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Spartan 10 Phonostage

Introduction

Still in the prototype stage, the Classic Audio Spartan 10 phonostage will be Classic Audio's first product. Combining years of experience of pre-amplifier design for moving magnet cartridges with high quality components with a proven track record to deliver consistently excellent performance and last the test of time, very low noise and distortion, along with a novel new switch-able low frequency cross-feed arrangement that audibly reduces vertical bearing and surface rumble, the Spartan 10 promises exceptionally transparent reproduction in any high quality setup.


Spartan 10 Prototype

Spartan 10 Electronic Prototype

Classic Audio aims to make the Spartan 10 phonostage available to customers by October 2021. As can be seen it is still in development as of August 2021, with the enclosure end plates to be finalised before the first production run begins. Due to draconian and ill-advised ecoDesign regulations, the correct AC adapter must be purchased separately.


Features

It has long been known that switching to mono brings an audible reduction to low frequency noise caused by surface imperfections in the record and bearing rumble of the turntable and even cutting lathe. This low frequency 'road noise' can be particularly distracting when listening with headphones as it sits across the stereo field, pulling the listener's attention to the sides of the stereo-field. Due to inherent limitations in the stereo groove system, all bass content has to panned to the centre of the stereo field to prevent the stylus from jumping up and out of the groove, so there is no stereo separation at low frequencies. Wouldn't it be nice if there was a way to correspondingly merge the two stereo channels down to mono at the low frequencies where it is already merged by the record cutter and cancel out the low frequency noise while still maintaining separation at higher frequencies? The Spartan 10 does this being the first commercially available phonostage to implement 'low frequency crossfeed'. And it really works!

If the best results are going to be had from mono discs, then a switch that puts the phonostage into mono mode is a must. By switching to mono, all stereo information is cancelled, which on a mono only disc will consist only of extra noise and distortion, the latter being particularly unpleasant as like the low frequency noise described in the previous paragraph sits distractingly across the stereo field. The mono switch can also be used on very worn or distorted stereo pressings, reducing distortion and surface noise enough to make otherwise un-listenable discs enjoyable again.

When records are cut, RIAA equalisation is applied to match the frequency content of recorded music to the groove displacement and acceleration limitations of the format. In order to reproduce accurately across the audible frequency, the equalisation curve must be faithfully reproduced. If it isn't reproduced accurately then the effect will be much having leaving the tone control on a pre-amplifier off-centre, resulting in a coloured, sub-optimal sound. Most phonostages on the market use cheap 5% tolerance capacitors, rounded to the nearest available standard component value to equalise the recording curve, resulting in a playback accuracy of ±0.5dB (5%) or worse, potentially causing a total deviation of 1dB across the mid-band which may be audible in itself. This then makes the phonostage the weakest link in the analogue signal path in terms of frequency response distortions. Most of this deviation from the curve occurs in the middle of the audio band, where the 500Hz and 2122Hz turnover points of the RIAA curve occur and the ear is most sensitive. While it may not seem like much, these little deviations can quickly add up in a sound system and cause an audible effect to be apparent resulting in an overly bright, dull, boomy, thin or mid-heavy 'shouty' characteristic to be added to the sound. This was the reason why many electrically asymmetrical tone controls that exhibited similar frequency deviations were condemned back in the day.

So as to avoid distorting the RIAA curve, the Spartan 10 therefore employs a very carefully tuned precision RIAA equalisation network utilising 1% tolerance polypropylene film audio capacitors and metal film resistors to attain a superb guaranteed RIAA playback accuracy of ±0.1dB all the way down from 35Hz to 22kHz. This ensures that you hear only the sound of the record as intended when it was cut, and not that of the phonostage.

Even with a perfect setup, subsonic disturbances (erroneously referred to as 'rumble' - it's not audible on its own), can seriously upset loudspeakers, headphones, and even some amplifiers. With the worst levels of subsonic content being in the 8-12Hz region they can easily push the unloaded drivers of bass-reflex loudspeakers past their precious few millimetres of linear excursion at the most moderate listening levels, promising severe 'shimmering', and 'fluttering' intermodulation distortion to be occur when audio is present creating a vague and unstable sound with an ill-defined stereo centre. A sure sign of this is movement of the loudspeaker cones during quiet sections of the record; if you can see it, you can be sure that it is going to be causing distortion, burying low level detail and directional cues under a sea of insidious amplitude-modulated artefacts. The same is true for the delicate drivers in headphones, especially open-back types that don't have the 'springiness' of the air behind the driver to prevent excessive excursion and distortion when faced with subsonic disturbances. Contrary to popular belief, on a well-adjusted setup the worst source of disturbance is in the surface of the recorded itself and can be easily verified by observing the changing distortion of reflected light on the surface of the record as it spins on the turntable.

