OBSERVATIONS AND INSIGHTS ...
The famous “Tubes VS. Transistors” controversy:
Tubes and transistors DO “sound different” ...or DO they? I used to think so. They certainly did
in the beginnings of transistor applications. Now...pay attention ...I am going to use this platform,
as I will do many other times in many other sections of these reports ...to convey a DUAL
MESSAGE! In this case, the “why” of early transistors getting a bad reputation will also serve to
point out and illustrate one of the most important parameters in audio electronic circuit design ...
and, that is...
HEADROOM!
Headroom simply means ... how much audio signal “room” is available between the average
operating level and the “CLIPPING POINT”. Now, for those who may not happen to know what
“clipping point” is, don’t worry we’ll cover it in more detail later. Just grasp the idea that every
audio circuit has one ... a clipping point. This is the point where the audio amplifier “runs out of
gain” or amplification capability, and the signal peaks no longer contain their soft, rounded,
sine-like characteristics but, instead, become FLATTENED (when viewed on an oscilloscope).
SIX
Since the peaks are essentially “clipped off” and the wave has become flat ...the result is called
...(duhhh!) ...”Clipping”!
Now, as you will learn later on, one of the most significant determinants of just how much signal
an amplifying component can “swing”, is it’s “FUEL” or it’s power supply voltage. In general, the
more voltage the amplifier can utilize the greater it’s signal handling capability which means
larger (louder) signals can be passed through and/or amplified with more “headroom”.
In the very early days, the first transistors, aside from assorted other problems (like noise), could
handle only very low power supply voltages lest they “breakdown” and self destruct. Well,
obviously this should have confined their use to things like toy “transistor radios”, which were all
the craze in the late fifties and early sixties, where they could be happy with their very own newly
developed “nine volt transistor battery” to power them.
But, as you might well expect, experimenters started trying them in assorted other audio circuits
and, before you could say “no headroom”, some idiot was trying to use these noisy, low power
culprits in audio mixers for semi-pro applications. The first such audio mixer I ever heard was a
nightmare! At all but ten feet or more away from the monitor speaker it sounded like eggs frying
and signals coming through it contained awful distortions (ODD harmonics) that we had not
heard before! No wonder transistors were gathering a BAD reputation right out of the starting
gate!
So, aside from being audibly noisy, transistors offered very LOW HEADROOM capability and
therefore they would distort very quickly. By quickly we mean at very low signal levels. Not only
that, they also wasted no time going from reasonable sine shape to flattened clipped peaks!
Unlike tubes which would GRADUALLY loose their peaks and “gently fatten” ...transistors would
just go ahead and CLIP ... all of a sudden without warning, without “graceful bending”, no in-
between ... just ...
“ Ooops dere it is”!
Now, it’s kind of like gossip in a small town. Once you get the reputation going around
...well...you know how it goes, it gets stuck in people’s minds from now own. This is largely the
reason that, even today (when many new transistors offer more voltage swing than tubes ever
could and are capable of phenomenal low internal noise) many still insist that transistors “sound
bad”. It has a lot to do, still, with that early bad reputation handed down in one form or another
over the years. Believe me ...this is true!
Tubes Sound “Warmer” ...
Well ...maybe.
When I began designing our first “InnerTube” processor back about 1980, this old dog learned
some new tricks! I found that you really had to go to a good bit of trouble to get a single tube
stage to sound any different, in A/B comparisons, than a comparable single transistor stage!
Now, if you don’t believe this statement, at some point in time during your audio electronics
experimentation, try it for yourself! Give a 12AX7 tube a healthy supply and do the same for a
2N3904 transistor ...or, even a dreaded op-amp for that matter ...and COMPARE them. I mean
REALLY compare them and REALLY listen. Have a trusted assistant map out a secret
SEVEN
switching routine for switching between the two and let you close your eyes, listen and choose.
See what you come up with! I’ll bet all $25.00 of this week’s profits that you won’t be able to
even come close to batting a thousand!
Of course, ONE of the “KEYS” to all of this is the fact that SUBTLE signal variances usually add,
algebraically, in a signal chain. This means that little differences in a SINGLE amplification
stage will often add directly in every succeeding stage. This has a lot to do with why I had
trouble picking up the subtle “textures” of just a SINGLE tube Vs. transistor stage.
