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audio gain

R

RG

Jan 1, 1970
0
Hi,

anyone got a simple way to vary the gain of an audio stage, as in the old
cassette recorders?

Low to moderate distorsion is not an issue, just want to keep the speech
audio level reasonably constant.
No ICs, just discrete transistors, FET or bipolar.

Ron
 
E

EEng

Jan 1, 1970
0
Hi,

anyone got a simple way to vary the gain of an audio stage, as in the old
cassette recorders?

Low to moderate distorsion is not an issue, just want to keep the speech
audio level reasonably constant.
No ICs, just discrete transistors, FET or bipolar.

Ron
I once had a client I designed a product for. When I was done he was
very impressed with the product, how flawlessly it performed, etc, but
he took serious issue (in fact became quite angry) with the fact that
he did not understand the schematics. He refused to pay because there
were chips he didn't understand. He insisted I re-design to use ONLY
discrete transistors, FET or bipolar and nothing else or he would sue
me for fraud. To this day he still owes me $4400.

Considering how weird and rare that request is, and that you posted
one here almost word for word identical, could you be him? If so, you
know who I am, so cough up the dough already...you lost the court case
you moron and next time you hire someone to do something you can't,
have the intelligence to recognize that you hired someone because you
couldn't do what they could. Do you ask your doctors to use stone
axes and spears because you don't understand scalpels?

If you're not him, then ignore this post but I suggest that when you
ask people for help, take their advice and let them do what they do
best, and what you weren't able to do yourself.
 
T

Tom Bruhns

Jan 1, 1970
0
Sure...just put a resistor in series with the audio input, and a jfet
from resistor output to ground. Keep the DC level zero. Amplify the
r-jfet node, and rectify the amplified signal to generate a control
voltage which turns the FET more "on" as the signal level increases.
If you use a P-channel jfet, you can use a positive bias to keep it
off till the signal is large enough. You get to work out the control
loop details (attack and decay times, operating voltage levels, ...)
I'd try to keep the audio voltage at the fet to around 100mV max, to
keep distortion down, but if you really don't care about distortion
you could go higher than that.

Cheers,
Tom
 
W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
Sure...just put a resistor in series with the audio input, and a jfet
from resistor output to ground. Keep the DC level zero. Amplify the
r-jfet node, and rectify the amplified signal to generate a control
voltage which turns the FET more "on" as the signal level increases.
If you use a P-channel jfet, you can use a positive bias to keep it
off till the signal is large enough. You get to work out the control
loop details (attack and decay times, operating voltage levels, ...)
I'd try to keep the audio voltage at the fet to around 100mV max, to
keep distortion down, but if you really don't care about distortion
you could go higher than that.

Cheers,
Tom

I'd say something like this one is a simple circuit. I think the 407k
resistors are really 470k. http://www.belza.cz/audio/alc.htm

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R

RG

Jan 1, 1970
0
Ban said:
That circuit is crap, will have more distortion than moderate, apart from
using ICs. If you use a Fet instead that diode connected bjt as suggested by
another poster, it might be somehow in the right direction.

BTW why is it that you want to use discrete components? Is it a homework
exercise or do you live in the past?

ciao Ban

Dear God!
I may have posted to the wrong group or should have posted to an "antique"
newsgroup.
What I want to do, as well as a other technician friends of mine, is
practically the same as people starving in a jungle for a week just to see
if they can survive "without" modern facilities or people still designing
amplifiers with vacuum tubes, etc.
Why sometimes people do something out of "normal" ways? Just for fun,
curiosity or challenge.

It is indeed an exercise or challenge a group of technicians at work came up
with when discussing about old audio circuits and how well some of these
perform without any ICs.

Anyway I remember an old cheap cassette recorder of the 70s with fairly good
autolevel control. The circuit was amazingly simple but I forgot the details
and I'm just searching for ideas.

Ron

N.B.
 
R

Roger Johansson

Jan 1, 1970
0
RG said:
Anyway I remember an old cheap cassette recorder of the 70s with fairly good
autolevel control. The circuit was amazingly simple but I forgot the details
and I'm just searching for ideas.

Look for AGC (automatic gain control) circuits, often used by radio
amateurs. The ARRL Handbook for radio amateurs.
Also sustain circuits used for electric guitars.

Some ideas:

Use BF981 type double gate mosfets, use two such stages in series, use
one gate for audio and the other for gain control, connect the control
voltage to both mosfets gain control gates together.