A subsonic filter that rejects the troublesome 8-12Hz disturbance region by a factor of 10 or so while not affecting the response at by more than 0.1dB at 40Hz, the lowest reasonable expected recorded frequency on the record, should therefore be considered mandatory for high quality reproduction. The Spartan 10 implements a 18dB/octave subsonic filter with a -3dB turnover at 22Hz, attenuating subsonic disturbances by over 20dB (a factor of 10) at 10Hz while ensuring response flatness within 0.1dB all right down to 35Hz, preserving the bass response while stopping insidious subsonic content from causing problems further down the signal path. 5% tolerance polypropylene capacitors are hand matched to within 1% of each other for each unit, ensuring excellent channel balance and flatness. Even better, and quite uniquely, the subsonic filter is implemented before the final 7dB of the total 41.6dB of gain is realised, so the subsonic content can't eat into the final overload margin of the phonostage, like it does in all other designs that implement the filter after all the gain has been applied. Splitting the gain this way also shares the burden of amplifying the signal between the Spartan 10's two amplifier stages, improving the overall linearity.

Many small phonostages today use an external switching power supply that can inject nasty audio frequency currents straight from the mains into the audio ground path between the phonostage and power amplifier resulting in unpleasant buzzes, hums, and whines adding a most unwelcome accompaniment to the music. With these supplies it is not possible, without the further addition of more noisy switching electronics inside the enclosure, to derive a proper split power supply for the audio electronics to work off, and a single supply arrangement where the sensitive audio ground has to be precariously derived from half the power supply voltage, sharing the same ground path as the power supply rails. With the standard 24V DC single supply this means that only 7V RMS of headroom is available, or even less than 5V RMS in some cases where the 24V supply is regulated down to 18V in an attempt to reject some of the switching noise, seriously increasing the risk of obtrusive overload when attacked with subsonic content and surface scratches.

Unlike the majority of small phonostages available, the Spartan 10 uses a tried-and-true ±15V split linear power supply that affords far better isolation from the mains, for use with an AC adapter that takes the power transformer and its associated hum-inducing magnetic field out of the enclosure and down to the mains power socket. Alongside the front panel power switch, a switch-on muting relay is employed so there are no loud thumps through the speakers when the Spartan 10 is switched on, only a satisfying 'click' from inside the enclosure after the second it takes for the power supply to stabilise. Having a true split supply of ±15V means that the Spartan 10 can handle an internal level and line output level of over 9V RMS, realising not only a very comfortable overload margin, but also allowing the gain to be increased so that the output with a standard moving magnet cartridge can subjectively match that of a modern DAC without the risk of overload!


Specification

As can be expected, all of the features listed above lead to a stellar performance that puts the Spartan 10 well ahead of its class at the price point advertised. See the table below for a comparison of the Spartan 10 with a well-known similarly UK based competitor (Graham Slee Era Gold V, as of August 2021):


Parameter Spartan 10 Era Gold V
RIAA accuracy 0.1dB, 35Hz to 22kHz 0.5dB, not specified
Channel balance 0.1dB, 35Hz to 22kHz 0.2dB, not specified
Channel separation 80dB, 20Hz to 22kHz 64dB, not specified
Signal to noise ratio 78dB, at output ref 5mV cartridge load 65dB, at output, not specified
THD + Noise <0.001%, 35Hz to 22kHz, 9V RMS output 0.02%, not specified
Maximum output >9V RMS, 35Hz to 22kHz 4.9V RMS, not specified
Maximum input at 1kHz 75mV RMS 39mV RMS
Maximum input at 10kHz 350mV RMS Not specified
Overload margin, ref 5mV 23.5dB 17.8dB
Gain at 1kHz 41.6dB, 120x 42dB, 126x
Output impedance 100Ω 910Ω
Minimum load impedance 2kΩ 10kΩ
Input impedance 50kΩ, ±1% 47kΩ, not specified
Mono switch Toggle, front panel Toggle, rear panel
On switch, muting relay Toggle, front panel None
Subsonic filter 22Hz, 3rd order None
Power supply Split linear ±15V SMPS 24V DC, single 18V rail
Advertised price £300 £430

As can be seen from the table above, the Spartan 10 offers an exceptional performance to cost ratio compared to one of many others of its contemporaries!

Build quality

Sitting inside a 3 piece extruded aluminium enclosure, the Spartan 10's PCB features a double sided ground-plane layout populated entirely with high quality through-hole components. Distortion free polypropylene film and C0G capacitors ensure that no low-level detail is lost in the ultra-accurate RIAA network and subsonic filter. Panasonic FM series electrolytic capacitors are used throughout, widely regarded as the best available with exceptional reliability and longevity. Professional standard audio amplifiers are employed in turned IC sockets to amplify the delicate low-level input signal with very low noise and almost unmeasurable distortion throughout the signal path. Hard wearing nickel plated connectors are used on the rear plate to make reliable connections, with a rear low voltage AC power connector that can accept both 2.1mm and 2.5mm centre pin barrel jacks.

The Spartan 10 is carefully hand made in the UK, using a UK made enclosure and parts sourced from UK suppliers.


Any questions?

If you have any questions or comments relating to this product and its development, then feel free to contact classic audio .