HMV Model 328A

Amplification

All of this section is preliminary - I am still working on some of the design and acquiring parts.  This represents my current thinking and things may change!  In the meantime, writing this helps to clarify some of the design details.


Power Requirements for the HMV 328A

This is a 7.1 system.  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.  For the reasons stated in Some Thoughts About Amplifiers, I am leaning towards higher power amplifiers.

Response CS-2354 Subwoofer

The Response CS-2354 12" subwoofer I purchased is a 4 ohm speaker rated at 300W, so I need one amplifier module rated at 300W into 4 ohms.  It needs to be a gutsy amp with a high damping factor, backed up by a solid power supply.  To retain tight control over the speaker, the amp 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 for an exotic high slew rate ultra low THD design.

As a stereo system, it would be nice if the system could deliver around 150W per channel into a normal set of 8 ohm stereo speakers, configurable to work either full range or in conjunction with the subwoofer.  This would be a respectable stereo power amplifier in its own right, capable of rocking it up at parties!

For 7.1 mode we need another 5 amplifier modules and if all of these had similar power ratings, I would have to think about massive power supplies, massive electricity bills and a truck to transport the thing in!

As explained in Subwoofer Design, the subwoofer requires a powerful amplifier due to its low efficiency and the massive amount of low frequency boost required to achieve extended low frequency performance.  The power requirements for the other channels would be relatively low to achieve comparable sound levels.  For 7.1 sound, movie theatres and mixing rooms are supposed to be calibrated at a sound pressure level (SPL) of 85dB per speaker at "Reference Level" which is -20dB FSD.  This corresponds to a PEAK level of 105dB for 0dB FSD.  The surround speakers are supposed to be calibrated to 82dB per speaker.  Domestic listening levels are often around 10dB lower than this.  To achieve these acoustic levels in a small living room, typical power requirements are not that high.

My solution is to build 7 smaller identical amplifier modules for the main channels.  Normally, each module runs into an 8 ohm speaker.  Alternatively, the two pairs of surround modules can be configured to run an additional set of stereo speakers.  In bridged mode, these amplifiers can deliver four times the power into an 8 ohm load.

The 7 small amplifier modules have single transistor pair output stages, whilst the Subwoofer Amplifier module has four sets of transistor pairs in parallel.  The Subwoofer Amplifier module runs on double the voltage of the small modules and therefore delivers four times the power into a given load.


  Big
Module
Small
Module
Small
Module
Bridged
Power into 16 ohms 75W 18.75W 75W
Power into 8 ohms 150W 37.5W 150W
Power into 4 ohms 300W 75W -
Target module power into various loads


Amplifier Configuration

The HMV 328A supports both a set of 7.1 speakers and a set of stereo speakers.  The 7.1 speaker system consists of the subwoofer in the cabinet, a centre speaker mounted on the cabinet, with outputs for the remaining 6 speakers (L, R, LS, RS, LB, RB).  In addition, there are outputs for a separate set of stereo speakers.

Exactly how the second set of stereo speakers would be used depends on the application.  In one scenario, you might have the 7.1 system plus some near field monitors in the same room, both using the subwoofer.  In another scenario, you might have full range stereo speakers in another room.

Amplifier configuration, speaker selection, muting during power up/down and fault protection is done by relays on both the inputs and outputs of the amplifier modules.

As a fail-safe measure, when the relays are de-energised, all amplifier module inputs are muted, all amp module outputs isolated and all speaker connections grounded.  When output relays are used for speaker protection, it is important that they not only disconnect the speaker from the amplifier output, but that they also short the speaker terminals.  This minimises the risk of damage to the speakers if there is a severe DC fault condition and the relay contacts have a flash over.  Amazingly, some commercial amplifiers I have seen fail to do this.


Amplification Block Diagram
Amplification Block Diagram



The relays can configure the amplification system to work with the two speaker systems in different modes, analogous to the speaker selector switch often found on old hi-fi amplifiers (A, B or A+B).  These modes are described below...


7.1 Mode

In 7.1 Mode, the system drives the 7.1 Speaker System.

 
7.1 Mode
Simplified Amplification Block Diagram in 7.1 Mode
 

The Left, Right, LS, RS, LB and RB channels are each fed via a high pass filter and trim pot to an amplifier module.  The trim pots, which are accessible from the rear of the unit, allow the relative loudness levels of each channel to be calibrated, with an adjustment range of ±10dB.

The Left, Right, LS, RS, LB and RB channels are also summed and fed to a common Low Pass Filter.  The LFE channel is then summed with the output of the Low Pass Filter.  The reason for this arrangement is that the LFE channel can contain frequencies as high as 120Hz, whilst the crossover frequency would normally be lower than this.  The relative gain of the LFE channel is 10dB higher than any other individual channel, in accordance with the Dolby 5.1-Channel Music Production Guidelines.  This is because the LFE channel is normally mastered 10dB below reference level.

The summed low frequency signals are fed to the Subwoofer Equaliser.  Note that this is not a room equaliser - this equaliser compensates for the low frequency rolloff of a speaker in a small box by boosting the low frequencies with a curve inverse to the rolloff, thus extending the low frequency response.

