If the goal were to achieve the lowest possible signal to noise ratio, then just about any suspended belt drive turntable would be an obvious choice over any idler/belt-idler or any direct drive design.
And there was a time during the sixties...and seventies when conventional wisdom suggested that what a turntable needed to do was produce the lowest possible signal to noise ratio while at the same time producing the least amount of wow/flutter. After that the "real job" was up to the tonearm and cartridge to reproduce the musical goods.
Let say you can produce a TD124 in perfect "as new" mechanical condition and compare that directly to a TD321. Given appropriate tonearms and cartridges. Optimal conditions being met for either deck. Which would you prefer? Is that the question?
I can't bear witness to that comparison because I haven't heard any of the TD3xx decks. I have heard the TD124. And I have compared it against a Teres 145 and a few different Thorens TD150 and TD160 decks.
Clearly these players all have a different signature sound when compared against each other.
Any deck powered by a synchronous motor and is belt driven will have a recognizable sound. It may not be noticeable to you until you have spent time listening to decks with different drive systems. But the sound of a synchronous motor powered drive train, to me, is one of absolute timeliness.
Always and forever on the mark and relentlessly driving the pace of the music forward. To me this is a good quality.
On the negative side of this design is the reality of a cogging synchronous motor. Each motor revolution is divided into a series of forward impulses, like little kicks in the rear that propel the motor around and around. These little kicks are sharp enough to minutely stretch the elastic drive belt as it pulls against the driven pulley at the platter. The effect is one of an elastic belt that stretches and contracts several hundred times per platter revolution. Thusly chopping up the flow of the music. The effect is so minute that you may not notice it, until you have listened to turntables of different drive designs.
The Teres, with its limp-wristed DC motor and its minimally intrusive controller will have a certain sense of flow to the music. No cogging whatsoever. There is a sense of liquid flow to the music. The Teres also enjoys a substantial bearing and platter. One of its forte's is the ability to extract high amounts of detail from within the record groove. Detail, air and flow. But the Teres is also laid back sounding. Not the most dynamic deck out there. Not when compared head to head against a TD124.
The TD124, with its shaded pole induction motor drives a large idler pulley that is --damped-- and pitch-controlled by means of an eddy brake from underneath. This eddy break will tend to smooth whatever cogging its motor experiences. This cogging is further damped by an additional element in the drive chain, the idler wheel that gets its drive off the capstan shaft at the top of the idler pulley. So by the time power is delivered to the driven 9 lb iron platter, flow is relatively smooth and free of the cogging effect.
The td124 can sound, when at its best, timely and with a very smooth un-interupted sense of flow to the music...or it can sound, when in need of service, as if it has no sense of timing whatsoever. It can sound ponderous and completely without pace. It all depends upon the mechanical condition of the drive train components.
Also, if you crave detail and air, the Thorens TD124 can deliver a surprising amount of it, but only when in optimal mechanical condition. As the motor bushings wear, as the idler tire hardens, as the platter bearings get sloppy, this quality gradually goes away.
TD124 and noise: Bearing rumble, drive train rumble. When in worn condition, such noises are high. When in optimal mechanical trim, such noises are quite low. Surprisingly low. Better than you'd expect.
Due to its idler drive dna, the TD124 delivers a sharper dynamic component. Those explosive moments within the record groove get propelled into and out the speakers with a more efficient delivery. Therefore you should notice a sharper, keener sense of rhythm. Drums leap out into the room with more suddenness. etc. This quality can be addictive. You only get this with an idler drive...!
On the other hand; with a suspended belt drive turntable, the only wear parts are:
These are much less complicated to maintain. In most cases, just routinely replace the drive belt, keep some oil in the platter bearing and don't accidentally bend the the motor shaft. Performance remains consistent from decade to decade. Let us not forget to periodically adjust the spring suspension for vertical bounce...!
But the TD124 needs a mechanic. Like a bicycle mechanic...only it is for turntables. Perhaps the work is a cross between clock repair, bicycle repair and light transmission repair.
When considering the prospect of investing in either of these directions one has to consider it like this:
TD124, requires the most maintenance to sound its best. Can you provide this?
Mitch cotter designed suspended chassis, synchronous motor, elastic belt drive, AR, Thorens, Linn, etc. Minimal maintenance but does require belt replacement, mininimal lube, periodic chassis adjustments, etc.
Teres: Even less maintenance requirement when compared to all of the above.