Phono stage and preamp overload


I’m in the process of setting up my first analog rig (i.e. trying to rely upon my digital gear for ease and building an analog system that’ll offer a different experience).  I’ve read as much as possible without asking some questions, so here goes…Basically, I’m trying to understand how to set the phono stage at an appropriate gain such that my line stage is not “overloaded”.  

 

Here is my relevant gear and numbers:

Line Stage - Bel Canto Pre3 (input voltage overload = 10 vrms; gain = 20 db)

Phono Stage - Pass Labs XP-17 (gain of 50/60/70 unbalanced or 56/66/76 balanced)

Cartridge - Dynavector 20x2 (0.5mv)

Amp - Pass Labs x250.8 (2.24 v at full gain; not sure if this is relevant or not)

 

I’ve read that dB gain = 20 log (Vout/Vin).  How do I calculate the appropriate phono gain so that I retain a reasonable amount of headroom and don’t overload my line stage?  My simple math leads me to setting a phono gain of 66, or 20xlg(1/0.0005), but I have no idea how that relates to the input voltage seen by the line stage nor do I understand whether or not Vout = 1 is the correct value).

It seems easy, but I remain perplexed…please help an analog newbie escape digital!

mikek1

You are seriously over thinking it. Especially since overload is much less likely than having too much. Your typical phono stage with 45 to 65 dB gain will output a normal 3 or so, right in the sweet spot of your preamp input range.

The volume range on virtually all preamps is so wide it just isn't a problem. Almost always, the worst that will happen is you have to turn the knob a lot more, or a lot less, than with your line level sources. This is where you stop and remind yourself, the position of the volume knob means nothing. It is nice if all the sources are at about the same level, because then you don't have to fiddle around as much. But that is pure convenience and nothing to do with sound quality. Do it a time or two, you will see.

Thanks, millercarbon.  Truth be known, your passion for analog and distaste for digital factored in to me going down this path.  Your input/insight is much appreciated.  Best,

Well the stuff you have listed there is excellent. I would lean Koetsu but in truth once you get much over a grand it is hard to find a bad cartridge and a good one at this level is really good. They do keep getting better but already you are at a level where it pays big time to put that cart on a top flight arm and table. Frankly the table and arm are more important. Especially once you have an excellent phono stage, which you do.

(I’m not sure if this is what you have or what you’re considering?)

What table and arm are you using?

Or on the other hand, let’s over think things a bit too :)

Yes, 66dB gain on top of 0.5mV results in exactly 1.0 Volt output. You did the math correctly! In reality, there are some additional factors to account for:

  1. Loading a cartridge (increasingly lower load resistances on the phono stage) will start to lose some of that source signal as the load resistance approaches the cartridge’s DC coil resistance. This is because of Ohm’s law. For example, if you had a 5 ohm coil cartridge (e.g. your Dyna is 5 ohms) and you fed it into a phono stage loaded at 10 ohms, the voltage would be divided such that only 10 / (10 + 5), or two-thirds of the cartridge’s voltage, would be available to your phono stage. That represents a loss of -20 * log(2 / 3) = 3.5 dB of signal from your loading, which is reasonably significant. It’s also why there’s a rule of thumb "load resistance should be AT LEAST 10x your coils" so that this loss is limited to well under 1 dB. In your case, 10x would be a 50 ohms loading. However I’ve gone lower than 10x, and it can sound fine in certain situations until you start going below 5x (other bad things start to happen besides the signal loss, like a changing frequency response). This is also the reason why stacking a SUT (step-up transformer) into another SUT won't get you anywhere - the net gain starts to move backwards from the loading losses.
  2. It’s hard to represent the output level of any analog device with just one number. Your cartridge will be able to put out much greater peaks than that 0.5mV. Some say up to 10x more. That’s why it’s OK to do the cartridge & phono math, but you need to leave a lot of room for overhead. MC is right that for preamps you generally have the volume control guarding the input stage, which helps protect its circuitry from overload (i.e. you just set the volume lower as necessary). However some preamps could be configured differently, with an input stage and THEN a volume control - these would be susceptible to overload.

In your case, a 1V "typical" output from phono stage is fine. Usually your concern should be overload margins within the phono stage itself, not the downstream preamp. But sliced a different way, your 66dB gain on 0.5mV is LIKE 20dB + 46dB which is analogous to a 5mV MM cartridge (20 dB is 10x which takes 0.5mV to 5mV) into a 46dB MM stage, which is quite normal. Either 66dB or 60dB is likely to be optimal for your system.

I also agree with MC that most cartridges over $1K these days can sound excellent. Proper installation and system matching are crucial, as always. And Koetsus are particularly lovely. And for the money, I’ve also become a particular fan of the current Benz lineup (Zebrawood L, Ebony L).