it=E2=80=99s been awhile. a tough bunch a months.
at the time of writing this there=E2=80=99s an election looming, in =
the usa, that=20
will set the course of the next generation and it=E2=80=99s economic =
future. there=E2=80=99s =20
=E2=80=9Cfrankenstorm=E2=80=9D on the american east coast and an =
earthquake and tsunami on the=20
west: all matter of unnatural and natural catastrophe. in the usa, very =
few=20
seem to realize there are just a few small mistakes left between =
generally=20
crappy and a major meltdown. that this was caused by the former =
republican=20
president and his advisors has been both completely forgotten and =
successfully=20
made vague. =E2=80=9Cit=E2=80=99s the black guy=E2=80=99s =
fault=E2=80=9D. the de-distribution of american=20
wealth that began under bush=E2=80=99s watch has continued helter =
skelter. never in the=20
history of personal wealth have the wealthy been so rich and in my =
opinion, so=20
undeserving of it. many of them make nothing. they speculate, gamble, =
con or=20
just plain steal. like goldman sachs. like morgan stanley. like =
citigroup. like=20
bain. on the flipside, poverty is now almost mainstream. it is crazy, =
to me, no=20
one seems to care how this happened. most just want a slice off the =
flank. it=E2=80=99s=20
business as usual. those responsible for informing the public have been =
downsized, both teachers and journalists. propagandists have replaced =
them, and=20
are actually much more profitable. conspiracy theories, the price of =
gas and=20
who gets to boss the vagina around gets more attention than why =
it=E2=80=99s that way=20
in the first place. the mainstream media=E2=80=99s open racism and =
radical revisionism=20
of all history, and even of science, has tainted the process to the =
point where=20
it is actually hard to find out what the fuck IS going on. you have to =
watch=20
the fake news to make any sense of it (thank goodness for john stewart =
and=20
stephen colbert). comedians, like chris rock, make fake public service=20
announcements that are way more honest and useful than any political=20
organization. it should be funny=E2=80=A6
hanging over the american election: corporate money has utterly =
corrupted the=20
democratic process. if romney wins, he will have proved that elections =
can be=20
bought in the usa.
and things are hard everywhere! the storm that trashed the east coast =
just=20
now, trashed the caribbean before that. there=E2=80=99s no electricity =
or safe place to=20
sleep, or sanitary way to take a shit, or a shower=E2=80=A6 for lots of =
people there.=20
the eu=E2=80=99s money problems persist and inequality there threatens =
to split the=20
union. the arab spring has collapsed. syria, jordan and lebanon are =
primed to=20
blow together! beyond the false pride, economics and human costs of the =
wars we=20
just can=E2=80=99t seem to =E2=80=9Cwin=E2=80=9D, corporate greed and =
avarice have become status=20
values. the answer. things to take comfort in. =E2=80=9Cat least our =
billionaires are=20
richer than yours=E2=80=9D, =E2=80=9Cwe=E2=80=9D must be doing =
something right? this is how we do=20
business. god bless.
it feels a little odd and even a bit shameful to be thinking about =
circuit=20
hacking under the circumstances=E2=80=A6 about art and craft. about =
making weird=20
machines. although it is a nice place to escape to. i have friends who =
have no=20
lights, heat or water at the moment. it=E2=80=99s getting cold and =
there=E2=80=99s a nor-easter=20
on it=E2=80=99s way to nyc. i guess i want to say that i feel =
compassion, sadness, and=20
anxiety for the people over there. i am grateful i have a job and a =
roof over=20
my family=E2=80=99s head.
i want to go back to a discussion of current amplifiers and also of =
current=20
feedback. i=E2=80=99m a little rusty at the blog thing=E2=80=A6. this =
will be a good way to=20
ease back into it.
