Summ-thing for Everybody: DIYRE SB2 Passive Summing Mixer

Today, I'm going to tell you about an inexpensive way to get into analog summing, provided you already have some other necessary gear in your studio.  If you've looked into analog summing in the past, the word “inexpensive” probably isn't the first to come to mind, but by using passive components and outsourcing the assembly process to you, DIY Recording Equipment (DIYRE) has managed to produce a high quality summing mixer for the rest of us.  


What is Analog Summing Anyway?


Before we dive into an explanation of analog summing, it's helpful to understand what summing is.  Summing is the process by which all of the tracks and busses in a song are combined into one stereo track.  This may sound a lot like the definition of “mixing,” but summing is a different process than mixing.  Mixing is the work that a mixing engineer performs through level adjustments, panning, equalization, and so on in order to make a mix ready for release.  Summing is simply the work that your DAW or console does to turn the different tracks that you have prepared with EQ and other effects into one stereo track. 


For many people, the summing process occurs completely in-the-box and happens while bouncing down a final mix.  You can choose to bounce it down in real time or expedite the process, but in either case your DAW is crunching 1’s and 0’s to produce a stereo file.  This is digital summing. 


Analog summing is another way of producing a stereo track and would traditionally happen through the use of an analog mixing console bouncing down multiple tracks to a two-track tape machine.  A traditional mixing console has many different components that are not related to summing -- preamps, equalizers, and pan pots, etc.  These features are helpful for a recording studio to have nonetheless, because they enable recording and mixing. Because analog summing mixers don’t come equipped with these features which are found in large format consoles (preamps, EQ, etc.), they are sometimes jokingly referred to as “mixers that you can’t mix through.”  This is perhaps an important consideration when looking into summing mixers for your studio: they are designed strictly for hybrid mixing setups (sometimes called “out-of-the-box mixing”) where most of the mixing is done in the box, but stereo groups are sent to the summing mixer and bounced down to a stereo track.  If you want a full-featured analog console, look elsewhere. 


It might also be helpful to mention what analog summing does in terms of sound.  As I’ve said -- your DAW can already perform summing.  If digital summing gets the job done, why would anyone care about analog summing?  Proponents of analog summing often talk about certain sound qualities that can only be achieved in the analog domain.  Using analog summing, the mix opens up in a way that can’t be achieved digitally and the different tracks seem to have a bit more separation, depth, and what is often referred to as a “3D” quality.  While they can provide a nice touch to your tracks that you wouldn't be able to achieve otherwise, the effects of analog summing are all-in-all rather subtle.  Compared to digital summing, we are certainly not talking about a night and day difference, or even the 5 PM/7 PM difference that is sometimes likened to different microphone preamps.  In terms of workflow, you can route your groups through a summing mixer at the end of your mixing process, but it is probably more helpful to set it up at the beginning of a mixing session so that you can mix through it.  


Is Analog Summing Really Worth It?


There are some downsides to having a summing mixer, which we will cover before talking about the benefits.  In terms of workflow, using a summing mixer adds an extra step to the mixing process and can be a pain to work with if you want to go back and tweak your mix after recording the output of the mixer.  From both the hardware and software sides, it takes a few minutes to set up a summing mixer, and you will always have to record the output of the mixer in real time.  If you do not have a large number of hardware outputs on your audio interface and you want to use hardware effects in addition to the summing mixer, you may have to make compromises in the number of busses you can send to the summing mixer.  If you’re like me and you like to print out different versions of a mix, the time spent waiting for a mix to print in real time adds up when multiplied by dozens of mixes.  It may not seem like a huge deal to wait a few minutes for a mix to print, but it can become an annoyance if you are used to the speed of digital summing.


