Archive of the Ask Eddie Category
My friend wants to connect multiple headphones to a power amp in the easiest and safest way possible. There are many ways to create a headphone system, depending on needs, budget and reliability. This is ONE of many ways.
My friend is using Sennheiser HD-280 cans, which are rated @ 1/2 watt and 64 ohms. I am not sure if 500 milliwatts (mW) is the max power or the nominal (expected safe) power. I did my calculations based on a 100 watt per channel amplifier, which is overkill but what is available (translation: laying around).
The power formula is: WATTS = VOLTS (squared) / OHMS
0.5 = volts (squared) / 64 (we’re solving for volts)
64 * 0.5 = 32, the square root of which = 5.65-volts RMS (Vrms), the amount of voltage required to drive the cans at their rated wattage.
If the power amp is rated for 100 watts into 8-ohms, then 100w = voltage (squared) / 8-ohms = 100 * 8 = volts squared = 800, the square root of which is 28-volts RMS. We now know the amp’s voltage output at rated power and load along with the voltage required for the headphones to dissipate 500mW. The ratio of these two can be converted into the amount of attenuation (in dB) as well as helps us ballpark the series resistor required to safely drive the headphones.
The formula to convert a voltage ratio to the more familiar decibels is…
dB = 20 log (new volts / reference volts)
The resistive ratio will be 5.6 / 28 = .2 (the log of which is -0.69897000433601880478626110527551) times 20 = -13.9 dB (it’s good to know the attenuation in a familiar quantity).
That same 0.2 ratio, in resistive terms = 64 / 64 + ‘X’ (‘X’ is the series resistor we want to find).
We need to isolate and solve for X. To do so we need to get it out of the denominator by swapping 0.2 with ’64 + x’ so that 64 + ‘x’ = 64 / 0.2 = 320 -64 = ‘x’ = 256-ohms.
We then need to calculate the current through the resistor in series with the headphones, which is the same current through the headphone voice coil. 28-volts from the amp will be across 256-ohms + 64-ohms, that’s 28-Vrms / 320 = .0875-amps. Power dissipated by the 256-ohm resistor that’s in series with the headphones = current (squared) * ohms = .0875Amps (squared) * 256-ohms = 1.96 watts. This resistor needs ‘headroom’ so it will provide reliable service and have an adequate safety margin (a.k.a. no fire). (You may not find these exact values but anything close will work, for example 250 ohms @ 5 watts.
Just to confirm that I made no mistakes, let’s calculate the power dissipated by the headphones = .0875amp (squared) * 64 = .49 watts, which is close enough to 1/2 watt.
My resistor values should get us in the ballpark, tho you could figure out your amp’s power and work backwards as I did to know the range (and for the sheer enjoyment of doing the exercise). To test using the amplifier in this example, have your DAW generate 1kHz @ 0dB full scale, send that to your power amp and adjust the gain so that a clean 28-volts appears at the speaker terminals.
My friend was also concerned about whether the amp’s outputs were balanced or ‘bridging,’ reading on a message board that a bridging amp would not like each channel’s ‘low side’ tied to the sleeve of a quarter inch jack. To test, connect a speaker to the amp, play some music at a low-to-moderate level, lift the PLUS wire from its terminal and connect it to a screw on the chassis. If no signal, you’re good to go. When doing the same to the black wire, the signal level should not change – this assumes the MINUS or ‘low’ side of the amp’s output terminals is (indirectly) connected to chassis ground.
Last point is to buy some 256-ohm 3 to 5 watt resistors – whatever is common – then prototype, test and adjust to taste. I would also get some 64-ohm 1/2 watt resistors wired to the 1/4-inch jack normals so that, when no phones are plugged in, the amp sees the same load. This way, plugging and unplugging cans doesn’t change anyone’s level.
