Unbalanced to Balanced Audio

There is a large number of things that amazes me on an almost daily basis.  To wit: a local mom and pop radio station called me because they couldn’t get their computer program to work right.  I decided that I’d give them an hour or two, in exchange for my hourly labor rate, and see if I could fix their problem.  The issue at hand was loud hum and other noise on the input source.  I knew before I even looked at it that the likely culprit was a ground loop.

It was worse than I imagined, with several unbalanced and balanced feeds improperly interconnected, line level audio going to a microphone level input and so forth.  I explained to the guy about putting line level into a mic level input, something akin to plugging a 120 volt appliance into a 240 volt outlet.  Improperly terminated balanced audio nullifies all of the common mode noise rejection characteristics of the circuit.

In any case, there are several ways to go from balanced to unbalanced without too much difficulty.  The first way is to wire the shield and Lo together on the unbalanced connector.  This works well with older, transformer input/output gear, so long as the unbalanced cables are kept relatively short.

simple balanced to unbalanced audio connection
simple balanced to unbalanced audio connection

Most modern professional audio equipment has active balanced input/output interfaces, in which case the above circuit will unbalance the audio and decrease the CMRR (Common Mode Rejection Ratio), increasing the chance of noise, buzz and so on getting into the audio. In this case the CMRR is about 30 dB at 60 Hz.  Also, newer equipment with active balanced input/output, particularly some brands of sound cards will not like to have the Lo side grounded. In a few instances, this can actually damage the equipment.

Of course, one can go out and buy an Henry Match Box or something similar and be done with it.  I have found, however, the active components in such devices can sometimes fail, creating hum, distortion, buzz or no audio at all.  Well designed and manufactured passive components (transformers and resistors) will provide excellent performance with little chance of failure.  There several methods of using transformers to go from balanced to unbalanced or vice versa.

Balanced to unbalanced audio using 1:1 transformer
Balanced to unbalanced audio using 1:1 transformer

Using a 600:600 ohm transformer is the most common.  Unbalanced audio impedance of consumer grade electronics can vary anywhere from 270 to 470 ohms or more.  The 10,000 ohm resistor provides constant loading regardless of what the unbalanced impedance.   In this configuration, CMMR (Common-Mode Rejection Ratio) will be 55 dB at 60 Hz, but gradually decreases to about 30 dB for frequencies above 1 KHz.

Balanced to unbalanced audio using a 4:1 transformer
Balanced to unbalanced audio using a 4:1 transformer

A 600:10,000 ohm transformer will give better performance, as the CMMR will be 120 dB at 60 Hz and 80 dB at 3 KHz, remaining high across the entire audio bandwidth.   The line balancing will be far better into the high impedance load.  This circuit will have about 12dB attenuation, so plan accordingly.

For best results, use high quality transformers like Jensen, UTC, or even WE 111C (although they are huge) can be used.  I have found several places where these transformers can be “scrounged,” DATS cards on the old 7300 series Scientific Atlanta satellite receivers, old modules from PRE consoles, etc.  A simple audio “balun” can be constructed for little cost or effort and sound a whole lot better than doing it the wrong way.

A brief list, there are other types/manufactures that will work also:

Ratio Jensen Hammond UTC
1:1 (600:600) JT11E series 804, 560G A20, A21, A43
4:1 (10K:600) JT10K series 560N A35

Keep all unbalanced cable runs as short as possible.  In stereo circuits, phasing is critically important, so pay attention to how the transformer windings are connected.

A broadcast console makers perspective

I received a great email from Michael “Catfish” Dosch, console designer for Telos / Axia Audio Systems.  The email was sent in response to a comment I posted on the WEBE WICC Studio Build Out post.  I though the email was very interesting and informative, presenting a perspective that most broadcast engineers do not often see or appreciate.  I asked Mike if I could use it as the basis for a blog post and he agreed.  I am not going to blockquote the entire thing, but here is the unedited email and pictures.

Quote:

“Ken said you had a concern about the ruggedness of our consoles as compared to the old PR&E boards. You might not know this, but I was with PR&E before joining Telos. In fact, I designed many of those old PR&E boards. I guess that makes me an old console designer. Ahem.

The Element design is more modern in construction and styling, but it is no less rugged than those old PR&E boards. In fact, you could stand on it if you wish. The top is 1/4 inch machined aluminum plate supported by structural aluminum ribs on the backside. The chassis itself is made of custom extruded aluminum structural pieces, machined aluminum side panels. The flat sheet metal on the bottom is not structural, it’s only a cosmetic cover. You’ll see a lot of folded sheet metal in other consoles because it’s cheap and easy. But it’s not as rugged as the Element approach which is why we chose to go with a more complex and expensive mechanical design.

Telos Axia console cross section
Telos Axia console cross section, Courtesy of Axia Audio / TLS corp

One very visible difference between Element and a PR&E consoles is the use of lexan on the front panels (PR&E would use aluminum or steel on the top panel). This might seem less rugged, but it is actually chosen because it is a more durable surface than painted and silkscreened metal. It is more scratch resistant and it is rear-printed so that the markings never wear out. Silkscreens would wear off under heavy use — particularly next to faders and monitor controls — and look horrible over time. These lexan panels will look just as good after 15 years as they do now.