I have seen how the gain can be controlled in an op-amp circuit by
grounding the input via a capacitor and a transistor, to make the AC
input signal smaller when the control voltage rises.

The capacitor is connected to the audio signal path and to the
collector of a bipolar transistor. The emitter to ground.
The base controls the dynamic resistance of the transistor and shorts
the signal to ground via the capacitor, so this does not affect the DC
conditions on the signal line, it just grounds the signal, dampening
it in a controlled way.

In all these circuits you get the control voltage by rectification of
the audio output signal.

By applying the control voltage to more than one stage the audio
signal passes through you get more regulation dynamics.

To regulate the amplification in a common emitter bipolar transistor
stage you can drag down the base towards ground with another
transistor. Collector to base of the first transistor, emitter to
ground. Sensitive circuit, you don't have to drag down the base much
before it stops working completely.
 
R

RG

Jan 1, 1970
0
The Handbook for radio-amateurs is a really good suggestion. Plenty of
circuit examples.
A friend of mine has a collection of these, I'll ask him to take a look.

Thanks!
 
W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
That circuit is crap, will have more distortion than moderate, apart from
using ICs. If you use a Fet instead that diode connected bjt as suggested by
another poster, it might be somehow in the right direction.

BTW why is it that you want to use discrete components? Is it a homework
exercise or do you live in the past?

ciao Ban

I'm not aware of where you live, but some people, especially in other
countries, don't have easy access, or access at all, to the selection
of parts that some of us have. So you have to realize that others
might be trying to do something with a limited stock, or even a
limited budget. Hey, when I was a kid, it was _hard_ to come by
transistors, so we had to make do with what we could get. Now you can
buy a handful of really decent transistors for the same price that a
single Germanium transistor cost back in the '50s. And money was
worth a helluva lot more back then.

So please try to view others in more than just a limited subset of the
real world that you live in.

This reminds me of the thread on the LM3909. Here you have just the
opposite, the 'crew' is trying to reverse engineer the chip so that it
can be synthesized from discrete parts. Necessity is the mother of
invention.


--
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W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
Dear God!
I may have posted to the wrong group or should have posted to an "antique"
newsgroup.

Yeah, he seemed a bit harsh.
What I want to do, as well as a other technician friends of mine, is
practically the same as people starving in a jungle for a week just to see
if they can survive "without" modern facilities or people still designing
amplifiers with vacuum tubes, etc.
Why sometimes people do something out of "normal" ways? Just for fun,
curiosity or challenge.

It is indeed an exercise or challenge a group of technicians at work came up
with when discussing about old audio circuits and how well some of these
perform without any ICs.

Anyway I remember an old cheap cassette recorder of the 70s with fairly good
autolevel control. The circuit was amazingly simple but I forgot the details
and I'm just searching for ideas.

Yeah, I remember those. They used a germanium transistor on the
microphone input. I thought that it was odd that there was no DC
flowing thru the transistor like normal, yet they worked well without
the DC. I'm going to try to find a schematic of such a circuit and
post it to the alt.binaries.schematics.electronic newsgroup. I'll be
looking thru my extensive collection of schematics for one; may take
hour, days, weeks..



--
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###Got a Question about ELECTRONICS? Check HERE First:###
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goes directly to the trash unless you add NOSPAM in the
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
[snip]

Problem with this is that the DC levels shift, causing a varying low
freq signal. That tends to make your speaker cone do wild gyrations.

One way it's done is to put a FET or transistor in series with the
emitter bypass capacitor. The gain can be reduced to nearly unity if
the emitter resistor is large.



--
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goes directly to the trash unless you add NOSPAM in the
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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R

Roger Johansson

Jan 1, 1970
0
Problem with this is that the DC levels shift, causing a varying low
freq signal. That tends to make your speaker cone do wild gyrations.

This assumes a common emitter stage which is DC-isolated from the next
stage with a capacitor, so the level change does not matter.

Bloggs mention of LDR-LED is a very good one I forgot in my article.
I have used that several times, use a tube of black plastic and put
the LDR and LED in it. It usually gives good time constants
automatically.
 
W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
- and -


The level control based on the LED/ LDR combination is excellent- see :
http://www.wireless.org.uk/circuits.htm
The OA's are just single-ended amplifiers- replace them with discrete
transistor subcircuits. You can also use the LDR as the series element
in a simple audio resistive "L-pad" to control the attenuation of the
through signal. If LEDs are too modern then go with an incandescent lamp.

By the time the loud signal has peaked, the lamp is still just getting
warmed up, and not ready to damp the peak. So the circuit then needs
some clipping to do the job while the lamp is warming.