I’m sitting in my control room, listening to Beck’s SEA CHANGE in surround while working on graphics for a SIGNAL FLOW class…
SEA CHANGE creates wonderfully interesting textures by mashing up traditional rock instruments – guitars and synths, dynamic and organic drums – plus really cool orchestrations, some with the Indian tinge. No matter whether listening closely or casually, surround or stereo, Beck takes me on a sonic journey. I’ve even created a ‘Beck channel’ on Pandora so I can listen while cooking.
The August issue of MIX is about Mixing and to me that starts with the arrangement. All technical stuff aside, the most interesting mixes are deceptively simple and in that simplicity is space – the sonic real estate that allows the effects and the processing to be subtle and enveloping. I love when the musician / arranger carefully chooses the instrumentation and the frequency range of the notes – it’s so much more efficient than ‘engineering the space’ with EQ. Instruments shouldn’t fight each other!
Recordings that have depth draw the listener in – and especially for engineers, it’s like listening with fresh ears. It’s somewhere between ‘how did they get THAT sound, OR, how can I get THAT SOUND. Back in the day, The Beatles, Steve Miller, The Moody Blues, The Beach Boys and Sopwith Camel are just a few of the bands that come to mind. Their records were experiments on multiple levels.
The idea of the ‘concept album’ while not new, emerged, flourished and was shared back when FM radio was in its brief free-form (underground) period. This was before ‘corporate’ came up with Album Oriented Rock (AOR) and other niche formats that ultimately destroyed personality-driven radio and created the narrow genres that in some way have hurt music by not expanding the listener’s boundaries.
Truth be told, I do miss the DJs that wove the oddest things together – from spoken word to early blues, topical comedy and off-the-beaten path modern. Social media has replaced the creative radio DJ in that people can easily share their playlists.
Of course, Audio is not just about music. At one time we could be myopic about our respective crafts, but these days there’s a lot of multimedia cross-pollination, and that’s a good thing. Music that is created for films and games is like FM and AM, Classical and Pop.
I’ll be back to update this after classes…
THE HAPPY ACCIDENT
As some of you know, I am all about creative recycling, so when I found an empty AMEK console frame in my school’s basement several years ago, I put wheels on it and laid a mid-seventies Raindirk (and later a Mackie mixer) on top. It was not the prettiest picture, but the end result was a console we could stand at – similar to how live mixers work – and I’ve found it works very well in a classroom environment. Students are more comfortable in numbers when no one person is in the ‘hot seat.’
Many of my ‘designs’ start out as quirky temporary solutions that turned into ‘happy accidents,’ so when our school started talking about a control room upgrade, I asked them to consider a work surface that supported stand-up operation. I have both stand-up and sit-down work benches in my shop as well.
NPR coincidentally did a piece about the ‘vertical office’ in May of 2012 – about how standing instead of sitting at your desk is better for you – just as I was about to move my happy accident to a new location…
We already have a control surface in furniture by Sound Construction and Design. Like most off-the-shelf solutions, it is designed for sit-down operation. The ‘desk area’ is flanked by – and mounted on – a pair of equipment racks. While space efficient and structurally sound, the racks are intended for minimally accessed equipment such as an I/O interface. Rack location and height may not be well suited to ‘conventional analog devices ,’ a.k.a. gear with knobs that can’t tolerate a strike by a wayward roll-around chair.
We are going to experiment with raising the existing furniture up to see how instructors and students respond to the alternative. If it works out, we’ll compare the cost of ordering custom furniture versus having a local cabinet maker aesthetically modify the existing furniture.
I integrate records into all of my classes – including electronics. One of my students brought in a recently pressed Vinyl LP by Dutch Artist, Arjen Lucassen, that included a companion CD.
While I don’t know whether Arjen’s mastering engineer attempted to reconcile or embrace the differences between the CD and vinyl, I know for sure that my students preferred the record.
The CD was obviously brighter than the LP – about 3dB brighter on the outside and about 4.5 dB brighter on the inside. (I calibrate my preamp and cartridge to a DIN test record.)
