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Phono Cart Impedance Question

I have a little bit of hiss through my 6 ohm, 0.28mV cart but only at a volume pushing the upper end of my listening volume...and it's only audible before I drop the needle, and even then not from the listening position.  I realize I therefore likely have a "non problem"!  

But I still have a generic question on to the sensitivity of producing hiss as it relates to cartridge impedance alone.  If one kept all other cart specs identical (including output of course) would you expect hiss to go up or down at the same volume and system gain if I used an 8 ohm or 12 ohm cart instead?  It seems like the hiss would likely go up just like moving to a higher impedance speaker under the same gain setup but perhaps I'm looking at this incorrectly? I realize moving to a slightly output cart could very well reduce the hiss.

Thoughts?   Sorry if this is a ridiculously obvious question and answer.

128x128three_easy_payments
uberwaltz's avatar
uberwaltz
13,381 posts
 
I did not realise it was "that" Roger ... Lol.
Just had email reply as did not see 6922 sln tubes online but he has them at $90 each.
Times 4.
Ln are $70 a pop.
Wth, it's only money.
You have to pay to play
mijostyn's avatar
mijostyn
8,438 posts
 
Yup, that Roger. Hope you like them. They made a big difference in the noise level of my ARC and lower noise means better sound in some respects. They work so well I have not felt the urge to change phono amps ever since I started using Roger Modjeski's (hope I spelled that right) tubes. Go for the SLN tubes. It would cost you a lot more to get a quieter phono amp!
Enjoy,
Mike
lewm
12,809 posts
 
Just to note that running the phono stage with no cartridge connected is very likely to produce volume control dependent levels of hiss. Most of our phono stages will behave that way.
intactaudio
300 posts
 
If I had to guess, the broadband hiss is from a noisy tube gain stage and since it changes with volume setting tubes would be a good place to look. Check to see if the noise is the same type / level in both channels and if there are differences swap the tubes left to right and see if the noise follows the tubes. Writing down some basic notes during this process helps a lot.

The noise types you typically get from a cartridge / SUT combo are:

  • low frequency line hum (50/100, 60/120) from EMI which can usually be modified by SUT / cable placement.
  • more of a buzzing sound that points to ground connections and use of an unshielded cable from the SUT to the phono input.
My test to see if any type of noise is an issue is to listen to an album side at what you would consider a loud level and then lift the needle and check to see if the noise is audible from the listening position. In general i find the line frequency stuff to be rather benign, the buzz to be unacceptable and the hiss tends to kill dynamics.

dave


three_easy_payments's avatar
three_easy_payments OP
2,097 posts
 
@intactaudio 

Thanks for your thoughts.  My own troubleshooting suggests the same culprit - noisy tube in the gain stage.  Upon setup I had addressed other issues you identified including placement of the SUT (which was sensitive to proximity to toroidal transformers) and by using a very well shielded cable between the SUT and the phono.  I tried several cables and I'm getting the best performance from an AQ Colorado with its active noise suppression.  It beat out some shielded Cardas cables I had.

I'm left now with no buzz or hum.  I also don't think the SUT is contributing to the "white noise"/tube rush.  It's exactly the same even when I remove the SUT from the chain and run the cart leads directly to the MC inputs on the phono.  Noise appears to be equal in level on both channels.

I'm just waiting to hear back from Keith before proceeding further.
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