But lexan for all of its durability has its own limitations. The edges can crack under abuse. This is why you see many older Wheatstone consoles (they have used lexan overlays for many years) with cracks and tears at the very edges of the plastic. This is particularly troublesome in the fader slot. A frayed edge on a faders slot can cut your fingers. That is mighty unpleasant! So when we decided to use lexan, we wanted to have all the benefits and none of the drawbacks.

So we designed a machined recess on each channel allows the lexan insert to have its outside edges protected by the aluminum. More obvious is the bezels around each button and even the fader slot. Look carefully and you will notice that all of the control bezels edges are above the lexan. The edges of the lexan are not exposed and therefore not prone to cracking, chipping or splintering.

Axia Audio console control surface, courtesy of Axia Audio / TLS corp
Axia Audio console control surface, Courtesy of Axia Audio / TLS corp

In this drawing, you can see the panel without the lexan. The machined pocket to protect the outer edges of the lexan, plus the raised edges of the button and fader bezels to protect the edges around the holes. These button guards are also designed to prevent accidental actuation of the buttons. And while the guards are designed to protect accidental actuation, they never hinder deliberate activation. Notice the guards at the sides of the ON/OFF buttons and not on the top and bottom. Even operators with long fingernails will have no problems with these controls. The small round keys are engaged with a light touch of the fleshy pad of the fingertip.

Yes, I think we built great consoles at PR&E. But Axia was a fresh start, a chance to raise the bar even higher, by retaining many of PR&E’s better attributes and improving upon some of the weaker areas. DIPswitch configuration has been replaced with the convenience of the web browser. Spill-prone motherboards and electronics have been eliminated from the control surface. Unreliable monitor pots have been replaced with optical rotary encoders rated for 5,000,000 rotations.

And you asked about the faders. This is a particularly important component in a broadcast console. PR&E used Penny & Giles faders for many years. We used their Series-4000 faders in the X-Class consoles (BMXIII, AMX, ABX and STX). This was their top-of-the-line fader at the time and performed beautifully… for a year or two. Then our clients started experiencing field failures at a very high rate. We worked with P&G on a return/rework/replace program that took years to clean up. Our clients were disappointed and we spent a fortune making things right. It was that experience that caused us to begin searching for alternatives.

The market for high-end faders is quite small. There are tons of consoles out there for live sound, home recording, etc., but these products are sensitive to costs and generally use very cheap faders. There just aren’t enough high-end recording consoles or broadcast consoles being built to attract a lot of fader vendors. After a lengthy search, I disqualified all but two fader companies: P&G and a Japanese firm by the name of Tokyo Ko-on Denpa (TKD). I assigned one of our engineers to create a set of environmental and life-cycle tests to see if the TKD faders could keep up with the P&G faders. We were all shocked by the results.

Out of 100 of each type tested in various environmental conditions and physically cycled for the accelerated equivalent of 10 years of heavy use, we had only one TKD fader failure, compared to more than half of the P&G Series 4000 faders! We defined “failure” as any deterioration to specifications or any discontinuities. All the failed units had discontinuities (audio dropouts). We were able to clean the failed TKD fader and it passed the retest. About half of the failed P&G units were cleaned and passed the retest. So in the end, the practical results were TKD 100% good and P&G 75% good. Not what I expected at all.

We then designed a TKD fader into the Radiomixer. We watched the customer support logs carefully for problems. Out of the first 1,000 console channels shipped, we saw one TKD fader failure during the first year. Warranty replacement of course. The failure rate did not increase with use as you would normally expect. We were seeing consoles with 3 or 4 or 5 years of heavy use with no fader problems at all. I have heard of 20 year old Radiomixers with original faders still working great.

One particularly elegant feature of the TKD fader used in Element is a side loaded wiper arm. This prevents liquids or other foreign matter from spilling into the fader slot and directly into the fader element. This feature alone is probably responsible for extending the useful life of the faders by a considerable amount. Of course, these can be disassembled and cleaned just like a professional fader from P&G, they just don’t need it so often.

Axia Audio TKD faders, courtesy of Axia Audio / TLS Corp
Axia Audio TKD faders, courtesy of Axia Audio / TLS Corp

Some have the misconception that if a fader is not P&G, it must be cheap. Actually, these are very expensive faders, about the same cost as P&G. But they are so well made, I think they’re worth every dollar. I know there are still some folks out there who remember P&G’s glory days when they made bullet-proof faders. I remember fondly those days as well. But in my experience, the TKD fader is superior to the equivalent P&G fader. We feel so confident, that we warrant all Axia consoles for 5 years, including all components….”

End Quote

That is a great explanation of what goes into one of these consoles right from the designer.  The pictures are courtesy of Axia Audio / Telos Corporation and special thanks to Mike for taking time out to give us a glimpse into the mind of a console manufacture.

