Category: Technical

Categories
Guitar Equipment History Technical

Fkkng magnets. How do they work?

I had often heard of primitive ‘field-coil’ speakers, but it was not until i was confronted with a pair of them that I actually had to come to grips with this ancient technology.

Consider how a basic modern speaker driver works.  See this excellent animation for a quick example.

There is a (usually) paper cone with some wire wrapped around a center post.  The wire coil sits roughly inside a ring of magnetic material (either ceramic or metallic).

An electrical-signal is sent into the wire coil, and this causes it move relative to the fixed magnet.

OK so we all know what a paper cone is.  And we all know what a coil of wire is.  But what about this magnet?  Where did it come from?

Well, it turns out that modern speakers use what are called ‘permanent magnets.’  As-in, the magnet has a permanent charge.  The material which composes the magnet is always magnetic, regardless of any other influence.  Hold a key up to the back of any raw speaker driver and you will see that yes this is in fact a magnet.  And a pretty powerful one.

Permament magnets possesing enough magnetic power to function in a speaker driver are not naturally occurring materials, though.  They had to be invented.  And they were, largely as part of American WW2 engineering efforts.  These new, powerful permanent magnets were engineerd from an alloy of aluminum, nickel, and cobalt, hence their name:  Alnico magnets.  In the 1950s, newer ‘ceramic’ permanent magnets were engineered, and these became the norm owing to their even greater efficiency and lower cost (cobalt is expensive as a raw material).

But what about all the speakers and guitar amps designed BEFORE the invention of this wonderful Alnico substance?  These devices (and it’s rare to find one that is still in good working condition) use similar looking speakers, but with a very different type of magnet.  They use Electromagnets.  Meaning:  they use magnets which are made of a material which only become magnetic when a large DC current is passing through it.

Exactly where the audio device creates this large DC current, and exactly what effect this arrangement has on the total system, are interesting issues to explore.  This piece is a still a work-in progress.

I hope to have it completed soon, and I will post some audio examples of this antique technology at work.

Categories
Custom Fabrication Guitar Equipment Technical

some recent pieces

Last week was busy in the shop.

This is a 2-watt per-channel stereo power amp built for our friends K&D.  It is tiny (only 10 inches wide).  The form of the amplifier was driven by the styling of these beautiful black-lucite vintage speakers which complete the set.

The speakers were made by Speak-Easy Intercom of Kansas City, likely in the early 50’s.  They are handmade from individual pieces of black lucite.  I found them with the original base-station for the intercom, all in flawless condition.  I replaced the drivers in the speaker cabinets.  This tiny tube-audio system is intended as a casual background-music-listening setup for the home or office.

—   —   —

A couple more ‘Recycled’  Tweed Champ Clones were born last week as well.

I have decided on the name ‘Cassius’ for these and all future small single-ended 5watt guitar amps.  ‘Cassius’ being the name of the most famous Champ of the 20th century.

Like the previous Recycled Champs, these are freshly-built  guitar amplifiers, built with largely new components, into antique cabinets.  In this case, factory/school PA speaker boxes.

I built these with identical circuits, very true to the circa 1955 5E1 schematic.   They use a 6V6 output tube with a 12AX7 preamp tube.  They have choke transformers and a feedback circuit.  The output transformer is mounted directly on the 8″ Alnico speaker in order to conserve space.  I used a 6X5 rectifier tube rather than a 5Y3 so that i could use a slightly smaller-sized power transformer.  The 6X5 is an excellent tube which was used in a great amount of pro-audio equipment in the 50s.  These amps also will also accept 12AY7 preamp tubes for a different breakup.

These amps are very small and lightweight. They sound fantastic.  Extremely dynamic.  The range of sound that you can generate using the volume knob on the guitar is outstanding.  The small Alnico speaker magnet make these amps less efficient than some other 5 watt amps, which makes these ideal for apartment dwellers (or anyone who lives with other people!).

Up next: this week I will be building a similar ‘recycled’ guitar amp, but scaled up: a 12-watt “Tweed Deluxe-type” amplifier with a 12″ field coil speaker.