There is also a lot of equipment needed to use a summing mixer.  For instance, if you have a passive summing mixer (like the SB2), it won't be able to provide enough gain to bring your mix back up to an appropriate level.  You'll need a pair of mic preamps for that, and preferably ones with stepped controls.  This can represent a costly, and otherwise unneeded, upgrade for some studios.  There are more hardware considerations if you want to set up a passive summing mixer -- what if your converter or interface only has two physical outputs for your monitors?  You'll need at least 8 for many summing mixers, and the effects of analog summing are cumulative and more apparent with higher channel counts -- so 16 outputs and up may be ideal.  Again, an expensive upgrade may be in order.  What about all of the cabling needed to hook everything up?  You can see how there can be many obstacles to setting up even a simple passive summing mixer and the cost of all the extra equipment can quickly balloon far beyond the cost of the actual mixer.  Part of the reason I hopped on the analog summing train is that I already had a lot of the necessary gear lying around.  If you are happy working with an audio interface that doesn’t have the slew of analog outputs needed to operate a summing mixer, or do not see a good reason to buy a pair of stepped preamps, then maybe the cost of upgrading isn’t worth it. 

Signal Flow Chart for the SB2.

Signal Flow Chart for the SB2.

Considering all of the required hardware, combined with the fact that, with the advent of digital summing, analog summing is no longer necessary, it isn’t surprising that many choose to relegate the task job of summing to a DAW.  While critics would say that digital summing is cold, lifeless, and doesn't have the “3D” quality that analog summing does, they're missing something important.  Digital summing is super utilitarian, and accomplishes the task of summing with little to no fuss.  Plus, if you want to stay in the box, but bemoan the fact that your tracks aren't running through a vintage Neve console, there are even some plugins out there that claim to reproduce the effects of analog summing.  I have tried out Slate VCC in particular, and in my opinion it sounds quite good.


Of course, there will always be purists, and some people insist that analog summing sounds better no matter what, and that no plugin will ever be able reproduce its sound.  There are plenty of people on both sides of this debate, but in the interest of helping you to make an informed decision, I’ll tell you about my personal experience and thoughts on analog summing.  Since I started using the SB2 passive summing mixer, I have noticed more separation and clarity in my mixes. Being able to pair it with different mic preamps for different sounds offers versatility, and, although subtle, has a greater effect on the sound than I would have guessed.  There are even more tonal variations when I drive my preamps, and connecting them directly to an outboard bus compressor is often what I need to glue a mix together and give it a finished quality. I've noticed that using my CAPI VP-28s to bring my mixes back up to level does something to the low end that I don't think I could achieve digitally.  There is a certain weight that good analog gear imparts to audio that plugins don’t quite replicate, and for anyone who doesn't want to compromise on their mixdown chain, I'd have to recommend a summing mixer and some quality preamps to help you achieve this effect. Of course, it is up for debate how much of this sound can be attributed to the preamps and compressor, and how much is truly due to the SB2.


In the end, there are pros and cons to analog summing, and people ought to weigh them on an individual basis.  You can certainly make great mixes while staying 100% in the box, and there are many amazing mixing engineers such as Dave Pensado who mix entirely in the box.  It should come as no surprise that you’ll get way more mileage out of honing your mixing technique than from buying a summing mixer and that using a summing mixer will not fix a bad mix, or bad mixing technique.


Putting It All Together


So far we've talked about summing in pretty general terms while skirting around the subject of this post: the DIYRE SB2.  Although building your own summing mixer may sound like a daunting task, the SB2 is an incredibly simple design that would make an ideal first DIY project.  Because it is passive, you will not have to worry about building and testing a power supply.  Plus, unlike some passive summing mixers, there are no pan pots, further simplifying the build (panning with the SB2 is accomplished in the box).  Don’t, however, mistake the lack of features for a lack of quality.  Barebones as it is, the SB2 sounds great and opens up a realm of sonic possibility that may have previously been out of reach for many home and project mixing engineers. 


