2. analog
  3. 173826-37344

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
lewm
12,645 posts
 

Upon further thought, I may be wrong. In a full function preamplifier (phono and line inputs) the line level inputs connect to the attenuators. So depending upon the type of attenuator, there would be an effect on overload. Sorry. For phono, the attenuators do not affect phono overload, is what I was thinking.

itsjustme
681 posts
 

It sounds like you have solid info above ( a rarity!)

In general, people struggle with too little photo stage gain, especially from low output moving coils where the max may be something like 1/2 mV, rather than too much.

In the good old days, manufacturer’s targeted a nominal interface at the preamp line of 1V rms = 2.83V peak to peak. Today this has been obliterated, mostly by DAC manufacturers wanting to have their products double as preamps. 2.2 and even 6 and 10 (!)V rms are not unheard of. Yes, i’m talking to you Theta., Rant off.

 

But 1V is still a solid target. So a MM cartridge that has, say an 5mV peak output might require a phono stage with 1/.005 > 200X gain (@ 1 kHz) or 46 dB. Bingo. A MC with 0.5mV peak output would similarly require 10X more, or 2000X (beginning to see why moving coil amps must be low noise???) or 66dB.

 

A 10V overload (rms or peak?) is huge.

 

As noted, in the vast majority of preamps, the first thing the RIAA stage output signal sees is the (passive?) volume control. So the point becomes moot. EXCEPT!!!!! for those that first:

 

a) hit a buffer

b) have monolithic resistor arrays as a volume control (like my prototype*)

c) have other similar circuitry first

 

In high end, which values simplicity and a pure signal path these are rare.

 

* note: the monolithic chip has been a learning experience. As a postulated it is in fact superior to most conventional volume controls with huge UI benefits, but it also carries lots of pitfalls, this being but one.

itsjustme
681 posts
 

Upon further thought, I may be wrong. In a full function preamplifier (phono and line inputs) the line level inputs connect to the attenuators. So depending upon the type of attenuator, there would be an effect on overload. Sorry. For phono, the attenuators do not affect phono overload, is what I was thinking.

Right ( I think i understand your point). Most of the time the flow is:

  1. Phono stage with lots of gain and equalization -->
    100X -> 2000X @ 1 kHz
  2. volume attenuators, which will lower this level commensurate with your desired volume level, and always therefore within the overload range
  3. active line stage with a little amplification/gain, maybe 2-6X.

Its very rare to have too much phono gain. Its hard to get without noise and distortion, so most of the time one struggles for enough

Note there also may be a transformer at the very front end. Those are some fancy, expensive transformers.

 

 

More to discover