Categories
History Publications Technical

Service Call

Why do some of us (audio enthusiasts, recording engineers, musicians) choose to use old vacuum tube equipment?  The reasons vary widely, but very few users would cite ‘better reliability’ as a positive factor.   True, much old tube-based audio equipment designed for professional use (EG., Fender guitar amps) is designed so that it CAN be easily serviced.  I haven’t found a tube guitar amp yet that I couldn’t fix. But ‘serviceability’ is different from ‘reliability.’

Is vacuum tube audio equipment inherently less reliable than solid-state equipment?  I am not certain that this is the case.  But there are a few basic conditions of tube equipment that would seem to make it more prone to breakdown.

*High Voltage.  Due to the optimum operating points of many vacuum tubes, most tube equipment will have DC voltages upwards of 300 or even 500 volts present inside the chassis, flowing through the capacitors, resistors, pots, and transformers.  This voltage is often near the stated working limit of these components.  Compare this to solid-state, where 24volts is the most you will likely find.

*Filaments (tube heaters).  Man look at those tubes glow.  They look great, right?  Yes they do.  But remember that the glow is created by heaters.  Their function is to make the tube hot.  Very very hot.  And they make everything else in the area hot.  Drying out wires and insulation, causing potting wax to flow out, and generally contributing to the decline of the physical condition of all the components.  Consider the amount of heat in, say, a Fender Twin amp.  Each 6L6 tube is drawing (6.3Vx .9A)= 6 watts x 4 tubes.  That’s 24 watts of pure HEAT present whenever the device is turned on.  Solid-state equipment does not need heaters to operate.

*Layout. In order to conserve space in a chassis, and simplify the construction, tube equipment was often wired point-to-point, with lots of little hand-made solder joints.  Compare this to solid-state equipment, where the smaller (due to lower voltage requirements) components can all be assembled on a board.  Just take a look at the two devices here.  Which one do you think is more likely to develop a problem due to mechanical shock?

(web source)

(web source)

Anyhow, vacuum tube equipment tended to require more servicing than later solid-state equipment designed for the same purpose.  A natural consequence of this was…  more servicemen!  Radio/Television repair shops used to be a common sight in America.  Now, not so much.  Even beyond the greater need for servicing in the tube-era, there is secondary reason: cost.  Since so much modern ‘surface-mount’ equipment can be built by robotic automation, and produced offshore, modern equipment costs less.  It’s often just not worth repairing.  You simply throw it out.  This was not the case in the 1950s.

Consider the cost of a basic 17″ television in 1951.  $139.50.  This is today’s equivalent of $1170.  That’s a lot of money. If your $1170 TV breaks, you would likely get it fixed.   Whereas in 2010, a 17″ TV will only cost you $168. Would you spend time and money to have a $168 TV serviced?  Not likely.

A large range of publications once existed to cater to these legions of TV/Radio serviceman.  We will look at several of these titles in the future on this site.  For today, let’s consider Radio & Television News, which was the 1948-1959 title of the magazine that Ziff-Davis published for the professional readership market.   Wiki has a good article about the long and complex history of this publication.  Anyhow, the R&TN ran articles on electronic theory, service practice, general small-business professionalism, etc.  They also had several articles in each issue which offered circuit descriptions and schematics for various devices.   R&TN published several ‘Audio Feature’ issues that have some interesting projects outlined.  Today we will look at some of these projects which have potential use in the recording studio.

FOLLOW THE LINK FOR MANY INTERESTING C.1950 DIY AUDIO PROJECTS.,..

Continue reading “Service Call”

Categories
Custom Fabrication History RCA Technical Western Electric

TECH: antique theater equipment

It’s a sign of real accomplishment for an artist to have a monograph of their work published.  I would imagine that a few hundred are published worldwide by recognized publishing companies each year.  But much more rare is the collector’s monograph.  That’s right.  You have amassed a collection of (x) that is so stupendous that “let’s make a book about it!”  And the book costs like $60.

