Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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  #1   Report Post  
User-Friendly
 
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Default Phono Preamp

I recently picked up an inexpensive phono preamp for the purpose of
making cdrs of some old records on my computer. When I plugged in my
turntable and captured some audio in the editor
the sound was distorted and and I saw that the output of the preamp was
about half clipped - it looked like a horizontal bar with a few troughs
here and there regardless of the recording volume setting of the sound
card. So I guess either the preamp is defective or there is a mismatch
between the output of the turntable and the input of the preamp. I
don't think the turntable has a builtin preamp. I opened up the preamp
and looked at the board - it's a simple single 4558 IC circuit, just an
input and an output for each channel, no pots or switches. I want to
mod it to lower the gain so it doesn't distort. I'm not an engineer or
a tech, just a tinkerer who's built a couple guitar fuzzboxes and done
a few simple repairs. I couldn't find any mention of a schematic for
this particular device on the web, and it seems like such a simple
circuit that I probably don't need one. I thought about trying to trace
the circuit but I'd rather not if I can avoid it. I know the
turntable's okay because I can get a clean (albeit low) signal direct
into my sound card. So I'm trying to figure out at what point in the
circuit the signal's getting clipped. I'm kind of shaky on these
concepts, but would this result from an impedance mismatch between the
output of the turntable and the input of the preamp? Or maybe the gain
of the IC is too large and I could lower the value of the resistor in
the feedback loop? Do I have to be concerned about alteration of
component values affecting the equalization curve? Any info would be
appreciated.

  #2   Report Post  
Charles Schuler
 
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Default Phono Preamp

You just might need a voltage divider to decrease the amplitude. Two
resistors.


  #3   Report Post  
Michael Black
 
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Default Phono Preamp


"User-Friendly" ) writes:
I recently picked up an inexpensive phono preamp for the purpose of
making cdrs of some old records on my computer. When I plugged in my
turntable and captured some audio in the editor
the sound was distorted and and I saw that the output of the preamp was
about half clipped - it looked like a horizontal bar with a few troughs
here and there regardless of the recording volume setting of the sound
card. So I guess either the preamp is defective or there is a mismatch
between the output of the turntable and the input of the preamp. I
don't think the turntable has a builtin preamp.


Is the turntable fairly recent? It would seem they did start putting
preamps in recently made turntables, because the phono preamps disappeared
from equipment. One way to check would be to plug the turntable directly
into the soundcard; it will be a terribly weak signal and sound awful
if there's no preamp in the turntable.

You can't start messing with the gain of the preamp, because it's set
for certain standards. Which is an indication something is wrong, either
in the preamp or the turntable. The turntable has got a moving magnet
cartridge? Older and even really low end (as seen in many all in one
stereos from even 25 years ago) had ceramic cartridges that had high
output, and required little or no equalization.

Take note that you may have too much signal going into the soundcard,
and that's where the distortion comes from. Lower the level on
that input and see what happens. There will be a separate level for
the recording channel. Make sure you are feeding the signal into
the line input, not the microphone input. If for some reason the issue
is too much output from the preamp for the sound card (ie the preamp
is not broken so it distorts, or the cartridge is not outputing too much
for the preamp), then a volume control between the preamp and the soundcard
will let you set that level, but realistically something else is
the problem.

If anything, a 4558 will have problems supplying the needed gain, so
likely the problem is elsewhere.

Michael

  #4   Report Post  
User-Friendly
 
Posts: n/a
Default Phono Preamp

Michael wrote:
Is the turntable fairly recent? It would seem they did start putting
preamps in recently made turntables, because the phono preamps disappeared
from equipment. One way to check would be to plug the turntable directly
into the soundcard; it will be a terribly weak signal and sound awful
if there's no preamp in the turntable.


The turntable is from the early 80s. I'm pretty sure there's no preamp
because the sound
is weak and awful when it's plugged into the aux input on a newer
stereo I own that doesn't have a phono input.