the ee universe is a man=E2=80=99s world, sadly. yes, and a man =
worthy of the name,=20
pays attention to the volts. you think i=E2=80=99m joking?! i am simply =
stating a fact.=20
ours is a voltage-centric discipline. a =E2=80=9Cthevenized=E2=80=9D =
kind of bag. thevenin is=20
the ralph cramden of engineering attitudes, and there is nothing more=20
conventional and everyman than that. CAD relies heavily upon it too, as =
nearly=20
all equivalent circuit simplification starts there. if you are not sure =
of what=20
i am talking about, please go look it up. it is worth your time and =
will help=20
you in your work. i am simply going to the punch line because i need to =
get=20
some where else, but i want you to know why. because of thevenin, we =
tend to=20
analyze active devices in his terms. an ideal voltage amplifier is a =
voltage=20
source with an output impedance of 0 ohms. a short circuit. and it =
likes to=20
work UP, into it=E2=80=99s load, which has some finite value.
as with everything else under the sun, there is another side of the =
tracks=E2=80=A6 =20
and it is called norton. the bald headed goofy well-meaning =
idiot-savant.=20
emphasis on savant. well educated EEs will of course point out, and =
rightfully=20
so, that in the end, thevenin and norton are simply flipsides of the =
same=20
insight: that no matter what kind of electrical circuit you have, it =
can be=20
simplified into one relationship between volts, current and impedance =
right=20
where the source of power meets the circuit. they come to the same =
conclusion.=20
one pushes a bus and the other works in the sewer. yes yes=E2=80=A6 =
well said.
i say, there is a practical limit there which tends to be pushed =
aside in our=20
haste to get to the end of the job well done. there isn=E2=80=99t a =
practical=20
amplifying device made today that is a short circuit or reliably drives =
one.
on the other hand, norton has a different perspective of these =
things. the=20
current-centric view. and a current source can be viewed, ala` norton, =
as an=20
infinite impedance looking DOWN into a finite load. so what? as noted, =
you wind=20
up with the same result. the big difference is that all of the =
practical=20
devices we have to work with today are much more reliable, or how about =
=E2=80=9Chappier=E2=80=9D, as open circuits than they are as shorts. =
vacuum tubes in=20
particular, and especially tetrodes/pentodes, could be said to fit this =
view=20
very nicely.
voltage feedback and current feedback have some related twists =
relating to=20
thevenin and norton practically, but as my perspective is =
controversial, i am=20
going to save most of that for another day. it is uncontroversial to =
say that=20
voltage feedback and current feedback transform practical amplifiers in =
slightly different ways. there can be advantages to current feedback in =
the way=20
it handles impedance transformation=E2=80=A6 even though in the end you =
have the =E2=80=9Csame=E2=80=9D=20
result.
below is a simple example of one of the most basic ways to return =
negative=20
current feedback to the first stage. most modern voltage feedback op =
amps are=20
actually based on this exact arrangement. the main difference is that =
this is=20
half of the circuit. the op amp would have a differential input. but in =
this=20
case, the =E2=80=9Cinput=E2=80=9D for the feedback circuit is low =
impedance=E2=80=A6 the cathode. the=20
input for the signal is very high impedance, the grid. i think this is=20
accurately describes a current feedback amplifier. i also believe =
modern=20
current feedback op amps are different in that they respond to current =
at the=20
inverting input. but that shouldn=E2=80=99t change this discussion. =
we=E2=80=99ll see who yells=20
at me for all this=E2=80=A6
here, you see two grounded cathode inverting gain stages, plus a =
cathode=20
follower buffer, which returns it=E2=80=99s current variations (the =
swing through it=E2=80=99s=20
Rk) back to the cathode resistor of the first stage. these current =
variations=20
are in opposite phase to those in the first stage. consider what =
happens when=20
the cathode follower has to dump extra current into a load=E2=80=A6. =
loading it down.=20
there is less current variation in Rk. less CFB. more voltage into the =
buffer.=20
this tends to stabilize the output. this is classic negative feedback =
error=20
correction, done with current. one fabulous feature is that the error =
signal is=20
DC coupled.