Of all the cults and sub-cults of audio-equipment collecting, few are more rarefied and costly than collecting antique movie-theatre equipment; especially equipment made by the Western Electric Company (hf. WE).  I won’t go into WE; the company had such a complicated history filled with intense government regulation, so tightly intertwined were they with the communication industries in American life; check out wikipedia for the details.  Suffice to say that, along with RCA, WE was a main manufacturer of the equipment used to playback sound in movie theatres at the dawn of the sound-film era (late 1920s).   Since the equipment was designed for such purpose, quality and reliability was very high.    Also massive.

(from “Recording Sound For Motion Pictures,” McGraw-Hill, 1931)

Here’s RCA’s theater system from that era:

(from ‘Audels New Electric Library,’ Audel+ Co, 1931-1958)

Mr. Yashima had quite a collection of this stuff.

(scanned from “Makoto Yashima Collection,” Seibundo, Japan)

It’s hard for me to say what the value of these WE components is, but i can easily imagine single pieces trading in the 5 figures.

Getting back down to earth, WE stopped making theatre-sound equipment in the late 1940s due to anti-trust regulations (complicated, right?), but RCA kept on building it.

This brings us into the realm of more accessible (even downright cheap!) devices.  Even though this later hardware may be inexpensive nowadays, we are still dealing with equipment that is designed for ultimate reliability, and really very good fidelity.  After all, tens of thousands of people sat in these theaters every year, paying a good fee in order to watch and listen to the latest films…  this is a case where quality really matters.

I  picked up this circa 1960 RCA 9362 booster amp for…  maybe… $70?  on eBay a while back.  I had no idea what it was, but it looked like it might be useful in the studio.  And here is where it gets technical….

Continue reading “TECH: antique theater equipment”
Categories
History Technical

The Limits of Control

How much control over their audio do music-listeners want and need? At the very least, we can agree that they need to be able to turn the sound on and off.  Control over volume (level) is probably the next most important thing.  But beyond that… what is really necessary, what is really desired, and what is just marketing?

I came across this RCA  vacuum-tube reverberation system this past weekend at the flea market.  It was new, in the box, never removed from its packaging.  Likely an ill-advised Christmas gift from the Eisenhower era.   It cost me one dollar.

This device was sold as an add-on to certain RCA stereo hi-fi consoles of the late 1950s.  Owners of these certain consoles could purchase the unit, open up their console, install a few metal boxes, plug in some cable harnesses… and voila.

The listener would then be able to selectively add reverb (ie., artificial echo) to whatever audio they were listening to.  Overall, the whole operation is about as difficult as installing your own car stereo.

This concept seems patently absurd to me, and i love reverb.  I love reverb on my guitar amp, i love the reverb chambers and plug-ins that I use in the studio…  but the idea of adding reverb to a recording which has already been mixed a certain way…  it simply would never cross my mind.

Getting back to my earlier line:  beyond on/off and level, what do we really want/need as audio listeners?  Someone decided a very long time ago that Bass and Treble control was pretty much a necessity, and there knobs (or sub-menus, as the case may be) have graced pretty much every audio amplifier since.

These controls were originally marketed as a solution for ‘poor room acoustics.’  Really?  It seems a little fishy to me.  If I go into a room that is bright-sounding, do I attempt to speak in a bassier-voice?   Used properly, there is no harm i suppose.  OK so Bass and Treble (note the convenient binary; also the reference to the musical staffs) seem alright.

So once we’ve given listeners control over the relative amounts of low-and-high frequencies, the next thing that manufacturers introduced were these reverberation units.  And they made them for everything.  Hell, they even made them for CARS.

(web source)

So weird.  Here’s a few 1970’s Japanese-made units.  This was clearly not a short-lived craze.

(web sources)

So we’ve given listeners the ability to control frequency response.  We’ve given them the ability to manipulate the apparent size of the space in which the recording took place.  What’s next?  In the 1970s, the DBX corporation sold hundreds of thousands of units just like this.

(web source)

A dynamic-range controller for home audio listening.  Now consumers could elect to give their recordings more dynamic range (IE more volume difference between loud peaks and quiet passages).  The stated intent of this was to make up for all the dynamic range that is ‘lost’ in the recording process.  Really?  I am pretty sure that whoever recorded that album was conscious of the dynamics that were present.  Why mess with it?  Anyhow, certain of these DBX units could also compress the source material.  IE., give the audio LESS dynamic range.

