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Review and Measurements of MHz stereo balanced class A reference preamplifier

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#1
It has been5 years from purchase . have paired C1 Platinum and B&W800D2.

completely different style and a completely different sound orientation, but we have to believe in science

How about the product? The black appearance, heavy, ugly,simple appearance. come form Fumac Lab

A precision instrument laboratory famous for bragging

All AP measurements reviews are reprinted from the web

Thanks FUMAC for the measurement
Thanks GOOGLE for translation
Thanks 20396 for reprinting

正面.jpg
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Thread Starter #2
4 pairs of balanced input terminals, relay switching,
A pair of balanced output terminals,
All use Swiss neutrik sockets.
There is also a pair of 5-pin output ports, which are used to control the mhzpower4 power-on standby.
The power supply is 230v input, of course it can also be changed to 110v
背面.jpg
 
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Thread Starter #3
Inside the amplifier, it looks very simple
Several modules are merged.
They are signal selection module, high-speed buffer, volume control 1, voltage amplification, volume control 2.
Power supply filter rectifier voltage regulator module
CPU control and VFD display module
电路.jpg
 
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Thread Starter #4
a circuit diagram and not discuss it for now. Who can help me look at the English translation part by the way, please correct me, if I want to send a pre-level correction, please don't correct me.。。。
NFB-Buf: Discrete class A non-feedback high speed buffer
Class A fully discrete no feedback twin buffer amplifier
Q-log_network: Quad constant synchronization relay with passive impedance of front-end logarithmic decrement network system
Passive Logarithmic Attenuation Network of Quadruple Constant Impedance Front and Rear Synchronous Relay
Twin-A amplifier: Discrete class A twin design with ultra-low distortion and ultra-high-speed amplifier
Class A fully discrete ultra-low distortion ultra-high speed twin amplifier
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Thread Starter #5
The basic introduction is complete. The following is the parameter calibration statement based on the test results:

Model number: MHzpreamp II (Ref)
Product name: MHz stereo balanced class A reference preampilifier
Product Name: Fully Discrete Class A Stereo Fully Balanced High Speed Low Distortion Reference Preamplifier

Input interface: Balanced x 4 pairs (1-GND, 2-Hot, 3-Cold)
Output interface: Balanced x 1 pair (1-GND, 2-Hot, 3-Cold)
Control interface: MHzpower remote link can control mhzpower series amplifiers.
Power socket: 235V AC 50/60Hz (115V AC 50/60Hz optional)

Amplification mode: Class A Balanced (Class A Balanced Amplifier)
Input impedance: 100 kOhm(single) /200 kOhm(balanced)
Output impedance: 600 Ohm
Maximum gain: 5 times (low gain version)
Maximum output voltage: 24vrms 600ohm (@THD<0.008%)
Volume adjustment mode: 2x16 bit relay 0.1% precision resistance attenuation type, 100 files (logarithmic mode)

Noise floor range: 1.9uV~35uV (Typical vol=0~99)
Noise floor range: 1.5uV~22uV (A weight vol=0~99)
Typical harmonic distortion: THD<=0.0001%(<=-120dB) BW=30k / 600Ohm load
Nominal harmonic distortion: THD<=0.0003%(<=-117dB) BW=30K 600 Ohm load
Dynamic range nominal value: >=126dBrA(vol=99)

Intermodulation distortion IMD: <0.0007% (SMPT din 4-1, see the test chart and AP self-test coincidence,)
Channel separation: >=120dB (20Hz-20kHz 0dBv input, vol=99)
Linearity: -116dBv~+14dBv(+/-0.1dB 120dB rang)
5uV~5Vrms(+/-0.1dB 120dB rang) See AP test chart
Frequency response range: 10Hz~1.0MHz (-3db 1Mohm), 10Hz~1.2MH (-3db 50 ohm)
Square wave response: 8kHz/20kHz/80kHz no overshoot, no ringing, no distortion, see the test chart.
Conversion rate: >=100V/us (balanced output) see test chart

Dimensions: 320Dx235Wx100H (mm) (excluding protrusions)
 