above is a very fancy practical hybrid version with metal pentodes =
and=20
extraordinary performance. the 6SJ7 (triode wired) is set for a gain of =
24 dB.=20
and the C3g is a Gm amp, with a gain of 65 dB. that is roughly 90 dB of =
gain=20
for the two. the bandwidth is DC to -3 dB at 18KHz open loop. the =
buffer is a=20
depletion mode mosfet with 10mA going though it. that is probably not =
enough=20
but it is what i have in the supply. there are better parts for this =
job=E2=80=A6 yes,=20
this combination is sure to piss off a few! of course that=E2=80=99s =
why i do it. i=20
will wire up a version with submini tubes (solder leads) under the hood =
for=20
comparison. here, i have dialed in 58dB of current feedback for a =
closed loop=20
gain of 32 dB. the 20pF compensation cap is for stability. below is a =
view of=20
the prototype. the heatsinks are for the current source and buffer =
mosfets.
here=20
are some measurements for you. first is a 250 Vpp sine wave at 20KHz. =
it looks=20
like a sine wave but nice and big. 50 volts per centimeter. the next =
image is=20
much more impressive. it is an X/Y measurement of the same signal. note =
the=20
simple straight line. it is completely closed. without a distortion =
analyzer or=20
phase meter i can still say that this 250 Vpp is provided with well =
under 1%=20
distortion, and well under 1 degree of phase shift. probably under 0.1% =
and 0.5=20
degrees.
that this was accomplished with 5 active devices is pretty cool. the =
two gain=20
stages are tubed. it could be all tube, but that would put a bunch more =
holes=20
on the top panel, and there isn=E2=80=99t any real advantage. the =
current source and=20
the buffer don=E2=80=99t have to be tubes. also, to do this in only =
solid state is not=20
that easy. most of the practical devices available have too much input=20
capacitance (and the C is too voltage variable) and the high voltage =
would=20
cause problems. you could go to more devices and get to the same place =
where=20
this design is=E2=80=A6 cascode high voltage transistors, or mosfets. =
but there would=20
be a great deal more phase shift at 20KHz with any 2 high voltage =
transistors=20
and the same mosfets. the tubes i use here have very low input C and =
and are=20
voltage controlled.
most tube circuits aren=E2=80=99t expected to work this well. =
especially in the DIY=20
world. but also in the =E2=80=9Chigh end=E2=80=9D world=E2=80=A6 of =
course, there will be many who=20
immediately ask, =E2=80=9Cbut, how does it sound?=E2=80=9D. =
=E2=80=9Cfeedback! what the fuck!=E2=80=9D =E2=80=9Cwho=20
needs it to be that good!=E2=80=9D and, =E2=80=9Cit ain=E2=80=99t =
natural=E2=80=9D (for some reason i hear=20
jeffrey jackson saying this to me=E2=80=A6). look, electronics is the =
antithesis of=20
natural. i say, it all depends on what you use it for=E2=80=A6 i plan =
to drive some big=20
globe 50 tubes with it. i=E2=80=99m pretty happy about it. i=E2=80=99ll =
have more to say about=20
this=E2=80=A6 later.
the square wave pic simply shows that the X/Y plot is very real. it =
is boring=20
in it=E2=80=99s simplicity. no ringing, no rounded edges=E2=80=A6 =
it=E2=80=99s just a plain old 1 KHz=20
square wave. 22 Vpp with half a volt in.
now for an interesting variation, imagine making the 3rd stage a =
transformer=20
coupled grounded cathode stage=E2=80=A6 but what about the phase? it is =
an inverting=20
stage at the plate but the current variations through the tube are the =
same as=20
for the cathode follower. it is still negative cfb returned to the =
first stage.=20
but in this case, the output impedance at the plate will be increased. =
the=20
unbypassed resistance of the first stage=E2=80=99s Rk (which is in =
series with the=20
output stage=E2=80=99s Rk), times the gain against the error signal =
there. variations=20
in load impedance will have the opposite behavior of the typical =
voltage amp.=20
with the source impedance (the amp) higher than the load=E2=80=99s, the =
output current=20
will tend to be regulated by the feedback, and the voltage will track =
the=20
impedance.