In terms of assembly, there is a case, PCB, and a few parts.  The build manual provided by DIYRE is comprehensive and easy to follow, and if you make a mistake their customer support is nothing short of fantastic.  The hardest part of the build is soldering the DB25 connectors in place because there are many points to solder and they are close together.  This is the only potential snag I see for beginners, but if you are attentive to detail and patient, it shouldn’t come a huge challenge (you may want to pick up a magnifying glass for your soldering station if you are feeling nervous).  


As with all DIY gear, the real magic of the SB2 is two-fold -- saving money (ok, maybe that part’s more boring than magic), and inspiring you to learn and grow as an audio engineer.  If you’ve never built a piece of recording gear, I promise it will help you grow your confidence and make you a better audio engineer. For those reasons alone, you should consider the SB2 summing mixer.

Classic Audio Products Inc. VP-26

How I Learned to Stop Worrying and Love Saturation


I remember the first time I used the output knob on a microphone preamp.  I was interning at a recording studio built into a old farmhouse right next to the historic Antietam battlefield.  Although I try to make recordings that transcend place, I am also a firm believer in the power of location to shape performances and inform creative decisions.  Watching the fog nestle itself among the blades of grass in the morning helped me clear my head and prepare myself for the day.  Sometimes it felt like I worked in a place untouched by humanity, as ironic as that statement might seem.


A freshly minted CAPI VP-26.

The studio owner, for whatever reason, liked to keep his preamp tones as clean as possible and kept the output knobs in the furthest clockwise position.  This was around the same time I was trying to understand what saturation was, and the myriad ways in which it could be used.  It was hard to separate the facts from the marketing hype surrounding all of the products promising that magical “warmth,” “fatness,” “glue,” and the other buzzwords that are used to describe the qualities of analog gear.  Looking at the different plugin offerings, it was hard to discern whether this saturation came from tape, tubes, transformers, or some combination -- and whether the magic could really be reproduced digitally.  All I knew was that dirtying my tracks in some way could make them sound more like a record.  How this happened exactly was a mystery to me.


One day, I was recording acoustic guitar by using a Royer 121 pointed at the bridge, and a small diaphragm condenser aimed at around the 12th fret.  The musician was playing a folk tune with lots of intricate picking.  The R121 was picking up the body and warmth of the guitar, while the small diaphragm condenser captured the detail.  I turned the gain up on my preamps until I had a good level.  Everything was there, but something unquantifiable was missing.  I couldn’t place it until I trimmed the output knobs on my preamps and drove the gain some more.  The signal thickened in an incredible, musical way, and the harmonics came to life.  There it was -- the sound of a record.  Vibe.  Character.  Mojo.  These things that I was convinced were just bullshit marketing terms suddenly became real and meaningful.  I felt like I’d stumbled onto some deeply held secret about how to properly make records.  Needless to say, this experience will forever be one of my favorite, most eye-opening moments as a recording engineer.


How Does It Sound?


The preamps I used that day were CAPI VP-25s...and yes, I know this post is supposed to be about the CAPI VP-26.   Bear with me.  Classic Audio Products, Inc. -- or CAPI, for short -- is a company that produces DIY versions of preamp, EQ, dynamics, and other modules that were included with vintage API consoles.  These consoles are commonly regarded as some of the best ever made -- up there with Neve, Trident, and SSL.  Moreover, with their punchy, mid-forward sound, they are the quintessential consoles for rock music.  Sourcing original API modules can be an expensive proposition, but luckily Jeff Steiger, the owner of CAPI, has gone to great lengths to clone these classic circuits for the 500-series format.  With detailed building instructions available online, it’s relatively simple and inexpensive to order a kit from CAPI and build one of these wonderful modules yourself.  