LEDs are a lot faster.
--
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goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
 
R

RG

Jan 1, 1970
0
The level control based on the LED/ LDR combination is excellent- see :
http://www.wireless.org.uk/circuits.htm
The OA's are just single-ended amplifiers- replace them with discrete
transistor subcircuits. You can also use the LDR as the series element
in a simple audio resistive "L-pad" to control the attenuation of the
through signal. If LEDs are too modern then go with an incandescent lamp.

Mmmh! The LDR approach. It is probably the simplest and garantees low
distorsion.
Very easy to put in an opamp feedback loop. In the 80s, I have tested AM/SSB
radios using an weird part made of a small bulb and a LDR, all in the same
casing, about the size a one inch capacitor. It controlled the squelch and
the automatic modulation control.
 
W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
Mmmh! The LDR approach. It is probably the simplest and garantees low
distorsion.
Very easy to put in an opamp feedback loop. In the 80s, I have tested AM/SSB
radios using an weird part made of a small bulb and a LDR, all in the same
casing, about the size a one inch capacitor. It controlled the squelch and
the automatic modulation control.

Hamamatsu made those long ago, and some stereos used them for volume
control. The lamp would burn out, and then they had to be replaced.
Or sometimes the heat from the lamp would roast the CdS photocell, and
it would fail. In any case, it was a poor design, and weak point.
But it was the way that early stereos implemented remote control. And
people got easily attached to having a remote control. Just look at
them today.

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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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B

Ban

Jan 1, 1970
0
RG said:
Mmmh! The LDR approach. It is probably the simplest and garantees low
distorsion.
Very easy to put in an opamp feedback loop. In the 80s, I have tested
AM/SSB radios using an weird part made of a small bulb and a LDR, all
in the same casing, about the size a one inch capacitor. It
controlled the squelch and the automatic modulation control.

RG,
as the original poster you asked for an audio-AGC circuit, but now your
interest seems to have shifted to compressors. The AGC must have a fast
attack time(<10ms) to avoid overloading the cassette tape, but a very slow
release time(>>15s), to preserve the original dynamics. Compressors have
release times of .5s at most, usually much shorter even.

Nearly all circuits presented here are of the compressor type, useful for
guitar or bass to get sustain, but with speech there will be pumping and in
pauses the environmental noise will be creeping up more and more.
You'll have to modify with big caps, if at all it is possible.

I think for your limited resources idea using a LDR is quite a difficult to
obtain component, show me a place on earth where a TL072 opamp is *not*
available, but LDRs are.

ciao Ban
 
R

RG

Jan 1, 1970
0
RG,
as the original poster you asked for an audio-AGC circuit, but now your
interest seems to have shifted to compressors. The AGC must have a fast
attack time(<10ms) to avoid overloading the cassette tape, but a very slow
release time(>>15s), to preserve the original dynamics. Compressors have
release times of .5s at most, usually much shorter even.

Nearly all circuits presented here are of the compressor type, useful for
guitar or bass to get sustain, but with speech there will be pumping and in
pauses the environmental noise will be creeping up more and more.
You'll have to modify with big caps, if at all it is possible.

I think for your limited resources idea using a LDR is quite a difficult to
obtain component, show me a place on earth where a TL072 opamp is *not*
available, but LDRs are.

ciao Ban

My original request didn't mean opamps, ICs or even DSPs are not available,
as I said, it is more a challenge between technicians at my work. Maybe a
foolish one ;) Indeed opamps are a much better way to go but the line has
already been drawn.

You have pointed out an important difference I was not really aware of, I
mean the difference between AGC and compression. So far the circuits I have
seen use a feedback from the output and I wonder if compression occurs with
closed loop circuit and AGC with open loop circuit.

A guy came up with a circuit (open loop) that changes the current flow
through a 1N4148 diode as a control element.. The input signal is first
divided by about 50 with a resistor to that diode, such small signal (few
millivolts) at the diode seems to be necessary to maintain low distorsion
because of the diode non-linearity at higher signal level.
The current through the control diode is directly controlled by the input
itself (1Vrms max) through an 1N34A diode, an RC time constant and a
transistor. As the voltage rectified from the input increased, it drives the
transistor in a constant current configuration and its collector drives the
control diode. When some threshold has been reached, if the input signal
doubles, the current through the control diode also doubles, reducing by
half its "apparent" resistance and reducing by half the output signal thus
maintaining it to about the same level. The signal at the control diode is
then reamplifed by about 50 to be usable.
The guy says the transistor has to be biased near conduction point at no
signal to be sensitive enough and it makes the circuit somewhat instable.
Here opamps would cure the bias instablility problem and this is exactly
what the guy is about to try (cheating..), just to see how good this design
could be.
 