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Thread Starter #6
A simple calculation: the relationship between volume and output power
This picture gives users a reference to the correspondence between volume and output power.
Calculated according to the input 0dB standard level, the main use area is about 0~45.
When it exceeds 45, the maximum sound pressure will exceed the maximum output power of MHzpower4.
Different broadcasting equipment and content are different.
If single-ended input is used, then the volume value will rise slightly, look at the light part, about 0~55.
Considering that many recordings, especially foreign recordings, in order to keep dynamic and prevent overloading during recording,
If the normal volume is low, the actual usage may slightly exceed this range.
If you are playing big dynamic music, please use a low volume to comment on the trial broadcast, and then adjust it appropriately to listen at a high volume.
In terms of experience, it is generally between 15 and 35, which can meet most music appreciation needs.
It can be seen from the curve on the figure that the volume and output power are approximately logarithmic.
It conforms to the traditional knob-type B-type potentiometer usage habits,
There will be no problem of adjusting the volume without feeling the change for a long time.
Or the sound pressure of one gear will change suddenly.

MHzpreamp II (Ref) has a total of 4 sets of virtual 4-link potentiometers composed of 8 relays (2x16bit) in each group,
It adopts 0.1% precision low temperature drift resistor network. Therefore, the error can be kept very low.
The relay adopts a relay with sealed nitrogen-filled gold-plated contacts, so it can maintain stable performance for life,
There is no aging problem. A total of 100 adjustments,
There is enough resolution to allow users to adjust to the most suitable listening volume.
max out put power vs volume.jpg
 
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Thread Starter #7
Below, we will introduce low noise, harmonic distortion, linearity range, etc. test charts and photos respectively.

List of instruments used in the test:
1. ShibaSoku AG15B Low Distortion Oscillator
2·ShibaSoku AD725D Automatic Distortion Analyzer
3·Audio precision AP-2322 Audio Analyzers
4·Panasonic VP-7722A Audio Analyzers
5·Panasonic VP-7214A Low Distortion Oscillator
6·HP-3457a Digital multi meter
7·TEK-2466B Oscilloscope
8. EFC-3210 Function generator
&
9·Pentex k7 for photos and video

The relationship between low noise and volume of MHzpreamp II (Ref).

It has extremely low noise, 1.4uV-25uV (A weighting).
The dotted line is the background noise of the test instrument

Because of the original front and rear double attenuation logarithmic volume control,
So it can effectively suppress the background output noise at a low volume.
Unlike the traditional design, the signal-to-noise ratio will deteriorate severely at low volume.
This can maintain a huge dynamic and analytical power even when listening at a low volume,
Very weak signals will not be overwhelmed by noise.

According to user experience, in normal use, the volume is set between 10~40 (depending on speakers of different sensitivity),
So there is no need to worry about the effective ultra-weak signal being overwhelmed by low noise.

Even with 99 output at full volume, the bottom noise is only 22uV,
Assuming that the gain of the rear stage is 29dB (MHzpower4 400w mono rear stage gain),
Then the noise voltage output is only 660uV, and the output power is only 110 nanowatts.
In other words, the background noise of the front stage will not form an audible sound pressure on the speaker.

Regarding noise, it is generally weighted by A,
A-weighted the auditory weighting of different frequencies,
Accurately reflects our actual hearing

The blue line in the figure is a typical value,
Even if it is a typical value, the highest low noise is only 35uV.
noise floor.jpg
 
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Thread Starter #8
Here is another way to analyze low noise, small signal FFT
A -60dB 1k signal is input, and FFT analysis is done after output.
The noise distribution of the entire frequency band can be seen completely.

From this figure, we can see that the amplifier has a very strong power noise suppression capability,
Two-stage independent regulated power supply and built-in filter effectively prevent hum interference
(50Hz 100Hz 150Hz 200Hz 250Hz...)
Suppress to a level below -123dBv.
Therefore, even the super weak signal details of -120dB can be fully amplified.
The high-frequency noise band on the right is not elevated,
Well controlled below the -135dB horizontal line,
Also verified the ultra-low noise data of the above post.
fft spectrum.jpg
 
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Thread Starter #9
Linearity

Test the linearity of MHzpreamp II (Ref).
The greater the linearity, the smaller the voltage distortion of the machine.
It can effectively keep the original dynamic of the music without compression and exaggeration.
The light blue is the linearity of AP itself,
Dark blue is the linear error curve of MHzpreamp II (Ref).