The more of these that you string together, obviously, the more audible the variations become
with accumulation! This is why, we had to go to great extremes to make a SINGLE tube stage
deliver a “sound” when designing our “Inner Tube” circuitry!
[Abridge]
... I have often heard musicians claim that tubes sounded “an octave lower” on bass and, to some
extent, my little accidental listening experiment was bearing this out. The actuality of that was
impossible of course since the cassette low end response was flat barely down to 50Hz with a
drastic loss of low end from there down. Yet, this MARVELOUS, FULL Low End was filling my
car and the smooth high end was just as apparent (as well as all in between!). Now, this was a
dub from a worn, somewhat scratchy, record that had been played MANY times since purchased
about 1964, transferred to a medium grade cassette on a low end deck (albeit the best turntable
chain available which didn’t hurt) and the sound was TERRIFIC! When a much more recent hit
copy would come on right behind it, the loss of “fullness” and “warmth” (the world’s most
over-used term these days to be sure!) was so obvious by comparison!
Now, how much of this do we owe to the superb recording techniques of the engineers and
producers of that era and how much to the equipment that they recorded on? Of course it has to
be some of both but I have to feel that much of the illusory magic must be credited to the
masters of their craft, especially those who created the “Nashville Sound” ... people like Owen Bradley,
Billy Sherrill, Bob Ferguson and Chet Atkins.
That said, you must also consider a properly designed “Bass Trap” with a well-tuned, well placed,
well played acoustic “bull bass” inside it and a properly positioned RCA ribbon mic feeding a
TRANSFORMER balanced tube mic amp and many succeeding transformer/tube
stages of processing! Then, the vocals and, likely, most everything else in an acoustic session,
captured by the classic U47 tube mic feeding said signal chain! Top it off with the old EMT plate
reverb and you had your tools in place to work some magic! And then, of course, you have to
give a “little credit” to the performers themselves!
But ...getting back to the heading ... I have come to realize, over the years, that much of the
credit given to “tube warmth” ... is actually due to the “SOUND OF TRANSFORMERS”!
These reports will NOT go into very much detail in regard to all of the physics interworkings
taking place with transformer action and, believe me, there are PLENTY! If inductors are your
thing then there are some good volumes out there and we’ll point you to one or two before we
are through! Suffice it to say, that from the standpoint of one engineer with extensive
experience in trained listening and comparison situations, TRANSFORMERS ARE VERY
AUDIBLE at audio frequencies, particularly at the LOW FREQUENCY end of the spectrum!
This audibility, which in the case of several “medium quality” transformers of the type used in
1960’s vintage broadcast mixing consoles, can often be more readily perceived as low end
“mud”, as opposed to “warmth”, and is one of the main reasons that yours truly was one of the
first recording engineers of the late sixties advocating “get the iron out” of our signal chains!
I once offended an area studio owner/engineer when called into his studio to consult about
problems he was having in another direction when I told him, quite accurately, that I already
knew what console he was using because I could recognize the distinctive “transformer sound”
of that model “muddying up” the low end on his recordings! I suggested that he allow us to
replace the transformer front-end with our new (at that time) transformerless, active balanced
preamp. That went over with him about as well as his iron clad mic amps went over my ears ...
not very well!
[Abridge]
... You might ask, if all this is true, why were you saying that maybe the sound you described
earlier, which was so desirable, was in part attributable to transformers? Quite simply there are
TWO apparent reasons. The first and most significant is the TUBE/transformer combination
itself! Note that the “mud” which was so objectionable happened to be with transformers coupled
to a transistor base as opposed to a tube grid! Secondly, no doubt the transformers used in
these early major recording consoles were of a much better quality of design and construction!
[After an in depth discussion on the subtleties involved with Tubes, Transistors and
Transformers in audio applications ... the subject moves on to the foundation
for today's modern Op-Amp based circuitry. The little opening narrative
is designed to both inform and inspire the would-be circuit designer! ...]
" ... WHY I 'LUV' OP AMPS:
I used to sit in my struggling studio control room and dream about building my own recording
console. It was a NO EXPENSE SPARED dream! In the daydream, I would build a huge
awe-inspiring thing. In the tradition of Neil Diamond gazing out upon his “Brooklyn Roads”, I'd
gaze out on my southern roads and I would mentally construct a SOUNDCASTLE of a console
with multi-colored lights and every feature imaginable! However, with my limited knowledge of
that particular time it would have been just as impossible for me to BUILD a console as it would
have been to BUY one of the multi-bucks monsters of the era!