Unlike digital, the many varieties of analog have their challenges, in this case getting high frequencies to the disc as well the ability to play them back. Way back in the day, producers tended to place less aggressive songs at the end of each side. As recording and playback technology improved, inner diameter high frequency response and distortion became less of an issue, but it never went away.
I do know that Neumann monitored cutter head temperature, cooling the voice coils with Helium. A simple “S” could raise the temperature and a high-frequency limiter was integral to Neumann’s cutter amps to keep over-the-top high frequencies in check.
All during the quarter I played a wide range of music to give students a feel for what records sounded like back when I was a college student. From Gentle Giant to Johnny Guitar Watson, Herb Alpert to Weather Report, Frank Zappa to Frank Sinatra and Les Paul to Todd Rundgren – we covered alot of basses (and trebles).
I was surprised when one album in particular resonated with them – Johnny Guitar Watson’s REAL MOTHER FOR YA (circa 1976 or 1977) – apparently because it gets played at a bar where two of them work part time. Despite the impossibility of comparing vinyl in a controlled environment to a digital (mp3?) version played through a bar system, one student was insistent that the digital version did not hold a candle to the EMOTION he felt in our control room. If you don’t know this recording, it pushes the limits of bottom- and top-end on a record for that time.
To assist my students in their quest to emulate the sound of vinyl, I always take the opportunity to review frequency response curves of popular mics. I favor using less hyped dynamic and ribbon mics along with a more selective use of condenser mics as part of a ‘balanced approach’ to managing high frequency content.
Condenser mics typically have a 6dB lift somewhere between 5kHz and 12 kHz. Many dynamic vocal mics have a substantial presence peak between 5kHz and 8kHz. Lately I have been favoring the less obvious studio mics – like the EV-635A and the RE-50 – because they are OMNI (no proximity effect), have about about half the presence lift and a gradual 6dB per octave roll-off above 10kHz or so.
Ribbon mics have an either smoother upper mid-range response, and while I would love to be able to afford a Coles 4038, a Royer or any of Wes Dooley’s recreations, I have been very happy with what I can afford – the Cascade FATHEAD and VINJET are remarkably versatile.
Back when I was a college student, the difference between albums at home, versus the same song on the radio was substantial. But radio compression gradually became part of the mixing and mastering process until we got to the this century’s version of the Loudness Wars.
I imagine that the reverse is true for modern engineers in training. When they record real instruments, the resulting understated ‘organic’ sound must be a bit disconcerting – so far from how they imagine the final mix to sound.
I try to encourage student engineers to resist the temptation to over process for as long as possible, emphasizing live performance and arrangement during recording and balance via automation as a foundation to building a mix.
It’s a common and often repeated theme…
I teach at two schools in the Twin Cities area – IPR in Minneapolis and MMI in Edina – and this is the last week of IPR’s quarter and the first week of MMI’s semester, so I’ll be busy.
In the next two months, IPR will be moving to a new facility a few blocks away, with studios being built by Dave Ahl and his crew. IPR had grown very quickly in its early days, so it will be nice to have more good rooms to record and listen / mix in.
For the past several years I have been lucky enough to work out of a former studio space known as Metro and later Oarfin studio. My friend, Bob Jenkins – Arranger / Composer / Trombonist (who played with Billy May) – tells me he did a thousand sessions in that space when Tom Tucker was engineer. Outside of the studio was the lounge, a great raw space with a wonderful natural reverb that we got to use until vending machines raised the noise floor…
When I came to IPR in Fall 2004, the studio was full of desks and the control room was empty, so I asked about using the control room as a hybrid space – part science lab as well as its intended application. The ‘yes’ included the caveat ‘until we turn it back into a proper studio,’ which never happened.
Over the years, I put out the word to facilities manager Tommy Tucker Jr (and a few of my friends and clients) to ‘bring me your tired, your poor and your weary gear,’ which pretty much translated into ‘unused, abandoned and broken gear that no one wanted.’ I ended up with a room full of tape machines, enough so that a half-dozen students could learn how to edit – all at once! In the process, my analog recording students acquired multiple fine motor and listening skills, from threading a variety of machines (each one different) to rocking kick and snare drum hits at less than 1/4 normal speed.