Back to the VP-25s I was using to record that folk guitarist: these preamps are nearly identical to the VP-26s, the only difference is that the VP-26s have a slightly smaller 2623-1 output transformer, which was used in API console preamps.  The rest of the circuit is identical.  What does this mean for the sound?  Tonally, the VP-26 is the more “vintage” sounding preamp, with the VP-25 representing a halfway point between it and the more modern VP-312, the latter of which is a clone of the legendary API 312.  Both the 25 and 312 feature the 2503 output transformer.  I have not done a head-to-head comparison with these preamps, but from what I’ve read, the VP-312 is slightly more open sounding, whereas the VP-26 is bigger and tighter in the lows and lower-mids.  For those wondering -- don’t worry, the VP-26 can produce that same euphoric saturation I got from the VP-25.


To date, I have built 5 VP-26 preamps, 4 of which reside in my home studio.  I’ve used them to record a ton of sound sources: vocals, overheads, drums, acoustic guitar, electric guitar, electric bass, and upright bass.  Considering that it is a clone of a console preamp designed for everyday use, it isn’t surprising that the VP-26 performed well in all of these applications.  It can produce a pristine sound that’ll have you opening up the module to check if you accidentally installed a John Hardy 990 opamp, or you can trim the output and drive the input gain for a killer retro tone.  I find it particularly useful on drums and acoustic guitar.  On the former, it delivers the punch and aggression that is so strongly associated with API preamps, and on the latter it produces a smooth, well rounded tone with plenty of transient detail.  As a vocal pre, the VP-26 doesn't always sound quite as upfront as I'd like, but does produce balanced, usable results.  When I first built a pair of VP-26 pres, I was upgrading from the pres in my Avid Mbox.  It's worth noting that in any application, the VP-26 is a major step up from the preamps included with most audio interfaces, which tend to be IC-based and offer a flat, colorless sound.  CAPI preamps certainly made me a believer in transformers and direct opamps, and I'm willing to bet they'd make a convert out of anyone looking for a more analog-sounding recording chain.


Build Review


As of writing this, I've built 5 VP-26 preamps, and 4 of them worked perfectly right out of the gate.  The one that didn't (my first) had a resistor in the wrong place, which, after the unit was plugged in for a few minutes, burst into a tiny flame.  Now I get why they call it the “smoke test.”  Anyway, there aren't too many parts in the VP-26, and there are build tips and a bill of materials (BOM) available directly from CAPI, as well as a build guide (with full HD photos!) from DIY aficionado Chunger.  Out of all the preamps I’ve built, the VP-26 is the least complicated.  Part of the reason for this simplicity is inherent in all 500-series modules -- no power supply is required, since the 500-series rack already provides power.  Another reason why the 26 is so easy to build is that it is a single stage preamp, as opposed to a two stage preamp.  Lastly, this is a simple, tried-and-true design that works as well today as it did in the 1970s.  Personally, when I build a VP-26, I like to print out the CAPI BOM, arrange my parts on top of it, then follow along with Chunger’s guide.  Be careful that you check the support documents on CAPI beforehand -- there are several revisions of this preamp so make sure you have the most up to date BOM.  Schematics and overlays are also available through CAPI.  

The author building a VP-26.

Although the VP-26 is relatively simple to build, if you're a true DIY novice it might be a better idea to start with something with even fewer parts.  If you've already built some preamps, it's almost guaranteed that the VP-26 will be easier by comparison.  You will probably find the hardest part of the build to be the direct opamp, of which CAPI offers several choices for slightly different tones.  There's the GAR2520 (a clone of the famous API 2520 opamp), as well as the GAR1731 (a clone of the Melcor 1731).  Or -- if you're like me and don't feel like building an opamp -- the Scott Leibers SL-2520 is available pre-assembled for an extra charge.  I haven’t tried the other op amp choices, but the SL-2520 performs perfectly and has a great tone.  In addition to having different opamp choices, you can also choose to build your preamp with variable or stepped input gain controls.  The stepped option costs more and requires you to solder a few more resistors, but it may be a worthwhile investment for those who need recallability.


If you're looking to get into the world of high quality, external preamps, and want something with serious vintage character to boot, the CAPI VP-26 is one of your best options, not to mention the most affordable.  Plus you get to work on your DIY skills while you're at it. For more information: