weird(?) DC voltages at output of mic preamp

[tempus fugit]

Hey all;

I posted a message on this topic a few days ago (pops problems when preamp
is turned on). I've done a little more experimenting, and found these
results:

1. When I switch the mic preamp on, I get a little (1 volt or so) pop. I
assume this is pretty normal, and not much of a problem. Someone answered
with some great ideas for a mute circuit, which I'm sure will solve the
problem.

2. (this is the biggy) When I turn the phantom power switch on, I get a big
(10 - 15v) DC blast at the output. If I probe the actual output pin (i.e.
before the DC blocking cap) this DC voltage remains for several seconds.

3. When I turn the phantom power switch off, I get another big (-15v) DC
blast at the output. Again, it remains there for several seconds at the
output pin of the IC I'm using, but drops away fairly quickly at the output
jack (post DC blocking cap).

4. Both 2 and 3 only happen with a mic connected - there is virtually no DC
pop (<1v) with no mic connected. Also, there is a brief DC pulse at the
inputs (with a mic connected) of similar size, but it disappears quickly.

There are DC blocking caps at the inputs. Where is all this DC voltage
coming from? Any suggestions as to how to get rid of it?

Thanks




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[John Popelish]

tempus fugit wrote:

Hey all;

I posted a message on this topic a few days ago (pops problems when preamp
is turned on). I've done a little more experimenting, and found these
results:

1. When I switch the mic preamp on, I get a little (1 volt or so) pop. I
assume this is pretty normal, and not much of a problem. Someone answered
with some great ideas for a mute circuit, which I'm sure will solve the
problem.

2. (this is the biggy) When I turn the phantom power switch on, I get a big
(10 - 15v) DC blast at the output. If I probe the actual output pin (i.e.
before the DC blocking cap) this DC voltage remains for several seconds.

3. When I turn the phantom power switch off, I get another big (-15v) DC
blast at the output. Again, it remains there for several seconds at the
output pin of the IC I'm using, but drops away fairly quickly at the output
jack (post DC blocking cap).

4. Both 2 and 3 only happen with a mic connected - there is virtually no DC
pop (<1v) with no mic connected. Also, there is a brief DC pulse at the
inputs (with a mic connected) of similar size, but it disappears quickly.

There are DC blocking caps at the inputs. Where is all this DC voltage
coming from? Any suggestions as to how to get rid of it?

Thanks
You are seeing the amplified blocking capacitor charging and

discharging currents. Any time the DC voltage across a coupling cap
changes, that is a form of AC, so it passes through the capacitor with
an RC time constant of the capacitance times the resistance that the
current has to pass through. If this pop takes several seconds, a
coupling cap is way larger than needed to pass frequencies no lower
than about 20 Hz.

--
John Popelish

 

[tempus fugit]

Thanks John.

I would like to clarify a couple things: If this is an AC voltage, why does
it show up as DC on my meter? Also, the pop isn't audible for several
seconds, but my meter does show the voltage for several seconds (not sure if
that was clear in my original post). Are your reponses still valid in this
case?

Thanks


"John Popelish" <jpopelish@rica.net> wrote in message
news:41AA9582.4649FB12@rica.net...

tempus fugit wrote:

Hey all;

I posted a message on this topic a few days ago (pops problems when
preamp
is turned on). I've done a little more experimenting, and found these
results:

1. When I switch the mic preamp on, I get a little (1 volt or so) pop. I
assume this is pretty normal, and not much of a problem. Someone
answered
with some great ideas for a mute circuit, which I'm sure will solve the
problem.

2. (this is the biggy) When I turn the phantom power switch on, I get a
big
(10 - 15v) DC blast at the output. If I probe the actual output pin
(i.e.
before the DC blocking cap) this DC voltage remains for several seconds.

3. When I turn the phantom power switch off, I get another big (-15v) DC
blast at the output. Again, it remains there for several seconds at the
output pin of the IC I'm using, but drops away fairly quickly at the
output
jack (post DC blocking cap).