It can be seen from the figure that from -106dBv to +14dBv, a straight line is maintained, and the error range is +/-0.1dB.
In other words, MHzpreamp II (Ref) has an excellent linear working area of more than 120db.
Can keep the amplified voltage without distortion,
It faithfully reflects the original dynamic proportions of music.
fig5.jpg
 
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Thread Starter #10
Degree of separation

The result of testing the full-band separation between the two channels of MHzpreamp II (Ref).
It can be seen from the figure that in the state of full volume, the full-band separation has been kept higher than 120dB.

In other words, when one channel outputs up to 22Vrms signal and the other channel only receives 22uV interference,
If combined with the previous noise map to evaluate,
The signal detected by the test is not an interference signal, but is actually background noise.
Because the background noise is about 22uV at the maximum volume.

Therefore, in theory, it can be said that thanks to the excellent design of a separate voltage regulator circuit,
Although sharing a transformer,
There will not be any interfering signals between the two channels,
It can be used as two completely independent amplifiers.

crosstalk.jpg
 
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Thread Starter #11
Frequency response curve

The figure shows the frequency response curve of the tested MHzpreamp II (Ref) at different volume levels. The flatter the better.

The design bandwidth of MHzpreamp II (Ref) is 1.2MHz(-3dB),
The testable bandwidth of the AP is <200k, so only the frequency response characteristics in the 200kHz range can be seen.
The different level curves above are all straight and straight, and the level has no effect on the bandwidth.
There are 3 sets of bandwidth control filters inside MHzpreamp II (Ref), which can be switched by remote control.
They are: 22k, 33k, 66k, 1.2MHz.
There are 3 sets of low frequency compensation filters for speakers in MHzpreamp II (Ref).
Used to correct the low frequency volume between the speaker and the room. A balanced tone can be achieved through audition adjustments.
They are spk0, spk1, spk2, and spk3. spk0 is a pass-through option.

The figure below is the connection used inside MHzpreamp II (Ref). Because of the extremely wide bandwidth, a high-frequency cable is used as the connection.
Bandwidth has a very large relationship with slew rate and intermodulation distortion, please see the following related items.
fig6.jpg
 
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Thread Starter #12
Harmonic distortion
Test the harmonic distortion of MHzpreamp II (Ref) at different frequencies at different volume levels.
Each figure represents a volume state, namely 14, 24, 34, 44, 54, 64, 74, 84.
The test frequency ranges from 100Hz to 20kHz, and the test filter is an internal 30k filter.

Harmonic distortion is one of the most basic indicators,
The harmonic distortion of MHzpreamp II (Ref) is extremely low,
And the shape of distortion is very good, from 14 to 54 volume,
The distortion of the whole frequency band is lower than -120dB, that is, ≤0.0001%.
Because of the excellent circuit structure, there is no problem of violent climb of high frequency distortion.

Even at very high volume 84 (output 8vrms), the highest distortion can still be kept ≤0.0006%.
According to the previous power and volume relationship diagram, it is easy to find that under normal use,
The front stage can keep working below 0.0001% distortion,
It is equivalent to a signal line with driving force.
Even at a low volume, such low distortion is maintained.
It can ensure high resolution and good linearity at low volume.
It is not like some systems can only work at a certain volume to have excellent performance.
fig9-15.jpg
 
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Distortion shape FFT analysis

Is the distortion FFT analysis graph,
You can effectively see how many times the distortion is, whether it is even or fundamental, and what are the respective components
The first picture is the test picture of MHzpreampII (Ref)
The second picture shows the self-test map of AP2322 itself.

