All of this section is preliminary - I am still working on some of the design and aquiring parts. This represents my current thinking only and things may change! In the meantime, writing this helps to clarify some of the design details.
For this project, very high sound level capability is not a priority. But I do want the system to sound clean at moderate sound levels, so for the reasons stated in Some Thoughts About Amplifiers in the HMV 328A Concept section, I am leaning towards higher power amplifiers.
I have decided to build a 7.1 system to support the latest and future blu-ray releases. At the same time, it must be capable of performing well as a stereo system since the HMV 328A spends most of its time playing music.
The Response CS-2354 12" subwoofer I purchased is rated at 300W, so I need one amplifier module rated at least 300W into 4R. It needs to be a gutsy amp with a solid power supply, due to the heavy Extra Low Frequency (ELF) boost being used for the subwoofer. It should have a high damping factor to retain tight control over the speaker cone and will need to deliver heavy instantaneous currents to counteract the physical inertia of the cone. Since this amplifier will only have to deliver low frequencies, there is no special requirement here for an exotic high slew rate low THD design.
It would be nice if the Left and Right channel amps could deliver around 150W per channel into 8R. This would be a respectable stereo power amplifier in its own right, capable of driving a normal set of stereo speakers (with or without the subwoofer) and rocking it up at parties!
For 7.1 mode we need another 5 modules and if all of these had the same power ratings, we would have to think about massive power supplies and electricity bills!
For a start, I would think that very rarely in a 5.1 or 7.1 setup would you ever require full power simultaneously from all channels, or if you did, it would only be for brief periods such as an explosion. As explained in the Subwoofer Design Concept for this project, the subwoofer requires a relatively large amount of power due to the fact that it is relatively inefficient and the massive low frequency boost needed for extended bottom end performance. The power requirements for the other channels will be relatively low to achieve comparable sound levels.
My solution is to build 7 smaller identical modules for the main channels. Normally, each module runs into an 8 ohm speaker. Alternatively, the two pairs of surround modules are configured to run a separate set of stereo speakers. In bridged mode, the amplifiers can deliver four times the power into an 8 ohm load. In addition, the stereo speakers and the Left and Right 7.1 speakers can be run in stereo simultaneously. The stereo speakers can work with or without the subwoofer.
Exactly how the second set of stereo speakers would be used depends on the application. In my case, I'm thinking of a 7.1 system plus some nearfield monitors in the same room, sharing the subwoofer. In another scenario, you might have full range stereo speakers in a different room.
The 7 identical modules have single transistor pair output stages, whilst the sub module has four sets of transistor pairs in parallel. The sub module runs on double the voltage of the other 7 modules and therefore delivers four times the power into a given load.
Amp module configuration, speaker selection, muting during power up/down and fault protection is by means of relays on both the inputs and outputs of the amp modules. There are separate connectors on the back for the seven 7.1 satellite speakers and the stereo speakers. With all relays de-energised, all amp module inputs are muted and all speakers isolated.
The subwoofer is wired internally and is always used by the 7.1 system. The Stereo Speaker Configuration Switch on the rear panel configures the stereo speakers to run either through the crossover filters with the subwoofer active ("Xover"), or full range without the subwoofer ("Full").
Sub, Centre, Left, Right, LS, RS, LB, RB amp modules run the 7.1 speakers.
Subwoofer level controlled by 7.1 Sub Speaker Trim.
LS, RS, LB, RB amp modules are configured in bridged mode to run the stereo speakers.
When the Stereo Speaker Configuration Switch is set to "Xover", the subwoofer level is controlled by the Stereo Sub Speaker Trim. When set to "Full", the subwoofer is muted.
Left and Right amp modules run the Left and Right 7.1 speakers. LS, RS, LB, RB amp modules are configured in bridged mode to run the stereo speakers.
When the Stereo Speaker Configuration Switch is set to "Xover", the subwoofer level is controlled by Stereo Sub Speaker Trim. When the Stereo Speaker Configuration Switch is set to "Full", the subwoofer level is controlled by the 7.1 Sub Speaker Trim.
