Trying to understand Step-up for Seduction..some questions

[Balle Clorin]

As a happy new owner of a secondhand Michell Gyro and SME V arm  it is time to try MC pick-up with my Seduction+Foreplay III.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fcdn.whathifi.com%2Fproductimages%2F1216404cfth.jpg&hash=7b6c4c309496a62bb0460e2fae3ef9fb9f2c984e)


I have zoomed in on Audio Technica OC9 MLII. Techically it is superior even to  more costly cartidges... (50uH,12ohm DC, 0.4mV output, recommended load 20 ohm). Browsing the old forum and the net on the subject I find the following..

Xindax MC-10 step up at 10-20-31 (20dB-26dB-30dB) stepup ratio, in a convenient package.
http://www.xindak.com/en-us/product_detail_info.asp?id=400

Impedance Rule of thumb: "Typical load should equal 10x cartridge imp/ohm: 120 ohm in this case, much higher than manufacturer recommends, but not too far that other are reporting using with ATOC9 (50-80 ohm)

1. 1:32 stepup ratio gives too high gain with 13mV(overload?) to the seduction, and 47ohm load for the cartidge.(47000/(32^2)

2. 1:20 stepup gives 8mV to the Seduction, and 118ohm load for the cartridge, too high according to manufacturer. Can compensate by reducing Seduction input (grid) resistance from 47Kohm to 8kohm to meet 20ohm load.

3. 1:10  Stepup gives 4mV, and 4700ohm cartridge load.Way too high, need to reduce Seduction grid/input resistor to 2kohm to meet 20 ohm load.  

Question A
Alternative 2 seem attractice with high but not too high signal (I have 6mV from MM today). But will  record noise be a problem with this SUT gain??. (I use Foreplay with padded input.)  Is 8kohm input resistance OK for the Seduction?

Quation B
Alternative 3 still have a good signal. But I need to reduce Seducion input/grid resistance to only 2kohm.  How will this effect the Seduction???, is alternative 2 better then?

To confuse matters more I use a KT88 amp driving 90db speakers, but may change to 300B amp/96dB speakers.

Let see if the new forum works like the old...
Best regards
Balle

[Paul Birkeland]

I like the 1:10 option the best out of all that you presented.  You can easily buy some tiny 100k trimmer pots and replace the 47k resistors in your Eros, allowing you to fine tune it to match the SUT (Doc does this with tape heads often).  You may try it with the 47k resistors in there and determine that trimming isn't needed. 


Option 1 will overload the Eros, Option 2 is probably worth trying, but with your amp/speaker combo, I think option 3 is more realistic.

[Balle Clorin]

I will go for 1:10 then. I was a bit surprised that the grid resistor value can be as low as 2000 ohm., I am used to maxumum allowed values for the grid-to-ground resistor , so I also exepcted it to be a minimum acceptable value also.
Maybe not. A 1:10 will probably also suit an Eros better, if I upgrade to that....

Best regards
Balle

 

[Paul Joppa]

I will go for 1:10 then. I was a bit surprised that the grid resistor value can be as low as 2000 ohm., I am used to maxumum allowed values for the grid-to-ground resistor , so I also exepcted it to be a minimum acceptable value also. ...
Balle

For step-up transformers like these, there are optimum source and load resistances for the transformer. Resistances far from the optimum will result in problems with the high frequency response. These resistances include the winding resistance of the coils, which are not always trivial for MC step up transformers. There are six variables in addition to the turns ratio that determine the transformer's response -the source resistance (12 ohms for the cartridge selected), the primary winding resistance, the leakage inductance referred to the primary, the self-capacitance (mostly of the secondary but including the preamp input capacitance)) referred to the primary, the secondary winding resistance, and the load resistance. In most cases, none of these can be safely assumed to be out of the picture.

Unfortunately, it is rare to see these requirements and winding resistances spelled out in the sales literature for such transformers. I would be cautious about exactly how the loading is applied, if you are going to deviate from the transformer's design intent. That design intent should be specified as the design source and load resistances, plus a range of acceptable load capacitances.

I wanted to post this earlier, but I didn't and still don't have a good answer beyond the above cautionary note.

[ironbut]

Yeah, I've had an OC9 (original) for years and right off the top of my head, I think I ended up with 110 ohm loading. 
 I took a handful of resistors and found the loading resistance by ear going way too high then way too low and working my way in ( a nice pair of trimpots would've made this easier but I just didn't have one on hand).
The closer I got, the longer it took to decide (it took more and more albums to find something wrong with the sound) till I got to the point that I bought a small range of $$$ resistors to try living with.
I totally ignored the manufacturers recommendation.
In other words, I like the systems approach for these kinds of fine tuning.

[Paul Joppa]

I spent another couple hours with Reuben Lee's book on transformer design last night. I am fast coming to the conclusion that varying the cartridge load has more of an effect on the transformer than on the cartridge - validating ironbut's systems approach. At this point I would not set much store by anybody else's optimum cartridge load unless I was using the same stepup transformer.

If you are not using a transformer, then recommended cartridge load resistance would be repeatable and might be useful. That might for example be the case for which the OC-9 uses a recommended load of 20 ohms. If that load is applied at the cartridge, the net source impedance is 12 ohms in parallel with 20 ohms, or 7.5 ohms. Then you would want to seek out a stepup transformer which is designed for a 7.5 ohm source resistance, and load it with the transformer manufacturer's recommended load.

For a realistic and technically correct example, consider the Jensen JT-347-AXT. It can be wired for 8, 12, or 24:1 turns ratio. Though they do not make specific recommendations, all their meaurements are made with source impedances of 27-45 ohms, 12-20 ohms, and 3-5 ohms respectively. Assume you decide to use the 24:1 stepup ratio and into an unbalanced input like the Eros or Seduction. According the the specification, the transformer would present a load of 78.3 ohms, so a parallel input resistor of 27 ohms would give a net 20 ohm load seen by the cartridge, and would present a source impedance of 8.3 ohms seen by the transformer. In a separate 6-page application guide they provide a recommended transformer load values for the various impedances and turns ratios. In this case, you would use a secondary load of 10806 ohms in series with 162pF and then into a 47K resistance plus the RIAA amp input and cable capacitance of 100pF.

Note that I am not saying Jensen makes the best transformers, I am just saying that they provide the necessary detailed information to get the best response from them. This approach will allow you to try different cartridge loads while maintaining fairly uniform transformer performance, but it gets quite complicated to do it.

[Balle Clorin]

Many thanks
So this means that I have to put an adjustment load on BOTH sides of the transformer..

The primary has a 27 ohm resistor in p

[Paul Joppa]

... Should the 10806ohm+162pF  be at the outlet of the transformer or is it better p