102dB at 1Vrms) an undistorted output of around 112 dB can be achieved. And that is a very unhealthy sound level:

https://www.noisehelp.com/noise-dose.html

With low impedance headphones undistorted sound levels are even much higher.

At normal listening levels the clipping of the amp is not a problem at all. Would it be, then you would have read about it in one of the many reviews found in the internet.

The "culprit" of the distortion thus is the electronic multiplexer switch at the volume control of the Jazz. Of course I could have used a conventional potentiometer instead, but the use of electronic switches allows for a much better channel balance.

As a side note: With normal music signals the implementation of the FF-technology reduces the output voltage at the first amplification stage by around 6..8 dB (depending on the frequency contents of the music). As a result undistorted sound levels are higher accordingly. So for true headbangers the FF-version of the JAZZ is recommended.

When you drive a system to its limits, then it will respond adversely.
And yes, the limits of the JAZZ are more strict than with most other amplifiers.
However, at normal listening levels the amp performs well. The proof is in its popularity and in the appreciation of those who own this amp.

Some comments to some comments:

" I find the graphics preminative and unattractive. "

Beauty is in the eye of the beholder.

"I am not one for detail as you can tell from my posts but come on: why is "designed" not capitalized? "

Seriously??

" Frequency response was flat enough to 20 kHz but then took a dive above 30 kHz"

Please be noted, that due to the concept of the volume control the frequency response depends on the volume setting. But even at the highest gain factor the -3dB point is still beyond 40 kHz.

Most headphones do not even get to 30 kHz.

And our ears ......

"They didn't even get the CE logo right."

With audio equipment the manufacturer has to guarantee that the devices he sells do fullfill the CE-requirements. And the JAZZ does, please be assured of that! The fact that I may not have printed the logo exactly according to design ....

" this amp is required by law to be CE-certified"

No, this is only required for very specific products like medical implants. With low voltage audio-products the manufacturer is responsible for fullfilling the CE-requirements. A certification is not mandatory (and never done by any of the smaller manufacturers, as otherwise you no longer could afford these products).

The JAZZ is very safe. I know well of the danger of electricity to the human body. By origin I'm a biomedical physicist and my PhD thesis was on the subject of neurostimulation. I worked 10 years as research project leader with a pacemaker company.

The CE-requirements are very strict. Do you ever wonder why certain companies from the US or Asia do not sell their headamps through a distributor in Europe?

" The unit is marked as "Designed by ... Germany" not "Built by ... in Germany" so you guess where it was built ..."

The amp was build by Shanling. Not the worst factory in the world. I don't make a secret about that.

Even high-end stuff like Quad and KEF are nowadays entirely produced in China.

" I would love to see a response from Jan Meier in this thread. But some of the comments are a little aggressive and that will most likely discourage him from doing so."

No, I don't mind a discussion as long as it is honest, fair and respectfull.

" You won't find in a german forum not a single one thread where someone expresses the smallest doubt about his amps"

Well, be assured that I never tell people what to write. My customers have to decide for themselves whether they want to write something and what they want to write. No fake news and no alternative facts here! I do not do in politics!

" His gear, always fine, always close to perfect german quality."

Being a Dutchman that's not really a compliment! :)

"Specs say: Maximum output 15V / 300 mA"

Oops! This is the spec for the JAZZ-FF at low measurement frequencies. With the FF-version the undistorted output level will increase below 1 kHz and reach its maximum at around 200 Hz.

Cheers

Jan Meier
 
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#97
I drawed a (simplified) schematic of Jazz (without the feedback network). The volume control is achieved by changing the parallel feedback resistor between VolCtrl_A and VolCtrl_B. I don't think it is a clever design as it will change the gain of the first 2 OPAMPs during volume adjustment.
JAZZ.JPG
 

solderdude

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#98
Looks like the gain should be set on 'high' and volume control on 'maximum' to get to the max. output voltage.

Also appears that Jan is a believer in the 'output stages of opamps must be pulled towards one voltage rail a bit so it is supposed to work in class-A' theory.

The design indeed isn't well thought out but should work fine when a headphone is connected.
The whole 'active ground' theory seems like BS and complicates the output stage without any proven benefits.
 

solderdude

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I have no idea why he chose such high feedback values.
Don't know if it is on high BW setting but in such case an extra stopper between BUF input seems to be a good idea.

The simulation seems close to what Amir measured. Strange circuit. Probably designed just to be different than the rest ?
 
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