The Amp Control Board controls the amplifier input and output relays. The microcontroller on this board generates three logic control signals - Input Enable (InE), 7.1 Enable (71E) and Stereo Enable (StE). Transistor drivers on the Amp Control Board control the amplifier input and output relays. They respond to the three control lines coming from the microcontroller according to the following truth table:
RL1 (SPDT) is for the subwoofer. RL2 (3PDT) is for the 7.1 L, R and C speakers. RL3 (4PDT) connects the 7.1 LS, RS, LB, RB speakers to the LS, RS, LB, RB amp modules, whilst RL4 connects the Stereo speakers to the LS, RS, LB, RB amp modules in bridged mode.
--- MCU Outputs --- Sub, 7.1 LS,RS Stereo
InE 71E StE 7.1 L,R,C LB,RB L,R
Mode RB0 RA7 RA6 RL1,RL2 RL3 RL4
---------------------------------------------------------------
Spkr Mute X 0 0 0 0 0
Stereo Mode X 0 1 1 0 1
7.1 Mode X 1 0 1 1 0
Dual Mode X 1 1 1 0 1
X = Don't care
Relay driver logic:
RL1,RL2 = Stereo Enable OR 7.1 Enable
RL3 = 7.1 Enable AND NOT Stereo Enable
RL4 = Stereo Enable
--- MCU Outputs ---
InE 71E StE 7.1 Stereo
Mode RB0 RA7 RA6 RL1-RL4 RL5-RL7
-----------------------------------------------------
Spkr Mute 0 0 0 0 0
Stereo Muted 0 0 1 0 0
7.1 Muted 0 1 0 0 0
Dual Muted 0 1 1 0 0
1 0 0 0 0
Stereo Mode 1 0 1 0 1
7.1 Mode 1 1 0 1 0
Dual Mode 1 1 1 1 1
Relay driver logic:
RL1-RL4 = 7.1 Enable AND Unmute
RL5-RL7 = Stereo Enable AND Unmute
The amplifiers are powered by a dual power supply:
With such meaty power transformers, a soft start sytem is mandatory. I have put much soul searching into this subject and have decided to keep it simple. The approach is to use a relatively high value series resistor, rated for a worst-case scenario of it effectively being connected across the mains (for example, if a bridge rectifier went short-circuit). The idea is to "trickle charge" the main filter capacitors until the rail voltages are high enough to activate a relay which by-passes the resistor. Relays have a typical "must operate" voltage at around 80% of their rated coil voltage and a "must release" voltage of around 30%, so there is little risk of an unstable situation and no need for an auxillary supply. However, this simple design has limitations.
Firstly, it does not necessarilly reset itself quickly upon power failure and therefore is useless for very short power interruptions. In the HMV 328A project, the power is controlled by the Computer Power Sense Board. If there is a power disruption, the Computer Power Supply will shut down, cutting power to the amplifier power supplies. The system must then be restarted manually and therefore there is no special requirement for the soft start system to reset itself quickly.
Secondly, if signal was applied to the amps during power-up, they may draw too much current to allow the power rails to get up to the 80% activating voltage, thus keeping the power supplies in soft-start mode indefinitely (read resistors heating up!) In the HMV 328A project, the amp module inputs are muted during power-up by the Amplifier Control Board to prevent this situation from ocurring.
If there was a fault in an amplifier module resulting in it drawing excessive current at power-up, the power supply could remain in soft-start mode. If the unthinkable happens and the power supplies are held in soft-start mode due to heavy current draw, a thermal fuse permanently opens when the resistors get too hot. This would permanently disable the power supply.
For additional protection against high inrush currents during very short power interruptions when the soft-start resistor by-pass relay remains engaged, the mains inputs of the amplifier power supplies are protected by quick-blow fuses (as opposed to the normal slow-blow mains fuses used on most power supplies). Any excessive inrush current would blow these fuses instantly.
Neon lamps on the amplifier power supplies are used to indicate blown mains fuses, blown thermal fuses and soft start mode.