NAD 7150 Stereo Receiver/Amp Noise when Listen knob set to phono.

Hi I'm not a tech in any way, but before I take this for repair, I want to make sure it's not a simple connection/hookup issue.

I got this and a Technics turntable on Ebay, and I'm very happy with them so far. The receiver is hooked up, radio works great, audio from DVD/TV works great.

When I select Listen/Phono, I get a loud rumble through the speakers, whether the TT is on or not, and whether it is plugged into the back of the REC or not.

The noise goes up and down when the volume is turned.

The TT does work and play records, but the rumbling is underneath.

Also, the ( I assume) ground lead from the back of the TT, where does that get connected?

Reply to
nukemedtc99m
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I don't know what you mean by "rumble", but I assume you mean "hum".

Yes, if the TT is not properly grounded to the amplifier, there's a good chance of getting loud and annoying hum.

On the other hand, the fact you get this noise even when the TT is not connected suggests there might be something wrong with the amplifier's phono preamp.

I'd suggest starting your troubleshooting by making sure the TT is properly grounded to the amp. Check the ground wire to be sure it isn't broken.

Reply to
William Sommerwerck

The fact that the rumble goes up and down with the volume control, coupled with the fact that it is not present on other input selections, indicates a fault with the phono input preamp, which is usually a fairly conventional opamp stage. As you say "speakers" in the plural, I assume that the problem is present on both stereo channels. I have heard this sort of problem being caused by the opamp itself being faulty on a number of occasions over the years, but other causes could be a power supply issue - such as a bad decoupling cap on one of the opamp rails, or even a noisy regulator, that only shows when you are switched to a very high sensitivity input such as "phono".

As far as fixing this problem goes, it should be a simple matter for an experienced engineer, but if you have no personal experience of electronic service work, it may be beyond your capabilities. I wouldn't expect it to be expensive.

Any separate ground wire from the turntable, just needs to be connected to the amp's rear panel metalwork. Most amps which have a specific phono input, usually have a knurled screw to accommodate this ground wire. Depending on the overall configuration of the system, the effect of attaching this wire can be anything from un-noticable to profound.

Arfa

Reply to
Arfa Daily

Thank you to both William and Arfa I'll be looking for a service establishment. Thanks again

Reply to
nukemedtc99m

Good morning,

On the back of your NAD 7150 to the left of the phono input jacks are 3 items that require your attention. On top is a switch marked "MC MM" to set the phono preamp for Moving Coil or Moving Magnet type cartridges. Moving Magnet are more common, so that is where I'd try setting the switch first. Just below that is the ground screw where you can attach your turntable's ground wire. The lowest item of the 3 is the phono capacitance switch, which I would leave in the center position for now.

I suggest moving both of the switches back and forth a few times to clean up their contacts, as that could be your entire problem. Try the MM position first. The MC position will give more gain, but overload more easily. As for the phono capacitance, I'm not sure what setting to suggest, as it depends on your cartridge, so try the middle first, and see how it sounds. You won't hurt anything by trying the other positions for a sound you like.

The phono stage in this receiver does not use a common op amp, it uses discrete (separate) transistors, and uses an FET input differential pair. I'd agree with the other posters that if both channels have the same problem, then the power supply could be the cause too.

Lastly, there are 3 other switches on the rear of your receiver and I'd suggest you set the as follows (for normal stereo use):

Bridging: Off

Soft Clipping: On

Speaker Impedance: Normal

Regards, Tim Schwartz Bristol Electronics

Reply to
Tim Schwartz

Thanks Tim I switched to MM and cut the sound down by half or more. I have confirmed that the sound is only on the left channel, and I would descibe it not so much as hum as the sound of wind blowing on a vocal microphone. Anyway........ The other settings you recommended have no real effect, but I have them where you suggested.

Where from here?

Thanks again. Gene

Reply to
nukemedtc99m

Whoops, make that the right channel, I replugged the spkr wires bkwds

Reply to
nukemedtc99m

Since it occurs even if the turntable is plugged in or not, it must be a fault in the receiver.

Since the level of the noise changes with the volume control, it's prior to that point in the circuitry.

Since you say it occurs when you select Listen/Phono, and don't say that it occurs in other settings of the input control, it's probably in the part of the circuitry which is specific to the phono stage.

Since it's only in one channel, rather than in both, it's probably not in any circuitry common to both channels (e.g. the power supply).

Since you say it's a wind-like noise and rumble rather than a hum, it sounds like it's of a broad-band origin rather than being from

60-cycle powerline noise pickup.

Based on all of the above, I suspect that the fault may be a failed transistor (or op-amp) in the phono preamp circuitry - the "RIAA amp" stage. This is the low-level circuitry which boosts the phono input signal up to "line level" voltage, and also apply the RIAA equalization curve (which boosts the low frequencies and rolls off the highs). It's located between (in the electrical sense, and perhaps physically as well) the phono input jacks and the program-selector switch.

The fact that it's a wind-like rumbling lends a bit of additional evidence to the suspicion that it's in the RIAA stage, because this stage boosts the lows. If it was just a white-noise source after the RIAA stage it would sound more like a hiss than a rumble.

RIAA phono preamp stages often use op amps (ideally low-noise ones) and/or low-noise discrete transistors (e.g. 2N4401/2N4403). The use of low-noise parts is even more important in preamps that have a moving-coil operating mode, since MC cartridges have low output voltages and require more gain from the preamp (and are thus more subject to noise problems). I suspect that one such transistor or op-amp has gone noisy in your receiver.

I don't know what the 7150's input stage looks like, as I wasn't able to find a schematic or manual for free download.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

On 2/3/2009 2:25 PM Dave Platt spake thus:

I had exactly this problem with a receiver years ago, and it turned out to be a bad transistor somewhere in the preamp stages.

In my case, I was easily able to find the offending part by tapping on the transistors, as the bad one had gone microphonic. You might try this; turn the receiver on, turn the volume up just a little ways until you can hear the noise (not too high, now), then tap on all the transistors you can see in the preamp section (near the input connectors, probably). If you hear a big "thump!" when you hit one, it needs to be replaced.

In the case of my receiver, I was able to get a NTE replacement that was close enough. The thing still works fine 15 years later.

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Reply to
David Nebenzahl

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