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Author Topic: Healthy Dose of Math for a Sunday Morning  (Read 3071 times)

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Offline EScott

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Healthy Dose of Math for a Sunday Morning
« on: September 04, 2011, 08:38:24 AM »
I need some help calculating the signal value entering my pre amp through my phono chain.

My cartridge has an output of 2.5 mV and then goes into a phono stage with RCA inputs and outputs, that adds 40dB of gain @ 0pf and 47kOhm.

The signal then moves into my pre amp which has an internal gain of 11.8 dB.

This combination produced a very bright signal, and at first I attributed it to the manufacturer's recommendation of 300 hours break in on the phono stage. 

I put a fair amount of hours on the device and eliminated that as the issue.  Why would a solid state device require 300 hours of break in anyways?  It is simply a circuit board - I opened it up and it is not rocket science.

I have ultimately had to put a 12 dB attenuator into the line stage input on my pre amp.  It sounds great, but I have to question if the attenuator is then coloring the sound coming from a mid grade turntable / cartridge combination.

Do the RCA's add gain since they are unbalanced?  And how do I perform the conversion from mV to dB to actually understand what is coming through the signal line?

Thanks.
"You know the type loud as a motor bike
But wouldn't bust a grape in a fruit fight"

Offline Patrick

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Re: Healthy Dose of Math for a Sunday Morning
« Reply #1 on: September 04, 2011, 01:13:25 PM »
The output of your Phono stage is already line level, and shouldn't have to go into another Preamp,  unless you meant to type "amplifier" in your original post. 

The breaking in period for cartridges is necessary but won't make a dramatic tonality difference.  It will be a subtle change, any other issues you are hearing is not going to be fixed by burning in the cart.

What Phono stage are you using?  You shouldn't have to use inline attenuators...do that attenuation at your Phono Preamp if possible.  The passive (?) attenuator Isn't helping your signal, but also isn't doing that much damage except adding a little bit of noise. They are necessary if one of your components is being overdriven, though.

Your RCA cables won't add any gain to your signal path, unbalanced or otherwise.

Db/mv calculator: http://www.mogami.com/e/cad/db.html
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Offline EScott

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Re: Healthy Dose of Math for a Sunday Morning
« Reply #2 on: September 04, 2011, 06:10:59 PM »
Thank you for the reply and calculator.

@ 2.5 mV, the dB = 2.5 mV ->
 Audio industory:
  -49.8227 dBm
 Television industory:
  7.9588 dBm
 Radio frequency engineering:
  -39.0309 dBm

So, with 40 dB of gain in the phono pre amp, it is close to 0 coming into my Manely Shrimp pre amp.  The pre has 11.8 dB so the 12 dB attenuator cancels that gain out?

Is that right?

Here are the specs on the pre:

ALL-TUBE: Vacuum Tube all-triode design
Input Tubes: 2 x 12AT7EH large plate Electro-Harmonix Russian
Output Tubes: 2 x 7044 or 5687 GE JAN NOS
Five Line Level Inputs: unbalanced RCA
MUTE switch and Warm-Up muting delay
NOBLE volume control: Accurate tracking usually within 0.5dB. Smooth operation. (no detents)
NOBLE balance control: Center detented
Record Output: (AKA "TAPE OUT") Unbalanced RCA jacks, unbuffered, passive (always ON)
Main Outputs: Two sets of unbalanced RCA main outs for bi-amping or driving a powered subwoofer
Gain: 11.8dB
Input Impedance: 250 Kohm
Input Sensitivity: 200mV in yields 1V out
Maximum Input Level: 5V in produces 1.5% THD at output
Frequency Response: 10 Hz - 80 KHz; 20 Hz - 20KHz, -0.3dB
Noise Floor: Typically -83 dB 20-20K with volume control fully open
Noise Floor: Typically -95 dB A WGT 20-20K with volume control fully open
Noise Floor (shorted input): Typically -94 dB 20-20K with volume control fully closed
Noise Floor (shorted input): Typically -106 dB A WGT 20-20K with volume control fully closed
S/N Ratio: typically 95 dB A WGT 20-20K
THD: 0.02% at 1V rms output
Output Impedance: 50 ohms
Maximum Output: +23dBu @ 1.5% THD into 100K load (10.9V rms)
Maximum Output: +20dBm @ 1.5% THD into 600 Ohm load
"You know the type loud as a motor bike
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Offline EScott

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Re: Healthy Dose of Math for a Sunday Morning
« Reply #3 on: September 04, 2011, 06:13:23 PM »
dBV = 20 * log (0.0025) = -52dBV, to which you added 52dB gain, for 0dBV, which was probably 10dB hotter than you wanted.

But what troubles me is whether or not the phono preamp is properly applying the RIAA EQ curve, or whether the cartridge is appropriately loaded--which the term "0pF" might mean it is not.  That is more typically 100pF - 200pF, although that is dependent upon manufacturer recommendations, cable length, and such.

What effect would adjusting the capacitance have?  It has a jumper for 100pF.

Thanks.
"You know the type loud as a motor bike
But wouldn't bust a grape in a fruit fight"

 

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