AM Precision audio detector
Patrick Turner, of Turner
(Australia) has made considerable
improvements to the original comp. follower detector circuit. The
following has been abstracted from e-mail correspondence (19th Nov.
2015 - 24th Nov. 2015) with Patrick's permission. Note that this
information relates to an experimental project in progress, the aim of
which is to reduce THD in the envelope of of an AM signal by using a
precision detector for NFB. The reason for doing so is that passive
modulators are not as linear as is widely believed. More information is
expected to appear on the Turner Audio website.
" I built a version of your idea for AM detector using a discrete bjt
op-amp, where I could take audio output from two collector load
resistors between collectors and +/- rails. I'm making an AM modulator,
and want to detect the raw AM wave to give a NFB signal that is applied
to a diff. pair with input signal to other side. The output from one
side of the pair is to the AM modulator, and the NFB removes the
several % of envelope shape THD. This idea is old. HP used it with
tubes in the ancient 1960s HP606A; good for 50 kHz to 65 MHz carrier,
and quite a wide AF bandwidth for the 0.5 MHz to 1.7 MHz AM band. So
far, my op amp will allow 5 MHz of carrier, while your original
schematic I found did 1.7 MHz when I built it here. Probably, I could
have used better / faster bjts for the discrete op-amp, but I don't
know what type numbers to choose.
The circuit I built first to test the idea is:
Notice I have quite high currents
and low RLs for all bjts, but this gave reasonable detection to 1.7 MHz.
Higher Ic = higher bjt gm
= higher voltage gain.
Then I decided to use my favourite discrete op-amp with symmetrical PNP
& NPN devices all the way to the output.......
This one goes to 5 MHz with 1 V
rms carrier input. At 100% mod. the carrier has a maximum of 2 V rms,
and slew limiting flattens the +/- peaks of modulation, so 0.5 V rms
input is better.
I found that a 0.5 V
rms un-modulated carrier was -3 dB down at 10 MHz, a fair result from
parts bought at a suburban Jaycar
store for obsessive
hobbyists. In other words, the op-amp is a linear follower with pole at
10 MHz, but you can have only 0.5 V rms. So when you have 100% mod, you
have 1 V rms at R20 and the pole moves down to 5 MHz. But at 100 kHz,
then there's a lot more output available at R20 100 Ω. Higher
values for R16 and R19 than 330 Ω means that the peak swing
away from the rails can become too much....
But this works better
than all the IC op-amps I've been able to buy easily. PN100 is a
generic replacement for 2N2222, and I've found it quite a good bjt.
BF470 and BF472 are "video bjts" which I saw in a 1982 audio amp
schematic by David Tilbrook. Just how much faster it could it go with
better bjts is not known.
Both the above
schematics circuits require R&C critical damping at HF to
prevent oscillations above 6 MHz (see C6, 100 pF, plus R6, 100
Ω, to shelve the HF gain). Connection of a small amount of C
across output RL R20, say 150 pF, does cause some oscillations. So
success relies on using only R to load the amp, C causes extra phase
shift as f rises, so it oscilates.
But the output for AF
is NOT within the GNFB loop; this is the real benefit: no diode and C
across R20. The output is from a collector current
, not a voltage source enforced by FB, so
that the AF circuit looks like say 20 k feeding 330 Ω, and if
each rectified ½-wave was 1 V pk, then average V dc out =
0.315 V dc if capacitance across the 330 Ω is large enough to
shunt all of the ac.
The shunting of
collector loads does not upset bjt function. If C shunts RF, but not
AF, then the C would reduce Miller capacitance to the base inputs. The
peak to peak max for AF would be 0.315 V, so max V AF is 0.157
× peak half wave V ac. This type of detection doesn't involve
diode charging and discharge, so no time constants to worry about
causing slew dn. Its not as efficient as diode detection, but it seems
to work just fine. The value of R20 100 Ω seems high, but you
don't want R20 too low, and its peak current is applied to 330
Ω so you get 3.3 × peak V dc in 330 Ω,
thus compensating for ineffiencies. I like headroom, hence the use of
±15V supplies. Using just +12 V dc will work, but gives
lower max Vo, or, put another way, higher rails give more Vo with low
THD, or the same as for 12 V rail, with lower THD.
