Sony XM-GS4
A couple of months ago we did a deep dive on Sony’s high resolution car audio player, capable of handling high sampling frequency PCM and Direct Stream Digital music files for, amongst other things, a frequency response extending way past that available from CD-quality sound, let alone MP3 and the like. We followed that with Sony’s super tweeters.
High frequency content in a source doesn’t mean much if the speakers won’t handle it. And, of course, something must sit in between a head unit and the speakers. Specifically, a wide bandwidth power amplifier if you want to preserve those high frequencies. That’s where the Sony XM-GS4 amplifier comes in.
The wider the bandwidth…
The XM-GS4 is a 4-channel car power amplifier with a frequency response that extends from 10Hz to 100,000Hz (at the -3dB point). That definitely qualifies when it comes to delivering high resolution sound. Each of the channels is rated at 70 watts into 4-ohms across the full audio bandwidth at one per cent total harmonic distortion. There are various other power specifications provided under different conditions. For example, at 1kHz the amp will deliver two channels of 160 watts into 2-ohms on one per cent THD. The net effect of the numbers is simply that this amplifier will be plenty for quality music delivery in almost 
all circumstances.
It is highly configurable, able to run four, three or two channels with commensurate power increases where pairs of channels are bridged. The basic plan is front left and right and rear left and right. The manual suggests five options including a basic plan – simply amplifying the four channels of output from a head unit to drive four speakers. The next option is a 2-way system: a regular pair of speakers and deploying the ‘rear’ channel amps to drive a pair of subwoofers. Another: 2.1 channels, with the ‘rear’ channel amps bridged to drive a single subwoofer. And yet another: left and right for both ‘front’ and ‘rear’ bridged to drive two subwoofers.
Finally, there’s the arrangement that would be appropriate for a high resolution system. Two rear channels, and two front channels, but the front channels have regular speakers and super tweeters paralleled (with suitable crossovers used for the super tweeters). Although not mentioned, there’s no particular reason why the same arrangement couldn’t be used at the front with twin or single subwoofer using the other pair of amplifier channels. Each channel operating alone can drive speakers with an impedance rating down to 2-ohms. Pairs of bridged channels need at least 4 ohms.
The XM-GS4 doesn’t leave your music entirely in the hands of the head unit. It has a rather extensive set of controls for balancing and shaping the signal for the various channels. The scheme is duplicated for the ‘front’ and ‘rear’ pairs. There are eight trim pots in total. Each is adjustable only by means of a screwdriver, but the slots are large and only slightly recessed, so access is easy. Each gets a level trim control to adjust for the different sensitivity of, say, a set of subwoofers running off ‘rear’ compared with full range speakers doing ‘front’ service. Or they can be merely used to set the input sensitivity of the amplifier to be appropriate for the head unit output level. These are marked with input sensitivity levels of 0.3 to 6.0 volts.
There’s a bass boost control which is capable of adding up to ten decibels at 40Hz. And a ‘subsonic’ filter (technically, the word should be infrasonic) intended primarily to eliminate the extremely deep parts of the signal that may not be reproduced by a particular loudspeaker installation. This provides for settings from 70Hz all the way down to 6Hz.
Finally, there’s a filter adjustment. This one’s a bit tricky, because it can be either a low-pass or high-pass filter. That’s subject to the control of a switch which offers high-pass, low-pass off. If you’re running main speakers from the ‘front’ and subwoofers from the ‘rear’, you’ll switch the filter for the ‘rear’ to low-pass, and may choose to set the filter for the ‘front’ to high-pass. The filter controls are calibrated from 50 to 300Hz.
Sony XM-GS4
The eight speaker terminals are gold-plated plate-and-screw arrangements, providing for the use of decent thickness cable and ensuring a very firm grip on cables. Four RCA sockets are provided for the left and right ‘front’ and ‘rear’ inputs. There’s also an eight-cable port for speaker level inputs in case your preferred head unit doesn’t have line level outputs. An adaptor plug for this is included with the unit.
The amplifier is encased in a heavily ribbed aluminium enclosure. Cooling is assisted by a two-speed fan. In my use this rarely came into operation. The whole unit can be screwed down to a mounting surface. There’s a fairly bright white rectangular light on the top to show when the unit is on. An aluminium cowling screws onto the unit to cover the trim controls after you’ve finished setting up, ensuring they aren’t knocked askew. All the connections are on one side of the unit, along with its two fuses, making them fairly easy to replace if needed. The whole unit weighs 2.7 kilograms and is 272mm wide, 202mm deep and 51mm tall.
