Analog Summing: What It Does and When It Pays Off

• 05.09.2026
• Michael Krusch
• Story
• Basics
• TSM

You finish a mix in the DAW. Balance is right, nothing is harsh, the low end is under control. And still the whole thing sits between the speakers like a photograph: clean, correct, flat. Somebody in a forum tells you the cure is analog summing. Somebody else tells you that is the most expensive placebo in pro audio. Both of them are sure.

We build an analog summing mixer in Berlin, the Tube Summing Mixer, and the first magazine reviews of it date from 2006. So this is the explanation from the inside, including the part that sales pages usually skip: what summing is, why "digital summing sounds bad" is the wrong explanation, what actually changes when you sum through tubes and transformers, what reviewers have heard since 2006, when a summing mixer pays off and when it does not, how many channels you really need, and how to set the whole thing up without ruining your recall.

In this article: What analog summing is · Why "digital summing sounds bad" is the wrong explanation · What analog summing actually changes · What it sounds like: what reviewers heard · When analog summing pays off, and when it does not · How many channels you need · Analog summing with the Tube Summing Mixer: converters, levels, recall, inserts

What analog summing is

Every mix ends with the same operation. Twenty, forty or a hundred tracks are added together into two channels, left and right. That addition is called summing. On a mixing console it happens on the mix bus: the outputs of all channels are combined electrically at a summing point and the bus amplifier turns the result into the stereo signal that goes to the recorder. In a DAW the same thing happens as arithmetic. Sound On Sound's technical editor Hugh Robjohns put it in one sentence back in 2004: "In a digital system the same thing is achieved by adding corresponding sample values together."

A summing mixer is nothing more than the mix bus of a console taken out of the console and put into a rack unit. It has inputs, usually eight, sixteen or more, and one stereo output. No preamps, no EQ, often not even faders. You keep mixing in the DAW, with all your plug-ins, automation and edits, and send the tracks or stems out through your converters. The summing mixer adds them in the analog domain and you record the stereo sum back into the DAW. That is the entire idea.

Two families exist. Passive summing boxes combine the signals in a resistor network and lose a lot of level in the process; Sound On Sound's 2012 group test describes the Roll Music Folcrom as "a passive device, and needs its outputs to be amplified by a mic preamp to bring them up to a healthy signal level." That has a consequence: with a passive box, a large part of the character comes from the preamp you choose to put behind it. Active summing mixers have their own gain stage after the summing point. The Tube Summing Mixer belongs to the second family: analog mix stages and an active output stage built with tubes and transformers, so the unit delivers line level on its own and the character comes from inside the box.

One thing to know before anything else: a sum is louder than its parts. If you send twenty tracks at the same level into any bus, analog or digital, the result is hotter than any single track. Music is not a stack of identical sine waves, so the level does not rise by a fixed 6 dB every time you double the number of channels; with unrelated signals it is closer to 3 dB per doubling. But it rises. A console needs headroom on its bus for exactly that reason. The Tube Summing Mixer's internal mix bus handles up to +22 dBu, and the stepped output switch on the front exists to bring the sum back down to a sensible level for your A/D converter. More on that in the last section.

Why "digital summing sounds bad" is the wrong explanation

The oldest argument for summing mixers goes like this: DAWs add numbers, adding numbers loses something, so the mix inside the computer is degraded and an analog bus repairs it. It is a tidy story. It is also not true, and we would rather tell you that ourselves than have you find out in a null test.

Robjohns again, in the same 2004 answer: "There is nothing necessarily wrong with digital summing" and, done properly, it "is technically equal or superior to analogue mixing." He traces the reputation to early DAWs that mishandled the large numbers that appear when many samples are added, and to "name" engineers being reported in the press as preferring analog mixing, which is how the urban myth settled in. Modern DAWs calculate the mix bus in 32-bit floating point or 64-bit double precision, far more internal resolution than any converter can deliver. If your in-the-box mix sounds flat, it is not because the software added your tracks incorrectly.

That does not mean the two paths are the same. In 2021 Nicholas Messitte at B&H sent a 1 kHz test tone through a Dangerous Music 2-BUS LT summing mixer and inverted the result against the untouched digital tone: the two "cancel each other down to -38 dB" instead of nulling completely. Different, measurably, before a single note of music was involved. But the difference does not come from the addition. It comes from everything that happens to the signal on the way to the addition and after it, and that is what the next section is about.

Michael Krusch, who designed the Tube Summing Mixer, has said the same thing to Amazona.de in fewer words: "Whatever works for someone is right. Unfortunately the discussion is partly led by people who have never tried both variants side by side. […] My recommendation to everyone: try it. Maybe you'll discover something new." (translated)

What analog summing actually changes

Four things change when you leave the box, and it helps to keep them apart, because only one of them is the reason to buy a summing mixer.

