Convert cartridge output voltage to db gain

Hi John,

I second Doug's excellent comments, and I agree with him and Kal that the noise you are hearing is most likely being generated in the front end of the integrated amp.

Re aiming for a 2mv cartridge to match the level of your cdp: The actual number is 1.25 mv. 66db corresponds to a voltage gain of exactly 2000 (20log2000 = 66). 2.5V/2000 = 1.25mv. However, that still may be high enough to unduly limit your selection of a new cartridge (if you can't resolve the problem via tube rolling), and might still lead to the kind of impedance matching issues with your phono stage that Doug referred to.

1.25mv is about 14db greater than 0.24mv. A 10db increase in level (which was your initial rough guess) would correspond approximately to a 0.75mv cartridge (20log(0.75/0.24)= 9.9db).

Another thought altogether: Although the problem is most likely in the line stage section of the amp, I'm wondering if it might be noise coupling that involves the interconnect between phono stage and amp (assuming the interconnection is unbalanced; if it is balanced that is almost certainly not an issue). That may differ with the new amp compared to the old preamp + amp, due to differences in ac-related leakage paths. Are the ac power plugs of the phono stage and amp plugged into separate dedicated ac lines, or into separate outlets or separately filtered power conditioner outputs? If so, try plugging them into the same ac source (which will minimize differences in potential between the two chassis, which might otherwise result in extraneous inter-chassis noise currents flowing through the interconnect shields, in common with signal return currents). Also try unplugging the cdp completely, to make sure digital noise it may generate is not somehow coupling into the phono signal path. And consider whether there may be any nearby sources of rfi/emi, such as dimmer switches, that the new amp may be more sensitive to than its predecessor.

Best regards,
-- Al

Thank you Doug, Kal and Al for your very informative replies. I think I have some ideas of were to head here. First let me make a few points of clarification. Kal, you are right, I looked it up, and the preamp stage in my integrated amp has 11 db more than the gain of my seperate preamp. So more gain may be a bad thing.

Secondly,I have owned this integrated for over a year now, but due to personal reasons, have not listened to analog for about the last 8-9 months. As I recall, the last I listened to this setup regularly, the noise was more apparent than with my seperates, but not nearly as loud as the noise floor is now. Doug, the noise sounds like surface noise, steady hiss, easily audible from the listening chair when I turn the volume up to my normal listening levels (70-90 db). Also thanks for the JJ tube reco Doug, I may try that.

Here are my current thoughts. First, my phono stage is the Walker Audio Signature. Walker recommends that the DC offset be adjusted monthly. When I had the rig running regularly, I checked it about every 3 months, no issues after the original adjustment. However, I have lost the manual, and can't recall how to adjust the DC offset using the supplied DMM. I sent Lloyd an e-mail yesterday and I'm awaiting a reply. I think I should check the DC offset before tube rolling. Second, once I have made sure the Walker phono preamp is in spec, if the issue still exists, change out the preamp tubes. As far as everyone has said, and I tend to agree, one of these two steps should solve my problem.

I do not expect a $6500 integrated amp to be as quiet as $20K seperates, but I do remember it being enjoyable before, so I should be able to make it enjoyable again for much less than buying a new cartridge or phono stage.

I'll keep you posted once I've heard Lloyd's reply. Thanks again for all of your help.

Cheers,
John

BTW, all cabling is single ended, not balanced.

"Kal, you are right, I looked it up, and the preamp stage in my integrated amp has 11 db more than the gain of my seperate preamp. So more gain may be a bad thing."

Bingo. The key thing here. Your system in its new incarnation is amplifying the noise resulting from the slight mismatch (slightly too much gain) you have with the phono stage and cartridge.

Get your gain on the phono stage into the 63 db area or get a cartridge with an output of .17 mV and your noise problem will disappear. Things will sound better as well. Getting the gain right will be much easier, less costly, and less time consuming than coming up with a new cartridge!

Hdm, I respectfully disagree. Lowering the cartridge output will make things worse. It will result in the signal being lower relative to the level of noise that is being summed onto it, presumably in the preamp stages of the integrated amp. The lowered signal level will result in John setting the volume control at a higher setting than he is presently using, which will further boost the noise (assuming the noise is being introduced at a point in the system that is prior to the volume control, which appears to be the case).

What is called signal-to-noise ratio (s/n or snr) is what is important, and what needs to be improved, either by raising the signal level prior to the point at which the noise is being introduced, or by reducing the amount of noise that is being introduced, or both.

Too much gain or too little gain is only an issue if the volume control ends up being used near the top or bottom of its range, or if the input of a component is overloaded, neither of those situations being present here, or if the additional gain comes at the expense of degraded s/n ratio in the particular design (which may be the case here).

Regards,
-- Al

Correction. The last paragraph of my previous post should have read:

Too much gain is only an issue if the volume control ends up being used near the bottom of its range, or if the input of a component is overloaded, neither of those situations being present here, or if the additional gain comes at the expense of degraded s/n ratio in the particular design (which may be the case here).

Regards,
-- Al