In 2007, when Kevin Ryan and Brian Kehew unleashed RECORDING THE BEATLES, Tom Tucker Sr. gave us the go ahead to do two solid weeks – an 80-hour Beatle-centric recording class. THAT brought more gear into the space – including my Altec 604Es in utility cabs, powered by a modified Groove Tubes Dual 75 running KT-66 power tubes. Dave Hill gave us some tube gear to mate with two Ampex MX-10 mixers, a Manley Massive Passive and Variable Mu compressor limiter. Student musicians stepped up to the plate and spent a month rehearsing.
This past quarter, I shared many of those recordings with my current students as we explored mixing techniques in the pre-automation age. I am proud of my students and all the cool things we got to do together. I’ve posted some samples below…
Meanwhile, the motley assortment of gear than constituted the ‘happy accident’ is moving to a new location, the former Game Room at Flyte Tyme, the studio formerly owned by Jimmy Jam and Terry Lewis and now home to MMI – the Minneapolis Media Institute.
I have been very busy (and sore) from schlepping all of that gear cross-town and reconfiguring it in its new environment – tape machines weigh a lot more than a hard drive and a laptop – the new digs have been dubbed The Studio of Misfit Toys, thanks to Paul Peterson for the name!
Below are sample sessions recorded at IPR…
STUDENT MUSICIAN SESSIONS
Pictures of Matchstick Men (4-track)
My World Is Empty Without You
Local Band “Hookers and Blow”
Who’s Gonna Help a Brotha?
If You Want Me to Stay (this was a live multitrack session that used less than 12 tracks to capture)
We Can Make It If We Try (same band as above, with vocal ODs, this is the only in-the-box mix)
Have you ever shrugged off an idea that seemed too simple? Sure ya have, and about a year ago, I did too.
I had this idea to record a frequency sweep from 20Hz to 20kHz and then automate playback to reflect the sum of my system’s ability to reproduce and my ability to hear that range. I did the test, it worked as expected and another distraction came along…
When several student mixes were suffering from Bass Management Issues, I pulled this exercise out of the hat and did it in class. The resulting ‘automation curve’ mimicked the essence of the Equal Loudness Curve and the student response was phenomenal – the point was finally driven home.
There are a few ways to do this test. If your DAW has a SINE WAVE oscillator, sweep and label the octaves – 20Hz to 40Hz, 40Hz to 80Hz, etc. (details below) – so you’ll always know the range you’re listening to.
In class, I recorded a manual sweep sine oscillator, then made a copy of the recording so one track would feed a frequency counter and another fed the monitoring system – the track being automated.
Set the Studio Monitor level to where it normally lives (should be at least halfway up).
Set the global automation at minimum across the entire frequency spectrum
Be sure the automation level is all the way down before looping between 2560Hz and 5120Hz. (3kHz to 4khz is the ear’s most sensitive frequency range, where the fire alarms are.)
Adjust the automation level until this region becomes audible, add automation points if needed.
Loop the 20Hz to 40Hz region, and adjust the automation level until this region is audible. Repeat through each octave up to 20kHz, adding automation points if necessary.
Zoom out and confirm that the automation levels emulate the essence of the Equal Loudness Curve.
20Hz – 40Hz
40Hz – 80Hz
80Hz – 160Hz
160Hz – 320Hz
320Hz – 640Hz
640Hz – 1,280Hz
1,280Hz – 2,560Hz
2,560Hz – 5,120Hz
5,120Hz – 10,240Hz
10,240Hz – 20,480Hz
Pretty much all Audio tutorials start with the Ear and the Equal Loudness Curve. Everything in between the sound source and the ear – from microphones to electronics, monitoring systems to acoustics – is covered in some detail.
