Re: Phono Preamp


"Michael Black" <et472@xxxxxxxxxxxxxxxxxxx> wrote in message
news:dl0hkp$fl4$1@xxxxxxxxxxxxxxxxx
>
> "User-Friendly" (ribaldreader@xxxxxxxxxxx) 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


.

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