Phono Preamp

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

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

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.

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.

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.

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 -

2. 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

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????

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

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

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.

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

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.

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

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.

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

greg

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.