Step-ups and MC cartridges
The secrets of a successful marriage
Please note: I wrote this little guide using the small amount of knowledge I've accumulated until now. I don't consider myself a master in electronics. In fact, I understand nothing of that subject. Some of you may find that I’m using some inappropriate terms. The aim of this paper is to understand the way step-up transformers function, and the rules you should follow to select the appropriate step-up to use with your MC cartridge (if you want to obtain a happy marriage).
Now, things are complicated by the fact that I’m French and even if Jas took a very close look to this text, some translation problem may have occurred. You’ve been warned! I would like to thank Jas, for his proof reading and Emile and Mambojet of the LS3-5a forum for their correcting of the first French version of this paper. All errors remain mine.
MM (moving magnet) cartridge, it’s very simple: you plug it into the MM entry of your amplifier and listen.
MC (moving coil) cartridge, it’s more complicated. The characteristics you should take care of for this kind of cartridge to be correctly exploited by your system are:
- The output level, measured in millivolts
- The internal impedance, measured in ohms
- The load impedance, measured in ohms too
In this paper, we’ll ignore high output MC cartridges that can be used with a MM input and concentrate on MC cartridges with an output of less than 1 mV. (The output level of MM cartridges is between 2.5 and 7 mV).
The job of a step-up transformer is to raise the output of the MC cartridge in order for that output to be correctly handled by a MM phono section. At the same time, the step-up transformer adapts the impedance of the signal produced by the MC cartridge to an impedance that can be “read” by your MM input. You must understand that step-ups all have different characteristics that we’re about to explore. These characteristics are such that if you select a step-up and a MC cartridge at random, your probability of obtaining a happy marriage is close to zero.
So please, don't make a definitive judgment of a step-up or a MC cartridge. Before saying “this step-up is pure BS” or ”this cartridge is pure BS”, check the compatibility of your MC / step-up combo. Most of the time, if the sound isn't good it’s because of a compatibility problem.
So, you need to amplify the signal output from a MC cartridge? In fact, you use can two different kind of devices to do that job:
active devices : head amplifier of MC section on a phono section
passive devices : step-up
How can you tell that a device is active or passive? Very simple, on active devices, there is a plug! On step-up, no plug (so, it’s passive since there is no power supply).
Another difference, that makes things unclear for a newbie, is that on active devices the load impedance is indicated (100 ohms, for instance), on passive devices you don’t have a load impedance but an internal impedance; rather than a manufacturer giving a load impedance figure you will get a statement such as: “this step-up is designed for use with cartridges with an internal impedance of 40 ohms". We’ll ignore active devices. There is no fun there; cartridge manufacturers generally specify a load impedance, so you set up your active device (head amp, MC phono section) to that load impedance and listen; game over.
By reading this paper you will find that some cartridges cannot be used with a step-up, whatever its characteristics. For these cartridges you’ll have to use an active device.
Now, on a step-up you’ll find internal impedances indicated: for instance, take a look at a DENON AU320, two positions are possible: 3 ohms and 40 ohms. So this step-up should be good with cartridges of internal impedance of 3 ohms and 40 ohms. Now the problem is, that for the moment, the load impedance used in that step-up is not known (but we’ll soon know this very important information!). We’re only sure that this load impedance is superior to the internal impedance. And secondly, the load impedance given by a manufacturer for its cartridge is the one you should use on active devices; the one you should use on passive devices is not generally the same and is superior to the load impedance given by the manufacturer.
How can you tell that the impedance indicated on a step-up is not the load impedance?
Let’s do some calculus! If a step-up has a gain of say 1:10, this means that the output of the cartridge will be multiplied by 10. If the output out of the cartridge is 0.3 mV, you’ll have, after the step-up, 3 mV, perfect for a MM input. Now, your MM input has its own impedance generally equal to 47 Kohms. The signal emitted by the MC cartridge must be in accordance to that 47 Kohms. A step-up modifies the impedance of the output by a factor equal to the square of the amplification ratio. This is known as ‘natural impedance’. A word of caution: by itself, a step-up doesn’t have any impedance. The natural impedance is directly linked to the impedance of your MM entry and to the amplification ratio.
So, a step-up with an amplification ratio of 10 has a natural impedance and load impedance of 470 ohms (if your MM input impedance is 47 kohms: 470=47000/10²). Now, let’s go back to our DENON AU320: on the 40 ohms position, since the amplification ratio is 1:10, the load impedance is 470 ohms.
