[Russtafarian]

Over the weekend I wired up a pair of Cinemag transformers as step ups between my DL103 cartridge and Audio Research preamp.  The preamp’s phono stage has 45db of gain and a 47k input impedance.

The transformers are wired using the 37 ohm input tap according to the Cinemag schematic and I’m getting signal from the cart through the transformers to the phono stage.  But here’s where I’m stumped.  The cartridge output through the transformers is lower than when plugged directly into phono stage.

I checked the wiring multiple times to make sure it’s correct.  I’ve tried three different grounding schemes.  There are no hum issues at this point simply because there’s not enough output to hear any hum issues.

I’m hoping this is easily resolved due to an obvious mistake on my part.  Does lower SUT output indicate a particular wiring issue that I missed?

Here’s how I hooked it up:

Cart +: Brown/Yellow
Cart -: Red/Green (37 ohm tap)
Pre +: Purple
Pre -: Grey
Ground: Black/white & either Pre- or Cart –

Schematic: http://cinemag.biz/mic_input/CMQEE-3440A-Moving-Coil.pdf

Thanks for your help.

[mgalusha]

It looks right but I suspect connecting the ground to the preamp or cartridge is a problem. What happens if you connect the white and black wires to chassis ground and not the signal ground? They specifically point out that neither the outer rings of J1 or J2 should be connected to chassis ground and by connecting the blk/white wires to the pre or cartridge that is happening - assuming you have connected blk/white to chassis ground or the transformer can.

[Steve]

 The preamp’s phono stage has 45db of gain and a 47k input impedance.

The transformers are wired using the 37 ohm input tap according to the Cinemag schematic and I’m getting signal from the cart through the transformers to the phono stage.  But here’s where I’m stumped.  The cartridge output through the transformers is lower than when plugged directly into phono stage.

Schematic: http://cinemag.biz/mic_input/CMQEE-3440A-Moving-Coil.pdf

Thanks for your help.

Hi Russ,

If I read you correctly, you are measuring the cartridge output both times at the 47K phono input. The transformer turns ratio will cause a different cartridge output vs without the transformer in the system (47k ohms input).

Hope this helps.

[Russtafarian]

Thanks for the replies. 

Mike, thanks for taking the time to review ths schematic.  I am going to try some different grounding schemes tonight, including disconnecting the signal and chassis grounds from each other to see what happens.

Steve, I'm not so much measuring as I am listening.  With the transformers in line, I'm expecting from 20 to 25db of additional gain.  So I'm puzzled when the transformers are lowering the gain as compared to the cartridge connected directly to the phono input.

Is it possible that the Audio Research SP14 has a screwy phono grounding scheme that's tripping me up?  Unlikely, but might as well ask.

[Steve]

Thanks for the replies. 

Steve, I'm not so much measuring as I am listening.  With the transformers in line, I'm expecting from 20 to 25db of additional gain.  So I'm puzzled when the transformers are lowering the gain as compared to the cartridge connected directly to the phono input.

........

Thanks Russ, I misunderstood you. Wow, I thought a little lower but not that much. R1 in circuit (value?) or out? Any near shorts? Hopefully not a "shorted" turn in a winding.

Cheers.

[Russtafarian]

Well, I though I found the problem...

Despite the preamp schematic showing a 49.5k input resistor, it's actually a 99.5k resistor on the circuit board. Since I got the preamp used, who knows when and why that resistor got changed. So I put a 100k resistor across the secondary (purple & grey) to drop the input impedance to around 50k. Still getting the same result.

I should try the SUT on another phono stage, to remove that variable from the equation.

The quest continues...

[Steve]

Well, I though I found the problem...

Despite the preamp schematic showing a 49.5k input resistor, it's actually a 99.5k resistor on the circuit board. Since I got the preamp used, who knows when and why that resistor got changed. So I put a 100k resistor across the secondary (purple & grey) to drop the input impedance to around 50k. Still getting the same result.

I should try the SUT on another phono stage, to remove that variable from the equation.

The quest continues...

Do you have a scope, friend, or nearest college with one? If so I would suggest you might attempt to measure (or college professor) the output at the transformer secondary with the preamplifier phono input disconnected. This will eliminate the possibility of the phono input loading problem.

Cheers.

[amandarae]

Hello Russ,

I saw you post the same thing in Audiogon so maybe I can suggest something. 

Your wiring is correct.  Did you get the Cinemag's new?  At 150 ohms setting (~1:18) you should measure a DCR of around 13.1 ohms or thereabouts on the input and around 6.65 ohms for 37 ohm setting(your config).  The ouput DCR should stay the same for both irregardless and measures at ~3.88 ohms DCR. If you get these values, the problem is not on the SUT side I believe nor the connections.  Does your phono preamp have an inline step up transformer incorporated already?  If so, that could present a problem you are describing.


Abe

[Russtafarian]

Thanks Abe.  I'll check DCR.  And I plan to check it with another phono preamp.

[Russtafarian]

I think I've resolved this mystery.  The cinemag wiring is fine.  It's the Audio Research preamp that's causing the problem! 

I hooked the cinemags up to the mm input of a Denon HT receiver and it passed the signal through with the expected increase in gain.  It didn't sound all that great but it showed me that the cinemags are operating as they should.

Now to call Audio Research to see if they can tell me why the SP14 phono stage reacts to the cinemag SUTs the way it does.

Thanks for all your help and support.

Russ

[Russtafarian]

I have resolved the mystery.  I talked to the tech at Audio Research and he told me where to look on the underside of the circuit board to see if there were any load resistors soldered across the phono input.  Sure enough there was a pair of 100 ohm resistors in place.  I desoldered them and now I'm enjoying the Cinemag transformers with my phono stage.  Happy ending!