It's possible you have a 48 with the wrong faceplate which is the exact same machine minus the +4 in/outs. The performance of the 2 machines should be identical. If you must have it operate @ +4, matching outboard matching devices are available.
What??
No.
The 58 and 48 are entirely different machines. Almost nothing is compatible between the two, Rick. I’ve owned and tore into both machines…highly dissimilar. They use the same heads, and the R/P amp circuitry is similar, but completely different PCB assemblies, the tape path is completely different, the chassis and metalwork completely different…no cross compatible PCB assemblies at all. I think the only common parts aside from hardware would be the reel adapters, the heads themselves (not the headblock assemblies), and maybe the feet on the back…the round ones for when laying the machine on its back. Oh, and the XLR jacks and reel motors are the same. But that’s IT.
@tremichael the 58, when originally introduced, was unbalanced -10dBv only. It was just the “Tascam 58”. I have one of those early versions. The manual is different than the second generation machines. A key telltale sign of an early machine is the scrub wheel has no finger detent in it…this thing:
I don’t think the 1st generation lasted long though because Teac was gunning for a foothold in the professional video production market. That’s really what the 58 was developed for as far as the marketing folks were concerned…for video sweetening…had chase-lock sync capability to sync with VTRs, the available full-function remote for install in the machine room and full control at the console, but the early version was unbalanced only. Then with the 2nd generation we see the advent of the model designations “58-OU”, for unbalanced, and “58-OB” for balanced.
The 58 is all unbalanced -10dBv internally, so the balanced I/O circuitry simply branches off of the same I/O that feeds the unbalanced RCA jacks. There is a balance amp PCB assembly, a separate regulated power supply for that PCB, and of course the rear panel is punched for and has the XLR jacks mounted; conceptually simple.
I have the balance amp PCB assembly and power supply here in my spares, as well as the punched rear panel. No jacks. But here’s the thing…or actually a couple things…though you could kludge the balance amp guts into a 1st gen or “OU” 58 just like Teac did (no disrespect meant, but that is what it is…the additional guts are tacked on and make it hard to get to everything else inside the 58), I don’t know why anybody would do this. For one, keep in mind balanced audio doesn’t make the audio sound better. Okay? You understand that, yes? Unless we are talking about a problem you are having with environmental RF or UHF “noise” being induced into your interconnects. Then balanced interconnections DO make your audio sound better because, if implemented correctly, the balanced circuitry mitigates the unwanted noise. But that is all balanced signal does or is designed to do when implemented correctly, is mitigate induced environmental electronic noise. The concept comes from the telephone industry where it was developed…it was necessary with thousands of miles of wire. The concept was carried forward into the audio world many decades ago in professional studios, video and broadcast production environments, where you have hundreds or sometimes thousands of feet of audio cabling going from room to room or often spanning multiple-story buildings. Induced noise was unavoidable, so it had to be mitigated. Plus the higher nominal signal level standard helped with any signal loss in those long cable runs. But balanced circuitry, whether active electronic (like differential amp circuits) or passive (transformers) actually can have a detrimental impact on the signal quality because you are passing signal through additional circuitry or components…additional amp stages and coupling capacitors…resistors…additional noise and distortion is the result…or through transformers which impart non-linear artifacts on the audio. Granted some folks like those non-linearities, and for others the detriments are outweighed by the benefits of the noise abatement because they have a long cable runs or an exceptionally “noisy” environment as far as RF, UHF, etc., or both. Just look at the 58 specs. The frequency range goes up to 25kHz using the unbalanced I/O, but it is 22kHz using the balanced I/O. Is that a deal-breaker? Nah. But it’s important to be aware of it, and to realize that if your environment requires cable runs under 25’, and you use good quality shielded cable, you really don’t *need* balanced audio. And actually, at least on paper, the performance is better on the 58 at the unbalanced jacks. Of course the better the quality of the components and circuit design, the less of an issue it is having the signal traverse a more complex path. But that comes with a price…fancier “better” circuits typically cost more. Of course there are also times it’s just easier to interface devices together if most everything else in your studio is +4dBu standard, but I rarely run across a device that can’t accommodate unbalanced -10dBv signal.
So now you just have to ask yourself what YOU want to do…determine what you actually need. And if you need the balanced I/O, I think you should not monkey with your nice 58…leave it as it came from the factory. Sure I could sell you all the guts and the jacks are readily available…you’d have some wiring to do and some modifications in order to mount the power supply and balance amp PCB assembly, but, again, why do all that? The 58 is a relatively rare, interesting and desirable vintage 1/2” 8-track machine…I wouldn’t cut into it. As
@RRuskin stated, there are a number of outboard boxes you can get that will do the balance/unbalance -10dBv/+4dBu conversion for you…get a couple LA-40mkII units, or an LA-80 and an LA-81…mount them close to the 58, get the shortest good quality pair of 8-channel snakes you can get to interface the boxes with the 58, done…the HOSA snakes have been a good bang for the buck as far as good shielding and construction…reliable…those above noted LA series devices are probably cleaner than the OEM balancing circuitry for the 58-OB. They have lower headroom. This isn’t an issue for most folks. It’s not that they have “low headroom” per se, it’s just that the 58-OB, since it was designed for video production and broadcast work, was capable of interfacing with the +8dBu standard of those markets, so the power supply and amp circuits have some greater oomph to them. But this is usually not needed in a typical audio production environment. If you want the headroom of the original 58-OB balancing guts, then you can look for a pair of 1st gen LA-40 units…harder to find but the amp circuit is almost the same as the 58-OB circuit.
Hope that helps.