You’re not convinced? Let’s do more calculus. If your load impedance was 40 ohms, your amplification ratio, in that case should be equal to (47000/40)^0.5 = 34.3. With a cartridge output of 0.3 mV, you are sending 0.3*34.3 = 10.28 mV to your MM input. This is important; you may be overloading your MM input. Let’s do the same calculus on the 3 ohms setting. If it were your load impedance, the gain in that case should be (47000/3)^0.5=125.16. Again, with a 0.3 mV output cartridge, you’re about to send 37.5 mV in your MM input: this IS overload! A MM input is generally designed to accept an output between say 2.5 and 10 mV, not 37.5 mV! And the amplification ratios calculated here have nothing in common with the step-up specs given by manufacturers (as we’ll see in a few moments).
Come on! You forgot to talk about “resistor tuning”!
Don’t worry, I’ll talk about resistor tuning in a few moments. But, as an introduction, understand that there is a way to reduce the load impedance by adding resistors inside the step-up. This is not the way manufacturers did it. In fact, a step-up looks like this (drawing taken from an audio technical manual):
No resistors here. You can do some reverse engineering by dismantling your step-up: you won’t see any resistors unless you have an Ortofon T30 (among the step-ups examined in my table, I believe this step-up is the only one with resistors)
That scheme is really interesting, please tell us more!!!
My pleasure! So, for one channel, there are two coils: a primary (the ‘input’: the signal emitted by the cartridge arrives at this one) and a secondary (the ‘ouput’: the amplified signal will go out to your MM amplifier stage from this one). You will see that there are less turns on the secondary than on the primary. This is not a strange idea from the designer of this scheme. Your amplification ratio is a direct result of the turns ratio (primary/secondary) (ok, I know, I know, a perfect transformer doesn’t exist and things are not that simple but the rule indicated here is a good proxy). This explains why, on a step-up that can be used with several internal impedances cartridges, the gain differs according to the internal impedance you’re selecting. On the audio technica step-up represented by the scheme, the gains are 23, 26 and 34 db for respective internal impedances of 40, 20 and 3 ohms.
In a perfect transformer: N2/N1 = V2/V1 = x
|Where:||N2(1) : number of turns of the secondary (primary)|
V2(1) : output out of the step-up (in)
x : amplification ratio
|And||Z2/Z1 = N2²/N1² = x²|
|Where:||Z2(1): impedance of the secondary (primary)|
Another « detail »: the grounding of the step-up. This is very important. If you’re having grounding problems, or shielding problems, you’ll probably have “hum”.
But, why are only internal impedances indicated on a step-up?
The main reason is certainly the fact that, as I already indicate, natural impedances are not constant: they are a function of the impedance of your MM input. For instance, if your MM input has a 10 Kohms impedance, with a 1:10 amplification ratio step-up, the natural impedance (and your load impedance) is 100 ohms (10000/10^2).
What step-up for my cartridge?
This is THE question. If you’re looking for a step-up you can buy second hand step-up transformers, brand new (most Mc cartridge manufacturers produce step-up transformers) or build them yourself using parts from manufacturers like Lundhal, Sowtek, Hashimoto, Jensen.. (See links to those manufacturers at the end of this paper).
To make things clearer, we’re going to take a look at the specifications of available products. Why? Because these products were designed by MC manufacturers, let’s assume that they knew what they where doing. Specs shown here are compiled from various sources (internet or vinylengine inmates like Yosh (Hi Yosh!). if you have data relating to other products, contact me and I’ll add them to the table. All I need to know is the gain (measured in db or in amplification factor 1:x) and the recommended internal impedance of the cartridges to use with that step-up.
Data in bold type are manufacturers specs, other data was recalculated this way:
X factor = 10^[(gain in db)/20]
Gain in db = 20*ln(x factor)/ln(10)
Natural impedance = 47000/[(x factor)^2]
|Manufacturer||Model||Gain in db||x factor||Natural impedance||Recommended Impedance|
|T10 MK2||28||25,1||74,5||2-6 ohms|
|FR XF-1||30||31,6||47,0||4-18 ohms|
|XG5||34||50,1||18,7||< 3 ohms|
|Supex||SDT 3300||28,5||26,6||66,4||2-10 ohms|
|Rothwell||rothwell||22,0||12,6||296,5||< 40 ohms|
|Sony||HA-T110||26||20||117,5||3 - 40 ohms|
* ortofon T30 : the natural impedance is certainly used for 48 and 12 ohms position and is reduced by “resistor tuning” – if someone here can read the value of the resistors inside the T30, that could help us a lot!
What is there to understand here?
Let’s imagine I have a 3 ohms internal impedance cartridge. What are the devices I should buy?
Some devices from this table have a 3 ohms setting: FRT-4, Denon AU 320, Denon AU 340, AT700T, Ortofon T2000, FR-X1L, EAR MC4. What we can see is that the natural impedance of these devices (respectively 37,3 - 36 - 43 - 18,7 – 14.9 – 11.8 and 52,2 ohms) can be separated in two groups: Some of them have very low natural impedance (between 11.8 and 18.7 ohms). I think these figures are really too low and will limit this kind of step-up for use with very low internal impedance cartridges (1-2 ohms) especially the Ortofon T 2000. The remaining step-ups have a natural impedance between 36 and 52.2 ohms.
