Eros 2 with Rega Apheta II

[Chris H]

After a successful Crack with Speedball build (Wow!) I am seriously considering the Eros II as a phono stage upgrade.
I am running a Rega Apheta II (LOMC) so I understand this would require use of step-up transformers (SUT).
From what I can gather, either the Sowter 1480 or 9570 with 10k load resistor would be suitable for the Apheta.

I am a beginner when it comes to electronics so wanted to check:
  1. That a suitable SUT configuration can be implemented for the Apheta and any recommendations
  2. That I have understood impedance correctly (Apheta is 10 ohm requiring 100 ohm load which leads to 10k load resistor for 10x SUT).
  3. What about capacitance loading (1uF for the Apheta)?

Any advice would be greatly appreciated as I am keen to get order and get started.

 

[Paul Birkeland]

The Apheta has 350uV of output and 10 ohms of internal impedance.  I would still go with the Sowter 1990 in the 1:20 configuration with no additional loading resistance other than the 47K resistor that's in the circuit.  Although this would appear to step-up the cartridge level too much, in my experience a 1:20 transformer in practice doesn't give a perfect 1:20 step-up.  (I use a 0.3mV MC cart with a 1:20 1990).

 

[Chris H]

Thank you for the quick reply.  I assume from your response I should not be too concerned about capacitance?
As much for my understanding as anything else, the 20x SUT with 47k resistor would in theory give 117.5 ohm input load?
Not far off recommended of 100 ohm.
Also for my understanding, the internal impedance of the cartrdge would result in some voltage drop (10/127.5) which is lower than 10/110 so not detrimental to SNR?
This then gives 6.45mv output assuming perfect 20x SUT.
Assuming less than perfect SUT gives a reasonable output voltage where distortion is not going to be an issue?
I read somewhere that too high a voltage can be a problem but I have no idea what 'too high' is!

Please forgive the questions but I'm just starting to get into the technical side of things after building the crack with zero understanding.
It got me hooked but much of what I am reading is still like a different language.

In summary though, I should have no concerns about ordering an Eros 2 with my cartridge.
Thanks again.

 

[Paul Birkeland]

Yes, 117 ohms is plenty close to 100. 

The main driver of SNR will be how much voltage appears at the input of the Eros.  You can't go too nuts, but 6mV is a nice place to be. 

 

[Chris H]

Thanks again.
Useful to just start to understand some of the theory so not trying to be too precise.
I followed the instructions in the other thread on SUTs easy enough but now I actually understand why the changes are made.
I could move a resistor ok but now I see that the 47k resistor is being moved to after the SUT secondary and know why.
Basic stuff for some but a big step for me.

Order placed.  Will  take your advice with the 1990s.  Can always try the 10x config at a later date.
Now all I need is for the BeePre to be available for 240v and my my conversion to tube will be complete.
(with a Kaiju of course)

 

[Paul Birkeland]

We are working away on the BeePre!  I have a major presentation to give to the boss when he gets back into town on Friday, and we would like to give more specifics on what we are up to this month. 

 

[Chris H]

Best of luck with the presentation.

 

[Doc B]

Yeah, there'd better be coffee and donuts, and maybe a big pad of paper on an easel and markers. No PowerPoint, and none of those pretentious TED talk headsets.

 

[Paul Birkeland]

Yeah, there'd better be coffee

Not my department!

 

[Jameson]

Doc, I’ll help out with the coffee in exchange for a new BeePre

 

[Chris H]

I'll bring the donuts - same deal.

Ordered the Sowter 1990s at same time as ordering the Eros.
Glad I did - disruption this year plus being very busy means they have low stock
Estimated lead time of 12 weeks in case anyone else is looking at SUTs.

Not a problem for me as only just ordered the Eros and I'm a slooow builder!