Late Grand Prize MHzpreamp II (Ref) Design and Test Notes

Thanks to Fumac for the award of the HIFIDIY 9th Anniversary Guangzhou Offline Friendship Exchange: the preamp of the MHzpreamp II (Ref).

The MHzpreamp II (Ref) design and test notes (original author fumac), most of which reached or exceeded 120 dB. The following will be a complete test picture and record the experience of the development process.

Chassis style, all-aluminum CNC lunch box, plus a VFD display with remote control

Fully balanced amplifier, so only fully balanced input and output, sorry, no single-ended, if you need a single-ended to make a transfer interface, 4 pairs of balanced input terminals, relay switching, a pair of balanced output terminals, all use Swiss Neutrik socket. There is also a pair of 5-pin output ports, which are used to control the mhzpower4 rear-stage boot standby. The power supply is 230v input, of course, it can be changed to 110v.

Inside the amplifier, it looks very simple, and several modules are combined. 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.

Put a circuit diagram. Which one will help me to see the English translation part, please correct me, if you are about to send the pre-level, then please do not correct me.

NFB-Buf : Discrete class A non-feedback high speed buffer

Class A fully discrete feedback-free buffer amplifier

Q-log_network: Quad constant synchronization relay with passive impedance of front-end logarithmic decrement network system

Quadratic constant impedance front and rear synchronous relay passive logarithmic attenuation network

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

The following is a parameterization declaration based on the test results:

Model: MHzpreamp II (Ref)

Name: MHz stereo balanced class A reference preampilifier

Name: Fully discrete class A stereo full 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)

Zoom mode: Class A Balanced (Class A Balanced Amplifier)

Input impedance: 100 kOhm(single) /200 kOhm(balanced)

Output impedance: 600 Ohm

Maximum gain: 26dB (x20)

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

Harmonic distortion nominal value: THD<=0.0003% (<=-117dB) BW=30K 600 Ohm load

Typical dynamic range: >=117dBrA(vol=99)

Dynamic range nominal value: >=116dBrA(vol=99)

Intermodulation distortion IMD: <0.0004% (SMPT din 4-1, see test chart and AP self-test coincide,)

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 test chart.

Conversion rate: >=100V/us (balanced output) see test chart

Dimensions: 320Dx235Wx100H (mm) (without protruding parts)

Intermodulation distortion, IMD correction is at least 0.0007% and AP2322 test coincides

A simple calculation: volume and output power curve

This picture gives the user a reference between the volume and the output power. Calculated according to the input 0dB standard level, the main use area is about 0~45. Above 45, the maximum sound pressure will exceed the maximum output power of MHzpower4. There are differences between different playback equipment and content. If a single-ended input is used, the volume value will rise slightly, looking at the lighter part, about 0~55.

Considering that many recordings, especially foreign recordings, are overloaded in order to keep the recording dynamic, the normal volume will be low, and the actual use may slightly exceed this range. If you play big dynamic music, please use the low volume review pilot, then adjust it to listen at a high volume. From the experience of use, it is generally between 15 and 35, which can satisfy most music appreciation needs.

As can be seen from the curve on the graph, the volume and output power are approximately logarithmic. It conforms to the traditional knob type B potentiometer's usage habits. It does not cause the problem that the volume change is not adjusted for a long time, or adjusts a gear position. The sound pressure will be abrupt.

The MHzpreamp II (Ref) consists of a total of 4 sets of 8 relays (2x16bit) of virtual 4-connected potentiometers, which are constructed with a 0.1% precision low temperature drift resistor network. So you can keep very low errors. The relay uses a relay that seals the gold-plated contacts, so it can maintain stable performance for life without aging problems. A total of 100 adjustments, enough resolution for the user to adjust to the most appropriate listening volume.

Below, we will introduce low noise, harmonic distortion, linearity range, and other test charts and photos.

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

Has a very low low noise, 1.4uV-25uV (A weighted). The dotted line is the noise floor of the test instrument.

Because of the original double-attenuation logarithmic volume control, it can effectively suppress the background output noise under the small volume. Unlike the traditional design, the signal-to-noise ratio below the small volume will be seriously deteriorated. This keeps the dynamic and resolution of listening even at low volume, and the weak signal is not overwhelmed by noise.