4. Both 2 and 3 only happen with a mic connected - there is virtually no
DC
pop (<1v) with no mic connected. Also, there is a brief DC pulse at the
inputs (with a mic connected) of similar size, but it disappears
quickly.

There are DC blocking caps at the inputs. Where is all this DC voltage
coming from? Any suggestions as to how to get rid of it?

Thanks
You are seeing the amplified blocking capacitor charging and
discharging currents. Any time the DC voltage across a coupling cap
changes, that is a form of AC, so it passes through the capacitor with
an RC time constant of the capacitance times the resistance that the
current has to pass through. If this pop takes several seconds, a
coupling cap is way larger than needed to pass frequencies no lower
than about 20 Hz.

--
John Popelish

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Outgoing mail is certified Virus Free.
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[John Popelish]

tempus fugit wrote:

Thanks John.

I would like to clarify a couple things: If this is an AC voltage, why does
it show up as DC on my meter? Also, the pop isn't audible for several
seconds, but my meter does show the voltage for several seconds (not sure if
that was clear in my original post). Are your reponses still valid in this
case?

The only DC that capacitors block completely is steady, unvarying DC.
AC comes in two forms, sinusoidal and exponential. They can combine
as a decaying sinusoidal, also. AC frequency is naturally measured in
radians per second (2*pi of those per cycle), and the natural measure
of exponential frequency is its time constant.

Capacitors pass current in proportion to the rate of change of voltage
across them (I=C*(dV/dT)), regardless of whether it alternates
direction or only varies on one direction. The amplifier is probably
saturating at the beginning of the decaying pulse, so the output is
steady till the decay goes low enough that the amplifier comes out of
saturation. If the decaying pulse lasts long enough, your DC meter
can measure it as it decays (especially the starting saturated part).
Your solution is either to shunt the audio signal to ground while this
decay occurs so it doesn't show up at the output, cancel it with an
equal and opposite decaying voltage or shorten the duration by making
the coupling cap much smaller (making a high pass filter) so that the
pop is not long lasting enough to be a problem.

--
John Popelish

 

[tempus fugit]

Thanks again John.

I have 2 100v 100uF caps at the input (to block the 48v from the IC) - the
schematic on the data sheet (it's an SSM2017) shows 2 47uF caps. Could the
extra size be a factor here (I don't know how much smaller I could get away
with without rolling off the bass too much)? Also, I have a 470uF 250v cap
as a smoothing cap in the DC circuit before the regulator for the phantom
power. Is the added voltage handling of the cap a factor also? I also have a
1 uF cap as a blocking cap at the output (at 100v I think - I used a poly
cap here).
Also, if I reduce the cap values, will this reduce the amount of voltage
hitting the IC as well as the duration?

Thanks


"John Popelish" <jpopelish@rica.net> wrote in message
news:41AA9BB9.49601C3B@rica.net...

tempus fugit wrote:

Thanks John.

I would like to clarify a couple things: If this is an AC voltage, why
does
it show up as DC on my meter? Also, the pop isn't audible for several
seconds, but my meter does show the voltage for several seconds (not
sure if
that was clear in my original post). Are your reponses still valid in
this
case?

The only DC that capacitors block completely is steady, unvarying DC.
AC comes in two forms, sinusoidal and exponential. They can combine
as a decaying sinusoidal, also. AC frequency is naturally measured in
radians per second (2*pi of those per cycle), and the natural measure
of exponential frequency is its time constant.

Capacitors pass current in proportion to the rate of change of voltage
across them (I=C*(dV/dT)), regardless of whether it alternates
direction or only varies on one direction. The amplifier is probably
saturating at the beginning of the decaying pulse, so the output is
steady till the decay goes low enough that the amplifier comes out of
saturation. If the decaying pulse lasts long enough, your DC meter
can measure it as it decays (especially the starting saturated part).
Your solution is either to shunt the audio signal to ground while this
decay occurs so it doesn't show up at the output, cancel it with an
equal and opposite decaying voltage or shorten the duration by making
the coupling cap much smaller (making a high pass filter) so that the
pop is not long lasting enough to be a problem.

--
John Popelish

---
Outgoing mail is certified Virus Free.
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[John Popelish]

tempus fugit wrote:

Thanks again John.