.....I'd look at pictures of a NEVE console and I'd always see an
imaginary "R" at the end of the name ...N-E-V-E ...R
In those days (1970), there were very few "gap bridgers" around ... not only in terms of
cost/performance ... but in terms of ANYTHING! Like many other young. aspiring studio owners
with more talent than money (to borrow TEAC's line...GOD [and lawyers] FORBID!)...or, more
ambition than brains, to be more accurate, I had to "OUT-RIG" a make-shift custom console
from various available bottom line "pro" components.
In those days, that meant mostly Shure and Sony small mixers. These were"KINDA NICE" but
they were lacking in features and performance, especially with headroom and EQ. But,
nonetheless, I managed to fabricate a fairly decent "console" of sorts using the now famous Sony
MX-12 mixers (yes, I still have both of them and ...NO...I would NOT part with them at ANY
price!) in conjunction with the so-called "professional" mini-mixers and processors from Shure-
the M67s,M68s,a pair of M63s for stereo EQ and an occasional M62 for compression.
A bit later on, I bought some more components as they became available (a larger Sony M14
and a Lamb mixer) and incorporated them into my rig. Somewhere along the way the OSMOSIS
principle had started to click in and I had begun to learn a FEW things about electronics.
TEN
More to the point, I had learned about resistors and was able to wire in a few faders with
"BUILD-OUT" resistors (combining resistors) to tie all of my stuff together in some 'semblance of
a "normal" recording console.
This was a step forward but the bulkySHURE/SONY/LAMB/WHATEVERELSEICOULDFIND
console was becoming kind of costly for its modest performance while it was VERY inefficient
and certainly not much to look at!
In fact, I can remember doing a lot of fast talking whenever a potential client would come in and
look over this rig. He would often humor me and nod obligingly before scurrying off to
never-never (N-E-V-E ...R) land to record on somebody else's console which would be an
AUDIO DESIGNS, an AUDITRONICS or,heaven help us ... a NEVE itself! This was even before
the Harrisons and the MCIs or later innovators (such as today’s Mackie designs ...actually though
his Tapco company wasn’t too far behind the first innovators!)
I can remember sitting across from my "FRANKENSTEIN" console (one of the few really
accurate ways to describe the thing if you think about it) and wondering why I should have to
buy another WHOLE M-67 just to have access to another balanced mic amp. Why couldn't I just
go INSIDE the box and.....yep......that's what got me started!
Even though I started my quest for a working piece of custom built componentry not knowing the
first thing about transistors or even, in the beginning, what they even LOOKED like, I was never
short of courage (idiocy, maybe?)! It was just about the same time that I tore open my
first M-67 that I also tore open my first electronics book and never looked back!
I also bought my first soldering iron and clumsily began learning what is now second nature. It
wasn't long before I was able to build simple passive circuits like the faders in the passive mixer
described earlier. Then, with one eye on the iron and the other on a book or
manual, I could actually remove a mic preamp from a stock mixer and devise a simple power
supply to power it up.
I WAS THUS ON TO SOMETHING THAT WAS TO BE THE MOST IMPORTANT DISCOVERY
FOR MY FUTURE ELECTRONICS ENDEAVORS ....
I HAD DISCOVERED THE ALL-IMPORTANT “BUILDING BLOCK” PRINCIPLE!
This means , of course, “all-important” in terms of gaining a grasp of audio electronics and how
everything goes together in STAGES ...or....as “building blocks” ! I was also learning the most
important unwritten (until now) law of electronics... which I'll dub, Davis' First Law Of
Electronics... and that is:
'NO MATTER HOW MUCH YOU HAVE LEARNED...IT IS STILL NEVER
ENOUGH!"
[Abridge]
Obviously, the development of the IC (not just the Op Amp but ALL IC’s) has brought new ease
and standardization to circuit design that is a true joy to those who like to “roll our own” circuits!
WE WILL NOT GO INTO ANY GREAT DETAIL, HEREIN, REGARDING THE
MANUFACTURING PROCESS OF INTEGRATED CIRCUITS NOR WILL WE EXAMINE THE
INTERNAL SCHEMATICS OF THE IC AMPS THEMSELVES .
