Stereo phono input testing - low freq noise

Just got my QA403 and I’m trying it out testing a stereo. Had no trouble getting it to work well testing a line-level input (~150mV) with full power coming from the stereo amplifier.

However when testing the phono stage with a very low level input (~5mV) I get a strange low frequency noise when turning up the volume control. I’ve tried two different stereos now just to compare and they both do this exact same thing. I’m guessing this is something people have run into before but I searched and didn’t find anything exactly like what I’m running into. Any suggestions appreciated.

Picture below is with volume turned up to about 3 where the low frequency noise seems to jump up quickly. It looks pretty normal with the volume turned all the way down.

@brian- You should measure the phono stage at the tape monitor output if you receiver, preamp or integrated amp has one. If it does not, you need to measure it at your preamp output. Right now you are measuring the effects of the amp and preamp, and not just the phono stage. You made need to an earth ground to your amp/preamp as well to cut down on hum. You may want to apply the RIAA user weighting file “RIAA Playback Curve High Res.txt” when you do the frequency response.


Yes, @brian, agree with @var on, especially on ensuring you have a good ground.

The bumps you see there look to be about 15 Hz apart, don’t they?

Also, just to make sure I understand your post correctly, are you saying you are testing the phono pre in two different stereo receivers (with integrated amps) and you are seeing the same thing? Or are you saying you have a single phono pre, and you have connect that to two different amps and the result is the same??

1 Like

When I saw this on the first receiver (a Harmon Kardon), I tried out a second one (a Sansui) and got the exact same behavior. I’m just using their built-in phono stages. Putting a signal into the phono inputs and looking at the speaker outputs (connected across 8 ohm dummy loads). It seems like motorboating I think. Both receivers looked very good when I tested the aux inputs with a line-level input signal. I was able to turn the volume up to full rated power (and a little beyond) without the low frequencies going out of control like they do with the phono inputs. Maybe I just have enough low frequency noise getting into the phono stage through the cables that it’s causing it?

I was trying to get a good characterization of the total system but using the tape record output or pre-amp output will be ok for now. I’ll try grounding the receiver chassis. I’m going to try differential connections as soon as I get a couple more cables too.

Are you connecting the QA403 ground to both the phono input ground and speaker ground perhaps?

If so try using a differential measurement of the output so you are not creating a ground loop around the amplifier.

1 Like

That’s exactly what I’m doing and since it worked ok with the aux inputs, I thought it would be ok with the phono inputs too. I’m going to try differential measurement as soon as I get some more cables in the mail.

Phono inputs have maybe 30 to 40dB more gain though…

I was testing the aux with about 150mV input and the phono with about 5mV.

Whatever, its the gain and the ground loop that will send things unstable if my theory is correct (basically you created unintended positive feedback), whatever level you used isn’t important.

1 Like

Ground loop is what I was thinking too. But I just tried differential measurements and it didn’t make a difference. So maybe it is just signal from the speaker outputs getting into the input cables? Seems worse at low frequencies so that might make sense? I was using differential cables on both the input and output. I’m able to do good measurements of the phono stage using the tape record out jacks so that’ll do for now. If you have any other ideas for getting better isolation, I’ll try them. Thanks for your help.

My experience has been that you will almost never get a good response when measuring the phono response if it goes through the entire amp chain. Phono responses are often specified as being measured at the tape monitor output so that you are just measuring the Phono section. I do not test the phono stage input to the power amp output anymore. Keep in mind that things always look worse when measuring an MC stage through the tape monitor loop due to the very high gain of that stage.

1 Like

That’s good to know and matches my experience so far. I’m getting good results using the tape output so I’ll go with that for now.

@Brian- for reference, here are plots that I measured a few days ago on a preamp’s MM phono stage. It is one of the best I have measured:

1 Like

Was the cartridge inductance included in the measurement - many MM cartridges are in the range 0.1H to 1H with around 1k of winding resistance - to get meaningful noise floor its good to model this explicitly - the phono input’s current noise interacts with the source impedance.

1 Like

The measurements I do on the phono stage are done with my RCA test cables from the QA40x connected directly to the preamp/integrated amp/receiver’s phono input and no allowance is made for cartridge matching. The particular preamp above did allow for a range of capacitance to be shunted at the input (selected digitally), but it made no real difference in the frequency response.

1 Like

If that’s one of the best, I’d hate to see the bad ones… :wink:

Here is a 41 year old Denon preamplifier’s MM phono stage (roughly the same settings for comparison)

My Y axis is +/-1dB, yours is +1/-1.5dB

It’s 1kHz MM stage phono overload is a phenomenal 340mV (spec 320mV).

1 Like

I have no idea how to do that. Didn’t even know modeling this was possible in the QA403. Could you point me to somewhere to read up on how to do it?

@restorer-john- I cannot believe that your 41yr old Denon is performing better than the 14year old C48, but I suppose it’s possible. The spec is 88dB with A wtg applied, which I did not do, but don’t think it would get there. The flatness you are showing is outstanding- like a loop thru. Not sure why you are getting so much better results…

Simply add an inductor and resistor inline with the phono input… An old MM cartridge would be ideal in fact!

@MarkT- Thanks for the suggestion- I am guessing one could build a matching network ? Hopefully, @restorer-john may done that ? I will do some research into that as well