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Ortofon carts are too hot for my phono preamp?

When I need hardware advice, I always come to Audiogon for the true experts.  Thanks in advance and happy Easter!

So after much trial and error, I THINK I have deduced that my Ortofon 2M Bronze and 2M Black cartridges are too hot for my Avid Pulsus phono preamp.  Some softer recordings are fine, but I think I am hearing some distortion with loud transients and even sometimes on relatively loud vocals.

I have a Pro-ject tube phono preamp I swap in and I don’ t hear that distortion. I never hear that distortion with other sources. 

Given my investment in 3 Ortofon 2M carts (1 Bronze, 1 Black, 1 Black SE) and the Pulsus, is there anything I can do to step down that signal from the carts without doing any damage to the sound?  

Any other thoughts?

(BTW the 2 turntables I am using are a Rega P3/24 and an older Rotel.  Meridian preamp has variable gain settings on inputs and I can hear the distortion at any setting.)

jji666

Showing 4 responses by lewm

lewm
13,514 posts
 

Seems I followed up my own post on the overload phenomenon, yet somehow it fails to appear here. Probably my fault for not clicking the green button.  Anyway, I did some further reading on the phono overload phenomenon.  Seems it is a complex subject, because cartridge voltage output rises with frequency.  The standard spec is for a velocity of 5cm/sec at 1kHz, as most know.  The input of the Pulsus is said by HFN to be overloaded at a threshold of 46mV with a 4.52mV reference (meaning for a cartridge output of 4.52mV). Elsewhere I read that the acceptable spec is that the overload margin of a good phono stage ought to be 20db above the reference input voltage.  In this case, HFN measured 19.6db for the Pulsus, with reference to 4.52mV input, another way of saying the 46mV upper limit.  With the Ortofon 2M output at 5mV, that would slightly reduce the overload margin, but by a probably insignificant amount.

dynacolum, you wrote, "The Avid, at 48dB gain is apparently too sensitive to overloading by your not abnormally high output MM cartridges".  But the phono gain is not by itself a measure of the overload margin.  That depends heavily on the topology of the input stage.  Also, tube MM phonos tend to have a higher overload margin than solid state MM phonos.  And 5mV is not an abnormally high output for MM cartridges.  In fact, that value is often taken as standard for an MM cartridge.

lewm
13,514 posts
 

This review at HFN says the Avid has an overload margin at MM inputs of 46mV. 
https://avidhifi.com/storage/uploads/cms/pdf/85/6345755481a21/2012%20HIFI%20NEWS%20PULSUS.pdf

It’s not obvious to me that the phono stage per se is responsible for the “problem”, if it even is a problem.46mV is 10X the cartridge output at a stylus velocity of 5cm/sec at 1kHz. Decent headroom at least. Further, HFN auditioned it with a Shure V15, which I believe also has nominally a 5mV output. Viridian made a good point about capacitance. Check phono cable capacitance. Then check the recommended capacitance for the cartridge and the input capacitance of the stage. Input capacitance adds to cable capacitance.

lewm
13,514 posts
 

Bill, Not a good idea because of capacitance.  Capacitance will cause a HF roll off probably before you notice a loss of gain. That’s throwing the baby out with the bathwater.

"I just set inputs on the preamp to 2.8v - up from 2.0."  What does that mean? And are you referring to the phono stage or to your linestage?

If the cartridge actually puts out 8.6mV RMS at 5 cm/sec, that is quite a bit off of the factory spec of 5mV, presumably representing peak to peak voltage. "RMS" voltage I think if memory serves is calculated by dividing the peak to peak voltage by 1.414 or square root of 2.  That means peak to peak V was higher than 8.6V.  I don’t get it; I doubt Ortofon specs would be that far off from reality. Then I found this on the Steve Hoffman forum:

"Recently, I have attempted to make sense of some conflicting measurements of the output voltage of various phono cartridges from different sources. The infamous one here are the measurements of the 2M Black in particular, comparing the results of HiFi World (HFW) with those of Miller Audio Research (MAR). I ended up emailing HFW and got a response from them that may explain the discrepancy.
To recap, HFW said the output of the 2M Black measured at 8.6mV (5cms/sec). From my email exchange, it appears they actually measured at 3.54cms/sec and "converted up" to 5cm/sec. MAR’s test said 5.79/5.73mV (L/R 5cms/sec).  HFW thinks the difference is due to different test records (yes, I’ve experienced that) but also the differences between peak vs. RMS standards for measuring the output. Of course that wasn’t mentioned as far as I can see in either test result.Using a quick conversion calculator, it looks like 8.6mV peak voltage is equivalent to 6mV RMS. That’s pretty close to the MAR results, so this appears to make sense."
The next post says that HiFi World (HFW) probably erred in labeling their result "RMS".  So their number should be divided by 1.414. In other words, the HFW value of 8.6mV is probably not an RMS voltage and is in other ways erroneous.  Stick with the idea the 2M Black makes 5mV, like Ortofon says it does. Even the above explanation of why the HFW value is wrong is fuzzy.
 
 
lewm
13,514 posts
 

The Ortofon Black produces 5mV at a stylus velocity of 5 cm/sec.  The Avid Pulsus phono stage provides 48db gain on its MM inputs.  48db gain is roughly a 250X voltage gain. So the output of the Pulsus would be about 1.25V.  That per se is not going to overload your linestage high level inputs which ought to be capable of tolerating it. The question is what is the overload margin of the Avid Pulsus inputs on MM.  Which is to ask how much signal voltage above 5mV is needed to overload the circuit.  Usually phono stages can have a better overload margin on their MM inputs compared to the MC inputs of the same unit, but I cannot find the overload specs for the Pulsus. You might ask your dealer or Avid whether the 2M Black, a very popular cartridge, might overload the MM inputs. But my guess would be no. And of course all that other stuff mentioned by Viridian is worth thinking about. I don't recommend increasing VTF by as much as 0.5g on a permanent basis, if that increase leads to VTF higher than the upper value given by Ortofon for that cartridge.

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