OK, firstly let me clarify some terms:
Single ended is a reference to a type of circuit - it means that shield ground and signal cold are tied together, they are at the same potential. This is opposed to balanced, where the signal cold can float and shield is not necessarily tied to signal cold, that is the signal wiring can "float" or it can be run balanced, i.e. the signal hot and cold are equally spaced from the chassis ground potential.
An XLR is just a type of connector. It can be used for single ended or balanced connections. Another common type of connector is an RCA connector. They can only be used with single ended connections.
In the Tube Repro it happens that we can take a single ended signal (only) out at the second stage of the circuit. This yields a -10dB signal (50dB of gain from a typical tape head), suitable for most home audio gear. The output impedance is a little on the high side at 4000 ohms, so this output may not be best to connect to a passive preamp or a load of less than about 15K ohms. The reason we offer this is that the circuit before this output is direct coupled and there is only one output capacitor in the signal path at this output. Theoretically that means less stuff in the signal path to affect the sound and hence potentially higher resolution. In some systems with enough gain in the budget and with a relatively high input impedance for feeding the Repro into it should work quite well if cables are kept to a reasonable length. In fact that is how I am currently running my own Repro, into a new single ended preamp circuit we are developing.
The balanced output on the Tube Repro comes after that single ended output cap and includes an additional triode and output transformer that can run floating balanced out (meaning the shield is connected only to the chassis, not the transformer secondary). That output is there for 15dB additional gain, up to the +4db standard for studios, and to lower the output impedance to a studio standard 500 ohms so that very long cables can be driven. It also helps when a lower output playback head is used.
Now when you say quiet, again, we need some clarification. Quiet means a lack of noise. But what kind of noise? Hiss? Hum? One must define this before one can discuss the subject at any level of confidence.
Tube hiss is as I already mentioned due quite often to 1/F noise and it will be noted in relation to signal level, i.e. the louder the signal the lower the tube noise in relation to it. So the solution to this is quiet tubes and a preference for a higher output playback head when possible. Single ended vs. balanced outputs used on the preamp won't make any difference.
If the noise we are discussing is hum, then the higher gain outputs can raise the level of any hum coming into the inputs of the repro amp. This can be 60Hz, generally created by AC magnetic fields of power transformers, or 120Hz and higher, generally of electrostatic nature and typically due to ground loops or bad signal or shield grounds. If there is hum coming in ahead of the Repro, trimming the output level of the Tube Repro balanced out down 15dB will result in a similar level of hum and 1/F noise as the single ended output since the later low gain triode stage and output transformer don't contribute anything significant noise-wise. In theory XLR type cables should be more quiet since shield is not usually tied to signal cold and thus external EMI is kept off the signal cold. But there are a zillion ways to rig cables, so there is no one right answer. But for certain getting quiet head cables is paramount to good reel to reel playback, just as it is with tonearm cables and LP playback.
It's also good practice to keep small signal tubes like EF86s away from power transformer magnetic fields, as they can pick up 60Hz this way. So I suggest not stacking a Repro on top of another piece of gear.
Input cabling is only one possible source of hum. The other big issue is ground loops, as audioblazer discovered. This is a particularly tough issue to work through because it is different in every environment. We can have a tape deck and preamp here in the lab that is dead quiet that will suddenly develop a 120Hz buzz in a different room with different wiring, and the only way to work out the issue is by methodically trying every possible combination of ground loop hum reduction. In Song's case we went thorugh a pretty exhaustive checkout of the cable connections to isolate the issue and then determined that in his particular case it was a matter of connecting the ground posts of the two units (that's why they are there, sometimes you need 'em). In some cases we have had to use a cheater plug on the tape transport power cord. And in some cases we have seen technicians fail to understand the that shield of the head cable should not be connected to the tape deck chassis when using an external preamp. We have also experienced a situation where a tech had an intermittent noise problem on their AC that was being generated by a neighbor in their business park - not much we can do to control that! I've also seen a case where the electrician alternated the sockets around a studio between two different circuits, which caused a slight buzz on some midrange drivers that took two years to track down.
This is really just scratching the surface of the subject. After having been the head noise troubleshooter on numerous high end installations at shows and in studios the one thing I am certain of is that every case of noise is solved just a little bit differently.
Oh and a quick final note: you can use both the single ended RCA output and the balanced transformer XLR output of the Tube Repro at once. If the RCA out is running into a relatively low impedance load it may cut the output level down a bit, but no harm will be done.