It’s all in the HD
The amplifier was used and tested with a number of different speaker configurations. One was in a 4-channel configuration: front and rear, with the front supplemented by the Sony super tweeters via their supplied crossover networks. Another kept the same front configuration (what’s the point of HD without HD-capable speakers?), and a decent subwoofer using the ‘rear’ channels. And for another I used a pair of high quality, very revealing full range high fidelity speakers.
In setting up the system with the subs, I used the trim controls rather than the head unit’s settings as a kind of ‘worst case’ scenario. In theory using the digital processing within a head unit should give slightly better results when it comes to setting the crossover frequencies. But there wouldn’t be very much in it, quality wise, and there’s a lot to be said for using the filters on the amp itself. In particular, just being able to turn both the high-pass (for the main speakers) and low-pass (for the sub) controls at the same time speeds up the optimisation process enormously. In addition you’re not actually locked into the fixed frequency intervals normally imposed in head units. Analogue means more or less infinitely adjustable.
The cowling which hides the trimmers is secured by two screws, and so can be easily removed if future adjustments need to be made, but its presence ensures the settings you do make will remain protected.
The sound quality produced by this amplifier was simply excellent. Tellingly, when I used high quality home high fidelity loudspeakers, the result was not very far different from that when I used thousands of dollars of dedicated hi-fi power amplifiers, subject only to the volume level. While it  would fill up the room with high quality sound, at extreme levels the power limits became apparent.
But we should remember that the volume of air in this room was around 25 times that contained in the typical medium sized car. In that environment the output was not unlimited, but close enough for any desired listening level in which music is the priority rather than sheer noise production.
I listened to a very wide range of music – from Yundi Li playing Chopin Preludes through to Rage Against the Machine’s debut album – using this amplifier, much of it of CD standard, but with a fair sprinkling of high resolution. The first thing to note is that the amplifier exerted excellent control over the bass end, whether driving separate subwoofers or full range speakers. It’s my policy to not employ infrasonic filters unless problems become apparent. None did so I left the ones on the amp disengaged. The kick drum on all manner of rock was tight and full, utterly under control yet extended in depth.
I jumped briefly across to some of my favourite Bach pipe organ renditions – favourite in part because of the great bass – and even more so, it was clear that the amplifier could deliver all the depth that you could possibly desire. On more regular material, this performance was maintained. The elements of surprisingly deep bass guitar in “Good Thing” from Sam Smith’s In the Lonely Hour album were delivered with good authority and the kind of musicality that ties the fundamental bass frequency with its harmonics so that 
the bass guitar sounded like, well... a bass guitar, rather than a mess of vaguely coinciding tones. This natural delivery was equally evident on the DSD version of Dire Strait’s self-titled first album.
A similar performance lifted the soundtrack of the hit musical ‘Hamilton’. The hip hop elements rely so much on rhythmic accuracy. The amplifier tied subwoofers and higher frequencies together to audible perfection, delivering a sound that had plenty of bite 
when actually demanded by the source, but a fine smoothness as well, speaking of an absence of any distortion or any frequency balance anomalies.
There was no audible misalignment between frequency bands. The aforementioned Chopin was on piano, with a 24-bit, 96kHz capture of the signal. It was rendered to the amplifier by the Sony high resolution player. The attack on the piano notes was well-nigh perfect to my ears, and the control during the faster sections was excellent. The slower Preludes, particularly the C minor Funeral March, were rendered with the delicacy, sensitivity demanded of such a work.
So, with high resolution audio, plenty of power, good configuration options and a reasonable price, the Sony XM-GS4 ought to be on the shortlist of any car music enthusiast. 
Sony XM-GS4
Sony XM-GS4 in-car stereo amplifier
Cost: $499
+ Fine performance, Good configuration options, High resolution audio support
- Head scratching…
Type: Configurable amplifier
Features: 2-, 3- or 4-channels, high- and low-pass filters for front and rear, subsonic filters for front and rear, level trims
Frequency Response: 10Hz to 100,000Hz
Rated Power: 4 x 70 watts into 4-ohms, 20-20,000Hz, 1% THD
Contact: Directed Australia
on 03 8331 4800