The use of symetrical input pair goes back 30 years to innovative audio
circuitry, with NPN and PNP diff. pairs working in parallel, and a gain
stage with PNP & NPN bjts in series - so no R loads, so huge
gain, and this is a good driver for a mosfet amp. Many things are more
complex that what I came up with. My pages on power amps
(solid state) tell the
story of the 2 × 300W amp I made in 1996. I was inspired by Douglas
and others who wrote in Electronics World.
But the op-amp I made
here can only make 0.5 V rms at 10 MHz. At 1 kHz, it can do far more,
but the higher the Vo, the greater are the internal gymnastics to keep
the output voltage linear to input, because like all DD devices, they
slow down with f increase.
I built one good Vac analog meter
, OK for Vac 5 Hz
to 250 kHz at least, 1 mV to 300 V ac, in 11 ranges 10 dB appart. It's
good, and I built another, with 3 Hz to 300 V ac, but up to 6 MHz, with
the faster discrete op-amps. The secret was to use a bridge rectifier
with Ge diodes or IN5711 Schottky in the NFB loop, so Vdc to drive
meter is linear to the RMS signal current in NFB path. I am not sure if
the principle could be used to produce demodulated AM - maybe.
Maybe 60 years ago, in 1955, Selsted & Smith came up with an
"infinite impedance" AM detector for which they made lots of claims
(its in the rear end of the RDH4 - Radiotron Designers Handbook, 4th
edition). I tried it, using tubes of course, but it's not as hot as
I then invented a
detector using a 12AU7 cathode follower to buffer the last IFT output,
then a diode to 270 pF, with 1 MΩ to -50 V dc, then 100
kΩ + 50 pF LPF, and then a second CF to buffer the detected
audio signal (see Radio re-engineering
, - Other
misc. AM radio schematics). Well, this is
and I've done it with darlington connected bjt followers, and it's all
good, but there's a flat spot on the AF at more than 95% mod, because
of diode drop. Ge works well, with Schottky maye better (IN5711), but I
could easily get up to about 10 V rms AF with THD < 1% at 90%
mod. This detector is far better than in every accountant designed AM
radio I had brought to me for singing lessons. It also beats everything
in RDH4. However, at carrier Vc < 0.5 V rms, some THD creeps in
when mod % is high. But I didn't want good long distance AM reception,
and with strong locals the AVC network keeps the carrier and hence AF
quite high with low THD.
Notice the R added across the last
IFT LC, this is usually 100 kΩ to slightly reduce Q so
extending IFT bandwidth and hence AF bandwidth. The use of what is a
virtual CCS to discharge 270 pF does allow condsiderable RF ripple
without causing THD. The 1 MΩ gives idle 0.05 mA dc through
the diode, so it's permanently slightly 'on', so the forward V drop is
reduced and RF ripple voltage is fairly constant for the AF voltage
produced. Low diode self C is a must, so maybe IN5711 is better than Ge.
I've used this
circuit to detect AF after the output stage of an RF gene I made with a
12AU7oscillator and a 6BX6 output tube. Such simple cathode modulated
pentodes have typical envelope THD of 7% at 90% modulation.
So the use of triode
diff amp with - you guessed - 12AU7 gave open loop differential gain of
about 13, and envelope THD is reduced by 15 dB at all mod levels, so I
am not left guessing when I ask "is the receiver making the THD, or is
it my sig gene?"
It all falls apart
for 100% mod if the AF is over 5 kHz, but then if you get 5 kHz right,
and then use 50% mod, it goes to 20 kHz AF quite well, and the AF
response of the receiver can be properly measured. IFTs in tube sets
need serious tweaking to extend usual BW from 7 kHz, (3.5 kHz audio) to
say 10 kHz, with the same skirt selectivity. In bjt superhet tuners,
with two IFTs, there is only one high Q LC network. So AF is limited to
1.5 kHz; and boosting the treble does nothing because there is no
treble to boost.
Anyway, in my tube
modulator, some trial and error was needed to stop parasitics, but it's
a lot simpler than the HP606A which somebody gave me....
The HP 606A
is a fully tubed RF gene
for 50 kHz to 65 MHz in 6 switched bands, in box about 45 cm wide, 33
cm high and 33 cm deep . Internally, it is a masterpeice of high
quality electro-mechanical craftsmanship, knocked up by guys who'd
probably had to make all sorts of gear to win WW2. 2× 6CL6
are used for a push-pull RF amp with tuned anode tank. LCs for
different bands are on a large rotating turret switch, and there's no
sign of compromises by accountants. A 6B4 is used to cathode modulate
the 6CL6s, and the 6B4 grid signal comes from a triode pentode diff.
amp using a 6AW8, with AF in to the triode side and detected AF to the
pentode side. THD is <1% at 97% envelope - quite good for 1958.
detected AF is from a sec. winding on the anode LC tanks, which give
max 3.0 V rms carrier output with 50 Ω in series. But
additional sec. turns give 6 V rms to feed 2 series Ge diodes feeding a
R&C network - a very crude detector, but it works.