now why would that be cool?
well, imagine you are interested in driving a =E2=80=9Cfull =
range=E2=80=9D speaker (a=20
misnomer) or a compression driver connected to a horn. both of these =
speakers=20
have difficulties at the bottom (near resonance) and at the top (roll =
off from=20
the mass of the piston). a traditional voltage feedback power amp (with =
an =20
=E2=80=9Cinfinitely=E2=80=9D low source impedance) would have a good =
=E2=80=9Cdamping factor=E2=80=9D and would=20
dump more current into the load as it approached 0 ohms, and less as it =
increased. it would have to do this in order to keep the output voltage =
=20
=E2=80=9Cregulated=E2=80=9D as compared to the input signal. but, this =
would result in a loss of=20
bass and treble into both =E2=80=9Cfull range=E2=80=9D speakers and =
compression drivers, as the=20
impedance is rising at both of these frequencies=E2=80=A6 and the =
efficiency is fading=20
at the low frequency end, at the same time. a current amplifier, with =
it=E2=80=99s=20
higher source impedance, would look at these particular loads quite=20
differently. the output voltage would track the impedance of the =
speaker. and=20
you would have extension of both the bass and the treble.
this is interesting for me as i use manger drivers in my system. =
mangers are=20
complicated drivers. they are bending wave drivers above 400 Hz, really =
inefficient piston drivers below, and beamy laser beams above 1 KHz. i =
have a=20
love/hate thing with them, for sure. but they do one thing better than =
all=20
standard moving coil dynamic drivers: their impulse response is almost=20
perfectly in time with the drive signal. if you can cross them over to =
a bass=20
driver low enough, and keep the woofer output out of the midrange as =
much as=20
possible, and then deal with the beaminess=E2=80=A6 (i have gone the =
way of the=20
diffusor), you have something pretty special. at least for a small to =
mid sized=20
room. they do make excellent monitors for recording as there is no =
appreciable=20
group delay or phase shift over the range of 150 to 30KHz (!). they can =
reproduce a square wave. the number of speakers ever made that can, can =
be=20
counted on one hand.
if=20
i could get a bit more extension at the low end of the manger, it would =
help me=20
with the crossover to the woofer=E2=80=A6 i want as little woofer =
output as possible=20
mixed in with the manger. so i put the woofers on the sides of the =
baffle, with=20
an electronic crossover (18dB/octave). this will be steeper in practice =
with=20
the mechanical crossover of the baffle added in=E2=80=A6 also, i can =
use the back wall=20
as a boundary reinforcement, to get a boost in the very low =
frequencies. this=20
allows use of smaller woofers (2 of them). but the fact is the manger =
starts to=20
crap out at 400 Hz. and X max is 2mm. they are easy to blow up, and =
crazy=20
expensive. we have had a great deal of experience with them at =
silbatone.
so, back to the current amplifier. i have made one using an 807 beam =
tetrode.=20
it drives the manger safely to 100 Hz, and drops off. at the top, i get =
a 3dB=20
rise in output up around 9KHz with the amp, which flattens out when the =
diffusor is installed. it=E2=80=99s a touch dull with the regular amps. =
i=E2=80=99ve been=20
listening to it full range for a few months now and it is wonderful. a =
good mix=20
of compromises. the diffusor was designed by MJ Chung. it works very =
nicely. no=20
more laser beam.