At first glance, the test chart of MHzpreampII (Ref) has a 2k distortion signal.
But comparing the curve of AP self-test, we found that
This 2k distortion is the signal source of the AP itself.
Even so, the distortion did not exceed -126dB, that is, THD<0.00004%.
3rd harmonic is close to low noise level -135dB
fig7.jpg
 
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Thread Starter #14
Regarding harmonic distortion, if anyone needs to inquire, I can send the original picture.
During the test, I took a photo of the test process, about 7G, which could not be uploaded.
For example, you can arbitrarily identify the frequency point and the picture under the volume state

Below I will post a few pictures of the test site

vol=34 @1k distortion 0.0001%
vol34.jpg
 
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This is the distortion of 7K when the volume is 44. 0.00008%

If anyone else needs it, I can send the original picture directly,
I will make a video later, which is the video of my process of testing the distortion
70050.jpg
 
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Thread Starter #17
The previous distortion is harmonic distortion, although it is already very powerful
But because harmonic distortion is a static distortion type
Can't fully reflect the performance under the dynamic music signal

There are many amplifiers with very low harmonic distortion,
But the sense of hearing does not rise gracefully as the distortion decreases
It was later discovered that it was caused by dynamic distortion or large intermodulation distortion.

In the early days because of poor technical means,
Therefore, many development engineers can only indirectly judge the magnitude of dynamic distortion by observing the shape of the square wave.
Later, with the development of computer testing technology, APsystem of AP Company can quantify intermodulation distortion very conveniently.
And it can quantify the entire frequency band, so it is very convenient

Of course, in order to use various methods to test the mhzpreampII(ref) we developed in detail
We did all these tests once.
黑白1.jpg
 
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The relationship between working bandwidth and square wave response and intermodulation distortion

Intermodulation distortion has been proven to be the main cause of sound deterioration
For example, some external narrow pulse signals radiate to the input terminal of the transistor
Caused transient overload of the transistor
As a result, severe distortion occurs. This is intermodulation distortion.
This is one of the reasons why low-frequency audio amplifiers also need good shielding.
At the same time, a well-shielded wire is also very helpful in reducing the probability of intermodulation distortion.
This is an external solution.
However, the interference signal may also be generated by the internal signal channel, which will also cause intermodulation distortion.

For example, the signal source adopts a switching power supply, and the switching power supply has a carrier signal that works at about 100k.
This signal is serially connected to the input of the signal amplifier. Intermodulation distortion will occur when the amplifier bandwidth is not high.
Also, the residual carrier signal of the CD player dac, 44.1kHz 96khz 192khz, these are bad signals
Also, the interference introduced by the external mains
Such as mobile phone chargers, air conditioners, etc. Now the popularization of switching power supplies is an audio nightmare.

You can't use shielding to solve internal interference
In addition to the selection of the grounding point and the filtering of the power supply
More important is the bandwidth design.

If we have an amplifier with a very high bandwidth, linearity can be maintained within this bandwidth range
If we design a low-pass filter in front of the amplifier to limit the high-frequency interference signal so that it cannot enter the amplifier
Then, our intermodulation distortion can be reduced to the limit level.

This is how we designed it.
Inside the amplifier is a conversion rate of 200v/us
The whole machine is limited to 100v/us after adding the input filter
Therefore, the generation of intermodulation distortion can be extremely effectively suppressed.
Even so, we still maintained a 1.2mhz bandwidth,
Keep the audio range perfectly responsive

Moreover, different bandwidths can be selected through the remote control 22k 33k 60k 1.2mhz
You can slowly test the difference between the different filters.

This picture shows the intermodulation distortion IMD of mhzpreampII (Ref) tested by ap 2322, using SMPTE4:1 standard.
The light blue is the self-test of AP2322's intermodulation distortion.

In other words, AP2322 cannot measure the intermodulation distortion of our previous stage.
So it can only be defined as IMD=0.0007%, which is a bit of a disadvantage.
fig17.jpg
 
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Thread Starter #19
Conversion rate

This picture shows the conversion rate of MHzpreamp II (Ref) we tested
According to the test calculation, the balance input is 150v/us
We call it 100v/us
150v.jpg
 
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Thread Starter #20
Square wave response

The key to the square wave is straight
Please pay attention to the square wave diagram below, the voltage output is 25V
Not the 1v square wave test chart of those weak chickens

8K 25V square wave response graph
8k25.jpg
 
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