NEITHER IS OF MUCH PURPOSE TOWARD THE ACHIEVEMENT OF OUR GOAL. AND WE
HAVE A CLEARLY DEFINED GOAL HERE WITH THESE TECH REPORTS ... “TO LEARN TO
QUICKLY AND CORRECTLY, BUT SIMPLY, APPLY WHAT IS AVAILABLE TO BUILD OR
MODIFY WORKING AUDIO GEAR WITH SUPERB PERFORMANCE CHARACTERISTICS”.
As the old saying continues ...”Don’t cloud the issue with the facts”!
Almost ANY textbook on op amps and, yes, there are many, will give you the complete theory
and the math. But HERE, we will LIMIT ourselves to BASIC, SIMPLE, but ACCURATE
WORKABLE definitions and examples and not burden the process with complex details so ...
RELAX AND ENJOY!
[Abridge]
“IN THE BEGINNING THERE WAS ....”
Well, let’s now interrupt all of our “foundational, overviewed, generalized” introductory patter and
get down to the business at hand!
In the traditional “Rambling Style” of our earlier writings, these latest updates will also be
presented in the somewhat random fashion meaning....ALL SORTS of related, but not
necessarily categorized, comments and illustrations will be included all along the way. So, even
if you are more advanced than the “ZERO LEVEL” we promise to start from, then you to need to,
at least, “speed-read” though the beginning material since we are known for dispersing our
“GOLDMINE TIPS” (any of which, we maintain, is each worth the price of the reports with their
eternal usefulness) anywhere and everywhere as we go along!
[Abridge]
ALL “MAPPED OUT” ...
It is assumed that ANY of you reading these reports will have, at least, SEEN a basic signal flow
diagram at some point. Maybe you can even read and understand one. That would be nice
since a signal flow diagram is only one step away from a full schematic diagram.
Throughout your electronics life, you will ALWAYS correlate between the “real world” laid out
components and their representations on paper which are known as signal flow or schematic
diagrams. The term “Schematic” is used for short. So, it is appropriate for us to start with various
representations of a schematic diagram while we describe their real-life counterparts.
[Abridge]
So then , our most BASIC electronic “component” is a single piece of wire which may be
represented on our “ELECTRONIC ROAD MAP” (schematic or signal flow diagram), as a simple
STRAIGHT LINE! Thus, your FIRST electronic schematic symbol is:
_____________________________ a straight line.
Though there are exceptions, usually a schematic diagram is laid out like other documents
in that it is read from left to right and top to bottom. Therefore, unless otherwise designated or
made obvious in some way, you would read the schematic, and the signal flow, from left to right
and top to bottom. In the case of our above straight line...if it were to represent an AUDIO
circuit, we could assume that our input is at the beginning of the line at the left and that our
output is at the right end of said line.
[From the simple straight line ... the reader is taken through a
complete discussion of all relevant electronic
components and their schematic representations ...]
FEELING CONFUSED? DON’T WORRY ABOUT IT! Right now we’re only interested in being
able to RECOGNIZE a capacitor ( and a resistor) for what they are on a schematic diagram and
then to be able to PRONOUNCE the name so as to be able to order one ...That’s ALL!
But, just as capacitor types and markings are varied, so are the markings on a schematic
diagram somewhat varied. However, the capacitor is normally designated “on the map” as TWO
VERTICAL LINES separated by a gap.
Here are some examples of schematic representations of capacitors:
----I I---- -----) (---- -----) ]-----
An electrolytic capacitor is usually POLARIZED meaning it has a plus and minus, positive and
negative relationship. The schematic symbol will usually show a PLUS sign to designate a
proper oriented polarized capacitor ...
--------) ]+ ------
Whereas, on the actual part itself, particularly with electrolytic capacitors, the MINUS terminal
will normally be the one marked with an arrow and a minus sign pointing to the negative end!
[Abridge]
Take The Map To The Road ...
Now, beginners, with what we have given you so far, you should be able to, at least, recognize
most schematics and break them into sub-elements or “blocks”. If you connect the REAL,
touchable parts together in just the way that they are presented on a schematic diagram,
substituting hookup wire for the lines, IC op amps for the triangles etc. etc., then you can build
over 75% of the analog audio circuits in existence today!
