newb designing a sut project to go with Eros

[Jay]

I plan to make a step up transformer to go with the Eros I have ordered (I use a lomc cart).  I plan to use the Cinemag 1254's.  I want to wire it to be switchable between the 1:10 and 1:20 windings.  I plan to put the transformers inside an aluminum project box for whatever extra shielding it will provide rather than surface mounting them.  I plan to directly wire short lengths of blue jeans lc-1 cable with switchcraft rca connectors on the input side instead of female rca connectors.  On the out side, I plan to use female rca connectors.  Ideally, I'd like to include some sort of option to try different resistances across the output leads.
Comments?
My questions:
1. What kind of switch should I get for this to switch between the different windings?  Here is the wiring diagram: http://cinemag.biz/phono/PDF/CM-1254%20Hookup.pdf
I mean what do you call it, or what should I search for on Mouser?
2. From what I gather, resistances of between 5k and 35k ohms might be useful to experiment with between the output leads.  What is a good way to implement this?  A little jumper box?  Potentiometer? A couple of hard lugs and just solder and desolder different resistors?  Or does someone have a specific recommendation for this resistance?  Do I even need a resistor here?
3. What the heck do you call the grounding terminal doohickey - you know, drill a hole in the project box and screw it on.  A thumbscrew on the outside of the box for a ground wire and a soldering lug on the inside...I have searched Mouser, Digikey, and Partsexpress to no avail.  I guess I just don't know what that thing is called!

Jay 

 

[Jay]

ok, it's a dpdt switch ( thank you Sparkfun tutorial )

 

[fullheadofnothing]

I want to wire it to be switchable between the 1:10 and 1:20 windings.

You only mention one cartridge. One ratio is going to work MUCH better than the other. If you're going to run a bunch of carts through it, then there is a convenience factor. If you're not rolling through cartridges, putting a switch on very low level signals is something that is worth avoiding.

On a related subject, have you determined done the math to determine what you need for your cartridge? I found this guide very helpful when starting out with MC/SUT matching. I still frequently refer to it.

I plan to directly wire short lengths of blue jeans lc-1 cable with switchcraft rca connectors on the input side instead of female rca connectors.  On the out side, I plan to use female rca connectors.

This is backwards from the "normal" way of doing this. I guess it works if your arm is terminated with female RCAs, but if you ever want to use it with an arm that is male terminated, you have to introduce a bunch of unnecessary connections. Male terminating the output is a reasonable option (although in my system I have found having a jack there better so I can run STP interconnects with the shield only grounded at the preamp end better from a hum perspective, but YMMV).

Ideally, I'd like to include some sort of option to try different resistances across the output leads.

This is nowhere near as necessary as people tend to think. It is a much better idea to find the right transformer ratio for the cartridge. Again I will refer you to this guide for more information on this topic.

3. What the heck do you call the grounding terminal doohickey - you know, drill a hole in the project box and screw it on.  A thumbscrew on the outside of the box for a ground wire and a soldering lug on the inside...I have searched Mouser, Digikey, and Partsexpress to no avail.  I guess I just don't know what that thing is called!

It's called a binding post.

 

[Paul Joppa]

The Cinemag diagram shows only 1:20 and 1:40 ratios. It includes "NOTE:  Hook up primary
windings in series for 1:10 turns ratio." but that arrangement requires another switch. No mention is made of a 1:5 ratio which might be possible as well.

 

[Chris65]

Here's an SUT I built using the Cinemag 3440AH done Bottlehead style, to give you an idea. A switch for the two gain ratios & ground lift switch in a simple Hammond aluminium case (these come painted so saves some work). This unit works perfectly with no noise.

Fixed leads are ok, but as Josh mentions you would put them on the output & have RCA input jacks.
The ground terminal can be called a binding post, ground post, earth post, etc

Not heard the CM-1254, but the 3440 at one third the cost performs very well.

 

[Jay]

Chris65: i see you have just one gain switch.  Is it a 4 pole double throw? Each transformer occupies two of the poles?

Paul: per the cinemag wiring diagram, if I get it at all which I doubt, 1:10 would be center pin of j1 to brown wire, then orange wire connected to yellow wire, then blue wire connected to the outside/ground of j1.  That seems to utilize all of the available primary windings.  How would I get 1:5?