From here, a phase reverse amplifier and switch allows the phase of the subwoofer to be set in relation to the satellite speakers.

A trim pot, which is accessible from the rear of the unit, allows the loudness of the Subwoofer Amplifier to be matched to the satellite speakers, with an adjustment range of ±10dB.


Stereo Mode

In Stereo Mode, the system drives the Stereo Speaker System.

 
Stereo Mode
Simplified Amplification Block Diagram in Stereo Mode
 

In Stereo Mode the LS, RS, LB and RB amplifier modules are configured in bridged mode to run a separate set of stereo speakers.

The Stereo Speaker Configuration Switch on the rear panel configures the stereo speakers to run either through the crossover filters with the subwoofer active, or full range without the subwoofer.

A separate set of trim pots and phase reverse switch allows the stereo speakers and subwoofer to be calibrated independently of the 7.1 speakers.


Dual Mode

In Dual Mode, the system drives both speaker systems.

 
Dual Mode
Simplified Amplification Block Diagram in Dual Mode
 

In Dual Mode, the Left and Right amplifier modules run the Left and Right 7.1 speakers in stereo, whilst the LS, RS, LB and RB amplifier modules are configured in bridged mode to run the stereo speakers.  In Dual Mode, the 7.1 LS, RS, LB and RB speakers are not used.

Thanks to the separate 7.1 and stereo trim pots, the relative levels of all speakers can be calibrated.  When the Stereo Speaker Configuration Switch is set to "Xover", the subwoofer level is controlled by Stereo Sub Speaker Trim - in other words, the stereo speakers take priority.  When the Stereo Speaker Configuration Switch is set to "Full", the subwoofer level is controlled by the 7.1 Subwoofer Speaker Trim.


Filter Board

The Filter Board contains the Channel Summing Amplifier, the LFE Summing Amplifier, the Subwoofer Phase Reverse Amplifier and all of the filters shown in the Amplification Block Diagram above.

The crossover filters are 24 dB/Octave Linkwitz-Riley filters, based on the Elliott Sound Products Project 09.  Linkwitz-Riley crossovers produce a flatter response with lower phase shift around the crossover frequency region than the more conventional Butterworth filter designs.  The steep rolloff of these filters minimises interaction between the drivers.  This system is designed around a crossover frequency of 80Hz.  The components which determine the crossover frequency for the eight filters are built on headers so that these can be changed or modified in the future.

The Subwoofer Equaliser is a Linkwitz Transform Circuit, based on the Elliott Sound Products Project 71.  The component values are calculated on a spreadsheet according to the volume of the speaker enclosure and the parameters of the speaker used.  The values are therefore specific to this project and you will need to look at the original article if you are building one for yourself.


Amplifier Trim Board

The Amplifier Trim Board contains the gain trim pots for the individual amplifer channels.  These trim pots are accessible from the rear of the unit through holes in the rear of the Main Chassis.  This board also contains two phase inverters for operating amplifiers in bridged mode and the amplifier input muting relays.  In addition to muting the amplifiers, these relays are used to configure the system in 7.1 Mode, Stereo Mode or Dual Mode.


Amplifier Power Supply

The amplifiers are powered by a dual power supply:

These are based on a conventional unregulated design typically used in hi-fi amplifiers:


Basic Amplifier Power Supply

The capacitors must be large enough to filter the rough DC output of the rectifiers when delivering the maximum current output.  The output voltages of power transformers are usually specified at their rated maximum load and the output voltage of typical larger transformers rises by 10% under no-load conditions.  The capacitors store this energy which enables the amplifiers to deliver short-term bursts of additional power.  This is known as Music Power.  When delivering continuous power, the rail voltages drop and therefore the Continuous Power rating of an amplifier with an unregulated power supply like this is usually less than the Music Power rating.


Soft Start

With such meaty power transformers used in this project, a soft start system is mandatory.  Larger power transformers can draw heavy current at switch on, even without a load connected.  When the filter capacitors are fully discharged, they look like a short circuit across the rectifier diodes at switch on.  The only thing limiting the inrush current on the mains side at switch on is the inefficiency of the power transformer and rectifier diodes.  It is not uncommon for larger power supplies without soft start circuitry to trip circuit breakers or blow their rectifiers at switch on.  These problems are even worse when toroidal transformers are used, due to their higher efficiency.

Soft start can be achieved with a resistor wired in series with the mains active input to limit the inrush current at switch on.  A relay is used to bypass this resistor at the end of the soft start timing period for normal operation.


Power supply with soft start

There are a number of important considerations which need to be taken into account when designing a soft start circuit:


Implementation

In this project, power to the Amplifier Power Supply is switched by the mains relay in The Dog Box, which is controlled by the Computer Power Sense Board.  The bypass relays are part of the Amplifier Power Supply and are controlled by the System Control board.

To assist diagnostics, neon lamps in the Amplifier Power Supply are used to indicate blown mains fuses, blown thermal fuses and Soft Start mode.