Equalization control and artificial reverb augmentation still exist in most home audio amps today.

They have renamed the reverb control as ‘sound space’ or something like that, but it’s the same idea.  ‘Take an audio signal and put it in a different space.’  The do-it-yourself dynamics processors seem to have been largely phased out though.  I don’t consider myself an audio purist, but there is something about all of these devices that really seems nonsensical to me.  Overall i get the feeling that  designers  ran out of knobs to put on the boxes, so they had to make new boxes and populate them with more knobs.

There’s one more angle that i’d like to consider:  So over the years, consumers have been given the 3 main types of audio processing that recording engineers have used in the studio for 60 or more years:  those 3 categories are *frequency balance, *ambience, and *dynamics.  In the past 20 years, though, recording engineers have been given tremendous new control capability due to Digital Audio Workstations, e.g., Protools.  What could this mean for consumer hardware?  Will audio waveform editing, time compression/expansion, and automation control soon be available in consumer audio hardware?  Is it already?

Does anyone actively engage with the ‘sound-space’ controls on their Hi-Fi receiver?  Are they useful?

Categories
RCA Technical

TECH: RCA BE-100

The BE-100 is a plug-in equalizer module that was made for use with the RCA BC-100 mixing console.  I have never seen a BC-100 in the flesh, nor am i aware of any albums that were recorded with a BC-100.  Since it was a product aimed at the broadcast market, and fairly rare, it’s possible that no albums ever were made on one.  From what i can gather from the forums, the BC-100 was apparently a custom-built product, and the pre-amps in these consoles were apparently built by API for RCA (no word on RCA’s involvement with the BE100).  A few years ago i was at a local flea market and some guys had a box truck with the contents of a storage-unit forfeiture sale.  I bought about 25 lbs of electronic parts, mic parts, etc., for a few bucks.  I also got some very interesting AES journals (interesting insofar as who had owned them) which i’ll post soon.  Anyhow, one of the parts i got was this BE-100.  just the raw module.   I bought a copy of the BC-100 manual from a dude who had been selling other parts from these RCA consoles on eBay.  He was not in the business of selling manuals, but agreed to sell me one for $25.  a very fair price.  he even spiral-bound it!  Here’s the schematic for the BE-100:

I added an old NOS BUD case, +/- 15v powersupply, a NOS UTC line-to-transistor input transformer from the local electronics warehouse, and an output transformer i pulled from an RCA broadcast phono pre of the same period.

I was too lazy to look up the specs on the transistors used in the BE100 to confirm that i was using transformers with the correct impedances, but my ‘good guess’ must have been close enough:  it works just fine.

The unit sounds…  grungy.  Aggressive.  not subtle, and not hi-fi.  It is quiet (IE no hum, white noise, etc).  I have not measured the freq response, but it sounds like it’s pretty much full-range…  i think the transformers i used are pretty decent.  The most interesting feature is the 40hz low boost.  it sounds awesome.  this will definitely find some use in the studio.

Has anyone else used one of these?  any tips or suggestions?

Does anyone know any records/studios that used these?

Categories
Custom Fabrication Guitar Equipment History Icons Technical

Recycled Champs

one of the most famous electric instrument amplifiers of all time is the tweed-covered Champ amplifier made by Fender in the 1950’s thru early 1960s.  Here’s a image of one from 1959 that i pulled off the web:

Fender made true vacuum-tube champs until the late 1970s, but the tweed-covered Champ (and it’s close cousin the tweed-covered Princeton) differ from their later namesakes in a lot of ways.  In the case of the final Tweed Champs, the lack of bass and treble controls  means that there is roughly 20db more gain available vs the later Black Tolex-covered Champ.  This does not necessarily make for a louder amp.  This does, however, make for an amp that can get super distorted and generally Sound Like Awesome.