The electronics. Tubes and transformers are not linear. They add harmonic distortion, mostly low-order, and the amount grows with level. Transformers behave differently at the bottom of the spectrum and on fast transients than a straight wire does. A tube stage driven near its comfortable maximum rounds off the very top of a peak instead of passing it through untouched. Sound On Sound's group test describes it plainly: "many models use valves and transformers, which have the effect of adding harmonic distortion in much the same way as vintage consoles and tape machines do." Robjohns calls it the "'rich sounding' harmonic distortion and benign transient clipping that you can achieve by slightly overdriving an analogue mixer." This is the part that is worth money. It is a coloration, chosen on purpose, and it happens at the one point in the chain where the whole mix meets at full headroom.

The converters. Analog summing means one extra D/A conversion per channel and one extra A/D conversion on the way back. Good converters are close to transparent. Cheap ones are not, and they are the more likely explanation for a "different" sound in a low-budget setup than the summing box itself. The converter count is also the hidden cost of summing: sixteen channels need sixteen D/A outputs.

The level. An analog return path almost never lands at exactly the same level as the in-the-box bounce, and a mix that is a fraction of a decibel louder sounds better to almost everyone. Messitte's own converter channels are matched to within 0.02 dB; that is the level of care a comparison needs. If you compare without level-matching, you are testing your volume knob.

Your ears and your expectations. You paid for it, you patched it, you want it to work. That is human, and the only defense is a level-matched blind comparison, ideally with somebody else switching.

So the honest statement from a manufacturer is this: an analog summing mixer does not fix your DAW. It puts a chosen piece of analog circuitry at the point where your tracks become a mix, and lets that circuitry do what tubes and transformers do. Whether you want that is a question of taste and material, not of mathematics.

What it sounds like: what reviewers heard

The Tube Summing Mixer has been reviewed since 2006, by different magazines with different monitoring, and the descriptions overlap more than you might expect. None of the reviewers describes a different mix. All of them describe the same mix with the corners taken off.

"The result already sounds much more like the sound image you are used to from analog productions than the purely digitally played out mix. Your mix sounds as tight and crisp as before, just a bit 'deburred' and gently rounded at the corners" – Andreas Hau, Sound&Recording, November 2006 (translated)

"The individual signals appear contoured and lively and the sound image tastefully gains color and expressiveness. The spatial information becomes more apparent, which results in a deeper listening impression. The sound image appears more vivid and acquires more three-dimensionality." – Michael Nötges, Professional Audio, June 2007 (translated)

"The audio material loses corners and is significantly more compressed, especially in the midrange, than without analog summing. Short, percussive sounds are only slightly decanted and experience a comparatively small change in sound. On the other hand, pads or already heavily compressed material experiences a significant increase in pressure, it becomes softer and more balanced in frequency. It is also worth mentioning that the sound coloration does not affect the treble or bass range." – Axel Ritt, Amazona.de, January 2023 (translated)

Three details in those quotes matter for the decision below. First, Ritt locates the effect in the mids and on dense material; short percussive sounds pass almost unchanged. Second, Ritt also reports that the coloration does not reach the treble or the bass, which is exactly what people fear from tubes. Third, Nötges reaches for depth, space and three-dimensionality, not for "warmth" in the sense of a treble roll-off. That matches the design goal. In Michael's words to Amazona.de: "It has to sound. Not a clean unit, but transformers and tubes that add something positive to the overall sound, like all our units. Well balanced – so not a tube distortion box either, but the gilding, high-quality, refining tube sound that people love." (translated) The full review is summarized in Amazona.de reviews the TSM analog summing mixer, and if you would rather hear it, David Vignola walks through the unit in his video review.

When analog summing pays off, and when it does not

It pays off when your mixes are finished and still feel two-dimensional. If balance, EQ and dynamics are where you want them and the only complaint is that everything stays glued to the line between the speakers, you are describing exactly what the reviewers above describe as the change. Dense productions profit most: pads, layered guitars, stacked synths, material that is already compressed. Sparse acoustic recordings change less, because there is less for the electronics to grip.

It pays off when you want a fixed color you never have to think about again. A summing mixer without channel controls is set once and then simply part of the studio, like the console bus used to be. Every mix passes through it, every mix gets the same treatment, and nothing needs to be written down.

It pays off when you already own analog hardware and want a real center for it. Tracks or stems that go out to a compressor or an EQ anyway can come back into a summing mixer instead of into the converter, and the whole analog part of your chain ends in one stereo return.