According to the experience, during normal use, the volume usage is set between 10 and 40 (depending on the sensitivity of the speaker), so there is no need to worry about the effective ultra-weak signal being submerged by low noise. Even with a full-volume output of 99, the noise floor is only 22uV. Assuming a gain of 29dB (MHzpower4 400w mono gain), the noise voltage output is only 660uV and the output power is only 110 nanowatts. That is to say, the background noise of the front stage does not form an audible sound pressure on the horn at all.

Regarding noise, it is generally weighted by A. A weighting of different frequencies is aural weighting, which accurately reflects our actual sense of hearing. The blue lines in the figure are typical values, and even for typical values, the low noise is only 35uV.

The following is another way to analyze low noise, a small signal FFT with a -60dB 1k signal input, and an FFT analysis after output. The noise distribution of the entire frequency band can be seen in its entirety.

From this figure, we can see that the amplifier's power supply noise suppression capability is extremely strong. The two-stage independent regulated power supply and the built-in filter effectively suppress the hum noise (50Hz 100Hz 150Hz 200Hz 250Hz...) to below - The level of 123dBv. Therefore, even the ultra-weak signal details of -120dB can be fully amplified. The high-frequency noise band on the right side is not raised, and is well controlled below the -135dB horizontal line. It also verifies the ultra-low noise floor data of the top post.

First, test the linearity of MHzpreamp II (Ref)

The greater the linearity, the smaller the voltage distortion of the machine. Can effectively maintain the original dynamics of music, no compression, no exaggeration. The light blue is the linearity of the self-test of the AP itself, and the dark blue is the linear error curve of the MHzpreamp II (Ref). As can be seen from the figure, from -106dBv to +14dBv, a straight line is maintained with an error range of +/- 0.1dB. This means that the MHzpreamp II (Ref) has an excellent linear working area of ​​over 120db. The amplification voltage can be kept undistorted, faithfully reflecting the dynamic proportion of the music.

Second, the degree of separation

Test the result of the full-band resolution between the two channels of the MHzpreamp II (Ref). It can be seen that in the fully open volume state, the full-band separation degree remains above 120 dB. That is to say, when one channel outputs up to 22Vrms and the other channel only receives 22uV of interference, if combined with the front noise floor to evaluate, the test detected signal is not an interference signal, actually the noise floor. 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 the separate voltage regulator circuit, although a transformer is shared, no mutual interference signal is generated between the two channels, and it can be regarded as two completely independent amplifiers.

Third, the frequency response curve

The above figure is to test the frequency response curve of MHzpreamp II (Ref) at different volume, the flatter the better.

The MHzpreamp II (Ref) design bandwidth is 1.2MHz (-3dB), while the AP's testable bandwidth is <200k, so only the frequency response in the 200kHz range can be seen. The different level curves above are all straight extensions, and the level has no effect on the bandwidth. The MHzpreamp II (Ref) has three internal bandwidth control filters that can be switched by the remote control. They are: 22k, 33k, 66k, 1.2MHz. The MHzpreamp II (Ref) has three sets of speaker low frequency compensation filters for correcting the low frequency sense between the speaker and the room. A balanced tone can be achieved by audition adjustment. They are spk0, spk1, spk2, and spk3. Spk0 is a pass-through option.

The following figure shows the wiring used inside the MHzpreamp II (Ref). Because of the extremely wide bandwidth, a high-frequency cable is used as the connection. Bandwidth and slew rate have a very large relationship with intermodulation distortion. Please see the related items below.

Fourth, harmonic distortion

Test the harmonic distortion of the MHzpreamp II (Ref) at different frequencies at different volumes. Each figure represents a volume status of 14, 24, 34, 44, 54, 64, 74, 84, respectively. The test frequency is 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 distortion is very good. From 14 to 54 volume, the distortion of the whole frequency band is lower than -120dB, which is ≤0.0001%. . Because of the excellent circuit structure, there is no problem that the high frequency distortion rises sharply.

Even at the very high volume 84 (output 8vrms), the highest distortion can still be maintained at ≤ 0.0006%. According to the previous power and volume diagram, it is easy to find that under normal use, the front stage can keep working below 0.0001% distortion, which is equivalent to a signal line with driving force. Even at low volume, this distortion is kept as low. It can guarantee high resolution and good linearity under the small volume. It doesn't work like some systems can only work below a certain volume.