B

Ban

Jan 1, 1970
0
RG said:
My original request didn't mean opamps, ICs or even DSPs are not
available, as I said, it is more a challenge between technicians at
my work. Maybe a foolish one ;) Indeed opamps are a much better way
to go but the line has already been drawn.

You have pointed out an important difference I was not really aware
of, I mean the difference between AGC and compression. So far the
circuits I have seen use a feedback from the output and I wonder if
compression occurs with closed loop circuit and AGC with open loop
circuit.

A guy came up with a circuit (open loop) that changes the current flow
through a 1N4148 diode as a control element.. The input signal is
first divided by about 50 with a resistor to that diode, such small
signal (few millivolts) at the diode seems to be necessary to
maintain low distorsion because of the diode non-linearity at higher
signal level.
The current through the control diode is directly controlled by the
input itself (1Vrms max) through an 1N34A diode, an RC time constant
and a transistor. As the voltage rectified from the input increased,
it drives the transistor in a constant current configuration and its
collector drives the control diode. When some threshold has been
reached, if the input signal doubles, the current through the control
diode also doubles, reducing by half its "apparent" resistance and
reducing by half the output signal thus maintaining it to about the
same level. The signal at the control diode is then reamplifed by
about 50 to be usable.
The guy says the transistor has to be biased near conduction point at
no signal to be sensitive enough and it makes the circuit somewhat
instable. Here opamps would cure the bias instablility problem and
this is exactly what the guy is about to try (cheating..), just to
see how good this design could be.

That design will be always crappy, no matter how low the signal is. The
diode has always an exponential behaviour(in the audio frequency range) and
will induce distortion. There are special high-frequency PIN-diodes where
you can do that though. But the carrier lifetime is too short for audio.

If you attenuate the signal and amplify it again later, a lot of noise is
created, which makes the circuit questionable. Then there is the relativly
high current noise from the bias, modulating this noise spectrum.
Rather have a look at the transconductance amplifier I had drawn in another
post. Also here you should have a low level signal, but it can be at least
10 times higher for the same amount of distortion, because of the symmetric
behaviour.
Another issue is the DC-shift created by most of the bias arrangements. If a
loud pulse forces the current to change a lot, it will induce a DC-level
change, with 10ms this is right in the speech range. There will be a "plop"
and then nothing any more, because the 50x amplifier saturates and will need
some time to come out of the saturation.
Here you can do a complementary design, which reduces this issue to
acceptable levels.
This is the great advantage of the often mentioned LDR/LED arrangement. The
driving current is isolated and doesn't give levelshifts when done rightly.

The next difficulty will be to derive the control-voltage or -current.
let's analize it in another post.

ciao Ban
 
R

RG

Jan 1, 1970
0
Ban said:
That design will be always crappy, no matter how low the signal is. The
diode has always an exponential behaviour(in the audio frequency range) and
will induce distortion. There are special high-frequency PIN-diodes where
you can do that though. But the carrier lifetime is too short for audio.

If you attenuate the signal and amplify it again later, a lot of noise is
created, which makes the circuit questionable. Then there is the relativly
high current noise from the bias, modulating this noise spectrum.
Rather have a look at the transconductance amplifier I had drawn in another
post. Also here you should have a low level signal, but it can be at least
10 times higher for the same amount of distortion, because of the symmetric
behaviour.
Another issue is the DC-shift created by most of the bias arrangements. If a
loud pulse forces the current to change a lot, it will induce a DC-level
change, with 10ms this is right in the speech range. There will be a "plop"
and then nothing any more, because the 50x amplifier saturates and will need
some time to come out of the saturation.
Here you can do a complementary design, which reduces this issue to
acceptable levels.
This is the great advantage of the often mentioned LDR/LED arrangement. The
driving current is isolated and doesn't give levelshifts when done rightly.

The next difficulty will be to derive the control-voltage or -current.
let's analize it in another post.

ciao Ban

Just a few words about the DC-shift problem: the guy (I should say "we" as
I unwantedly got involved in his experiment...) added phase cancellation by
adding one diode in the emitter of the constant current transistor, that
diode connected to ground, its anode to a 100 ohm resistor then to the
emitter itself. The control diode is connected to Vcc and the collector.
Finally two 10K resistors are added: one resistor at control diode, the
other resistor at the emitter diode. Both other sides of the resistors
connected together (center point) where the output signal is picked up. The
DC-shift of both diodes cancel out nicely and only audio signal can get
through. Unfortunaltely this arrangement further reduces the signal .

