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Q&A on balanced vs. unbalanced amps.
Q1: Both VL20 and VS70 are fully balanced and especially designed for balanced signal transmission. Then it is mentioned that singled-ended connections will benefit from the circuitry. How? 

Q2: If I use XLR-to-RCA adapters to connect single-ended cables to the XLR sockets, will that achieve the same as connecting to the RCA sockets?

Q3: Some amps like McIntosh usually have two-stage differential voltage amps such that the XLR inputs can be easily converted to single-ended. How do the VS70 and VL20 do it? 

John Lam’s had to tackle Q2 and Q3 first.
To Q2: In an XLR interconnect there are 3 conductors, one for +VE, one for -VE and one for ground. In an RCA cable, there are only +VE and ground. In an XLR-to-RCA interconnect, the -VE conductor is simply discarded so this type of cable can be used. However it cannot convert RCA to XLR because it lacks a -VE signal. In principle the XLR-to-RCA cable and the RCA-to-RCA cable are identical. They may sound different because of different conductors and dielectrics.

To Q3: Regarding certain commercial vacuum tube amps with XLR inputs, not all are true balanced circuits. A true balanced power amp or preamp has to take a balanced input and offer a balanced output. The internal circuit must be fully balanced as well. Please refer to the following details.

Figure 1 at right shows the balanced and unbalanced amps.

Figure 2
shows how an unbalanced amp takes an balanced inputs.

To Q1: Figure 3 below shows how noise affects balanced and unbalanced amps. For unbalanced amps in figure 3a and 3b, we can see noise at the output. A fully balanced amp tends to cancel common-mode noise and is free of noise as shown in figure 3c. Even as an unbalanced signal is fed to the balanced amp as shown in figure 3d, the output noise is still lower than in figure 3a.

Therefore an unbalanced signal will still benefit from a truly balanced amp by suffering less noise than in an unbalanced amp nd by sounding better in a symmetrical circuit.

If unbalanced mode is selected in the VS70 and VL20, they only amplify the +VE signal. The -VE signal is discarded. Since +VE and -VE are identical except for polarity, there is no loss of information. Gain simply becomes different for balanced and unbalanced inputs.

Q4: How does the bal/unbal toggle switch operate on the VL20 and VS70? Are they switching input connections or the operational mode of the circuit?

The XLR pin configuration is pin 1 ground, pin 2 +VE, pin 3 –VE. The toggle switch selects between BAL or UNBAL inputs. In UNBAL, two things happen. The -VE conductor (pin 3 of the XLR socket) is grounded and the -VE input of the preamp or power amp is grounded. See Figure 4 below.

Q5: When XLR connected but set to UNBAL, the amp still operates. When connected to RCA and set to BAL, it also does. Some amps won't tolerate this and produce noise as for example NuForce. Why is that and what does it imply?

When connected through XLR but set to UNBAL, see my answer to Q4. Since the +VE signal remains connected to the preamp or power amp, it is amplified.

As the -VE conductor is grounded, it may affect the source because the -VE signal is shorted while operating. Thus XLR must be set to BAL. When connected RCA set to BAL, the -VE input of the amplifier sits idle. It's then best to connect it to ground to not pass unwanted noise. We thus recommended to set the toggle to UNBAL for RCA inputs.

Q6: The switch can be toggled back and forth under signal yet there is no popping noise. Why?

When properly designed, there needn't be any noise.

Q7: If source to preamp is RCA set to UNBAL, preamp to power amp XLR set to BAL, what happens? Would it be better if both preamp and power amp were unbalanced throughout?

It is best to be fully balanced throughout. However, there is no problem with an RCA connection between CD player and preamp. Preamp and power amp should always be connected balanced. If some CD player's RCA mode sounds better than XLR mode, it is better to use the RCA connection. That's why our preamp offers this flexibility.

On the VL20's back panel, you have a printed warning to not connect XLR and RCA simultaneously to the same input. What would happen?

As mentioned in my answer to Q5, when XLR connected but set to UNBAL, the -VE signal conductor is shorted to ground.

Q9: What about the VS70? Is it safe to connect both XLR and RCA from the VL20 to the VS70 and use the toggle switch to compare BAL and UNBAL performance?

Do not connect XLR and RCA simultaneously for the same reason. The VL10.1 preamp can take both RCA and XLR at the same time but its design is a lot more complicated than the VL20. Since the VL20 is a low-cost version, we simplified the design.

Q10: Somewhere it is mentioned that the XLR and RCA outputs of the VL20 can be used for bi-amping. Will the two pairs of outputs be different? Would you recommend XLR for the low-frequency amp and RCA for the high-frequency amp?

Yes, the VL20 can be used for bi-amping. As shown in figure 4(a) above, the RCA output lacks the -VE signal. Therefore there is 3dB less gain. For power amps with a gain setting to adjust their input level, the VL20 can drive two power amps at the same time.  Since the balanced mode has better dynamics and transient response, it is preferable for the bass amp.