but this isn=E2=80=99t the only thing i am working on=E2=80=A6 i =
think the most useful place=20
to use a current amp is on compression drivers. the big advantage now =
shifts to=20
the high frequency end of things=E2=80=A6 because of mass roll off, =
many compression=20
drivers, especially older ones, are limited to a fundamental range of =
about 6=20
to 12 KHz. think of a WE555 or an altec 288. modern drivers have wacky =
tricks=20
to extend the range by use of resonance and peaking. it has never =
impressed me=20
as a sound approach. the classic approach was equalizing the driver and =
using a=20
super tweeter. a current amp offers a middle way. the output rises with =
impedance. the amp equalizes the output. it is very natural and not =
anywhere as=20
harsh as what you get with the modern beryllium peaking types. i have =
only=20
heard this on a an altec 288A. but plan to test out some things soon on =
the=20
WE555 and WE594. more on that, later. the real interest for me is with =
full=20
range horns. and drivers like the WE555. the vitavox low resonance =
freq.=20
compression drivers are also very interesting to me.
obviously, if you are into =E2=80=9Cfull range=E2=80=9D drivers, =
current amps are something=20
to check out. by the way, nelson pass has been into this for some time. =
way=20
longer than me. he has a bunch of really interesting stuff on his =
website along=20
these lines. i do have to control my gag reflex when it comes to =
lowthers, but=20
many people like them. a current amp could (nothing can save those =
pieces of=20
shit) help=E2=80=A6?
here is the 807 current amp design adapted for 2A3s. i have =
substituted more=20
common tubes than i used. i can recommend the 2A3 EH (made in russia), =
but=20
6B4Gs or any of the extended family would do=E2=80=A6 or even a 6AV5 or =
6550 could be=20
crowbarred in there. i used one electron output trannies for this test =
and they=20
didn=E2=80=99t disappoint. you can use whatever you can get=E2=80=A6 =
fun, and interesting.
addendum!
after a great deal of trouble driving the output stage with the =
current=20
feedback driver stage shown above, i had to back up and get some =
help=E2=80=A6 2 quick=20
conversations with morgan jones and frank bl=C3=B6hbaum (at etf) helped =
enormously.=20
i blew up a bunch of fancy depletion mode mos, and normal run of the =
mill=20
hexfet enhancement mode mosfets too. it was hard to see what the =
failure=20
mechanism was (morgan was extolling the virtues of inexpensive digital=20
oscilloscopes for the ability to record events=E2=80=A6 i guess now i =
have one on my=20
christmas wish list). it eventually turned out to be the gate charge =
and the=20
extremely high impedance of the gM amp (the C3g stage). you learn =
something new=20
every day. i had diodes and up to 5k gate stopper. boom. especially on =
big=20
transients. i ended up using an IRF730 with a 10k gate stopper (!), a =
12 volt=20
zener from gate to source, and a 100 ohm source resistor in order to =
make it=20
reliable. but the performance is affected. the much lower C in of the =
IXCP45MS=20
allowed a large amount of current feedback (58 dB) with relatively =
modest=20
compensation. the much larger value of input C on a hexfet changed =
everything.=20
it was much harder to manage it without oscillation. i ended up =
increasing the=20
compensation cap to 420pF! it had been 20p. and it is still just stable =
at=20
clipping=E2=80=A6 i will work on making the fancy mos more reliable. =
while this=20
measures and sounds excellent, it can be better. it makes me appreciate =
tubes=20
all the more.
i switched to a submini tube as a cathode follower (because a solder =
lead=20
tube fit under the chassis and i didn=E2=80=99t need to drill any more =
holes) and=20
everything was perfectly stable with much less compensation. it is so =
easy to=20
compensate tubes, at least if you know them=E2=80=A6 but the insertion =
loss was 50=20
volts! the mos dropped 5. my 250 volts of swing became 200 just like =
that.=20
tubes just don=E2=80=99t have the transconductance to make really good =
simple buffers.=20
they need help. they sound good, don=E2=80=99t get me wrong, but solid =
state does=20
buffers without losing so much level. at line level, who cares. but for =
lots of=20
volts, they eat up swing.
