LOMC / TREBLE PEAK / CAPACITOR CURE

bimasta

AK Subscriber
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I have 3–4 fairly good LOMC carts, all NOS and working fine. I like them BUT some have a nasty treble peak often associated with MCs (+6dB starting at around 6KHz and really bad by 10Khz). Luckily some of them are flat, but I can’t bear listening to the peaky ones, they’re like a dentist’s drill into my brain.

All are “vintage”, the newest I had was a Lyra Parnassus 20 years ago, also too peaky. It was new and I sold it because I knew wouldn’t use it and its resale value was high.

Loading them way down in my preamp helps the harsh highs but hurts everything else.

I read an old Hi-Fi News (UK mag) from the early ‘80s*, and it offered a “fix”. Putting a 1.5micro-farad cap in parallel flattens the peak. It says Fidelity Research did this by putting a tiny tantalum cap inside the cartridge. Obviously I can’t do that, so it will be an inline contraption — tonearm leads into RCA female inputs, to RCA male outputs, with the cap somewhere in between.

I’m a total amateur at even simple electronics. But if it works, I’ll have 3–4 “new” MCs I like, at no extra cost, for my golden years.

Question 1: How do I wire it? I may seem simple, but not to me. “Parallel” with what? What would it look like? A schematic won’t help, I may misread it. A simple sketch for a layman would help a lot.

Question 2: Should it plug into the SUT, or after the SUT into the preamp? After the SUT has a stronger signal: is that a factor?

I’ve probably left things out. Any guidance is welcome!


* Here’s the Hi-Fi News article cited above. A good read about some famous carts of yore...

https://www.vinylengine.com/turntab...5&start=0&postdays=0&postorder=asc&highlight=
 
This is putting a high capacitive load on the cartridge, done by putting one cap across each of the + and - terminals coming out of the cartridge. Most phonostages already have 100pf or so. Simple to do.
 
I was testing out a new capacitance meter and was grabbing all sorts of cables, 1 of them was 6 times the others
"capacitance" I think I tossed it in the trash. Might have been useful in your case as a quick point of reference.
 
hard to understand how adding 1.5uf would have any affects on a MC, let alone a LOMC. might check those numbers again. The temperature variations in the room will move the cables +/- of that
 
hard to understand how adding 1.5uf would have any affects on a MC, let alone a LOMC. might check those numbers again. The temperature variations in the room will move the cables +/- of that

1.5uf is 1,500,000 pf. Quite a load!
 
You say you are using a SUT. What’s the turns ratio?

Something else is amiss here as there shouldn't be a peak in the response of a LOMC that can’t be easily tamed with some load resistance.

BTW it is easier to put a load on the preamp side of the SUT than the cartridge side.
 
All I know is what I read in HFN, published over three decades ago, when LOMCs were "the new thing" in Audio and interest, like the treble, was "peaking". IIRC, HFN was a pretty serious journal back then, with high credibility, and far more scientific analysis in its reviews than I ever see in reviews anywhere today. Today it's about the sizzle, few care about the steak.

So all I have to do is connect a cap to both + and – pins of each channel (or center-pin of RCA and outer ground) meaning 2 caps total — did I get that right? I have the parts so I can try it.

Any chance it could damage the cartridge, or anything else? That would be my main concern. If it doesn't work, fine. But I don't want to risk damage, for what may be folly.

I'll wait to see if other comments come in, from an abundance of caution.
 
You can't hurt anything. You're correct, right across center pin and ground. I'm not knowledgeable on SUTs but W9TR says to put it after the SUT.
 
Make sure the capacitor is fully discharged before you connect it or you could damage the cartridge from the residual charge on the cap. LOMC carts are fragile devices. You can even damage them by trying to measure their resistance with an ohm meter.

Also if you put a cap on the sut secondary (preamp end) the value would be much less as the impedance seen at the secondary will be higher by the square of the turns ratio. If your sut is 10:1 then the cap on the secondary would be smaller by 100 times, 0.015uF

Keep us posted on your experiment, and good luck!
 
You say you are using a SUT. What’s the turns ratio?

Something else is amiss here as there shouldn't be a peak in the response of a LOMC that can’t be easily tamed with some load resistance.

