AN201 - Sorting Out Hum and Buzz in the Sound Studio

Sorting Out Hum and Buzz in the Sound Studio

Hum and buzz have been demons of sound and music studios since the early days (1940s ish) but probably got worse in the 1970s due to the proliferation of music studios to record all the new rock n’ roll made popular by the Beatles.  From about 1968 and all through the 1970s music studios sprang up in Europe and USA like mushrooms after a rain and many were only partially designed by engineers and technicians that knew what they were doing.

Some historical perspective is needed:  From the 1930s through till about 1965 music and sound studios were designed by engineers who knew what they were doing and a lot of their knowledge was built upon proper knowledge and experience of telephone industry tech professionals.  It was also the case that all sound equipment like mic pres, equalizers, compressors, recording and mixing boards as well as amplifiers had input and output transformers  (as did the telco gear).   Because transformers are so good at preventing hum and buzz they did so even when gear was not all properly connected to the same ground.  Back in the old days electronic sound equipment had only 2 prong power cords and thus did not get ground from the third pin, aka ground pin, of power cords that became common in the  1970s.  All that 2 prong gear got “grounded” together by the metal equipment racks in which they were mounted.  When two or more equipment racks were in use and/or a stand-alone recording/mixing board a (roughly) 12 gauge wire was run from one to the other screwed tightly to the rack or board chassis.  

That “old days” scenario was not perfect for various practical reasons, for instance, ground wire connecting screws could loosen up, gear in racks may not have been screwed in really tightly, and the gear itself was not always designed for it’s rack mounting ears to be a good, solid connection with the internal circuit ground.  But it worked due to the almost miraculous ability of transformers to pass signal from gear to gear while not picking up any hum or buzz from the imperfect grounding. 

I will note at this point that grounding is not Earthing...one can have equipment all grounded together and yet not Earthed, that is, connected by wire to some metalic structure well burined in the Earth.  Typically in the old days that might be the cold water pipes.  Equipmenr ground would be connected to “ground rods”  and “ground plates”  (actually “Earth rods” and “Earth plates”) only if and when radio interference was getting into the sound system.  Radio stations were often located adjacent to transmitting towers and so they always Earthed the ground, as doing so prevented radio waves from entering or leaking into the audio gear and wiring.  (Just how  Earthing prevents radio and televison signals from  leaking into into audio gear requires it’s own separate explanation...for now it’s best to accept that doing so works).  

So it was that due to fairly good grounding and the use in equipment of input and output transformers the sound and music studios in the old days had few buzz an hum issues.  However there was a smallish hum problem that was just accepted on pragmatic grounds.  The vacuum tube equipment of that era did not have regulated power supplies due to cost concerns.  The unregulated power supplies always imparted some hum onto the signal passing through.  Tube filament voltages, also being unregulated dc and often ac voltages, also imparted some hum to the signal passing through the tubes.  Tubes used for high amplification, like in microphone preamps, amplified the tiny power supply hum along with the music...not the best situation by far but accepted on pragmatic grounds.  Typically the hum imparted by tube equipment power supplies was considered acceptable in part because the playback equipment, home hifi, tv’s or commercial sound equipment also imparted some hum.  And with the advent of the popularity of the electic guitar, the pickups of which picked up a lot of hum and buzz, the hum in sound and music recording equipment was swamped out by that of the playback systems and guitars.   

But all that changed in the late 1960s and 70s when the  studio business mushroomed and good grounding practices were often not followed because those (often hastily) building those new studios often just didn’t know what they were doing (and may have been smoking weed).  And as huge profits were to be made some details just got overlooked.  

The replacement of tube-and-transformer equipment with transistor equipment introduced new hum and buzz issues.  While technically speaking the transistor was invented in 1947 but they were not very good and reliable until the early 1960s and so studios continued to buy and use vacuum tube equipment well up until about 1970.  Early to mid 60s actually good pro audio equipment built with transistors often mimicked the vacuum tube designs with both input and output transformers and so all was still good, hum-wise, at least as good as it had been.  As transitor based equipment came to have lower price tags than vacuum tube gear it naturally proliferated. Contributing to the proliferation was the huge reduction in the size and heat generated by transistor based gear, as compared to vacuum gear,  and also the ability to pack more punch into a smaller space in the way of more features, such as on recording and mixing boards, an eq on every channel and multiple echo and effects sends.   It was that a plethora of eqs and sends became actually needed as such was part of the new generation of popular music. 

As manufacturers always need to cut costs the accountants began casting a dim eye on input and output transformers since they were a big part of the build budget and added to the cost of shipping.  Output transformers especially got the dim eye since they were heavier than input transformers.  It was the case with vacuum tube equipment that output transformers did triple duty: they prevent hum and buzz due to imperfect grounding among equipment; they transformed vacuum tube circuit high impedances to low impedances needed to send signals down often long wires; they offered human safety by acting as separation barrier between the high vacuum tube circuit voltages...200 to 350 volts...and the signal wires and patch cords that people handled.  

But pro audio transistor circuits used power supply voltages in the range of 15 to 28 volts...door bell voltages...and thus the human safety factor of output transformers was moot.  And by the late 1960s new transistor circuits were invented for low impedance  outputs. Thus two of the three good reasons for using transformers in pro audio equipment were eliminated, leaving only one: buzz and hum prevention.  More and more the accountants won out and output as well as input transformers in pro audio gear fell out of favor…and the instances of hum and buzz between interconnected gear went up.  The transition was gradual and the problem not noticed at first mainly because when a transformerless piece of equipment was connected to one with transformers those transformers saved the day.  As it turns out just one transformer, input or output in one piece of interconnected gear will have more or less the same hum and buzz prevention ability as a transformer in each of the interconnected equipment.  

As it was, some of the best and most expensive recording/mixing boards and tape recorders continued to have input and output transformers even as much of the outboard gear did not.  So there were still a lot of  transformers in the signal paths coming to the rescue.