Single ended is a reference to a type of circuit - it means that shield ground and signal cold are tied together, they are at the same potential. This is opposed to balanced, where the signal cold can float and shield is not necessarily tied to signal cold, that is the signal wiring can "float" or it can be run balanced, i.e. the signal hot and cold are equally spaced from the chassis ground potential.
An XLR is just a type of connector. It can be used for single ended or balanced connections. Another common type of connector is an RCA connector. They can only be used with single ended connections.
In the Tube Repro it happens that we can take a single ended signal (only) out at the second stage of the circuit. This yields a -10dB signal (50dB of gain from a typical tape head), suitable for most home audio gear. The output impedance is a little on the high side at 4000 ohms, so this output may not be best to connect to a passive preamp or a load of less than about 15K ohms. The reason we offer this is that the circuit before this output is direct coupled and there is only one output capacitor in the signal path at this output. Theoretically that means less stuff in the signal path to affect the sound and hence potentially higher resolution. In some systems with enough gain in the budget and with a relatively high input impedance for feeding the Repro into it should work quite well if cables are kept to a reasonable length. In fact that is how I am currently running my own Repro, into a new single ended preamp circuit we are developing.
The balanced output on the Tube Repro comes after that single ended output cap and includes an additional triode and output transformer that can run floating balanced out (meaning the shield is connected only to the chassis, not the transformer secondary). That output is there for 15dB additional gain, up to the +4db standard for studios, and to lower the output impedance to a studio standard 500 ohms so that very long cables can be driven. It also helps when a lower output playback head is used.
Now when you say quiet, again, we need some clarification. Quiet means a lack of noise. But what kind of noise? Hiss? Hum? One must define this before one can discuss the subject at any level of confidence.
Tube hiss is as I already mentioned due quite often to 1/F noise and it will be noted in relation to signal level, i.e. the louder the signal the lower the tube noise in relation to it. So the solution to this is quiet tubes and a preference for a higher output playback head when possible. Single ended vs. balanced outputs used on the preamp won't make any difference.
If the noise we are discussing is hum, then the higher gain outputs can raise the level of any hum coming into the inputs of the repro amp. This can be 60Hz, generally created by AC magnetic fields of power transformers, or 120Hz and higher, generally of electrostatic nature and typically due to ground loops or bad signal or shield grounds. If there is hum coming in ahead of the Repro, trimming the output level of the Tube Repro balanced out down 15dB will result in a similar level of hum and 1/F noise as the single ended output since the later low gain triode stage and output transformer don't contribute anything significant noise-wise. In theory XLR type cables should be more quiet since shield is not usually tied to signal cold and thus external EMI is kept off the signal cold. But there are a zillion ways to rig cables, so there is no one right answer. But for certain getting quiet head cables is paramount to good reel to reel playback, just as it is with tonearm cables and LP playback.
It's also good practice to keep small signal tubes like EF86s away from power transformer magnetic fields, as they can pick up 60Hz this way. So I suggest not stacking a Repro on top of another piece of gear.
Input cabling is only one possible source of hum. The other big issue is ground loops, as audioblazer discovered. This is a particularly tough issue to work through because it is different in every environment. We can have a tape deck and preamp here in the lab that is dead quiet that will suddenly develop a 120Hz buzz in a different room with different wiring, and the only way to work out the issue is by methodically trying every possible combination of ground loop hum reduction. In Song's case we went thorugh a pretty exhaustive checkout of the cable connections to isolate the issue and then determined that in his particular case it was a matter of connecting the ground posts of the two units (that's why they are there, sometimes you need 'em). In some cases we have had to use a cheater plug on the tape transport power cord. And in some cases we have seen technicians fail to understand the that shield of the head cable should not be connected to the tape deck chassis when using an external preamp. We have also experienced a situation where a tech had an intermittent noise problem on their AC that was being generated by a neighbor in their business park - not much we can do to control that! I've also seen a case where the electrician alternated the sockets around a studio between two different circuits, which caused a slight buzz on some midrange drivers that took two years to track down.
This is really just scratching the surface of the subject. After having been the head noise troubleshooter on numerous high end installations at shows and in studios the one thing I am certain of is that every case of noise is solved just a little bit differently.
Oh and a quick final note: you can use both the single ended RCA output and the balanced transformer XLR output of the Tube Repro at once. If the RCA out is running into a relatively low impedance load it may cut the output level down a bit, but no harm will be done.
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