You can't start messing with the gain of the preamp, because it's set
for certain standards. Which is an indication something is wrong, either
in the preamp or the turntable. The turntable has got a moving magnet
cartridge? Older and even really low end (as seen in many all in one
stereos from even 25 years ago) had ceramic cartridges that had high
output, and required little or no equalization.


I don't know what kind of cartridge the turntable has. I'm thinking
moving magnet, here's why. I was able to capture acceptable audio by
running both channels of the turntable directly into the mic input of
the sound card. The levels were okay but the sound was very tinny like
you'd expect from a phono signal that hasn't been eq'd. I applied
equalization in software to mimic the RIAA curve and the sample sounded
fine. I want to capture from the line in jack though so I don't lose
the stereo.

Take note that you may have too much signal going into the soundcard,
and that's where the distortion comes from. Lower the level on
that input and see what happens. There will be a separate level for
the recording channel. Make sure you are feeding the signal into
the line input, not the microphone input. If for some reason the issue
is too much output from the preamp for the sound card (ie the preamp
is not broken so it distorts, or the cartridge is not outputing too much
for the preamp), then a volume control between the preamp and the soundcard
will let you set that level, but realistically something else is
the problem.


I know I'm not clipping the input to the soundcard. I could see from
the editor that the sample was in range, just the peaks of the signal
were chopped off.

If anything, a 4558 will have problems supplying the needed gain, so
likely the problem is elsewhere.


Michael


  #5   Report Post  
Arfa Daily
 
Posts: n/a
Default Phono Preamp


"Michael Black" wrote in message
...

"User-Friendly" ) writes:
I recently picked up an inexpensive phono preamp for the purpose of
making cdrs of some old records on my computer. When I plugged in my
turntable and captured some audio in the editor
the sound was distorted and and I saw that the output of the preamp was
about half clipped - it looked like a horizontal bar with a few troughs
here and there regardless of the recording volume setting of the sound
card. So I guess either the preamp is defective or there is a mismatch
between the output of the turntable and the input of the preamp. I
don't think the turntable has a builtin preamp.


Is the turntable fairly recent? It would seem they did start putting
preamps in recently made turntables, because the phono preamps disappeared
from equipment. One way to check would be to plug the turntable directly
into the soundcard; it will be a terribly weak signal and sound awful
if there's no preamp in the turntable.

You can't start messing with the gain of the preamp, because it's set
for certain standards. Which is an indication something is wrong, either
in the preamp or the turntable. The turntable has got a moving magnet
cartridge? Older and even really low end (as seen in many all in one
stereos from even 25 years ago) had ceramic cartridges that had high
output, and required little or no equalization.

Take note that you may have too much signal going into the soundcard,
and that's where the distortion comes from. Lower the level on
that input and see what happens. There will be a separate level for
the recording channel. Make sure you are feeding the signal into
the line input, not the microphone input. If for some reason the issue
is too much output from the preamp for the sound card (ie the preamp
is not broken so it distorts, or the cartridge is not outputing too much
for the preamp), then a volume control between the preamp and the
soundcard
will let you set that level, but realistically something else is
the problem.

If anything, a 4558 will have problems supplying the needed gain, so
likely the problem is elsewhere.

Michael


You should be able to feed a magnetic cartridge directly into the " MIC "
input of a soundcard, without need for a preamp. Both the sensitivity and
the impedance of this input, in general, match the output level and
impedance of a ' standard ' moving magnet cartridge ( 1-5 mV into 47k ). A
ceramic cartridge would overload this input and present a serious impedance
mismatch, but would be a better match to the line in. To get a proper level
with this setup, you would want a matching-amp with a gain of perhaps 5 -
10.

Some commonly available Aiwa phono decks from recent years, do have a switch
selectable preamp built in. The switch is located under the turntable at the
back, and is accessible through a hole in the turntable, visible after the
mat is lifted off.