Of course, learning some “technique” is required along with a few other details but, importantly,
you ALREADY have the basics you need to COPY somebody else’s design via their schematic
and prepare your own version!
Getting it Laid ...
There is another important step that is usually employed before actually building a circuit from a
schematic diagram, however. That step is the “LAYOUT DIAGRAM” . While many of us can
rapidly lay down a working circuit from a schematic, normally an intermediate drawing is
involved whereby the miniature “pictures” of the actual components are laid out in the exact
order, on paper, as they will be used on the circuit boards. (Our British cousins draw their
schematics much like a PCB layout so their method requires less need for the intermediate
stage).
But, as for the traditional American way, you’ll see plenty of examples of this herein with
hand-drawn layouts offered to support their respective schematics particularly in the section on
“BUILDING BLOCK” circuit design which will be forthcoming!
Meanwhile, here and now, we are going to teach you to build your very first, working, useful
audio circuit! Now that we are blessed with the knowledge of what a resistor is ...not what it
DOES exactly ... just what it IS ... and the way it is represented by “ZIG-ZAG” lines on a
schematic ... we are now ready to present your first circuit ... using only two resistors and some
wire.
[The first circuit is a resistive pad so more foundation must be laid before
getting to the first, basic "BLOCK"...]
... To understand where we are headed with this ... we need to interrupt to talk about GAIN/LOSS
RELATIONSHIPS in audio and thus we need a measuring stick so .... we have to introduce the
infamous ..... “dB” or “Decibel”.
,p.
dB’S ...SIGNAL LEVELS ...GAIN ...HEADROOM ....NOISE FLOOR ...PROBABLY THE MOST
IMPORTANT AREAS OF AUDIO ELECTRONICS YET THE LEAST COVERED, THE LEAST
FULLY UNDERSTOOD, AND THE HARDEST TO GET A “FIX” ON BY BEGINNERS! BUT
WE’RE ABOUT TO CHANGE THAT!
To begin with we must consider “STANDARDS”! Now, old-timers, please bear with us and
marvel at the new way we teach this stuff to beginners ... who knows, you might just be like a few
other “old-timers” who picked up a few pointers from this section!
As you might be aware, most of the early pro audio standards we have come to know and
....(gulp)...”LOVE”....were handed down like they were carved in stone (MAYBE THEY WERE!).
Most of these standards were developed for the only REAL practical use for audio of the time...
that being, of course, TELEPHONES and TELEPHONE LINE TRANSMISSION.
Now, without getting TOO DEEP into all of this historical sidetracking (you can read about Mr.
Bell and his namesake the 'deci-bel' in an encyclopedia somewhere) we'll let it suffice to say that
a “dB” is simply a UNIT OF REFERENCE used to compare one "piece" of sound to another
"piece" (or unit) of sound. These UNITS are used to designate a CHANGE from one reference
point to another (much like a MILE is used to designate travel....10 miles to Grandma's....2 miles
to Hotlips'...and so on).
The dB...decibel....represents a UNIT OF CHANGE in audible signal level and is, thus, a
standard measurement reference for sound level. Now the OLD measurement standard was
based on everything moving around on/in 600 ohm terminated lines (the old phone system
again).
In order for standards to work we have to define a reference point. In the case of sound levels,
it was decided that "O" dB would be the center reference point for measurement with signals
swinging to either side of the center reference in +/- fashion. It was established that this "O"
point would correspond to "0 DBM" ...meaning the level point whereby 1 milliwatt of power would
be dissipated across a 600 ohm resistive "load" in an audio circuit.
By defining such a point, we could now, mathematically, divide this into sub measurements
corresponding to the fluctuating sound levels. (IMPORTANT NOTE FOR NEWCOMERS: A
"LOAD", audio or otherwise in electronics, is something "hanging across" a line from the "high
side" (positive side) to the "low side" (usually GROUND these days, or the minus, or low, side of
a balanced line if not ground).
