Help needed: Question about the impedance on an SUT?

ShawnZH

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Sep 20, 2020
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I'd like to get some advice how to read/use the impedance on an SUT to match up carts. Some of the SUTs mark the ratio (for example x20) and the db gain (for example +26db in the case of x20 ratio), while some will put an impedance along (1ohm, 3ohm, 40ohm for example, while higher impedance usually represents lower ratio and lower gain).

My question is how to read/match the impedance? For example, I have a low self impedance (1.5ohm for example) but high output cart (0.6mv for example) and I don't need too much additional gain. In this case, it looks that I have to choose 40ohm, which is usually x10 ratio. But should I go for the lower impedance such as 3ohm or so? I might end up getting too much gain (x30 ratio) if I choose low impedance.

Another extended question is how to calculate the actual output when applying a SUT.

For example when I put 1.5ohm cart of 0.6mv output into a x20 SUT. How much actual output I will get? 0.6mvx20=12mv seems to be very high.

Thanks
 

bazelio

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I'd like to get some advice how to read/use the impedance on an SUT to match up carts. Some of the SUTs mark the ratio (for example x20) and the db gain (for example +26db in the case of x20 ratio), while some will put an impedance along (1ohm, 3ohm, 40ohm for example, while higher impedance usually represents lower ratio and lower gain).

My question is how to read/match the impedance? For example, I have a low self impedance (1.5ohm for example) but high output cart (0.6mv for example) and I don't need too much additional gain. In this case, it looks that I have to choose 40ohm, which is usually x10 ratio. But should I go for the lower impedance such as 3ohm or so? I might end up getting too much gain (x30 ratio) if I choose low impedance.

Another extended question is how to calculate the actual output when applying a SUT.

For example when I put 1.5ohm cart of 0.6mv output into a x20 SUT. How much actual output I will get? 0.6mvx20=12mv seems to be very high.

Thanks

SUTs should have both a winding ratio (e.g. 1:20 or "x20") and a primary impedance. The problem is that most SUT manufacturers just spec something like L, M, or H for impedance and they don't tell you what numerical value it is, much less at what frequency (its value is a function of frequency). Your best bet with a SUT is indeed to use a low impedance cartridge (which 1.5 ohms is), and then you don't need to worry too much about SUT primary impedance as most are sufficiently low. Higher impedance SUTs would normally be by special request. Now you really only need to care about cartridge loading and step up ratio. Your cartridge should have a loading spec, say for example, 100 ohms. To determine what load your cartridge sees through a SUT, divide the phono stage input load by turns ratio squared. Example 47000/(20*20) = 117.5 ohms. So, with a 47k input load on the phono and a x20 SUT, you'll be loading the cart at around 117 ohms. A lighter load can only be achieved by using a "smaller" SUT. E.g. on the same phono, a 1:10 SUT loads the cart at 470 ohms. Lastly, yes, you should be concerned about overloading the phono input stage. 12 mV may or may not be too high. I'd suggest being under 10 mV unless you're sure. So in your case, a 1:14 may be more suitable.
 
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dave slagle

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For example when I put 1.5ohm cart of 0.6mv output into a x20 SUT. How much actual output I will get? 0.6mvx20=12mv seems to be very high.

In reality you will get a bit less than what the math suggest but you are best to be safe and use the 12mv as a worst case. With the current trend of MC carts with higher output levels, dynamic high frequency overload is becoming a much more common occurrence and bazelio is correct above when he suggests to keep things below 10mV. For the output impedance the cartridge Z gets multiplied by the turns squared (1.5? * 400) for 600?.

In reality the impedance numbers are fairly soft so there is some wiggle room which tends to lead to a lot of confusion on how to match a SUT to a system. One of the grave errors I repeatedly see with SUT's is choosing a turns ratio to set the load the cartridge sees. I laid out my approach at the link below.

My Take on SUT's

dave
 
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groovemaster

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Now you really only need to care about cartridge loading and step up ratio. Your cartridge should have a loading spec, say for example, 100 ohms. To determine what load your cartridge sees through a SUT, divide the phono stage input load by turns ratio squared. Example 47000/(20*20) = 117.5 ohms. So, with a 47k input load on the phono and a x20 SUT, you'll be loading the cart at around 117 ohms. A lighter load can only be achieved by using a "smaller" SUT. E.g. on the same phono, a 1:10 SUT loads the cart at 470 ohms. Lastly, yes, you should be concerned about overloading the phono input stage. 12 mV may or may not be too high. I'd suggest being under 10 mV unless you're sure. So in your case, a 1:14 may be more suitable.
In principle, you are describing a basic problem that constantly occurs in transformer operation.

The 47K MM input impedance is a fixed value and that is the real problem. There is recently a solution which offers 7 different input impedances for MM input.

The values are 100K/47K/32K/24K/20K/16/and14K.
If you use a 1:20 transformer as an example, this results in an impedance of 35 ohms at 14K, which your MC system sees. Of course, this fits perfectly.

Ask at Audiospecials.de for the Phonolab. It should soon be available with an MM/SUT input that offers exactly these many different impedances for MM operation.

groovemaster
 

dave slagle

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There is recently a solution which offers 7 different input impedances for MM input.