We will consider this range as a good range for loading a 3 ohms internal impedance MC cartridge. The amplification ratios associated with these step-ups are between 30 and 36 (gain is between 29.5 and 31 db).
Now, there are some other step-ups in my table that have a natural impedance between 36 and 52 ohms: EAR MC3 (4 ohms position), Ortofon T3000 (2-10 ohms position), FR XF-1 (4-18 ohms position), FR X1-M (4-18 ohms position), FRT-3 (10 ohms position). Conclusion: you can use that step-up at the position indicated with your 3 ohms cartridge.
One could add to this list the Ortofon T10, T20 and T30. The natural impedance is equal to 30 ohms (a little low for me – I would couple those transformers with very low impedance cartridges of say 1 - 2 ohms). On one hand you could say “come one, this is Ortofon, worldwide specialist of low impedance cartridges, they know what they doing”. But on the other hand I must add a very important point to understand why Ortofon may have changed their minds. These models are rather old (70’s 80’s). It seems that manufacturers realised in the 80’s that designing a high gain step-up transformer is really difficult. The more gain you have, the more difficult it is to have a flat frequency response. The difficulty of producing a quality high gain step-up with a linear response is translated in the actual price of these step-ups: for instance, the price of an Ortofon T3000 is 1625 euros (are you sure you really want to buy a low impedance cartridge?)
But, from this table, you can find some other transformers that are said to treat correctly a 3 ohms internal impedance cartridge. In fact, for these transformers, manufacturers provide a range of possible internal impedances for the transformers.
Things are now a little more complicated and one could ask if we can really trust the manufacturers’ recommendations? Let’s take a look at the Denon AUS1, Ortofon SPU-T100, Ortofon T1000, FR XG5, Ortofon T5, Nakamichi MCB100, Sony HA-T110, Ortofon T20MKII, Ortofon T10 MK2 and Supex SDT 3300.
The natural impedance for these transformers is between 66.4 ohms (Supex SDT 3300) and 278 ohms (Denon AUS-1) : there must be a problem for the Denon unit. A 3 ohms internal cartridge plugged into a Denon AUS-1 is certainly producing very amplified highs. So, when a range is indicated for your step-up, there is always some kind of compromise since they cannot be good everywhere. The risk is to have a too high a load impedance for a cartridge with an internal impedance equal to the minimum of the range (too bright sounding) or too low a load impedance for a cartridge with an internal impedance equal to the maximum of the range (no high frequencies in this case).
The Denon AUS-1 should work well with a 40 ohms internal impedance cartridge but no good with a 3 ohms internal impedance cartridge. The Ortofon T5 might not work very well with a 40 ohms internal impedance cartridge or a 3 ohms internal impedance cartridge, but may be good with say, a 10 ohms internal impedance cartridge. Let’s take a look now at what is proposed for 40 ohms internal impedance cartridges: there is a consensus on a natural impedance of 470 ohms (biased by the fact that we’re having a lot of Denon products in the table). Now, from the table, you can see that the FRT-4 (100 ohms position) and the Ortofon T30 (48 ohms position) will give you the same natural impedance. With transformers proposing a range of internal impedance including 40 ohms, the natural impedance is generally lower, from 118 ohms (Sony HA-T110, Ortofon T5) to 296 ohms (FR XG-5) – for the Briston TF1, 40 ohms is the minimum of the range : the natural impedance (and then the load impedance) is much to high (1052 ohms).
Let’s assume that the loading of a MC cartridge with a transformer is not an exact science.
Loading a MC cartridge is not an exact science? Why? Simply because manufacturers never produce a unique load impedance for a given internal impedance and because, for a given natural impedance, there is frequently a range of internal impedances that suit the transformer. Now, for a given internal impedance, we should propose a range of load impedance. What I’m proposing now is just a starting point – by following my rules, I’m not telling you that you will obtain the perfect load impedance, I’m just telling you that you won’t make stupid mistakes.
So please, don’t contact me to insult me about my rule!
For this rule, we’ll use a spec given by the manufacturer: the load impedance. We’ll take this load impedance as meaning a load impedance recommended for active transformers (and not for passive devices). This data certainly includes things relavent to important characteristics of the cartridge we don’t know, but that must be considered.
A manufacturer generally gives you a minimum load impedance for active devices (or a range, but it’s less common).