Tweed Champs cost a fortune to buy.  Eric Clapton apparently used one for the Derek And The Dominos record.  This fact became widely known, and they have been very expensive ever since.  Fender now even makes a ‘re-issue’ (never a good idea) that sells for close to $1000.

champSchem

The schematic is posted above for anyone interested in checking it out. They are very very simple.   Anyhow, since these things do sound so great and they are so simple to build, it’s a lot of fun to build them into Any Old Thing that catches your eye.  Here’s a quick survey of some Champs and Princetons that i’ve built into found enclosures.  You’ll see recycled Intercom units, school PA system speakers, and 16mm film projector speakers.  The circuits in these are all new, built with new or lightly used parts; but the cabinets (that part you actually SEE and touch) are straight up ancient.  I generally take plenty of liberties with the circuit, changing parts, adding features (reverb, add’l gain control, EQ, etc), even building them using different (but similar) tubes, and i’ve yet to be disappointed.

Categories
Custom Fabrication History Icons Technical

Hey it’s one of those old horn speakers

Along the lines of the ‘Carbon Mic’ (see earlier post) are these early horn speakers.   They are visual icons that have become separated from their actual sonic function due to the fact that they do not interface with any other audio equipment that any normal living person would own.  But don’t those things look great?  yes they do.  They look very similar to the acoustic horns that are mechanically coupled to the needles of ancient record players, but in fact these are electrical.

here’s an example of a very old record player which has a horn mechanically coupled to the needle:

On the other hand, the early electro-mechanical horns were made for use with early Tube radios. They have drivers with permanent magnets attached to the base of the horn.  Driver:

These type of speakers date from the 1920s, and they are the earliest common electro-mechanical transducers.  I picked up this example, made by Music Master of Philadelphia, at a yard sale.

Here’s the base of the unit, which contains the driver.

i plugged the very frayed cloth-covered wire into an old receiver and…  sound!  it worked.  the volume level was very very low, tho, even with the 50 watt receiver turned up all the way.

Turns out that these early speakers require a slightly different sort of amplifier than we use nowadays.  Not surprising.  So i built something to do the trick.  I describe the process below for those who want all the bloody details.  Once i had this thing running properly, tho…  the big question… how does it sound?  well, when listening to music recorded in the 1920s (like my Blind Willie Johnson), it sounds fine.

Later music sounds pretty bad.  and not even in an interesting way.  just bad.  But old classical and gospel are cool.   Hearing those old recordings played back on the same sort of system that folks would have used 90 years ago…  wow.  it’s fun.  I have the horn (and it’s attendant special amplifier) hooked up to an Apple Airport Express which hides in the base of a corner cabinet in our house.

Other rooms in the house have their own full-range systems with their own Airports, so it’s really easy to switch up the playback systems depending on mood etc.  Love iTunes on the laptop.

Here’s the tech-y stuff for those of you who care.  So why did the speaker not play back at a decent level when used with my old SONY receiver? A quick bit of online research revealed that these old horn speakers have an effective impedance of 1000-2000 ohms.  WAY off from the 8 ohm speaker output of a contemporary receiver.   Anyhow, to confirm this, i inserted my handy University Sound universal impedance matching transformer (wired to couple 8 ohms to 600 ohms) and what do you know.  the speaker worked fine.   Decent volume level with the volume knob set at 10’o-clock.

Anyway, rather than run this thing all the time with a giant shitty receiver, i decided to simply build a tiny 5watt tube amplifier with the highest impedance that i could easily generate – 600 ohms.   I had some Edcor 5K/600 single-ended transformers lying around from a mic preamp project that i aborted because…  well…  the Edcors don’t have enough low end response to make a good mic preamp.  In this decidedly lo-fi application, though, they work just fine.  I used a 6J7 (for the old-timey look) into a 6V6 tube with a 5V rectifier.  I initially built the unit with a 6L6, but the edcor was getting REALLY hot (i guess they mean it about the 5watt rating) so i switched the tube to a 6V6 (and changed the cathode resistor appropriately).  It runs cool now.

