Eros 2 Step-up Transformers

[Paul Birkeland]

Wouldn't the external SUT be easier to shield? I'm thinking about using an external SUT instead of the Sowter/Jensen/Cinemag to avoid shielding issues.

I would suggest considering that we have tested the placement of the Sowter transformers on the Eros 2 to verify appropriate placement and that they don't interact with the power transformer.  You also won't need to deal with another set of interconnects and another drain wire to connect the Eros to the separate box that holds the transformers.

[Paul Joppa]

To amplify (sic) PB's points, Jensen warns that for best performance the secondary load must have very low capacitance - a couple feet of coax (100pF) is more than enough to harm the high frequencies. Eros is especially well suited to use transformers, because it uses a pentode input tube with about 4pF input capacitance.

This is different from moving magnet carts, most of which are designed for 100-500pF in parallel with the 47K input resistor. A 3 to 6 foot cable accounts for about half of that, and the typical triode input stage Miller capacitance accounts for the rest.

[maarten]

Ok I will go for the Sowter! Will try to find them somewhat cheap (in EU)

[maarten]

Just a small question here as my Sowter pair arrives.

In your early prototype picture, you have a 220 Ohm going from pin 9 to the jack. I guess this was changed to 90.9 Ohm later? (You also have an extra heatshrink wrapping on the Right side (in the manual it's only on Left) btw.)

That resistor is a 90.9 Ohm and goes to terminal 2/17 right? There's no need to change it to a 220 Ohm one?

And then, the 47.5k resistors  that used to go from ground to jack, just go to 1-2 (and 16-17).

Thanks for clarifying

[Paul Birkeland]

In your early prototype picture, you have a 220 Ohm going from pin 9 to the jack. I guess this was changed to 90.9 Ohm later? (You also have an extra heatshrink wrapping on the Right side (in the manual it's only on Left) btw.)
[/quote]
The pictures I posted are of a prototype Eros 2 that was built before we wrote the manual.

[maarten]

Thanks, I realise it was a prototype. Is the rest of my assumptions on resistor placement correct?

I suppose I will can get a gauge of the load impedance (when the device is off) by measuring resistance between the ground and the input jack for each channel?

Can I execute the other resistance checks that come in the manual after the Sowters are placed? Which checks would need adapting?

Thanks again for all your help, I'm almost done building and can't wait to hear the results.

[Paul Birkeland]

You cannot measure AC impedance with a DC meter. 

I did not find any immediate errors with what you posted, but as always it's a good idea to double check against the pictures I posted.

[nmatlin]

Hi folks,

I'm in the process of changing over to a Hana SL MKii w/ cartridge impedance at 8 ohms and recommended loading at greater than 80 ohms; output is 0.4mv. I have the Sowter 1990 step-up transformers.

I've read the Sowter guidance, and thus, have worked out that likely loading (with a 1:10 ratio) should be 8k ohms.

In terms of the directions (pasted below), I'm a bit unclear. Do I completely remove the 47K loading resistor (which, I assume, is conventionally used for MM cartridges) and then put an 8k resistor on each side across the secondaries (1/2 and 16/17)?

1.  Pop the resistors out of each RCA jack that go to pin 9 on each EF86 (leave them attached to the socket, just pop out the end sitting in the solder cup of the RCA jack).
2.  Remove the 47K loading resistor on each RCA jack.
3.  Reconnect them at terminals 2 and 17.

I'm new to MC cartridges and SUTs, so just wanted to confirm.

Thanks!

Nick

[Paul Birkeland]

Yes, you would remove the 47K resistors and place an 8K resistor across the secondaries. Reply number 4 and reply number 5 in this thread have pictures of this.

[nmatlin]

Thanks, Paul!

One more question: Is 1:10 the ratio you would recommend for the 0.4mV output on the Eros 2? I have the 1990s, so I could opt for the 1:20 ratio (and I suppose, in that case, 32k loading resistors), but I'm not sure what would be ideal.

Appreciate all your help!

[Doc B.]

A .4mV LOMC into 1:10 will give you a nominal output level of 4mV. Around 2.5mV-5mV nominal is right about where you want to be for the Eros. 1:20 will put out 8mV nominal which is a bit too hot and could push the Eros into clipping.

[nmatlin]

Thank you! I very much appreciate your help.