It does not pay off when the mix has problems. A summing mixer does not fix a bad balance, a boomy low end or an arrangement with too much in the midrange; it makes those problems slightly rounder. Some of the best-known mix engineers in the world work entirely in the box, and a piece of hardware does not close the gap between you and them. Skill does.

It does not pay off when the converters are not there. If you have a two-output interface, you have nothing worth summing. If you plan to buy a summing mixer and feed it from an eight-output interface that cost less than a good microphone, the interface will define the result more than the summing mixer. Budget the converters first.

And it does not pay off if what you actually want is a console. If you find yourself wishing for faders, pan pots, aux sends and monitor paths, buy a console. A summing mixer is the bus without the desk, on purpose. Why the Tube Summing Mixer has exactly one switch is explained in The Single-Knob Mix.

How many channels you need

The channel count is decided by two things: how many converter outputs you have, and how far you want to keep your tracks apart before they meet.

Eight channels means four stereo stems. That is how most people start, and it works: drums and bass on one pair, guitars and keys on the next, lead vocal on the third, backing vocals and effects on the fourth. The catch is that each stem is already a small mix that the DAW summed for you. The analog stage then meets your instruments already grouped, not one by one. It is the cheapest way in.

Sixteen channels means eight stereo stems or a mix of stereo and mono paths, and that is where most of the classic console groups fit: drums, bass, guitars, keys, lead vocal, backing vocals, effects returns, and one spare. The individual instruments are still grouped, but the groups are small enough that the analog path sees each family of sounds separately.

Thirty-two or forty channels means individual tracks, or close to it. Now the summing mixer really is the bus of a console: every track arrives on its own path and the addition happens entirely in the analog domain. The Tube Summing Mixer has 2×20 channels: 2×16 balanced inputs on four sub-D connectors in Tascam layout, plus 2×4 balanced inputs on stereo TRS jacks. If a project needs more, several units can be cascaded, which also allows analog summing of surround mixes.

Two practical rules follow. First, count your D/A outputs before you count summing channels; a forty-channel summing mixer behind an eight-output interface is a forty-channel summing mixer with eight channels. Second, if you only have two or four signals to combine, you do not need a summing mixer at all. What you want then is a stereo unit on the mix bus, for example the Creme bus compressor or the Vari Tube Compressor, and your DAW does the adding.

Analog summing with the Tube Summing Mixer: converters, levels, recall, inserts

Routing. Each track or stem in the DAW gets its own hardware output. The converter outputs go to the inputs of the summing mixer, the stereo output of the summing mixer goes back into two converter inputs, and a stereo track in the DAW records the sum. On the Tube Summing Mixer, mono sources are handled without a Y-cable: plug into only the lower TRS jack of a channel pair and the signal is automatically fed to both channels, so it sits in the phantom center.

Levels. A common convention is to calibrate the converters so that around -18 dBFS in the DAW corresponds to 0 VU, or +4 dBu, in the analog world; -20 dBFS is just as common, and your interface's manual tells you which one applies. Keep the individual outputs conservative. Peaks around -12 dBFS on the stems are a safe starting point, but with many channels the sum climbs, so let the meters tell you where you are, not the rule of thumb. The output switch reduces the sum in fixed -3 dB steps, and the two illuminated VU meters on the front sit behind that switch: they show the output level, which is exactly what your A/D converter receives. Find the setting that fits your music and then leave it there.

Recall. The classic objection to analog summing is that it breaks total recall: open the session next month and the channel knobs are not where they were. The Tube Summing Mixer answers that by not having any. No faders, no pan, no per-channel gain, just the stepped output switch. In Michael's words: "Total recall. You mix in the DAW and sum in the TSM. No watering-down of the summing concept with channel controls and the like. One rotary switch to match the overall level to the setup. Put it in the rack, wire it up and never touch it again." (translated) Whether you think that is a limitation or the whole point tells you whether this unit is for you.

Inserts. The Tube Summing Mixer has no insert points, and that is a consequence of the same idea. Hardware goes into the chain in one of two places: before the summing mixer, between the converter output and the input, on a single track or stem, or after it, on the stereo sum, where a bus compressor or a tube compressor does its work on the finished mix. Both ways keep the signal in the analog domain from the first converter to the last.

Comparing. If you want to know what the unit does for your music, record the analog sum, bounce the same mix in the box, match the two by level, and switch blind. That is the only test that counts, and it is the one we ask you to do. That is why every Tube Summing Mixer can be tested for 14 days in your own studio, with your converters and your mixes.

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