Table of Contents
For a long time my friend Heiner had expressed the wish that I build a multichannel preamplifier for him. Since the subject itself was very appealing to me, I started developing it in the third quarter of 2016. First of all the basic concept for the hardware, but also for the software, had to be created. I also wanted to include as much of my current circuit topologies as possible in this preamplifier. But in the end, I made detailed changes to all circuits and created new layouts.
The entire preamplifier is distributed over 2 housings
- the Control Unit consists of
- the 230V/AC input with line filters
- the unregulated analog voltages supplies
- the unregulated digital voltages supplies
- the controller board for control and operation
- the Audio Unit consists of
- 2 motherboards, each for 6 audio boards
- 12 audio boards with the actual preamps
The Control Unit contains all circuit parts which are not directly related to the processing of audio signals. In particular, the mains voltage is only fed into this part of the preamplifier.
The 230V/AC are fed into the housing via a mains filter with integrated fuses. This voltage is applied to the first printed circuit board via the mains switch. There you will find a capacity filter and then a 230V/AC DC filter. Behind these filters 2 external toroidal transformers for the generation of audio voltages and a toroidal transformer on the board for the digital supply voltages are placed. All transformers are additionally protected individually. The digital voltages are rectified directly on this board. Two unregulated DC voltages are available – μC board supply and relay voltages.
Furthermore there is a circuit part on this board to determine the actual phase position. For the first time I really thought about the functional safety of such a circuit and redesigned it. I now use a neon lamp which can be switched between one phase and earth by a relay from the microcontroller. If a defect occurs here, the probability of a danger to the user is extremely low.
Of course, the user’s safety is primarily guaranteed by the earthing of the housings.
The neon lamp is monitored by a photo resistor whose signal is connected to the μC PCB via a comparator. When querying the phase position, the controller switches the relay and evaluates the output signal of the comparator. The relay is then switched off again.
System Power Supply
Two further PCBs in this unit are responsible for providing the unregulated DC voltages of the audio circuit components. Here I use proven technology. Behind the secondary windings of the toroidal transformers snubber networks are placed, followed by rectifiers with discrete Ultra-Fast Soft-Recovery diodes. This is followed by a CLC filtering with 22mF capacitors. Behind these filters are capacitance multipliers with additional low-pass filtering towards the output. From here, the DC voltages are fed into the Audio Unit via a cable.
Analog Power Supply
As an additional circuit board, the microcontroller system is mounted behind the front panel, together with the LC display as a unit. Here I use my proven Atmel AT89C51ED2 x51 controller again. Communication with the Audio Unit is only possible via serial protocols – I²C and SPI – and is in idle mode as long as no switching is in progress. The preamplifier is operated by means of two rotary encoders or an RC5 remote control and the display.
The LC-Display has 2 lines and 20 columns with a character height of 12.7mm. It is – as mentioned above – screwed directly to the back of the μC board.
LC-Display at the back of the μC Boards
The picture below shows the complete Control Unit with the cover removed. All printed circuit boards and components described above are clearly visible.
View into the open Control Unit from behind
The Audio Unit is filled almost completely by 2 bus boards. These boards are equipped with discrete voltage regulators for the power supply of the analog circuits and six slots for audio boards. For each of these 6 slots there are 2 inputs and one output on the board. Therefore there is no wiring with the audio boards.
Furthermore, each bus board has a digital circuit part for controlling the relays and the digital potentiometers. The transition from the μC-system to the analog circuitry is made via a potential separation with devices from the ADuM-series of Analog Devices.
There are two types of audio boards. Both are identical in their circuit topology. The balanced input is routed directly to the first amplifier stage, the unbalanced input is balanced with the help of a DRV134/135. The symmetrical signals are then amplified with a gain of 6dB via an OPA1632. After this amplifier, the symmetrical signal is adjusted to its final volume by a digital potentiometer – the MUSES72320 is used. The final stage is a balanced output driver.
The two boards differ here in the technology. The first one is equipped with 2 operational amplifiers connected in parallel per signal phase (2× OPA1612).
OP Amp Preamp Board
The second board implemented discrete Diamond Buffer with servo controller instead. The OPA1632 and the MUSES72320 are placed on the back of the board.
Diamond Buffer Preamp Board
The picture below shows the complete Audio Unit with the cover removed from the back. A total of 4 audio cards are inserted.
View into the open Audio Unit from behind
For the Control Unit a housing with an inner height of 80mm and for the Audio Unit a housing with an inner height of 120mm is used. I need the higher case to get space on the rear panel for all connectors. In addition, the plug-in cards also need some height space.
The front view can be seen at the top of the page. The LC display is located in the middle. To the left and right of it you can see the two rotary encoders. On the left side you can see the integrated IR receiver.
The rear view can be seen on the following photo. Per channel you can see the two inputs (XLR & Cinch) and one XLR output. The digital signals and the digital supply voltage are transmitted from the Control Unit to the Audio Unit via the 25-pole Sub-D connection. The Hirschmann industrial connectors are used to connect the analog voltages between the two housings. The two connecting cables required for this are self-made.
At the Control Unit there is a 9 pin Sub-D connector to transfer the software to the microcontroller (ISP – In Ssystem Progamming), an input for an external infrared receiver (IR) and the output for the remote voltage (see above). Of course, the 230V/AC mains voltage input with filter and integrated fuses is also located here. The power switch is not to be found on the photos, it is located in the front right on the bottom of the Control Unit.
View of the complete preamplifier from behind.
How do the two audio modules compare to the VV5? I must say surprisingly well! However, I have only heard one afternoon and so the preamplifier modules need still time to reach their end performance. So my friend Heiner will run it for a while and then we will both report on it.
2018/04/03 / Ralph :
My first positive impression did not mislead me, it would be difficult for me to choose between this preamplifier and the VV5. The richness of detail of both preamps is comparable, there are minor tonal differences. But these are going towards taste and I don’t know what I would finally choose. All in all a quite successful construction that shows what modern audio IC’s are capable of nowadays.
2018/04/26 / Heiner :
Now that I have owned the VV6 for some time and have heard it for many hours, here is my impression:
Actually, all preamplifiers should sound the same or very similar, the measured values suggest this. Everything is very good and there are deviations in such small quantities that our hearing cannot detect them. Yes, but it’s not like that. The VV6 takes 1-2 hours to warm up. After warming up the VV6 plays in a way that I just want listen to the music. I listen to CDs from beginning to end and no longer just play my “test music”. Complex structures (e.g. tutti in classical music) are so clearly structured that they no longer appear complex. Or that it must be a piece of cake to dissolve them and present them as musical events. The VV6 can make any instrument sound natural, just as with small instrumentation. The three-dimensionality is very authentic. It’s really fun and listening to music is beautiful and musical and natural. I have treated my living room acoustically quite well and the reverberation times are fairly linear over the frequency response. The VV6 is now running in this environment and I always enjoy listening to it.
One must still admit that we both – and consciously independently of each other – give the module with the Diamond Buffer the preference.