This "load" can be a resistor, another piece of audio gear plugged up to the other end of the line
or a screwdriver shorting out the works when we're not careful! Today, in modern audio, 600
ohm lines are the exception rather than the rule and the term DBV has come to be utilized more
and more to replace the term DBM. This means we can still talk about the “DB” as a standard of
reference but without the “M” for “Milliwatt in 600 ohm” designation! Instead, we can reference it
to signal voltage swing and call it ... “DBV”. Without getting TOO DEEP here, let’s just ACCEPT
the fact that whether we are talking DBM or DBV, our signal VOLTAGE LEVEL (The thing we are
concerned with measuring) will be very close to the same thing with either designation
PROVIDING ... the associated amplifier can safely “drive the load” for transferring this voltage
level without being “loaded down” into signal loss, or distortion (more on this later!) by the “load”
of the next stage.
[Abridge]
... So, now, it is time to apply some of our “POOR MAN’S ENGINEERING EDUCATION “(meaning
nothing too “formal”) and, while skipping all the heavy math and traditional methods of teaching
the “DB” audio “handle”. we’ll try and come up with some alternative ways to further explain the
DB reference system for measuring audio signal levels.
Well, when you think about it, one way to explain or to begin to develop that all-important “feel”
for a system of reference (one thing relating to another thing) is to just JUMP IN SOMEWHERE
... ANYWHERE ...and GET STARTED!
All audio signals are actually miniature AC (alternating current) signals. Though certainly not
MINIATURE, the 110 volt AC power line signal at your wall outlet is an AUDIO signal! Usually it
is a pretty clean one (unless you live in my neighborhood supplied by my co-op!) being
generated as a 60HZ sine wave by that big "audio oscillator in the sky"! This is the SAME audio
signal that we are more familiar with in audio as "HUM" which can be a "bitch" to keep out of our
sensitive audio circuits!
This analogy should also serve to show something else. That is how easy it is for a signal with a
powerful potential (110 volts) to "crosstalk" into a nearby signal with a much lower potential,
ESPECIALLY if they share common paths or, heaven forbid, common ground references!
(We'll get into this in some detail a bit later in the section on "Crosstalk" and in "Grounding" as
well). Most audio signals, as you will learn, are less than 1 volt AC, so great care must be taken
to isolate them from the 110 volt power line! This report will, (WHEN?-A BIT LATER ON,
OF COURSE!) provide you with quite a few "tricks" on how to do just that!
[After more in-depth discussion and illustration to complete laying the foundation ...
the first actual circuit block is finally ready for presentation ...]
PROBLEM: Construct a working circuit to provide a match between so-called “pro” gear
operating at +4dbv to so-called semi-pro gear with an input rated at -10dbv. Now we assume
that the pro gear is using a low impedance drive stage, probably an op-amp, with a source
impedance of 100 ohms or less (actually any output drive impedance up to about 1 KOHM or
less would work without a large fluctuation in resulting performance) ... the lower the better.
Then we assume that the following “semi-pro” gear has a “non-loading” input impedance
anywhere from 10KOHMS up to 510KOHMS or above (the higher the better).
SOLUTION: We will refer to our gain chart and find that a “gain”, or ratio, of 5 to 1, will translate
to approximately 15 dB. We are trying to drop 14 DB of level to match the “pro” output to the
semi-pro input but we don not have to be exact. A loss of 15 dB will be close enough.
SO, IF WE WANTED AN AMPLIFIER TO HAVE A GAIN OF 5, (AMPLIFYING THE SIGNAL
FIVE TIMES), OR 15 DB, THEN WE’D NEED TO CONFIGURE OUR RESISTORS FOR A
RELATIONSHIP, OR RATIO, OF 5:1. Now, though, what we need is a simple PASSIVE
(meaning no active, powered circuitry) circuit to give us our “loss”, or attenuation, of 5 times ...
or 15 DB. SO, we still choose a resistor ratio of 5 to 1.
Of course, we could very well do the job with an op-amp configured for a loss since you’ll learn
that they can be used in this manner to provide active loss as well as active gain. But, another
DAVIS DOWN-HOME LAW states that we should “NEVER USE OVERKILL TO DO A SIMPLE
JOB”! Why spend the coins and the time for an op-amp based active circuit when a simple
PASSIVE circuit will do the trick almost as well? This is a practical example of applying the
philosophy of “KEEP IT SIMPLE” never using active circuitry when passive will get it done! That
is because, no matter how near perfect an amplifier is, it always adds SOME “color”...some
minute noise/distortion to the straight wire signal which always has the potential, if enough are
cascaded together, to become audible!
