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Phono amp output impedance/integrated amp input impedance problems

Hi everyone,

I have been shopping around for a new integrated amp for my all-vinyl system.  In another thread (asking for integrated recommendations), I learned that I should be careful with the impedance matching with my tubed phono stage (500 ohm nominal output impedance).  Our esteemed Almarg alerted me to the fact that I should probably be looking for an integrated with a line-level input impedance of 37.5k ohms to be safe, about 25k at a minimum.  I did some research over the last couple of days into some integrated amps, and I was shocked to find out that a 37.5k minimum input impedance rules out nearly every amp I investigated: the Creek 100A, Parasound Halo, Exposure 2010/3010, Conrad Johnson CA150, Shiit Ragnarok.   

Perhaps separates are a better choice?  Or a SS phono stage?  

Any thoughts on how to open up more options would be appreciated.  

Best, Scott



smrex13
yogiboy's avatar
yogiboy
2,678 posts
 
The rule of thumb is the input impedance should be a minimum of 10 times the output impedance of  the source. If you do the math, it looks like you should be safe with any of your choices. I'm sure Al will correct me if I am wrong!
smrex13 OP
423 posts
 
I think the problem is that the 500 ohms is only the nominal impedance of the phono stage, and tubed phono stages can actually have output impedances several times the nominal impedance, especially at lower frequencies.  


almarg's avatar
almarg
8,673 posts
 
I think the problem is that the 500 ohms is only the nominal impedance of the phono stage, and tubed phono stages can actually have output impedances several times the nominal impedance, especially at lower frequencies.
That’s right, Scott. Most tube-based phono stages and preamps (as well as some solid state designs) utilize a coupling capacitor at their output. The impedance of a capacitor increases as frequency decreases, and as a result it is not uncommon for the output impedance of such a design to rise from a few hundred ohms at mid-range frequencies to 3K or 4K at 20 Hz. (That rise can be reduced if the designer chooses a larger value capacitor rather than a smaller value capacitor, but doing so can have other downsides).

Since the 20 Hz output impedance of your Croft RIAA phono stage doesn’t seem to be indicated anywhere, and since it presumably uses a coupling capacitor at its output, I had suggested that to be safe a factor of 50x to 75x should be applied to the 500 ohm nominal output impedance, resulting in the 25K and 37.5K numbers you cited. Note that the 25K figure is not even all that conservative, as an output impedance rise to 3K at 20 Hz (which is not uncommon) would not meet the commonly stated 10x guideline, at that frequency.

What I’d suggest at this point is contacting Croft and asking if they can tell you specifically what the output impedance of the phono stage is at 20 Hz.  Or, alternatively, what they would recommend as the minimum load impedance that should be used with that phono stage.

Best regards,
-- Al
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