As far as altering the gain of the preamp goes, I see no problem with
altering the value of the feedback resistor. The gain is set by the ratio of
the feedback resistor to the input resistor. The input impedance is
generally accepted to be equivalent to the value of the input resistor, so
altering the feedback resistor will have little or no effect on that
parameter. The manufacturer's designed gain setting for the preamp is to
some extent arbitrary. The output level of cartridges does vary over quite a
wide range, as does the input sensitivities of various manufacturers' line
inputs. I accept that there are ' general ' design rules for these
parameters, but they are by no means stuck to universally, and I certainly
wouldn't trust soundcard manufacturers to do so. Therefore, I would have no
problem with altering the gain of your preamp to suit your circumstances.

I would dispute that a 4558 will have any difficulty whatsoever in supplying
any amount of drive that you may require. A 4558 is merely a dual version of
the ubiquitous 741, which is well known to have an open loop gain of at
least 100,000. So even if you are running it with a gain of 1/100th that, an
input of 1mV will produce an output of 1V, plenty enough to drive a line
input on a soundcard.

Arfa




  #6   Report Post  
sofie
 
Posts: n/a
Default Phono Preamp

How does the turntable sound when through the preamp you plug the preamp
output into the auxiliary input of your newer stereo??? I suspect that it
might sound fine indicating that the preamp is good and the problem may be
in your sound card????
- - - - - - - - -

snipped:
"User-Friendly" wrote in message no preamp in the
turntable.

The turntable is from the early 80s. I'm pretty sure there's no preamp
because the sound
is weak and awful when it's plugged into the aux input on a newer
stereo I own that doesn't have a phono input.



  #7   Report Post  
User-Friendly
 
Posts: n/a
Default Phono Preamp

Thanks for the informatative reply. Any guess as to how or where in the
circuit this clipping is occurring?

  #8   Report Post  
User-Friendly
 
Posts: n/a
Default Phono Preamp

When I plug it into my stereo the sound is distorted, same as the
samples that I capture on the computer.

  #9   Report Post  
Ray L. Volts
 
Posts: n/a
Default Phono Preamp


"User-Friendly" wrote in message
oups.com...

I don't know what kind of cartridge the turntable has. I'm thinking
moving magnet, here's why. I was able to capture acceptable audio by
running both channels of the turntable directly into the mic input of
the sound card. The levels were okay but the sound was very tinny like
you'd expect from a phono signal that hasn't been eq'd. I applied
equalization in software to mimic the RIAA curve and the sample sounded
fine. I want to capture from the line in jack though so I don't lose
the stereo.


Since you can achieve an acceptable result from your mic input, an
alternative approach might be to capture each channel separately (two
recordings) and then use your editor to combine the channels. Obviously,
you'd have to magnify the samples and perfectly sync the beginning points of
the channel samples. Anyhow, it might be worth a shot just as an
experiment.

Having said that, are you certain your mic input is mono? Some soundcards
have stereo mic capability, despite the mic or headset boom which may have
been supplied with them. Check the jack on your sound card. It could be
that you need only purchase the appropriate adapter(s) for your patch
cables.


  #10   Report Post  
Arfa Daily
 
Posts: n/a
Default Phono Preamp


"User-Friendly" wrote in message
ups.com...
Thanks for the informatative reply. Any guess as to how or where in the
circuit this clipping is occurring?


It should be easy to tell if the problem is originating in the preamp
itself. Just stick a pair of high impedance 'phones directly across the
preamp outputs. Typical aircraft 32 ohm jobs, or the sorts of earpiece that
come with Walkmans etc are fine for this. I keep just such items in my
workshop for doing just this sort of check.

Whilst 32 ohms is low in comparison to the several k intended output
impedance, you will never-the-less generally find that the preamp will
deliver plenty enough to be able to clearly hear its performance.