I have 2 100v 100uF caps at the input (to block the 48v from the IC) - the
schematic on the data sheet (it's an SSM2017) shows 2 47uF caps.

Do you have a link for this schematic?

The closest thing I could find was this replacement chip:
http://aes.sdsu.edu/documents/1510data.pdf

Could the
extra size be a factor here (I don't know how much smaller I could get away
with without rolling off the bass too much)?

It certainly makes the problem worse. The application note on my data
sheet (page 2) shows the phantom power connected to the mic side of
the 47 uf caps through 6k8 resistors, and another 4R7 after them
before the clamping diodes. So the caps charge with very nearly a
6K8*47u=.3 second time constant, while the diodes conduct. But after
the cap current falls to what will not drop 15 volts across the 10k to
ground resistors (about 1 time constant), the time constant becomes
(10k+6k8)*47u= .8 seconds.

So the inputs settle toward ground at that rate. Since the amplifier
has a differential input, it starts to work as soon as the common
voltage on the two inputs reaches the input common mode limit, which
is about 2 volts inside the supply rails. But the output will still
stay saturated a while after that if the two input capacitors are not
well matched, so that one input settles toward zero faster than the
other. You can test for this problem by looking at the DC polarity
out of the amplifier during settling, and then switch the two input
capacitors and see if the DC switches directions.

Regardless, the input caps have a long time constant with the input
loading (grounding resistors) compared to 20 Hz. 1 uf should be
enough. 2 uf plenty. And then you can use film caps with much better
leakage and tolerances than electrolytics.

Also, I have a 470uF 250v cap
as a smoothing cap in the DC circuit before the regulator for the phantom
power.

I doubt that it is involved in the problem.

Is the added voltage handling of the cap a factor also? I also have a
1 uF cap as a blocking cap at the output (at 100v I think - I used a poly
cap here).

If the amplifier is saturated, the output cap is not the problem. The
huge over voltage takes place at the input, before all that gain.

Also, if I reduce the cap values, will this reduce the amount of voltage
hitting the IC as well as the duration?

It won't affect the peak much, only the duration.

--
John Popelish

 

[tempus fugit]

Thanks John.

I don't think the chip is made anymore. I could email you the schematic or
post it on ABSE if you don't mind.

"John Popelish" <jpopelish@rica.net> wrote in message
news:41AD510C.178B206@rica.net...

tempus fugit wrote:

Thanks again John.

I have 2 100v 100uF caps at the input (to block the 48v from the IC) -
the
schematic on the data sheet (it's an SSM2017) shows 2 47uF caps.

Do you have a link for this schematic?

The closest thing I could find was this replacement chip:
http://aes.sdsu.edu/documents/1510data.pdf

Could the
extra size be a factor here (I don't know how much smaller I could get
away
with without rolling off the bass too much)?

It certainly makes the problem worse. The application note on my data
sheet (page 2) shows the phantom power connected to the mic side of
the 47 uf caps through 6k8 resistors, and another 4R7 after them
before the clamping diodes. So the caps charge with very nearly a
6K8*47u=.3 second time constant, while the diodes conduct. But after
the cap current falls to what will not drop 15 volts across the 10k to
ground resistors (about 1 time constant), the time constant becomes
(10k+6k8)*47u= .8 seconds.

So the inputs settle toward ground at that rate. Since the amplifier
has a differential input, it starts to work as soon as the common
voltage on the two inputs reaches the input common mode limit, which
is about 2 volts inside the supply rails. But the output will still
stay saturated a while after that if the two input capacitors are not
well matched, so that one input settles toward zero faster than the
other. You can test for this problem by looking at the DC polarity
out of the amplifier during settling, and then switch the two input
capacitors and see if the DC switches directions.

Regardless, the input caps have a long time constant with the input
loading (grounding resistors) compared to 20 Hz. 1 uf should be
enough. 2 uf plenty. And then you can use film caps with much better
leakage and tolerances than electrolytics.

Also, I have a 470uF 250v cap
as a smoothing cap in the DC circuit before the regulator for the
phantom
power.