The problem with this approach is the load at the input of the MM phono that terminates a SUT can have as much impact on the sound of the SUT as the cartridge. In my view, the transformer should be designed to work into a 47k? or greater value load and any additional loading needed by the cartridge should be added to the cartridge directly.

dave
 

bazelio

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My Emia phono has a fixed 200k-ish input impedance, which is much more convenient than 47k. I also do what Dave says and add a parallel load to the cart at the SUT primaries which can be any resistance value I want. Generally, I find that about 50 ohms separate audible differences in the range I tested for the Opus 1 cartridge.

PXL_20211104_151820463~2.jpg
 

groovemaster

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To evaluate the sonic effects of changing the 47K MM input impedance at the transformer, we did the experiment with AS Phonolab PRO on the Phasemation T-550 and the wonderful Ortofon SPU system.

Clearly the best result was the 14K setting on the Phonolab. This resulted in an impedance of low 35 ohms with the Phasemation T-550 transformer 1:20. These low values are what the SPU loves. So the SPU presented the best stage with the best transparency. There was simply more space and atmosphere between the instruments. Very emotional. The 47K standard setting didn't stand a chance against this.
groovemaster

_DSC1587.jpg
 

bazelio

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I think the point being made was that you are typically better off placing the additional loading at the SUT primaries as opposed to the secondaries. I.e. stay at 47k, and put a 50R load at the 1:20 phasemation primaries, which will then yield your desired 35R load for the cartridge.
 

Solypsa

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Both Groovemaster and Slagle bring up similar points- that the 47k standard can be changed to suit, it is not 'required'. In the case of the Phonolab they wanted a lower numerical load and got it with lower numerical preamp loading...for Slagle he seems to often like high ratio step ups and so uses higher numerical preamp loads to acheive the end load result. Same strategy different windings....
 

groovemaster

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The big advantage of the impedance adjustment directly at the MM input of the phonostage is, that each transformer of the market can be individually adapted to the pickup system. And this in the listening comparison. That is the point.

If the gain fits optimally, (here 1:20, and AS Phonolab at 14K), then really each transformer can be adapted by hearing. The eternal change and try with different transformers for the respective pickup system then has an end. Ultimately, the listening result decides.

groovemaster
 
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dave slagle

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The next logical step in your progression to complete things would be the following experiment. Compare the 35? load combo of the cart >> 1:20 >> 14k? with the same 35? load for the cartridge by using the 100k? setting with an additional 40? resistor at the cartridge directly. (100k?/20*20= 250?. 250?||40?=~35?). In a perfect world with an ideal transformer the sound of these two setups will sound the same.
 

dave slagle

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Slagle he seems to often like high ratio step ups and so uses higher numerical preamp loads to acheive the end load result. Same strategy different windings

Not the same thing... I see the reflected load via the transformer as a liability and it should be kept at as high as possible. Any further load required by the cartridge can then be added to the cartridge directly. As I have said above, using a load on the secondary of a SUT to effect the load on a cartridge has a more profound impact on the sound of the transformer than the cartridge.

dave
 
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Solypsa

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thanks for the correction :)


btw when you apply load to the 'cartridge directly' where do you like to ( physically ) attach the resistor?
 

dave slagle

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This becomes a tough one. I use resistors soldered to a RCA jack into an additional RCA socket in parallel with the primary of the SUT. In a pinch for experimentation a Y-connector can be used to allow the extra jack for the load resistor. I know this introduces another set of connections at a very vulnerable spot but it becomes a valuable tool for experimentation. After the experimentation is done a more permanent solution can be determined.

dave
 

bazelio

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The big advantage of the impedance adjustment directly at the MM input of the phonostage is, that each transformer of the market can be individually adapted to the pickup system. And this in the listening comparison. That is the point.

If the gain fits optimally, (here 1:20, and AS Phonolab at 14K), then really each transformer can be adapted by hearing. The eternal change and try with different transformers for the respective pickup system then has an end. Ultimately, the listening result decides.

groovemaster
I would think that if your end goal is to provide an optimal load for the cartridge, then you would NOT want to do so by altering the load of both the cartridge and the step up transformer. The transformer isn't ideal and is going to have measurable differences in response as you alter its load. Or at least, that's what common sense would dictate. So therefore, pick the right transformer for the task at hand and then alter the load on the cartridge only.
 

groovemaster

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Many roads lead to Rome.
In our community AS -Analog Spirit- , the transformers are so popular that a practical solution was desired without having to buy several transformers to achieve the best sonic result. This quickly becomes a never-ending story if you try several Transformers.

In the end, the impedance matching at the MM input of the phono stage proved to be the most practical, (without soldering !!!) , and also the most convincing in terms of sound. The assessment was made exclusively by ear! The field of step-up users is divided into two groups. Those with tube phonos have to use them because the noise of the MC inputs would otherwise be too high.

It gets really exciting with transistor phono stages.
Here, many use the step up to create their own personal sound character in combination with the pick up.

CSE (Colour, Stage, Emotion) is very popular with us. Here, everyone can realise their own personal preferences with only one step up and the individual impedance adjustment of the MM input. Very exciting.
We found the Phasemation T-550 very convincing. But Hashimoto and Ortofon are also worth a try.

Grooveamster
 

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