If the preamp proves to be delivering a clean signal, and the input level
seems to be within range of the soundcard control software, this still
doesn't necessarily prove that you are not overloading the soundcard. It
depends on whereabouts in the circuitry chain on the soundcard, that
recording level is carried out. It may be that there is an analogue fixed
gain input buffer immediately behind the input. If this is designed for an
input level of say 10mV p-p max, and you throw 20mV p-p in there, then that
stage will limit, giving a seriously clipped output. If this is then
followed by the software-controlled record gain circuit, this will be able
to turn down the ( already distorted ) output of the buffer, until the level
appears to be within range of the software.

I wonder if the sound capture software that you are using, has its own fancy
level controls and GUI, or whether it just hijacks the standard Windows
soundcard control panel ? I would feel inclined to check the basic Windows
control settings, making sure that you select " Recording " under the "
Options " tab, then shut off all unused inputs, and play with the record
level control on your selected input. You should be able to find an option
to allow you to monitor the signal from the soundcard output at the same
time as you are inputting. If you are able to ultimately achieve a clean
sound, but the record level control has to be way way down, this will
generally indicate that you are putting excess signal into the card.

In this case, either pot it down between the output of the preamp, and the
input of the card, as has been suggested by other posters or, better in my
opinion, reduce the gain of the opamp IC by altering the value of the
feedback resistor.

Luck. Arfa




  #11   Report Post  
tempus fugit
 
Posts: n/a
Default Phono Preamp

Have you tried just leaving your turntable plugged into your stereo and
running a phono cable to your line in on your soundcard? That's how I do
vinyl capture and it works perfectly.


"Arfa Daily" wrote in message
news

"User-Friendly" wrote in message
ups.com...
Thanks for the informatative reply. Any guess as to how or where in the
circuit this clipping is occurring?


It should be easy to tell if the problem is originating in the preamp
itself. Just stick a pair of high impedance 'phones directly across the
preamp outputs. Typical aircraft 32 ohm jobs, or the sorts of earpiece
that
come with Walkmans etc are fine for this. I keep just such items in my
workshop for doing just this sort of check.

Whilst 32 ohms is low in comparison to the several k intended output
impedance, you will never-the-less generally find that the preamp will
deliver plenty enough to be able to clearly hear its performance.

If the preamp proves to be delivering a clean signal, and the input level
seems to be within range of the soundcard control software, this still
doesn't necessarily prove that you are not overloading the soundcard. It
depends on whereabouts in the circuitry chain on the soundcard, that
recording level is carried out. It may be that there is an analogue fixed
gain input buffer immediately behind the input. If this is designed for an
input level of say 10mV p-p max, and you throw 20mV p-p in there, then

that
stage will limit, giving a seriously clipped output. If this is then
followed by the software-controlled record gain circuit, this will be able
to turn down the ( already distorted ) output of the buffer, until the

level
appears to be within range of the software.

I wonder if the sound capture software that you are using, has its own

fancy
level controls and GUI, or whether it just hijacks the standard Windows
soundcard control panel ? I would feel inclined to check the basic Windows
control settings, making sure that you select " Recording " under the "
Options " tab, then shut off all unused inputs, and play with the record
level control on your selected input. You should be able to find an option
to allow you to monitor the signal from the soundcard output at the same
time as you are inputting. If you are able to ultimately achieve a clean
sound, but the record level control has to be way way down, this will
generally indicate that you are putting excess signal into the card.

In this case, either pot it down between the output of the preamp, and the
input of the card, as has been suggested by other posters or, better in my
opinion, reduce the gain of the opamp IC by altering the value of the
feedback resistor.

Luck. Arfa




  #12   Report Post  
Asimov
 
Posts: n/a
Default Phono Preamp

"User-Friendly" bravely wrote to "All" (10 Nov 05 15:54:16)
--- on the heady topic of " Phono Preamp"

Us From: "User-Friendly"
Us Xref: core-easynews sci.electronics.repair:348122

Us When I plug it into my stereo the sound is distorted, same as the
Us samples that I capture on the computer.