So...yesterday night, I checked his "marvel". He didn't tell me there is an
audible hiss on speakers... as you expected. In my headphone, it sounds like
a tape hiss from a cheap tape player. For distorsion, not so bad
surprisingly but not hi-fi for sure, acceptable for speech. This simple way
to control audio would probably be acceptable in a cheap cassette recorder
with high tape hiss, low frequency response and moderate distorsion.

I will not consider the LDR/LED arrangement, too mechanically oriented. On
the other hand, the transconductance amplifier looks more interesting. I
remember few magazines like Popular Electronics with projects using OTAs.


Ron
 
B

Ban

Jan 1, 1970
0
RG said:
Just a few words about the DC-shift problem: the guy (I should say
"we" as I unwantedly got involved in his experiment...) added phase
cancellation by adding one diode in the emitter of the constant
current transistor, that diode connected to ground, its anode to a
100 ohm resistor then to the emitter itself. The control diode is
connected to Vcc and the collector. Finally two 10K resistors are
added: one resistor at control diode, the other resistor at the
emitter diode. Both other sides of the resistors connected together
(center point) where the output signal is picked up. The DC-shift of
both diodes cancel out nicely and only audio signal can get through.
Unfortunaltely this arrangement further reduces the signal .

So...yesterday night, I checked his "marvel". He didn't tell me there
is an audible hiss on speakers... as you expected. In my headphone,
it sounds like a tape hiss from a cheap tape player. For distorsion,
not so bad surprisingly but not hi-fi for sure, acceptable for
speech. This simple way to control audio would probably be acceptable
in a cheap cassette recorder with high tape hiss, low frequency
response and moderate distorsion.

I will not consider the LDR/LED arrangement, too mechanically
oriented. On the other hand, the transconductance amplifier looks
more interesting. I remember few magazines like Popular Electronics
with projects using OTAs.


Ron

Hi Ron,
since I see you can follow a more technical description, lets put some
numbers.
You mention 1Vrms as max. input voltage.
Now let's want 40dB of regulation range. A hell of a range, much more than
the cheap cassette recorders had.
Actually we have to do a division,
Ue/Ûe(this should show as a circonflex) to get a constant output level. The
transconductance S = dIc/dUbe = Ic/Ut is proportional to the current. To get
a linear relation it will be necessary to 1/x the control signal(~Uemax).

This would be needed to make a forward control, not dependent on the output
signal. Possible would be a divider working with the time-devision method,
which could be made without ICs working linearly over that 40dB range.

Since we always want the same output level, we can substitute the division
by a subtraction and derive the control voltage from the output. This
linearizes the control curve sufficiently, but will not allow a ratio of
infinity, though very close to it.

So I made up this peak/hold two-way rectifier, made from another
differential amp. The time constant would be 60s with the 220u cap and 250k
pot +10k in series.


+------+---+---+------+---------------+-------o+15V
.-. | | | .-. .|
4k7| | | | | | |4k7 .-.
| | | | | | | | |1k
'-' | | | '-' | |
| |/ | + \| | '-'
+----| 220u |----+ current |
| |> --- <| | source |
| | | | | |<
| +---+---+------)-------------|
| | | |\
| release .-.10k-270k | | Icontrol
| time | |<-- | | o
| | | | | |
10u | '-' | +---+ |
#| |/ | \| | | |
o-#|--+--| === |---+---+ | |/ |/ current
#| | |> GND <| | +------| mirror
.-. | ___ ___ | .-. | + |> |>
| | +----|___|-+-|___|----+ | | ### | |
4k7| | 470 | 470 4k7| | ---10u .-. .-.
'-' .-. '-' | | | | |
| | | | | 1k| | | |1k
=== | | === === '-' '-'
GND '-' GND GND | |
+------------------------------+----+o -15V
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

enough for now
ciao Ban
 
W

Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
The audio signal must be controlled so that it does not overload the
audio amplifiers. Therefore, what is usually done is to allow the raw
microphone audio to be attenuated only when it is at a high level, and
as the level drops to a low level, the attenuation is reduced or
eliminated. This is normally done at he input because with loud
sounds, the microphone output might be much greater than a volt,
instead of the normal few millivolts.



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