Joshua: Thank you for the link.  If I'm making sense of it, my cartridge which has 0.3 mv output and 5 ohm internal resistance would like a loading of between 20 and 70 ohms. The cart mfg says >20.  That means a 1:30 turns ratio, but that would send 9 mv to the Eros which is a bit much.  The 1:20 would load the cart to 117 ohms and send 6 mv to the Eros.  The 1:10 would load the cart to 470 ohms and send 3 mv to the Eros.  So, my take is that the 1:20 is best if no resistor is to be used.  Bottlehead recommends between 1:5 and 1:20.  Using the 1:10 with a 5.6k resistor gets me to a cart loading of 50 ohms.

I think I'm staying up too late

Thanks for input y'all

Jay

 

[Chris65]

Chris65: i see you have just one gain switch.  Is it a 4 pole double throw? Each transformer occupies two of the poles?

Yes, just the one gain switch between the 37.5/150Ω taps of the transformers (in the case of the 3440AH).
DPDT switch, connects signal in to each tap as per the Cinemag schematic.

 

[Jay]

Yes, just the one gain switch between the 37.5/150Ω taps of the transformers (in the case of the 3440AH).
DPDT switch, connects signal in to each tap as per the Cinemag schematic.

I'm confused.  I truly hope I'm not saying stupid stuff.  The way I read the 3440 diagram, you'd need two dpdt switches, one for the left and one for the right channel.  One for each 3440.  That's what Shannon Park's budgie sut looks like:  http://parksaudiollc.com/budgiesut.html .  How does it work to switch both channels with one dpdt switch?  I must be missing something.

 

[Paul Joppa]

...
Paul: per the cinemag wiring diagram, if I get it at all which I doubt, 1:10 would be center pin of j1 to brown wire, then orange wire connected to yellow wire, then blue wire connected to the outside/ground of j1.  That seems to utilize all of the available primary windings.  How would I get 1:5?
...

Sorry, I was reading too fast last night. I believe you are correct about 1:10, and there is no 1:5. My bad.

 

[Jay]

So, to continue my saga, here's where I'm at in my thinking based on Joshua's link, this link http://www.hagtech.com/loading.html , and the info on the Eros description page.  If somebody would say either that I don't get it and need to try again, or that I'm on the right track, I'd appreciate it.

My cart is .3 mv output and 5 ohm internal resistance, so an estimate of the desired load to the cart is between 20 and 70 ohms.  A 1:20 gets me 117.5 ohms and about 5.7 mv to the Eros.  A 1:10 with its lower gain would be less subject to noise and distortion, but loads the cart with 470 ohms and sends about 3 mv to the Eros.  Adding a 5.6k resistor to the 1:10 wiring puts me back at a 50 ohm load to the cart.  Adding a 34.8k resistor to the 1:20 wiring also gets me back to 50 ohms

So, a question.  Which is more important: the cartridge loading, having the lowest possible turns ratio, or avoiding resistors?  Assuming my cart really likes a 50 ohm load, can anyone say which of the above options is most likely to work well enough?

Thanks again,
Jay

 

[Chris65]

I'm confused.  I truly hope I'm not saying stupid stuff.  The way I read the 3440 diagram, you'd need two dpdt switches, one for the left and one for the right channel.  One for each 3440.  That's what Shannon Park's budgie sut looks like:  http://parksaudiollc.com/budgiesut.html .  How does it work to switch both channels with one dpdt switch?  I must be missing something.

Not sure what those switches are on the Parks Budgie SUT, maybe single pole?
Anyway, DPDT switches have 6 terminals, two centre common used for L&R signal in, then one side is connected to the 37.5Ω taps of each transformer, other side connects to the 150Ω taps of each transformer.
This diagram from the web maybe shows it better.

 

[Jay]

Thanks for that diagram!  I see what you are saying about the dpdt switch now.  But to me, the diagram you posted is different from the Cinemag diagram ( http://cinemag.biz/phono/PDF/CMQEE-3440A-Moving-Coil.pdf ).  For instance, in the Cinemag diagram, the red and green wires for the 37.5.ohm tap are on separate terminals, shorted only when the switch is in that position, but in the other diagram, they are always shorted on the same terminal.  From the Cinemag diagram, I assumed that each channel would require its own dpdt switch.  Your way is more efficient.
Thanks,
Jay

 

[Chris65]

Yes, I see what you mean. It is a slight variation or an interpretation of the Cinemag schematic. As you say, it is a more efficient way of achieving the same result. A web search will show many DIY Cinemag SUT's wired like this with a single switch for the gain.
Would like to say it was mine, but I just found it on the web.