Balanced or single-ended inputs: what matters in a tube system

Most tube amplifiers and DACs arrive with RCA sockets, and many can be ordered with XLR as well. The two connectors carry the same music in different ways: an RCA lead is a single-ended, or unbalanced, connection, and an XLR lead is usually a balanced one. This guide explains what the difference does, when it matters in a home system, which Demograf Audio models have which connections, and what to check before you choose.

“Single-ended” has two meanings in audio. Here it describes a connection between two components. A single-ended amplifier is something else: an output stage with one tube per channel. Either kind of amplifier can have either kind of input.

What a single-ended (RCA) connection is

A single-ended connection uses two conductors: the signal wire in the centre and the screen around it, which is also the return path and is tied to the ground of both components. The receiving input measures the signal against that ground. It is simple, needs one amplifying path per channel, and has been the standard for home audio for decades.

Its weak point is the shared ground. Any interference that reaches the signal wire, and any small voltage that appears along the screen because current flows in it, is added to the music and amplified with it. On a short lead between two components on one rack this is usually too small to hear. On a long lead, near mains cables or power supplies, or in a system with several earthed components, it can become a hum or a buzz.

What a balanced (XLR) connection is

A balanced connection uses three conductors. Two of them carry the signal, the second in opposite polarity to the first, and the third is a screen that carries no signal. In a three-pin XLR plug the convention (IEC 60268-12) is pin 1 for the screen, pin 2 for the positive (“hot”) leg and pin 3 for the negative (“cold”) leg.

The receiving input responds only to the difference between pins 2 and 3. Interference that reaches the cable reaches both legs almost equally, so it appears as the same voltage on both and largely cancels at the input. This is called common-mode rejection, and it is why studios and concert halls run balanced lines over tens of metres.

Two points are often missed:

• The connector does not make the circuit balanced. Rejection depends on the two legs having matched impedances at the output, in the cable and at the input. An XLR socket wired to a single-ended circuit is still single-ended.
• Levels differ. When both legs carry the full signal, the balanced output of a device is commonly twice the voltage of its single-ended output. Switching from RCA to XLR on the same pair of components can therefore raise the volume at a given knob position; compare them at matched levels.

Balanced inputs and tube amplifiers

A single-ended tube amplifier is single-ended inside: each channel has one signal path from the input tube to the output transformer. To accept a balanced signal it must convert it at the input, either with an input transformer, which responds to the difference between the two legs, or with an active differential stage. A fully balanced, or differential, design keeps the two halves of the signal separate through every stage, which needs two of every stage per channel.

So an XLR input on a tube amplifier is an extra stage or an extra transformer, chosen and built for that amplifier. For our models the XLR input is an option agreed before the build; tell us what your source has and we will say how it would be done in the model you have in mind.

Which Demograf Audio models have which connections

The table lists the line-level connections from each model’s page. “To order” means the connection is fitted when it is agreed in the quotation.

The Hector preamplifier comes with RCA inputs and outputs, and further RCA or fully balanced XLR inputs and outputs can be added to order. In its passive transformer version it adds no gain, which matters with sources of about 2 V; our guide to passive and active preamplifiers explains why. The amplifiers are on the pages of Endymion, Ether, Helios, Atlas, Solaria, Eurybia, Tantal and Tantal Senior; the converters on those of Prometheus, Reference DAC, Hyperion, Hades and Bellerophon.

Cable length

Length is the clearest reason to choose one or the other.

• Components on one rack, leads of a metre or two. RCA is the usual choice, and a balanced connection has little interference to remove.
• A long run. When the DAC or preamplifier sits by the listening chair and the amplifiers stand by the loudspeakers, the interconnect may be several metres long and pass mains cables on the way. This is where a balanced line does its work.
• Monoblocks. Two amplifiers placed beside their loudspeakers usually mean two long interconnects, one per channel, rather than long loudspeaker cables.

With RCA, keep the leads as short as the layout allows, and keep them away from mains cables, or cross them at right angles.

Hum, buzz and the ground

A steady hum at mains frequency in a system is most often a ground loop. Two components are each earthed through their mains plugs and also joined by the screen of an interconnect, so the screen becomes part of a loop in which a small current flows. A balanced connection reduces the effect, because the screen carries no signal.

Before changing connectors, try the simple steps: plug all components into the same mains outlet or distribution block, re-route interconnects away from mains cables, and disconnect sources one by one to find the one that brings the hum. Endymion, Ether, Helios, Atlas, Solaria, Eurybia and Tantal have a ground-lift switch for this. Never remove or tape over the earth contact of a mains plug: it is there for safety.

Mixing XLR and RCA

Systems often mix the two, and adapter leads exist in both directions.

• RCA output into an XLR input. The RCA centre goes to pin 2, and the RCA screen to pins 1 and 3. The connection works, but it is still single-ended, so it brings none of the rejection of a balanced line.
• XLR output into an RCA input. Pin 2 goes to the RCA centre and pin 1 to the screen. What to do with pin 3 depends on the output stage: some balanced outputs expect pin 3 to be grounded, others must not have it grounded. Check the manual of the source before you use such a lead. The level drops to about half the balanced level.

What to check before you choose

1. Look at the source. If your DAC, phono stage or preamplifier has only XLR outputs, order the XLR input. If it has both, RCA is usually enough on one rack.
2. Measure the run. Note the cable length between source and amplifier in your room, and whether it crosses mains cables.
3. Count the earthed components. Several mains-earthed components in one system make a ground loop more likely.
4. Plan for the next component. A balanced input fitted before the build costs less effort than one added later.

Questions

Does a balanced connection change the sound?

It changes how much interference the cable lets through. Where there is hum, buzz or noise from the interconnect, a balanced line can remove it; where there is none, it has nothing to remove. Compare RCA and XLR at matched levels, because the balanced output is often louder.

Can I connect a DAC with XLR outputs to an amplifier with RCA inputs?

Yes, with the right adapter lead or with an XLR input ordered on the amplifier. Check in the DAC’s manual how its balanced output may be connected to an unbalanced input.

Can an amplifier be fitted with XLR later?

Write to us with the model and its year. It is simpler to agree on the XLR input before the build, while the chassis layout is still open.

Write to us by e-mail or WhatsApp with your source, the distance between the components and a photo of your rack, and we will tell you whether RCA is enough or an XLR input is worth ordering.