Is your turntable's pickup a moving magnet or a ceramic type? A
ceramic type will probably overload a moving magnet type preamp. If
this is the case you can connect the turntable directly to the card's
line input. Perhaps you are using a moving coil type preamp. These are
about 100 times more sensitive than a moving magnet preamp. Be sure
you aren't connecting the preamp output to the mic input. There might
be a serious overload level mismatch in this case.

Another cause of difficulty may be the turntable being improperly
adjusted. Perhaps you have a bent or misaligned needle shank. Perhaps
the stylus is loose in the shank. Perhaps the cartridge is old and the
elastomere shank mounts are hard as rock.

Have you grounded the turntable? Many turntables will make an
incredible noise if the chassis is not grounded. There is typically a
separate wire for this along with the phono cable. Look carefully for
a screw labeled gnd if available.

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.

  #13   Report Post  
Mark D. Zacharias
 
Posts: n/a
Default Phono Preamp

Is the preamp battery-powered? If so, the battery could just be low.
Otherwise the opamp may be railing out. Either a bad IC or maybe a capacitor
or resistor bad.

Mark Z.


  #14   Report Post  
GregS
 
Posts: n/a
Default Phono Preamp

In article .com, "User-Friendly" wrote:
I recently picked up an inexpensive phono preamp for the purpose of
making cdrs of some old records on my computer. When I plugged in my
turntable and captured some audio in the editor
the sound was distorted and and I saw that the output of the preamp was
about half clipped - it looked like a horizontal bar with a few troughs
here and there regardless of the recording volume setting of the sound
card. So I guess either the preamp is defective or there is a mismatch
between the output of the turntable and the input of the preamp. I
don't think the turntable has a builtin preamp. I opened up the preamp
and looked at the board - it's a simple single 4558 IC circuit, just an
input and an output for each channel, no pots or switches. I want to
mod it to lower the gain so it doesn't distort. I'm not an engineer or
a tech, just a tinkerer who's built a couple guitar fuzzboxes and done
a few simple repairs. I couldn't find any mention of a schematic for
this particular device on the web, and it seems like such a simple
circuit that I probably don't need one. I thought about trying to trace
the circuit but I'd rather not if I can avoid it. I know the
turntable's okay because I can get a clean (albeit low) signal direct
into my sound card. So I'm trying to figure out at what point in the
circuit the signal's getting clipped. I'm kind of shaky on these
concepts, but would this result from an impedance mismatch between the
output of the turntable and the input of the preamp? Or maybe the gain
of the IC is too large and I could lower the value of the resistor in
the feedback loop? Do I have to be concerned about alteration of
component values affecting the equalization curve? Any info would be
appreciated.


Experiment using an amplifier and a speaker instead of the computer.
If that sounds OK, then the computer input may be overloading.

greg
  #15   Report Post  
DaveM
 
Posts: n/a
Default Phono Preamp

"User-Friendly" wrote in message
oups.com...
Michael wrote:
Is the turntable fairly recent? It would seem they did start putting
preamps in recently made turntables, because the phono preamps disappeared
from equipment. One way to check would be to plug the turntable directly
into the soundcard; it will be a terribly weak signal and sound awful
if there's no preamp in the turntable.


The turntable is from the early 80s. I'm pretty sure there's no preamp
because the sound
is weak and awful when it's plugged into the aux input on a newer
stereo I own that doesn't have a phono input.

You can't start messing with the gain of the preamp, because it's set
for certain standards. Which is an indication something is wrong, either
in the preamp or the turntable. The turntable has got a moving magnet
cartridge? Older and even really low end (as seen in many all in one
stereos from even 25 years ago) had ceramic cartridges that had high
output, and required little or no equalization.


I don't know what kind of cartridge the turntable has. I'm thinking
moving magnet, here's why. I was able to capture acceptable audio by
running both channels of the turntable directly into the mic input of
the sound card. The levels were okay but the sound was very tinny like
you'd expect from a phono signal that hasn't been eq'd. I applied
equalization in software to mimic the RIAA curve and the sample sounded
fine. I want to capture from the line in jack though so I don't lose
the stereo.

