A question about cartridge loading and the spec on Seduction

[Pfenning]

So I came across an Ortofon MC20MKII cartridge and MCA-76 step up box I've had for years. I set it up on my Technics SL-1200, and plugged it into my Seduction. It sounds very good to me, it really gets my toe tapping. After reading some around the internet about cartridge loading, I thought I should look into making sure the Seduction is loading it correctly. Here's the cartridge specifications I found online.

Specifications:
- Output Voltage: 0.2 mV
- Frequency Response: 20Hz - 40kHz
- Tracking Force: 1.6-2.0g
- Mass: 10g
- Channel Separation: > 25dB
- Channel Balance: < 1dB
- Load Impedance: 10 ohms
- Output Impedance: 5 ohms
- Stylus Tip: nude FG80 5/80um
- Dynamic Compliance: 16x10-6cm/DyneI

What was the Seduction loading set at from the kit? Is it a simple resistor change to adjust the loading? Does the MCA-76 ad any loading? How sensitive in the interconnect selection between the cartridge/MCA-76/Seduction/Yamaha C-85 preamp?

Thanks, Pfenning

[Pfenning]

After posting this last night, I stumbled onto this thread discussing proper loading of cartridges.

https://www.vinylengine.com/turntable_forum/viewtopic.php?f=19&t=6674

I don't belong to that forum, so I can't access the pictures, and I'm only on page 3 of the thread, but there seems to be some good info here. Looks like I need to learn how to measure capacitance of my tonearm and interconnect cables. Is there a spec sheet for the Seduction that includes capacitance and ohm load in stock format? Can anyone explain to me how to adjust the ohm loading on the Seduction?

I have a Dynavector 20x cartridge that SoundSmith retipped for me years ago, that I've always found harsh and not much fun to listen too. I'm hoping that adjusting the loading to the cartridge will bring out the positive sounds I read about in reviews, but have never experienced.

I'll take any help I can get on this topic.

Thanks,
Pfenning   

[EricS]

Just about any capacitance meter can help you out here, it depends how deep you are/want to become in DIY electronics.  A quick Amazon search reveals capacitance meters for as little as $25 that will get you what you need.  If you have more money/interest/etc, a more feature-rich LCR (inductance, capacitance, resistance) meter might serve you better with future projects.  Expect a decent one to cost closer to $200.

To measure the capacitance of your setup, remove the cartridge and measure capacitance across the RCA terminals (inner pin to outer barrel on each channel) from your turntable.  This will give you the total capacitance of the RCA interconnects including that of the tonearm wires right up to the headshell.   The measured amount of capacitance will be very small, so it is critical that you don't touch the meter leads with your fingers - this will change the reading.

Most cartridges out there will come with a "Recommended Load Impedance" of somewhere near 47,000 ohms, though some may vary.  Another specification for every cartridge is the "Recommended Load Capacitance."  For my cartridge, this is 100 – 200 pF (picoFarads).  Getting back to measuring the capacitance of my turntable, I measured somewhere near 375pF at 1kHz and 425pF at 120Hz for each channel.  This is WAY higher than the recommended 100-200pF loading recommended for the cartridge.  As a result, I ended up rewiring my SL-1200.  I replaced the tonearm cables (kit available through "KABUSA") and replaced the stock RCA cables with Blue Jeans Cable.  This stuff has capacitance of about 12pF per foot.  The result is that my turn table tonearm and RCA cable now has a measured total capacitance of about 56pF per channel.  This is well below the recommended load for my cartridge.  Then, I experimented with adding different amounts of capacitance until I got within the recommended range and results that I liked.

In my phono stage, I added two sockets that would let me play around with some changes.  Do an EBay search on "Break Away Female Headers - Swiss Machine Pin" and you'll find plenty.  Just snap off what you need, solder it down into the PCB and you insert the resistor or capacitor legs into the socket.  It is an easy way to experiment with different values on a PCB without having to futz around too much.  They grip quite tightly to the legs of various devices.

I'll leave comments specific to the Seduction to others, though I suspect there is a 47k resistor close to the RCA inputs.


 

[Pfenning]

Thank you Eric. I ordered the break away female headers, now if I can figure out if 10 ohms is the correct loading for my MC20MKII, I'll get some resistors on order, and start rolling them through.

Pfenning

[2wo]

Keep in mind that the step up box loads the cartage and the seduction (47K) loads the box...John 

[Pfenning]

John, I have attached the specs for the Ortofon MCA-76 step up box. Assuming the Seduction is currently set at 47k ohms, does this mean I should adjust it to something closer to 140 ohms?