[Next, after VERY extensive discussion of op-amps and various
building blocks in conclusion of Part three ...]
Pots & such ...
To the right side of the last diagram, you will note a schematic representation as well as a layout
drawing of the 1 MEGOHM potentiometer, or “pot”. As already explained, a pot is simply a
variable resistor with a movable tap that slides along the element. It normally has three
terminals, one at either end and a center tap for the “wiper” or slider. As you can see from the
diagram, the schematic representation is the same “ZIG ZAG” with an arrow representing the
moveable element. In this case, the arrow is shown tied back to the first terminal. This is known
as “strapping” and is often used for a variable gain control (variable resistor) in this manner.
A “pot” is the most basic and often used audio device as it performs as a volume control, gain
control, tone control and mixer control to name a few applications. After many years of using the
terminal designation as shown in the diagram, 1,2,3 in left to right fashion, I later learned that the
industry standard was “BACKWARDS”. Well, rather than change the logical way of looking at a
pot and numbering it, some of us decided to just keep our own logical “standard” of 1,2,3 instead
of 3,2,1 (Craig Anderton, one of our recommended authors, also designates pots terminals in this
“backwards” fashion as do many other contributing authors!). Chet Atkins would often joke that
buy the time he learned the “proper” way to tune a guitar he was “too rich to quit” doing it his
way. Well, riches (don’t I WISH!) have nothing to do with my stubbornness but I can’t force my
old brain to change something as basic and intrinsic as...left to right ... 1,2,3!
As it happens, about the worst that would occur should you accidentally wire a pot “backwards”
would be that it would work that way! That means, turning it counterclockwise would turn the
volume UP instead of DOWN and vice-versa. Also, your EQ controls could be wired to work
backwards ... a good prank to pull on someone maybe but not much use otherwise!
[Part Four continues to chronicle the evolution of op amps and
other audio ICs and discusses
which specific parts were turning up in which big name brands of gear ...]
[Abridge]
You may have heard about the craze of the early nineties whereby a few “ parts sharks” had
convinced a large percentage of the pro audio industry that they needed to ...”UPGRADE” to
so-called “Video High Speed OP Amps” ..for a nice profitable fee to the sharks of course ...!
This sickened me because I have never “sold out” for a quick buck to the gullible yet these guys
were taking “money for nothing” right and left and being praised for it besides in many cases!
Ahhh ... the “sophomores” of the “pro” audio industry! It just goes to prove what my Ma always
said about “ a little learning” being a dangerous thing!
Well, one reason why the “upgrade” often sucked was because .....
[This section then goes on to list the "why's" of the failure of
many upgrades based on plugging in
high speed op amps into sockets on PCBs without the proper, or with the IMproper, support circuitry ... ]
[Another narrative is presented along the way to illustrate many faulty
instructive articles being published in trade magazines ...]
The lesson here is ...THAT GUY IS TYPICAL OF A LOT OF "CONTRIBUTING AUTHORS"
TO TECHNICAL MAGAZINES SO ..WATCH OUT FOR PSEUDO-EXPERTS! The real trick is
to learn as much as you can about the particular "quirks" of the various types of op-amps and the
various families of the devices in general. Hopefully our in-depth discussion has armed you with
the bulk of all you'll ever need to know! But...remember Davis' First Law and keep on
doing the reading and your own research!
[Abridge]
Part 5
Other IC Devices/Manufacturers ...
Before we leave the family history section of op-amps, let me present a kind of “digest” regarding
some of the esoteric devices and companies supplying them. I wanted to lead you up to today’s
devices by giving you a solid foundation in the evolution of audio IC’s toward today’s
“state-of-the-art”. Because of the huge variety of devices and families of devices now available,
to cover the each of the specific parts from the huge array of devices available today in both
standard packaging, as well as micro surface mount devices and LSI devices, is not possible
within our scope herein and will not be attempted. However, we will point you in the direction to
find out more from each source should you wish.
[Then the long awaited Basic Ciruit BUILDING BLOCKS ARE PRESENTED
and described in extensive detail ...
Then, in the following section ... CIRCUIT CONSTRUCTION TECHNIQUES
are revealed and examined] ...
Part 6
[the Basic Building Blocks section is concluded and then ....on to] ...
GROUND RULES!