Denon DN-950 FA Cart CD player

These were all the rage when they came out some 23 years ago or so. They were specifically made for DJs that were used to shuffling carts in and out of cart machines.  The idea was to use familiar motions and procedures so DJs could easily perform their shift using CDs without relearning studio dynamics.  The only downside, a DJ could remove the CD that was playing by accident whereas pulling a playing cart out of a cart machine is difficult to do.  Later Denon versions made it more difficult to remove playing CDs.

This is a promotions photo circa 1987.

Denon DN-950FA cart CD player
Denon DN-950FA cart CD player

This machine is still in use 22 years after it’s manufacture date.  Over the years the top cover has been removed countless times, no doubt to replace the KSS-210A optics and bearings or to periodically clean them.  The Phillips head screws are so worn a screw extractor is nearly required.

Denon DN-950FA back
Denon DN-950FA back
Denon DN-950 FA optics and platter
Denon DN-950 FA optics and platter

They are located under a circuit board, which has to be removed.  Again, the DN-951 series CD players did away with this, making maintenance easier.  These CD players could and often were affected by RF especially when the studios were co-located with an AM transmitter site.  One such symptom was randomly speeding up and slowing down while playing.  It made for some interesting sounding songs and even more interesting commentary by the morning show.

Every time the optics and bearing were replaced, there was a pretty involved alignment procedure that took some time to get right. I remember some funny Japanese to English translations in the service manual.

Of course, nowadays if there are any issues, you just chuck the computer and get a new one.

The Pacific Records and Engineering BMXII console

I snapped these pictures at WICC in Bridgeport, CT. It is an older PR&E BMXII console, 26 channels, I believe.

Pacific Recorders BMXII 26
Pacific Recorders and Engineering BMXII 26

These were manufactured starting in 1985, I installed one in 1990. It is a testament to their durability that this one has lasted 23 years.  They were expensive when purchased, and all of them were purchased directly from PR&E, Carlsbad, California.  The beauty of these things is their modularity.  All of the major components are replacable, including the module face inlays.

Penny Giles conductive plastic fader
Penny Giles conductive plastic fader, PRE BMXII console

The faders, Penny and Giles 4000 series, are fully rebuildable.  The part that wears out the most is the nylon bushings that slide along the metal rails.  The contact fingers sometimes also need to be replaced.  These are 10 Kohm conductive plastic linear faders.  P&G does not make these anymore, they have been replaced by the 8000 series, which has a edge connector instead of a wiring harness.  Since the top of the fader is open, it also tends to accumulate dust, dirt and other debris.  The fader board itself should be cleaned off with warm water, light soap may be used if needed.  Do not use alcohol on these because it eats into the conductive plastic and ruins the fader.

PRE BNXII line input module
PR&E BMXII line input module

One of the great things about this console is the fact that all the modules are hot pluggable.  If one needs to be serviced, it can be pulled out of the main frame while the console is on the air and a new module plugged into it’s place.  Only the line output module replacement necessitated taking the station off the air, and then only for a few seconds.  It was a great concept which is now standard in almost every broadcast console.

There were several basic module configurations.  On the input side, line level, mic level and telco were standard console inputs.  There was also a passive remote line select button set.  Out put modules consisted of line level output, control room monitor, and studio monitor modules.

PR&E consoles were top of the line gear, but expensive.  Most radio stations could not afford them and went with less expensive models like Wheatstone, BE, LPB, Autogram, Radio Systems, etc.  The fact that some of these BMXII consoles are approaching 30 years of age and still in service is a testament to their construction.

In the early 1990’s, PR&E began branching out into the lower priced market with their product line.  They produced the Radiomixer and Productionmixer consoles, however, mid market sized radio stations were slow in adopting them because PR&E had the reputation of being expensive.  After all, if you can only afford a Chevrolet, why bother looking at the Mercedes?

WQXR control room
WQXR control room

This is a grainy promotions photo from the early 1990’s showing what I think is the WQXR master control room, nick named “The Bridge.”  I took a tour there around 1993 or so and it was a fantastic facility, of course the New York Times spared no expense.  I really felt like Willie Wonka in the Chocolate factory.

Later in that decade, they changed the name to Pacific Research and Engineering, and the went public.  I think going public was the death knell, soon thereafter they sold the entire product line to Harris Broadcast.  The final non-Harris console was the Airwave, which is a good medium duty modular console, incorporating some of the traditional PR&E designs.  The later consoles stopped routing audio directly through the faders, using voltage controlled amplifiers instead.  This solved some of the channel drop out problems which sometimes occurred in earlier consoles.  The Airwave consoles are much less durable than the BMX series, however, with the advent of voice tracking, perhaps 24/7 durability is not that necessary anymore.

Harris has dropped support of much of the early PR&E line, but there are those that soldier on, buying up parts and rebuilding these things.  Mooretronix has a good selection of BMX and ABX parts.

The WICC/WEBE  installation is about to be refurbed, which means these consoles will be headed out the door.  There are three of them in fair condition.