Specifications
Frequency response: 20Hz to 50kHz

Voltage gain: 34dB

Input impedance: 75ohms

Output impedance: < 140ohms

Maximum input: 6mV RMS

Harmonic distortion: 0.04% (2nd harmonic)

Hum level: 120dB below max output

Signal to noise ratio: >69dB

Sub sonic filter: -20dB

CD-4 filter: -20dB

Power requirements: 90-132 or 180-264V AC

Weight: 1.1kg

Dimensions: 40 x 60 x 224mm

[EricS]

Having that step-up box complicates some of this analysis a bit.   The general rule (and I'm straying a bit from my knowledge zone - most of my understanding of this comes from solid state stuff which may not directly translate to tube equipment) is that you want the input impedance of your preamp (phonostage here) to be at least 10x greater than the output impedance of the source component (your step up box).  When you are less than this "10x rule" some sort of impedance matching buffer is typically required. 

Since the output impedance of your step up box is ~140R, you shouldn't need to make changes to the input load in your Seduction (which likely has an input impedance of 47k).

In general, changing the input load resistance and capacitance in your phono stage alters the shape of the frequency response from your cartridge - this is tied to the specifics of the coil windings in the cartridge.  In my case, reducing the capacitance of the turntable cables reduced the rising treble response of the cartridge, thus solving a problem for me.   Without a clear understanding of how this particular cartridge/step up box combination behaves or without good measuring equipment and a test album with test tones / sweeps / etc, you may be more likely to be shooting in the dark than anything else.

Having said that, I'm also a fan of experimenting.

[Paul Joppa]

EricS has provided much of what I was going to say.

But I do have a bit more. The input impedance of the MCA-76 is 75 ohms. The best way to reduce this is to place a resistor across that input. To get the desired 10 ohms, use an 11.5 ohm resistor. You can do this anywhere along the connection - at the turntable, at either end of the interconnect, or inside the MCA-76.

The capacitance of the cables is not important in this setup, because both cartridge and stepup are low-impedance sources for whatever is downstream of them.

[Pfenning]

Thanks for the reply Paul. When you say place a reducer across that input,I assume you mean between signal and ground (center pin and outside sleeve of the interconnect). Does the loading on the Seduction even matter after the signal passes through the MCA-76?

[Brent]

Correct me if I'm wrong, but isn't the MCA-76 an active device rather than a step up transformer? Thus the loading at the cartridge would not be affected by the loading at the phono preamp inputs, and standard 47Kohm loading at the phono pre input should be correct to load the head amp as is.

Maybe this on everyone's mind, but I can't tell from comments on the thread. Just trying to help here.

[fullheadofnothing]

You're right brent. I've deleted my post.

[Paul Birkeland]

Dude, this box is built for the MC-20, plug it in and use it! You're nit picking design aspects that have already been optimized by your cartridge by Ortofon.  There aren't a whole ton of cartridges available with an active step-up device that's built for only one cartridge. 

[Pfenning]

Blackplate, I probably wasn't clear, but I assumed the interface between the MC20MKII and the MCA-76 was correct being they were built for each other. I'm trying to understand how the loading at the Seduction effects the sound quality. After the discussion on this thread, I wonder why I need any loading on the Seduction. How are the load resistors effecting the signal from the MCA-76?

Let me add another facet to this discussion. For years I have had a Dynavector 20x that was retipped at Soundsmith. It's the high output version, so I ran it straight into my Seduction. The sound quality has never been great, it has always seemed harsh to me. I picked up another table recently, and mounted the Ortofon because I had it, and wanted to give it a try. I was blown away by the sweet sounds directly into my Yamaha C-85. After listening to it for a couple weeks I decided to try the Seduction as the phono pre instead of the C-85. What was really good before became even better. Noticeably better. My new turntable was in a different system, so I assumed it was the speakers that were making the 20x sound harsh. I grabbed my old table and hooked it up to my other system. 20x to Seduction to Yamaha C-85 aux input. Same harsh sound as before. Then I plugged the 20x directly into the Yamaha C-85, and mistakenly set the loading to 100 ohms. The 20x sounded wonderful, and very close to the Ortofon through Seduction. So now I don't know what to think. I switched the loading to 1000 ohms, and instantly harsh sound, switch back to 100, sweetness comes back. It's not like the highs are rolled off at 100 ohms. I can still hear the drum stick hit the cymbal, and hear the ring and decay very clearly. My next move is to try the 20x on the MM input, through all the different settings, and make notes on what I hear. This second system is very generic.

Yamaha C-85 preamp into Yamaha M-65 power amp into Polk Monitor sevens with recapped cross overs and a tweeter upgrade.

Pfenning   

[Paul Birkeland]

Ah, yes, the Ortofon active step-up doesn't really care what load the Seduction presents (within reason).

If the SS retip is high output, you can do a 1:5 step-up with a 100 Ohm resistor across the inputs to accommodate your cartridge.

If you ever want to get rid of that MCA-76, I currently run an MC-30 and would be interested in hearing it.