BTW it is easier to put a load on the preamp side of the SUT than the cartridge side.
The problem is not the electrical resonance, but the cantilever resonance. The load R has nothing to do with it.
Adding a shunt cap across the output of the cartridge (input of the SUT) adds a low pass (mostly) made up of the series R of the cartridge coil (3 to 20 ohms or there about) together with the shunt cap. Start with a value significantly lower than you think is necessary and work up.
Taming the HF resonance usually generates a dip in the 4-5kHz region due to the action of the low pass, and the sound will change as the cap is increased.
 
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Oh, bye the way I have measured L/R and C of numerous LOMCs over the years and never damaged one.
Their fragility is over stated.
 
Make sure the capacitor is fully discharged before you connect it or you could damage the cartridge from the residual charge on the cap. LOMC carts are fragile devices. You can even damage them by trying to measure their resistance with an ohm meter.

Also if you put a cap on the SUT secondary (preamp end) the value would be much less as the impedance seen at the secondary will be higher by the square of the turns ratio. If your sut is 10:1 then the cap on the secondary would be smaller by 100 times, 0.015uF

Keep us posted on your experiment, and good luck!
The caps are new, unused, so I assume no residual charge. You asked for the specs. I found these on another forum about the same SUT; if that info is accurate (I trust the source, also a member here) the turns ratio is 1:10; gain is 20dB; impedance 40ohms.

I planned to put the cap in the preamp, not the SUT, thinking the higher gain might be a benefit. But isn't that the same as putting it on the secondary, which you warn about? After all, isn't the preamp fed by the secondary? I could put it on the primary instead — the FR cart with the cap inside (if true) is putting it on the primary, isn't it? It's also easier — I could put it on the tonearm cable's RCAs (which means no messing around inside either the preamp or SUT). Is that correct?

The reason I use so many question-marks is I'm truly ignorant in this area, and going on logic alone, so I'm seeking confirmation my logic is correct. Is it?

Thanks again to all of you, not only for your advice but your patience.
 
So if you want to put the cap across the cartridge the value of 1.5 uF is a good start. If you want to put it across the secondary or your 10:1 step up (the input to your preamp) the value needs to be 0.015 uF. See wyn palmer's post #11 and my post #9. The SUT not only provides gain, but it transforms the impedance as well.
 
The problem is not the electrical resonance, but the cantilever resonance. The load R has nothing to do with it.
Adding a shunt cap across the output of the cartridge (input of the SUT) adds a low pass (mostly) made up of the series R of the cartridge coil (3 to 20 ohms or there about) together with the shunt cap. Start with a value significantly lower than you think is necessary and work up.
Taming the HF resonance usually generates a dip in the 4-5kHz region due to the action of the low pass, and the sound will change as the cap is increased.
This is correct. Adding a capacitive load to the cartridge will not flatten its frequency response - at least its electrical response - but rather skew its response in a crude attempt to correct its mechanical shortcomings. Still, it won't do any damage to the cartridge of anything else, so it's worth a try and might make the cartridges a bit easier to listen to.

However, I do not recommend adding a capacitor to the output side of a step-up transformer. Step-up transformers generally don't take kindly to excessive capacitance loads and their HF response suffers. Having said that, if you're attempting to skew the frequency response to offset cantilever resonance I suppose it's worth a try. Again, you won't do any damage to anything.
 
This is correct. Adding a capacitive load to the cartridge will not flatten its frequency response - at least its electrical response - but rather skew its response in a crude attempt to correct its mechanical shortcomings. Still, it won't do any damage to the cartridge of anything else, so it's worth a try and might make the cartridges a bit easier to listen to.

However, I do not recommend adding a capacitor to the output side of a step-up transformer. Step-up transformers generally don't take kindly to excessive capacitance loads and their HF response suffers. Having said that, if you're attempting to skew the frequency response to offset cantilever resonance I suppose it's worth a try. Again, you won't do any damage to anything.

