Design Overview
We have had the good fortune of testing some of the
industry's best multi-channel amplifiers.
If you are from the camp that all amps sound the
same, then don't waste your time reading another amplifier review.
The truth is, most amps
sound similar if
they: a) have wide enough bandwidth, b) low enough noise floor, c) are not
driven beyond their linear operating region, and d) are capable of sourcing enough steady state power
without clipping dynamics to satisfy your listening needs.
The question remains however, "what is good enough"?
Thule seems to think their design approach meets these
criteria nicely and, best of all, does so in a reasonably-sized efficient package that likely
won't invoke a hernia to the installer.
Their solution is a multi-channel amplifier that
can be configured for 5- or 7-channels or be bridged for an ultra high power two-channel amp.
With rack space being a premium these days, and heat being of paramount concern, this amplifier design
certainly piqued my interest.
I was eager to find out how this Danish-made amplifier would
perform in the
Audioholics -
Premier Showcase
system
.
Design Overview
Thule refers to the PA350B
as a "Virtual Class A"
amplifier design.
Upon inspecting their literature and also
popping the top cover off, the best I could make of this claim is that the amp is a rail switcher which
is biased into Class A (100mA of bias current is always flowing through the transistors according to
the manufacturer) to eliminate cross over distortion caused by the on/off switching of a push pull
transistor scheme found in most traditional Class B amps.
This is nothing revolutionary as most
modern linear amps these days are of a Class A/B design where the amp operates in Class A for low power
levels.
Although Thule was able to achieve a higher bias into Class A while preventing excessive
heat build up by rail switching at higher power levels.
This amp employs a rail switching scheme where at low power levels it operates on +-
20V rails and switches over to +-40V rails for high output conditions. This is a typical design scheme
of many modern Class G amps such as those from Outlaw Audio, Emotiva, Sherbourn, etc and if executed
properly, can significantly increase efficiency and reduce heat dissipation without any audible audio
penalties.
Thule takes this design approach one welcomed step further by executing a TRUE fully balanced input to output topology from line level all the way to speaker level. They also caution the end user NOT to connect a ground reference to the speaker outputs. I paid careful attention to ensure that my Audio Precision Audio Analyzer was in balanced mode when testing this amplifier. There are a number of advantages to designing a fully balanced amplifier topology including, reducing distortion (cutting it in half) and noise (reducing it up to 6dB). The downside is increased circuit complexity and parts usage that most manufacturers opt not to undergo.
Editorial Note on the PA350B Amplifier Design
- The PA350B is basically a Class G rail switching amp design.
- It contains two 500VA toroidal power supplies each with their own capacitor banks as indicated below.
- It has 2 x 15,000uF 50V caps for the +-40V HIGH rails and 2 x 15,000uF 25V caps for the LOW rails assigned to each power transformer. The manufacturer claims a whopping 120,000 uF of total power supply capacitance but since they operate each pair of capacitor banks on different rails, I consider the total effective supply capacitance to be ½ of this stated claim.
- The PA350B has a total of 36 output devices (2 pairs of which are reserved for upgrading this unit to 7 channels). I would presume that 2 output device pairs are allocated per channel while the amp gangs up 4 pairs per channel when bridged into 2CH high power mode. Only 18 output devices are visible in the top view picture, though I suspect the other 18 are below the circuit board mounted to the heat sink.
- In order to do 350wpc a minimum recommended cap voltage of sqrt(2)*53 + 5 = 80V should be used. But because of the balanced amplifier topology, the required working voltage of the power supply and Caps is only ½ that. Thus 50V caps for the HIGH rail has sufficient design margin to achieve their sustained high power specification.
- The bridged output topology of this amplifier greatly increases efficiency and reduces heat dissipation compared to conventional amplifier designs.
- Care must be taken however when driving low impedance loads as most bridged amplifiers cannot safely drive loads below 2 ohms. Thule warns about this scenario as it will cause the amp to turn off if you present it with an impedance below 2 ohms.
Editorial Note on Balanced vs Unbalanced Connections
I measured the voltage gain of the Thule PA350B via balanced and unbalanced inputs and found the following:Unbalanced: Av = 29.8dB (about 0.8dB higher than THX standard)
Balanced: Av = 23.8dB (6 dB lower than unbalanced)
This is a standard practice in most consumer and pro audio. The preamp therefore usually has a +6dB boost via the balanced outputs to compensate. If you are using unbalanced to balanced connection scheme that I used in order to interface my receiver with this amp in bridged mode, you will likely find that you have to increase the channel trims of your main speakers to +6dB to compensate for the level offset.
My personal criteria for amplifier gain structure is that it should be able to hit full power when driven with 2-3Vrms. The PA350B fell well within this criteria
Check out our Balanced vs. Unbalanced Connections Article for more information.