so back to the mosfets. i have been compensating the driver stage by =
using a=20
shunt cap to ground from the C3g plate. this is supposed to help define =
the=20
pole associated with this stage and prevent it from screwing with the =
other=20
poles=E2=80=A6 particularly the buffer and opt tube. i have switched to =
using a=20
=E2=80=9Cmiller=E2=80=9D compensation scheme (basically the cap goes =
from plate to grid)=20
instead and everything works much better=E2=80=A6 why throw bandwidth =
away for=20
stability without getting anything for it? miller effect is negative =
feedback.=20
you get the compensation needed AND distortion is reduced in the =
attenuation=20
band. now, even the fancy depletion mos is stable and i don=E2=80=99t =
need more than=20
22p of C.
i meant to drive a 50 tube and test something i have wanted to for =
years.=20
50=E2=80=B2s are tough to drive because they have a largely variable =
input admittance.=20
for this reason, they expressly do not recommend fixed bias and most of =
the=20
amps you will ever find that used them were transformer coupled to the =
grid=20
(low impedance) and self-biased. the grid current varies from negative =
to=20
positive quite a bit from cutoff to saturation. with fixed bias, there =
is=20
nothing to prevent runaway if the bias shifts due to clipping or =
asymmetrical=20
waveforms of large amplitude (drums and synth pulses from dance =
music=E2=80=A6). i want=20
to push the envelope and see what i can get away with. the problem is =
the=20
resistor connecting the 50 grid to the source of bias: for cap =
coupling, this=20
is usually on the order of 100k to 500k. this allows a cap of =
reasonable size=20
to be used, typically .047uF to .47uF, for low frequency performance. =
this=20
maximum requirement is always reduced for fixed bias=E2=80=A6 an EL34 =
in fixed bias=20
shouldn=E2=80=99t have it=E2=80=99s Rg exceed 50K. most amp =
manufacturers have no clue. this is=20
a major source of failure in the musical instrument amp business. a 50k =
grid=20
resistor would need a minimum of 1uF and probably more like 2uF for =
anything=20
quality in an RC coupled fixed bias amp, if you wanted a reasonable low =
frequency performance. that is a big and expensive part.
i have always got around it by direct coupling. no cap is the best =
cap. but i=20
wanted to know just what the range was=E2=80=A6 and my contrary nature =
began to tug at=20
me: who says it can=E2=80=99t be done! so i want to make a reliable cap =
coupled fixed=20
biased 50 amp. driven with metal pentodes and very high performance. =
fixed bias=20
gives a bit more power. when you are talking about a 4 watt amp, an =
extra watt=20
doesn=E2=80=99t hurt.
so here it is. after a bunch of head scratching, it works fine. not =
only is=20
it fixed bias, but cap coupled. it sounds fabulous as it =
should=E2=80=A6 it makes just=20
under 6 watts at 2% thd. almost 1 and a half more watts. i even tested =
an=20
electrolytic coupling cap (4.7uF 400V, bypassed with a silver mica =
.001) and it=20
still sounds and tests excellent. look, i just can=E2=80=99t help =
it=E2=80=A6 tell me i can=E2=80=99t=20
or shouldn=E2=80=99t and i just have to try. panasonic won again, by =
the way, over=20
sprague and illinois. but now i am using a 2uF film and foil. i worry =
about=20
such big AC swings across an electrolytic=E2=80=A6 if it =
weren=E2=80=99t such a rare output=20
tube, i would let it rip. the grid resistor is 12K. i started out with =
5k (and=20
the 4.7uF electrolytic). the drop across the resistor at clipping is =
0.4 volts=E2=80=A6=20
not enough to worry about. at 50k, this would be 3 volts less bias. =
100K would=20
be 5 or 6. you see the problem. the grid current and the voltage drop =
across=20
the grid resistor would destroy the tube.
and=20
finally, here is a look at the top. now i have to make the other =
channel=E2=80=A6 bleh!=20
i need an intern. more soon=E2=80=A6