I doubt that it is involved in the problem.

Is the added voltage handling of the cap a factor also? I also have a
1 uF cap as a blocking cap at the output (at 100v I think - I used a
poly
cap here).

If the amplifier is saturated, the output cap is not the problem. The
huge over voltage takes place at the input, before all that gain.

Also, if I reduce the cap values, will this reduce the amount of voltage
hitting the IC as well as the duration?

It won't affect the peak much, only the duration.

--
John Popelish

---
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[Glenn Gundlach]

"tempus fugit" <toccata@no.spam.ciaccess.com> wrote in message news:<d9krd.89$lW6.4@fe51.usenetserver.com>...

Thanks John.

I don't think the chip is made anymore. I could email you the schematic or
post it on ABSE if you don't mind.

"John Popelish" <jpopelish@rica.net> wrote in message
news:41AD510C.178B206@rica.net...
tempus fugit wrote:

Thanks again John.

I have 2 100v 100uF caps at the input (to block the 48v from the IC) -
the
schematic on the data sheet (it's an SSM2017) shows 2 47uF caps.

Do you have a link for this schematic?

The closest thing I could find was this replacement chip:
http://aes.sdsu.edu/documents/1510data.pdf

Could the
extra size be a factor here (I don't know how much smaller I could get
away
with without rolling off the bass too much)?

It certainly makes the problem worse. The application note on my data
sheet (page 2) shows the phantom power connected to the mic side of
the 47 uf caps through 6k8 resistors, and another 4R7 after them
before the clamping diodes. So the caps charge with very nearly a
6K8*47u=.3 second time constant, while the diodes conduct. But after
the cap current falls to what will not drop 15 volts across the 10k to
ground resistors (about 1 time constant), the time constant becomes
(10k+6k8)*47u= .8 seconds.

So the inputs settle toward ground at that rate. Since the amplifier
has a differential input, it starts to work as soon as the common
voltage on the two inputs reaches the input common mode limit, which
is about 2 volts inside the supply rails. But the output will still
stay saturated a while after that if the two input capacitors are not
well matched, so that one input settles toward zero faster than the
other. You can test for this problem by looking at the DC polarity
out of the amplifier during settling, and then switch the two input
capacitors and see if the DC switches directions.

Regardless, the input caps have a long time constant with the input
loading (grounding resistors) compared to 20 Hz. 1 uf should be
enough. 2 uf plenty. And then you can use film caps with much better
leakage and tolerances than electrolytics.

Also, I have a 470uF 250v cap
as a smoothing cap in the DC circuit before the regulator for the
phantom
power.

I doubt that it is involved in the problem.

Is the added voltage handling of the cap a factor also? I also have a
1 uF cap as a blocking cap at the output (at 100v I think - I used a
poly
cap here).

If the amplifier is saturated, the output cap is not the problem. The
huge over voltage takes place at the input, before all that gain.

Also, if I reduce the cap values, will this reduce the amount of voltage
hitting the IC as well as the duration?

It won't affect the peak much, only the duration.

--
John Popelish


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Outgoing mail is certified Virus Free.
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What is the reason for turning the phantom power off and on? I've used
dynamic mics on phantom powered preamps with no ill effect. Obviously
the condenser mic requires the power. I've also had no problems
plugging the condenser mic into the preamp 'hot'. Just curious.
GG

 

[Glenn Gundlach]

"tempus fugit" <toccata@no.spam.ciaccess.com> wrote in message news:<tbwrd.152$lW6.87@fe51.usenetserver.com>...

Well, the unit has to be switched on and off in the first place, so I'll get
the DC problems whether I have a separate switch for phantom or not.

What is the reason for turning the phantom power off and on? I've used
dynamic mics on phantom powered preamps with no ill effect. Obviously
the condenser mic requires the power. I've also had no problems
plugging the condenser mic into the preamp 'hot'. Just curious.
GG


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True, but a simple power on mute system would take care of things
without having to have a 'smart' mute that would mute, toggle phantom
power, un-mute. The mute system would be more complicated than the
actual preamps.

GG