I analyzed and found
that about 15 dB to 20 dB of NFB is used. One reason for stability is
the tuned LC anode tank; it excludes both LF and HF parasitics, which
can happen with untuned circuits within thre NFB loop. Full circuit
details are in the HP606A Operation and Service Manual
I don't know what is done nowdays with AM transmissions; but here the
ABC here puts out a splendid signal with 9 kHz AF, and it sounds better
than my FM tuner full of ICs I use to get FM in the kitchen, using an
AM radio AF amp. This is all despite networking where a programme in
Sydney goes to a satellite encoded by digits and VHF and then back down
to a whole lot of stations for re-broadcasting by remaining Steam
Powered AM Transmitters, SS FM transmitters, or to digital radio, or to
the Internet for pod-casting etc.. At one time, they used to receive AM
transmitted from Sydney in "special quality" receivers in Melbourne,
over 1,100 km away, then re-transmit it. The quality was good enough
not to fetch complaints. Just what those special receivers were is a
mystery, but I heard AWA was involved, and they kept stuff like that
My cathode follower detector really needs to make at least 2 V rms of
AF average to avoid THD caused by diode drop and CF losses.
The detector of yours does a good detect job even with Fc at 25mV, and
100% mod. This better suits my present purpose to make a wide band AM
modulator without any tuned circuits, although I have used hand wound
RF toroid chokes of about 20 mH with CT, and shunted at modulator with
low value RL to flatten response to as wide as possible. I don't know
if I can get away without a tuned LC because initial tests with a
passive detector gave all sorts of parasitics.
I finally got my long tail
modulator with 2 × BF469 to give 100 kHz to about 5 MHz of
flat bandwidth, with 33 Ω across each half of the15 mH L with
CT. I have an amp. with gain of ×12 following, and I get two
phases of 0.5 V rms and at Vc 200 kHz, THD in +/- envelopes is 2.5% at
95% mod without any NFB loop. The THD is < 1% at 50% mod. At 2
MHz, with bjts beginning to struggle with f, the THD = 5% at same
levels, so at 4 MHz I expect more THD.
The detector worked fine, up to 2 MHz at least, and then I tried using
detected AF as NFB signal with simple long tail pair with gain of 10.
However: each and every attempt to apply more than 3 dB NFB gave dismal
results with parasitics popping up in many different ways, and I can
say that this path to a working linear modulator, ( THD <1% at
95%, for all f ) not going to work, because the modulator does not have
tuned circuits so it can easily oscillate.
The detector does have a high
amount of carrier
harmonics which need reduction, much more HF harmonics than you get
with simple old diode+C+R. So any LPF filter, even with pole at 50 kHz,
with high slope seems to create a path for low level carrier harmonics
to cause mayhem, and for phenomena such as oscillating at 10 kHz with
no external mod signal applied, above some low threshold of level, far
lower than wanted, and fiddling with time constants does not seem to
help. Like all cascaded circuits including lots of stages with gain and
GNFB, things that you hope won't oscillate just will oscillate, at
whatever f is possible if there's just enough phase shift.
AND, after getting it to work slightly with only a useless 3 dB GNFB,
using a 1 kHz square wave gives horrid amounts of ringing and
So I'll have to settle for a the passive modulator which is actually
better over all than another tubed modulator I made with NFB in
The HP606A RF gene is so much simpler, and with tuned anode tanks
there's no chance of stray f away from fo ever showing up in anode
circuit, which can only
work at the tuned f. So
much for trying to get away from radio tuning gangs. I've spent days
trying with this thing, and have about 5 times the number of parts used
compared to HP606A, which does 50 kHz to 65 MHz, 6 ranges, nicely
calibrated, full 97% mod with envelope THD quite hard to see; it is a brilliant
simple thing from the past......
Regarding digital methods: almost nobody online has published anything
useful about really linear modulators. Probably, there is a function
generator chip that does it better. I once had a Topward
very fragile, but that did a good AM wave, and it was broad band, so
you could AM modulate 100 Hz with 5 Hz.
27th Nov. 2015.