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Confused about gain: phono + pre, or just phono?

I'm trying to figure out how "low I can go" on my phonostage. I understand that the ideal numbers aren't necessarily "written in stone" in that the cart will still work even if the number isn't exactly hit. I'm looking to try to understand the ideals.

So, when trying to sort out what your phonostage will support, I've seen the formula that takes your cart's output in mV, divides that into 1, takes the log of that, and multiplies it by 20. The result is the targeted minimum that your phonostage should provide. Said another way:

Targeted phono gain (in dB) = 20 * [log * 1/(your cart's output)]

My question: is that resultant just what your phono stage should support, or what your phono+pre supports?

For example, say you have a .25mV cart.

1/.00025 = 4000
Log10 of 4000 = 3.6
20x = 72

So, you need 72dB.

But what if your phono does 60dB and your pre does 12dB? Are you good to go, or do you need 72dB from the phono by itself?
socrates7
hdm
971 posts
 
www.kabusa.com

Click on phono preamps at the top of the page and then scroll to the bottom of that page for the "phono preamp parameter calculator".

That gives you the ideal gain number for the phono preamp. Unless you are running a passive preamp, my advice to you would be to focus entirely on what you need at the phono stage, which is what the KAB calculator does.

FWIW, I have found it to be pretty much dead on and having either too much or too little gain can be problematic and result in less than optimal sound quality. As you see from the KAB site, optimal gain for .25 mV is around 62 db. 72 db would, IMO, be quite high and in many cases result in overload and compromised sound quality.
mihaitaa
111 posts
 
I might be wrong, but usually the preamps have 500mV input sensitivity, which will mean 66dB of gain required for the phono stage to bring 0.25mV to 500mV.
Going 62 dB instead will load the pre with 315mV at the input instead of 500mV.
Of course, the pre will pick up the task of amplifying to whatever the input level of the power amp is.
I dare to say that going with less than 66dB of gain at the phono stage level will work, only 66 dB will do better, signal to noise ratio wise.
socrates7 OP
76 posts
 
Interesting -- two responses, and contradictory. ;-)

I've heard about the 500mV target. But I see the KAB calculator is targeting 375mV.

I understand that a .25mV cart "will work" with a 62dB (375mV target) gain phono pre -- but will a 72dB phono stage (500mV target) be "better" in some way (and why)? Or will I be cranking the dial on a 62dB phono in ways that would be suboptimal?
mihaitaa
111 posts
 
My answer was only looking at the mere numbers and factoring in the main principle that, considering the noise floor of any amplification stage, loading it in its optimal zone will improve the S/N signal to noise ratio.
Thus there is no fundamental contradiction, so to speak, KAB targets a lower preamp input voltage, hence recommends less gain.
It could very well be that KAB reached the conclusion that the sweet spot of preamps input is less than 500mV? Don't know, I tackled this from the numbers angle only.
Many preamps indicate that they handle overloads very well, over the specified 500mV and harvest better S/N ratio in the process.
socrates7 OP
76 posts
 
KAB sells pre-amps and phono pre-amps -- is their calculator then based on their own as-designed input sensitivity of 325mV (not 375mV, I mistyped above)? The question then is, is the 325mV "standard" or do some target a 500mV input sensitivity (or something else)?
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