Take note that you may have too much signal going into the soundcard,
and that's where the distortion comes from. Lower the level on
that input and see what happens. There will be a separate level for
the recording channel. Make sure you are feeding the signal into
the line input, not the microphone input. If for some reason the issue
is too much output from the preamp for the sound card (ie the preamp
is not broken so it distorts, or the cartridge is not outputing too much
for the preamp), then a volume control between the preamp and the
soundcard
will let you set that level, but realistically something else is
the problem.


I know I'm not clipping the input to the soundcard. I could see from
the editor that the sample was in range, just the peaks of the signal
were chopped off.

If anything, a 4558 will have problems supplying the needed gain, so
likely the problem is elsewhere.


Michael



Check the recording level setting in the sound recorder. It may be waay to
high. Bring it down while recording a session and see what effect it has
on the recording. (What sound editing software are you using?)

Others have given good advice, but obviously you haven't found the right
clue yet. I need to ask which input on the sound card you're plugging the
preamp output into. The preamp output should always be plugged into the
line input of your sound card, NOT the microphone input. Although the mike
input gives you a better signal when you plug the turntable directly into
it, it's obviously not the right one. The preamp applies RIAA equalization,
(assuming that it's a PHONO preamp, not a MICROPHONE preamp). The
microphone preamp will not apply equalization, and will make the sound
awful.

Next, assuming that the preamp is a phono preamp, what's really important is
that you determine the type of cartridge you have in the turntable, and how
old it is. If the stylus is original from the 80's, it should be replaced,
without question. If you can see any numbers on the cartridge, you might
post them here... someone can tell whether it's a moving coil vs. moving
magnet. Also, what kind of turntable are you using (make & model)? That
might give a clue to the nature of the problem.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!




  #16   Report Post  
Posted to sci.electronics.repair
Dana
 
Posts: n/a
Default Phono Preamp

I was going to say the same thing. Hook the turntable up direct and see
what happens.

On Thu, 10 Nov 2005, User-Friendly wrote:

Michael wrote:
Is the turntable fairly recent? It would seem they did start putting
preamps in recently made turntables, because the phono preamps disappeared
from equipment. One way to check would be to plug the turntable directly
into the soundcard; it will be a terribly weak signal and sound awful
if there's no preamp in the turntable.


The turntable is from the early 80s. I'm pretty sure there's no preamp
because the sound
is weak and awful when it's plugged into the aux input on a newer
stereo I own that doesn't have a phono input.

You can't start messing with the gain of the preamp, because it's set
for certain standards. Which is an indication something is wrong, either
in the preamp or the turntable. The turntable has got a moving magnet
cartridge? Older and even really low end (as seen in many all in one
stereos from even 25 years ago) had ceramic cartridges that had high
output, and required little or no equalization.


I don't know what kind of cartridge the turntable has. I'm thinking
moving magnet, here's why. I was able to capture acceptable audio by
running both channels of the turntable directly into the mic input of
the sound card. The levels were okay but the sound was very tinny like
you'd expect from a phono signal that hasn't been eq'd. I applied
equalization in software to mimic the RIAA curve and the sample sounded
fine. I want to capture from the line in jack though so I don't lose
the stereo.

Take note that you may have too much signal going into the soundcard,
and that's where the distortion comes from. Lower the level on
that input and see what happens. There will be a separate level for
the recording channel. Make sure you are feeding the signal into
the line input, not the microphone input. If for some reason the issue
is too much output from the preamp for the sound card (ie the preamp
is not broken so it distorts, or the cartridge is not outputing too much
for the preamp), then a volume control between the preamp and the soundcard
will let you set that level, but realistically something else is
the problem.


I know I'm not clipping the input to the soundcard. I could see from
the editor that the sample was in range, just the peaks of the signal
were chopped off.

If anything, a 4558 will have problems supplying the needed gain, so
likely the problem is elsewhere.


Michael



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