V. Distortion morphology FFT analysis

From the FFT analysis of the distortion, it can be effectively seen how many times the distortion is even or base, and what is the respective components. The first picture is the test picture of MHzpreampII (Ref), and the second picture is AP2322 itself. Self-test map. At first glance, the test chart of MHzpreampII (Ref) has a 2k distortion signal, but compared with the AP self-test curve, this 2k distortion is the signal source of the AP itself, even if the distortion does not exceed -126dB, that is, THD <0.00004%. 3rd harmonic is close to low noise level -135dB

Below I send a few test site maps:

Vol=34 @1k distortion 0.0001%

Vol=34 @5k distortion 0.00005%, count the wrong, 4 0

This is the distortion of 0.00K% at 7K when the volume is 44.

The distortion in front is harmonic distortion. Although it is already very powerful, because harmonic distortion is a static distortion type, it can not fully reflect the performance of dynamic music signals. There are many amplifiers with low harmonic distortion, but the sense of hearing does not follow. The distortion is reduced and gracefully rises, and it is later found to be caused by dynamic distortion or intermodulation distortion. In the early days, due to poor technical means, many development engineers could only indirectly judge the size of dynamic distortion by observing the shape of the square wave. Later, with the development of computer testing technology, AP's APsystem can easily quantify the intermodulation distortion, and It is very convenient to quantify the entire frequency band.

Of course, in order to test our developed mhzpreampII(ref) in various ways, we have done all of these tests once. Add a work photo when testing mhzpreamp II (Ref) with AP.

Relationship between working bandwidth and square wave response and intermodulation distortion

Intermodulation distortion has been shown to be the main cause of sound deterioration. For example, some external narrow pulse signals radiate to the input of the transistor, causing the transistor to be instantaneously overloaded, thus generating severe distortion. This is the intermodulation distortion. This is one of the reasons why low-frequency audio amplifiers also need good shielding, and well-shielded wires are also very beneficial to reduce the incidence of intermodulation distortion. This is an external solution. However, the interfering signal may also occur in the internal signal path, which will cause intermodulation distortion. For example, the signal source uses a switching power supply, and the switching power supply has a carrier signal, which works at about 100k. This signal is serially connected to the input end of the signal amplifier. When the bandwidth of the amplifier is not high, intermodulation distortion occurs. Also, the carrier of the cd machine dac remains. Signal, 44.1kHz 96khz 192khz These are bad signals, as well as interference introduced by external power, such as mobile phone chargers, air conditioners, etc., now the popularity of switching power supply is a nightmare of audio.

The internal interference can not be shielded. 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 very high bandwidth, we can maintain linearity in this bandwidth. If we design a low-pass filter in front of the amplifier to limit the high-frequency interference signal from entering the amplifier, then Our intermodulation distortion can be reduced to the limit.

We are designed this way. The internal conversion frequency of the amplifier is 200v/us, and the whole machine is limited to 100v/us after adding the input filter, so the intermodulation distortion can be effectively suppressed. Even so, we still maintain 1.2mhz. Bandwidth, the audio range to maintain a perfect response, and, through the remote control you can choose a different bandwidth 22k 33k 60k 1.2mhz, you can slowly test the difference between the different filters.

This picture is the intermodulation distortion IMD of the ap 2322 test mhzpreampII (Ref), using the SMPTE 4:1 standard. Light blue is the self-test of the AP2322's own intermodulation distortion. In other words, the AP2322 cannot measure the intermodulation distortion of our preamp. Therefore, it can only be defined as IMD=0.0007%.

Conversion Rate: This figure is the conversion rate of our test MHzpreamp II (Ref). According to the test calculation, the balanced input is 150v/us, which we call 100v/us.

Square wave response: The key of square wave is Zhongzheng straight. Please pay attention to the square wave diagram below. The voltage output is 25V. It is not the 1v square wave test chart of weak chicken, 8K, 25V square wave response diagram.

Square wave response: 20K, 25V square wave, no burrs, no deformation, no ringing, no self-excitation