One reason the mixes that result from these ‘lessons’ do not often reflect the depth of understanding and discipline required to pull off a respectable rough mix – what I like to call ‘a foundation to build on’ – is that professionals spend years learning by successive approximation, by listening on many systems, by making mistakes and knowing how to stay within the limitations and by pushing the boundaries to expand the possibilities.
To teach such ‘accumulated knowledge’ requires many hands-on exercises, each one a small step in the process. Whether you are a student, an educator, an enthusiast or a professional, I think you’ll find this exercise worthwhile. I plan on starting all of my classes with it. Let me know how it works for you.
Here I am yet again, trying to squeeze in a few moments to share what I’ve been tinkering with – on the bench and at school. I had intended to share this a month go, but life and scheduling have been a little intense lately. I hope I can make up for this lack of weekly blogging with a more extended ‘tease’ of my next MIX column.
I recently had my students check out the Chris Lord Alge Classic Compressor video – primarily because it showed what HE is listening for – one facet of Ear Training exercises that lead to the acquired skill set than eventually defines a MIX Engineer.
Dynamics Processors are squirrelly beasts- Hardware and Software. If the magic doesn’t happen right off the bat, many engineers try something else. Every signal processor has some usable magic, especially – as Chris observed – the goal was to capture his hardware ‘while it still had a vibe,’ that magical sweet spot between brand-new-perfect and ‘broken.’
It is my observation, however, that while the allure of retro graphic models is their familiar face and sound (for those who have used the hardware) they are less likely to reveal what’s going on behind the face plate. DSP designers are trying to capture analog’s sonic fingerprint and through that process they know the numbers that the dials represent. The knobs on an 1176, for example, aren’t calibrated – you have to play with the Attack and Release parameters to figure out which direction is fast and slow. Of course, you could read the manual, assuming the hardware is within spec…
I know we’re supposed to be LISTENING, but, I kinda like to know what parameters make the magic! Attack, Release and Ratio settings are very application specific.
Some dynamics processors, like the 1176, are versatile enough to do both Peak Limiting and Compression Chores while the LA-2 is not fast enough to be a Peak Limiter. The next level of processor has separate Peak and Compression Side-Chains – this part of the circuit does ‘the math’ and creates a Control Voltage (CV) to tell the ‘Gain Cell’ what to do. The most sophisticated dynamics manipulators – like the CRANE SONG STC-8 – allow interaction between compression and limiting.
On my bench most recently have been the Sontec compressor-limiter (with Crest Factor for whiter teeth) and the EMT-156 broadcast compressor-limiter-expander. These are all very sophisticated processors, the calibration of which is what allows me to explain to their prospective users how they work and how to use them.
The EMT is stereo-only, has a single meter and its classic X-Y ‘transfer function’ graph. Color-coded knobs correspond to specific ares on the graph where the controls operate.
Some no-frills software processors – like the generic plugs from PT, Adobe Audition and SoundScape (now owned by SSL) – have incorporated that static graphic as part of their dynamic display. That plus calibrated controls and more precise metering help the tenacious user zero in on the parameter-specific values that define Limiting and compression.
ROLL YOUR OWN Peak Limiter for Snare Drum
ATTACK: As fast as it will go (less than 1mSec)
RELEASE: As fast at it will go
RATIO: As close to Infinite-to-one as possible.
THRESHOLD: Enough to achieve not more than 6dB of Limiting.
The trick is to save the preset and then AUDIO SUITE the track to prove that the processor is doing exactly what you want to do – in this case, give Snare Drum transients a military flat-top haircut.
I’ll have some before-and-after pix soon (famous last words). Meanwhile, you try it and get back to me, ok?
As usual I arrive, tail between my legs, because I have not kept up my end of the bargain. That said, Part-1 of my Fender Hot Rod Deluxe mods is now in the February issue. Part-1 implies Part-2 and, like Sourdough starter, I am already working on Part-3 even though Part-2 has not yet been put to bed.