Anyhow, this little amp also has the added feature of two RCA input jacks that passively mix to the input grid of the 6J7.  which is a necessary feature since i use this to listen to (stereo) music from iTunes via the Airport Express.  Also: super-nerdy but maybe worth mentioning – dig the old ‘screw-lug’ speaker connection.  i have been using these a lot lately and i think they add a little charm, even tho they do generally require some dremel-ing to the chassis in order to mount. (i hate the dremel and will do almost anything to avoid it.  Greenlee punches 4eva)

If you find one of these speakers for a good price (mine was $25, down from the sellers asking price of $80), and you can confirm that it works, you might want to pick it up.  One caveat: i apparently got lucky with the speaker that i bought.  Apparently, it’s common in these older units for the magnets to actually have lost their charge, and if that’s the case, they will need to be re-magnetized using wire and very high voltages.  Dangerous and irritating.  There are a few pages on the web that describe this procedure.  It’s pretty incredible to me that this technology is so old that the magnets have lost their charge.  crazy.  will this happen to all of our permenant magnet speakers some day?  will all of those coveted old Alnico drivers be useless at some point?  when?  2050?  Can’t wait for “The Day The Tone Died”  haha i can’t believe i said that….   awful.    Hate ‘Tone’ as a synonym for ‘pleasing sound quality in an electric-guitar sound reinforcement scenario.’

Does anyone out there use one of these speaker systems for music listening?

Anyone have a dedicated ‘antique’ system for listening to certain genres/ periods of recordings?

Categories
History Icons Microphones Technical

ICON: “gimme one of those real old mics…” :::UPDATED:::

When you are creating a set for a musical performance, nothing says ‘old school’ and ‘authentic’ like one of those mics…  those real big old mics with the springs…  what the F are they called?  Turns out that they are called Carbon Microphones, or more specifically, Double Button Carbon Microphones.

And while many a rapper, RnB singer, or songwriter type may favor them for their music video, i can promise you two things:  *)no one would be able to actually hook the mic up and use it, and *)if they did, they probably would not dig the sound.

Carbon mics are the oldest microphone technology still in (albeit limited) use today.  They actually pre-date vacuum tubes.  Wikipedia has a great article on their history and use, so no need for me to retread those waters.  Carbon mics are used in landline telephones, so we all have a basic idea of what they sound like.  midrangey, a little crunchy (distorted), compressed…  hey wait a minute!  that sounds pretty good to me!  Aren’t there like a million expensive DAW plug-ins in order to give you ‘that sound?’ Anyhow, we all know in general what they sound like… but how do those big old music-video props sound?

In order to find out, it turns out that it’s necessary to actually build a power supply.  Carbon mics need a few volts of a DC current moving through them in order to operate.  I found this handy schematic online and put it together.

I added a DC voltmeter so that i could monitor the effect of varying the voltage on the sound (the mic i have seems to like 6 volts).

I used a double-button mic input transformer salvaged from an ancient tube PA head that i had.  To the output of this transformer i added a second transformer to bring the impedance back down to Low-Z mic impedance so that i could use this whole rig with whatever mic preamp i wanted to.  The particular mic i have is a Lifetime Model Six.

I bought it years ago on eBay along with a little tube amp and some shitty speakers (and about a mile of useless rotten old speaker wire) for $150.  Anyhow, i won’t bless you with any vocal performances, but here is an acoustic guitar recording from my living room.  The left channel is the Carbon mic.  The right channel was recorded simultaneously with good equipment (414/omni into an API 512) so you can get a pretty good idea of what i was hearing in the room.  I put a little EQ on the Carbon mic to make it more audible (low pass at 3k, 5 db peak at 1.7K). No other processing was used.  Check it out.

carbonMicTest

Does anyone out there use double-button Carbon mics for audio production work these days?  Music recording? Sound design work?

I heard recently (maybe Tape Op mag?) that someone was making new-production ‘professional’ carbon mics.  Has anyone used these?  thoughts?

UPDATE: I recently had the chance to use this Lifetime Model Six Carbon Mic in a modern-recording context.  We tracked the following cut at Gold Coast Recorders, using the Lifetime for the vocals.  It sounds pretty outstanding…  I feel like you could get 90% of the way to this vocal sound with an SM57 and a fuzz pedal, but that extra 10%…  it’s a game of inches, ain’t it.   This is ATLANTIC CITY, my studio project with T.W.   Take a listen:  Ten Past Midnight