I , as a matter of course, add capacitance to the secondary side of my SUT because it's more convenient.
I've measured/characterized the SUT and the circuitry on both sides of the transformer and I've added the equivalent scaled caps to see what effect they have on the measured response of the cartridge using test records etc.
The secondary winding of the SUT that I have has c. 250pF of capacitance, and the RIAA preamp has 95pF of input capacitance without any additional cap.
The turns ratio is 20:1, so the cap load that the cartridge sees is 345*400pf=0.138uF without any additional caps.
The cartridge that I use most often is a Miyajima Madake- and the recommended cap load according to an e-mail exchange that I had with the designer, is 0.68uF.
The cartridge, without a cap, has a rising peak which is about +7dB at 18kHz and about +1dB at 10kHz due to the cantilever resonance.
I presently have 500pF added to the secondary which theoretically is equivalent to an additional 0.2uf at the primary, and so the total load is about 0.34uF, or about half the recommended value. I've measured and simulated the frequency response and the result (relative to 1k) is a dip at 5kHz of about -1dB and a 18kHz peak of about +3dB. The response is also up about 0.7dB at 100 Hz as a result.
The resistive load that the cartridge sees is about 60 ohms. The overall response loaded in this way is 20Hz-30kHz +/-2dB, which meets the cartridge spec. and, incidentally, equals or beats the frequency response spec. of many professional analog tape decks, such as Studer A810/A820, the Otari MX series etc.
I've tested the SUT response against an opamp based RIAA stage with theoretically equivalent loads and the response is very, very similar.
The Madake, despite its not-ruler-flat frequency response, is the most "real" sounding cartridge that I've ever heard- full yet at the same time delicate and powerful, with intimate, slightly forward voices and deep, natural sounding, bass. The extra cap load, to my ears, improves the sound. The designer prefers it without any additional load.
So, I repeat my recommendation. You can load the SUT at the transformer secondary/input of preamp. Just start with a small cap- say a few hundred pFs and work up until the sound is the best for you.
 
So, I repeat my recommendation. You can load the SUT at the transformer secondary/input of preamp. Just start with a small cap- say a few hundred pFs and work up until the sound is the best for you.
Yes, I'm not saying you can't load the SUT at its secondary winding, just saying that it will affect the frequency response. Even with a purely resistive source impedance of just 10 ohms the transformer's HF performance is affected by capacitance at its outputs. Clearly you have the knowledge and the test/analysis gear to measure the results, but most forum members don't, and the OP was considering the use of 1.5uF at the input to the SUT or 1.5uF at the output of the SUT. At the output of the SUT a much lower value of capacitor would have a similar result to a larger value at the input, as you say.
 
hard to understand how adding 1.5uf would have any affects on a MC, let alone a LOMC. might check those numbers again. The temperature variations in the room will move the cables +/- of that
The electrical impedances of a LOMC are generally pretty low, but not so low that the cap will not make a difference- for example the Miyajima Madake/cable combo that I have has a 16 ohm R, an 11.6uH inductance and an equivalent shunt cap of 75pF and is optimally loaded in 60-200 ohms with 0.68uF cap. A 1.5uF load is a bit high for this, but only by a factor of 2-4. Cartridges with lower R and L might require larger caps as might those with extremely undamped supersonic or near supersonic cantilever resonances, such as the Fidelity Research devices.
Temperature will have minimal effect on the capacitance. It does effect the cantilever resonance due to changes in the physical properties of the materials both in the cantilever itself and in the suspension/dampers, but nothing that is significant in a normal room environment.
 
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This has been an interesting discussion. Thanks all for the well considered contributions.

I guess both my cartridges don’t exhibit this cantilever resonance phenomenon or more likely it happens at a much higher frequency. I have a Dynavector Karat 17D3 and a Sumiko Palo Santos Presentation. They have diamond and boron cantilevers, respectively. I can get them very flat with just resistive loading on my preamp.

I use a B+K QR 2011, Analogue Productions Ultimate Test Record, and JVC TRS-1007 test records. They all give slightly different results but close enough.
 
This has been an interesting discussion. Thanks all for the well considered contributions.

I guess both my cartridges don’t exhibit this cantilever resonance phenomenon or more likely it happens at a much higher frequency. I have a Dynavector Karat 17D3 and a Sumiko Palo Santos Presentation. They have diamond and boron cantilevers, respectively. I can get them very flat with just resistive loading on my preamp.

I use a B+K QR 2011, Analogue Productions Ultimate Test Record, and JVC TRS-1007 test records. They all give slightly different results but close enough.

This has been an informative discussion. Do you use an O-scope with these test records? Interested in your methodology.
 
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