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Old 10th November 2010, 12:18 PM   #1
kmj is offline kmj  Sweden
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Default Help with tuning a SUT for low output MC

Well, I managed to get my hands on a Ortofon SL-15 mounted on a RS-212 tonearm, on a Thorens 125.

The thing is that I didn't get a SUT with it and I didn't feel like paying to much for a factory built one, so I ordered a pair of Lundahl LL1678 and wired them in 1:32 mode.

My goal is to use the SUTs to connect the turntable to my MM phono stage (Rod Elliots P06) and perhaps later to TNTs Indescrete RIAA. The input sensitivity of these to seems to be 47K resp 68K, but I'll start with P06.

Now, I did find some calulations online but I kind of got lost when I should evaluate the calculation I made. So, that along with some pointers for future problems would be nice .

Anyway. I got some different info when it comes to the cartridge. On the datasheet in Vinylengines library it says that output voltage for the SL-15 is 0,04mV but in their database is says 0,125mV. I'm hoping for the second alternative since I'm screwed if it happends to be the first one.

I thought that it would be as simple as calculating the output voltage after the SUT by this:
Voltage after SUT = 0.125 x 32 = 4mV

But, using the calculations shown in the analogdepartment here I got the following results:
R(Load_effective) = (47000 ohms / 32^2) = 45.9 ohms

Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))
45.9/(45.9 + 2)=0.9582

0.125 x 0.9582 = 0.1198mV
0.1198 x 32 = 3.8330mV

As to this point I would say that I am in the clear. However, it doesn't seem to be as simple as this.

After this there is all the impedance parts and it is here I am out on very thin ice with a pogostick. According to the datasheet (the one with 0.04mV) the recommended load impedance per channel is 1.5 ohms and this isn't even close to the 45.9 ohms I got earlier. My thought is to do it backwards, this way.
The recommended load of 1.5R gives on the secondary side:
1.5x(32^2)=1536R
To get 1536R I need to put a resistor (called X) in parallell with the secondaries:
(1/47000)+(1/X)=(1/1536)

X = 1588R

So, putting a resistor of approx 1588R in parallell with the transformer secondary should make the cartridge "see" a load of 1.5R.
Am I right or totaly in the dark?

Kind regards
Ken
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Old 10th November 2010, 10:04 PM   #2
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For 0.125mV you need a gain of 27.6dBs so a step-up 1:30 (29.5dBs) is enough.

Yes your calculations for parallel resistor to 47kOhms phonostage using a 1:32 step-up are ok 1587.9 ohms.

Cheers

Felipe

Last edited by merlin el mago; 10th November 2010 at 10:10 PM.
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Old 11th November 2010, 12:50 AM   #3
kevinkr is offline kevinkr  United States
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Check the secondary winding dcr as it may have a significant effect on the loading and the gain.

Ultimately your calculations are great, I went through this whole exercise a few months ago and ended up with something that sounded best but was not predicted by my calculations.

So be ready to try a variety of different secondary loads to find the one that best complements both your transformer and your cartridge. Don't be concerned if the best results are provided by values nowhere close to what you expect.

Given the high gain required out of this transformer I would expect it to be fairly obvious as you approach optimum loading.
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Old 11th November 2010, 05:06 AM   #4
kmj is offline kmj  Sweden
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Thanks guys, they it's vinyl time after work today!
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