Time to get “reverent”, dear reader, as you are about to enter
and explore ... HOLY GROUND!
... the most acclaimed section of our Tech Reports! This is the section that many say is WELL
worth the price of the reports many times over, all by itself!
Now that we have given you several working circuit diagrams for several VERY important audio
building blocks ... now's the time to inject another subject important to building your own "blocks"
from the GROUND up (pun intended)!
59
MORE GROUND RULES ...
The subject of audio ground systems could fill an entire book by itself! What is basically the
simplest THEORETICAL concept in electronics turns out to be the most COMPLICATED of all
the REAL WORLD electronics concepts! In other words...GROUND AIN'T ALWAYS GROUND!
Few things in the real world ever meet the theoretical ideal but few things are as unpredictable
and perplexing as audio ground systems. A grounding problem can often turn up where you least
expect it (and when you least need it!) and then drive you UNDERground as you try to unravel
the mystery!
But, take heart! Once again ...our TECH REPORTS to the rescue! Keeping in mind always that
"clean" audio begins with "Clean" grounds ...if you learn to accept a few basic principles as
second nature ..and that can mean running them in the ground ('nother pun)...until YOU are well
grounded in fundamentals ...it will help you design and construct systems with much fewer
*!#@?!%!!"
The IDEAL AUDIO GROUND IS ONE, SINGLE POINT! One single spot, preferably at the
power supply common, connected by conductors from the various system sub-elements. These
ideal conductors, of course, would have NO resistance and would, therefore become actual part
of the perfect ground system itself. Such is the theory behind the STAR ground system
approach.
There is one major flaw for accepting this theory however, and that is, in the REAL world there
“ain’t nothing” close to perfection!
Unfortunately, the wires connecting a system together DO have resistance. In fact, the metal
chassis has a slight resistance from one point to another! Anytime there is resistance in an
electrical circuit then there is current flow so, each ground lead, no matter how minute, has
resistance and carries current along it’s length. The KEY is, to minimize this as much as
possible! Otherwise, these minute currents and their respective voltage drops will invariably be
amplified somewhere else along the chain and can wreak havoc!
[Abridge]
So now, here is yet another of our many "GOLDMINE TIPS" (any ONE of which could be worth
the price of the complete info package and THEN some when you become involved with building
your own audio gear). In relation to that "ground follows signal" approach that we have just
previously described, there is one little "tweak" that will always guarantee superior results in
MIXERS and CONSOLES using the "virtual earth" mix amp (the active combining network
discussed in detail earlier)."
[Abridge]
CLEAN AUDIO STARTS WITH CLEAN GROUNDS! THE REAL WORLD (Not always a nice
place!) ALWAYS CALLS FOR COMPROMISES! YOU HAVE TO ADAPT AND DO THE BEST
WITH WHAT YOU HAVE!
The rules continue ...
Well, we have so far given you enough to get you started on the road to confusion, no?
“Busses”,”Stars”, “One Point”,”Follows Signal”- Any Contradictions? Not R E A L L Y ...READ
ON ...
[Abridge]
... Let me try and clear things up a bit by giving an example of how a signal chain should be
grounded and why. To do this, I’ll use a worst case scenario and I’ll venture outside of the audio
realm just a bit to do it. In this example, we will combine a “BUS” system...with the GROUND
FOLLOWS SIGNAL approach and, at the same time, we’ll keep our “family” of grounds together!
How is all of this possible? Just follow this “TRAIN” of thought (‘nuther intended pun!).
This will occur because our signal flow blocks will be laid out in families very much like a train
with all its cars.
[Abridge]
Well ...now that we are pretty well grounded in grounding technique ... let’s slip back briefly to
that subject of the “VIRTUAL ground” that we covered back in the section on the inverting
amplifier. This is the “later” that we said we would get to in regard to expounding on it
somewhat. If one of your goals in learning all of this stuff was to aid you in building your own
AUDIO MIXERS then, this next tip is one of the BEST ALL-TIME GOLDMINE TIPS EVER! And, I’ll bet
you won’t ever see it revealed anywhere else!
Here it comes ...
[This section ends with one of our most valuable ..."GOLDMINE TIPS"!]
All excerpts are copyrighted material from TECH REPORTS
©1982,1986,1999 by Hayne Davis
This web page ©2000 by DaviSound-All Rights Reserved