Zooming in on a guitar amp with a microphone has a tendency to reveal certain flaws that might not get noticed at rehearsal or at a live gig. As a recording engineer with geek abilities, I wanna know how to make ‘one of THOSE amps’ sound good in the studio, especially when my ‘clients’ are students on a limited budget. It’s not hard and it’s ‘mostly time,’ lol!
Meanwhile, in the first three weeks of Electronics Class, we covered voltage dividers, series and parallel circuits, Ohm’s Law and the Power Formula. Power Supplies and basic soldering skills are currently in the cue. We learned, about half wave, full wave, voltage doublers, filter capacitors and regulators.
When I showed up to class this week, students had drawn their power supply schematics 15 times in prep for a from-memory mid-term exam. Now we can ‘relax’ and stuff some components into the circuit board and solder away. Next week we start JFETs.
Here are some review links from my own site and afrotechmods.com
Series Parallel and Resistor Color Code Test
Some of these videos are not as well done as the stuff Afrotech does, some of the audio is absolutely horrendous! That said, our minds are all wired a little differently and it’s good to check multiple sources.
This one has terrible audio, but it seems relevant…
Recently a ‘cartoon’ pointed out that the penalty for sharing a Michael Jackson file was 5 years, while killing him was only good for 4 years. It points to the media industry’s heavy handedness – and laziness – like ‘scared straight’ for the down-loader instead of ‘the owners’ finding a more viable solution.
I propose a way to make every file sharer an asset instead of a liability. It was once tested on a small scale at the early part of this century…
A better file distribution system would thwart piracy. To make file sharing a viable business requires a DRM format that embeds paying customers’ ID into the files they share.
1.) Audition a file (music, video etc) to confirm it’s what you want – just like iTunes, Amazon, etc.
2.) Pay for it (this embeds your ID into the DRM for that file).
3.) Share the file with a friend. That person also gets to audition to confirm that the file is what it purports to be. Your friend pays, and you get a taste of said payment.
4.) EACH PERSON IN THE DISTRIBUTION CHAIN is compensated when someone pays for file. That gives everyone a vested interest in the system working.
5.) This system was attempted on a small scale in the early part of this century by Steve Turnidge. It was called WEEDSHARE. I had the exact same idea, Steve just had the ability to bring it to market, although my guess was that his project was under-capitalized. I was disappointed to see that it didn’t fly. Maybe it deserves a second chance?
Students often get ask about sub kicks and while I have not experimented other than to prove a speaker can be a microphone – I get plenty of low end from the kick without trying – I wanted to point out the where the sub kick resonance comes from to assist DIY’ers in their selection choices.
All ‘woofers,’ large and small, have a Free Air Resonance, or Fs. It is pretty much what you might imagine, the natural resonance of a woofer in free air.
There are a range of resonant frequencies that are useful in augmenting an overly dampened kick drum – really, whatever your preferred frequency might be, 65 Hz plus or minus 15 Hz is the ballpark.
To find an off-the-shelf woofer, all ya gotta do is go to web site that caters to loudspeaker components, like MCM, Parts Express and Madisound. When used for their intended purpose, better woofers manufacturers are more likely to provide more detailed specs. A sub kick doesn’t need much more than an Fs
Below is an 8-inch woofer available from MCM Electronics. Notice the Fs is 65Hz…
This 5 1/2-inch driver from Parts express has an Fs of 82 Hz…
And from Madisound, this Fostex product sheet goes into remarkable detail about all of the essential loudspeaker component parts. Fs is is determined by the cone’s mass – both material, thickness and larger diameter generally translate into high mass, lowering the Fs. More compliant surround material lowers Fs while stiffer surround raises Fs. Behind the cone is the ‘spider,’ which is used to center the voice coil in the magnetic gap. Not that the frequency response charts also show include an Impedance curve that reveals both the Fs and the effect the voice coil has on inductance (a gradual impedance rise at high frequencies).
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