If the load impedance (for active devices) is said to be > or equal to 20 ohms, the internal impedance is 3 ohms, you should try to load your cartridge at load impedances between:
- the load impedance given by the manufacturer (20 ohms)
- the load impedance given by the manufacturer + 10*internal impedance (50 ohms)
Now, for a 40 ohms internal impedance cartridge with load impedance for active devices of 100 ohms, you should try load impedances between 100 and 500 ohms. If your system is too bright (suppose you’re having triangle loudspeakers…), you will certainly be happy with something that will “cool down” your sound, so try something close to 100 ohms. On the contrary, you may be happier with something closer to 500 ohms.
Now, my opinion: I never hear satisfactory results when the load impedance on a passive device is equal to the load impedance indicated by the manufacturer (we understand as load impedance on an active device). You’ll try and you’ll tell me. We’ll see later the way to test different load impedances but you don’t need to test 100 values between 100 and 500 ohms : something like 3 or 4 value is enough and in our case, you could try 100 ohms (and you’ll tell me), 200 ohms, 300 ohms and 470 ohms (with a 1:10 transformer).
If the manufacturer is proposing a range of load impedance (say between 20 and 200 ohms), follow that range by testing say, 50 ohms, 100 ohms and 150 ohms.
Do you think things are over? No, they aren’t! The load impedance is an important thing BUT it’s only the first important thing to consider. Remember that the natural impedance is directly linked to the gain of your step-up. In the next table, you will see the maximum gain of your step-up to have a desired load impedance.
TABLE 1: Load impedance and maximum gain of the step-up
Impedance of the phono stage : 47000 ohms
How to use this table? Suppose you would like to test load impedances of 470, 300 and 200 ohms. Take the higher load impedance (470 ohms) and check the table. You now know that you’ll need a 1:10 step-up (a step-up with a gain of 20 dB). For load impedances of 300 and 200 ohms, see the resistor tuning section of this paper.
Things aren’t over! No, they aren’t!
We have not considered an important spec of your cartridge: its output level. You should not forget that the first objective of the step-up is to amplify the low signal coming for your cartridge to make that signal compatible with a MM input.
Jensen, on their website, states that the output, once amplified, must be between 2.5mv and 10mv to be handled correctly by a standard MM input. We’ll use this rule as a good starting rule. If the signal is too low, we’ll have to turn up your volume knob too high (and you’ll hear the noises coming from your amp), if it’s too high, you’ll overload your MM input. There is another difficulty here: the output level should not be considered as a constant. It seems (but someone will certainly be able to explain this better than me) that the output level is affected by your load impedance: the closer you are from the internal impedance, the lower your output will be – this phenomenon is the one that explains the under amplification / over amplification of high frequencies (now, it’s hard for me to be more precise – if someone is able to explain this in simple terms, he’s welcome!).
In the next table, you can observe as a function of the gain of your step-up, ranges of output levels coming from the MC cartridge that will let you to stay between 2.5 mV and 10 mV at the MM entry level.
Table 2: step-up gain – minimum and maximum output of the MC cartridge
|Gain in dB||x factor||Min output (mV)||Max output (mV)|
Things become complicated: as we say in France, you’re running after two hares at the same time (and we’ll add two more hares in a few moments):
- you must have a correct load impedance
- you must have a sufficient gain
For some cartridges, it’s mission impossible! An example? The Denon DL-1000A: the output is 0.12 mV, the internal impedance is 33 ohms, the load impedance is 100 ohms so, following the rules, you feel that this cartridge should be comfortable with say, 300 ohms.
To obtain 2.5 mV, the gain of the step-up must be, at minimum, 1:20 but with that kind of step-up, the natural impedance is 118.1 ohms: it’s lower than the desired value of 300 ohms! No passive device for this cartridge (in my opinion): find an active device!
Some cases are more interesting. For instance, let’s consider the Ortofon MC30 supreme: the internal impedance is 5 ohms, the load impedance is superior or equal to 20 ohms, and the output is 0.5 mV. Following the rule, you must try load impedances between 20 and 70 ohms.
If I only consider the load impedance criteria and by taking a look at the table 1, I’m about to order a 1:26 step-up. Now, 26*0.5mV = 13mV = overload!
“I’m about to have a nervous breakdown!” Come on! Resistor tuning will save you!
No, you don’t have to add large tyres on your turntable! Resistor tuning is a way to modify (to lower) the load impedance directly linked to the gain of your step-up transformer.
You can lower the load impedance by adding a resistor (in fact two: remember, stereo!) between the + and the – of the RCA plug (out of the step-up).
Suppose you have a 1:10 gain step-up. Your actual load impedance is 470 ohms (natural impedance). You would like to test a load impedance of 100 ohms. Let’s now calculate the value of the resistors you should add:
In fact, by adding resistors, you’re modifying the impedance of the MM entry (actually equal to 47 000 ohms - standard). To have a load impedance of 100 ohms, you need to have an MM entry impedance of 10 000 ohms (remember 10000/10^2=100).
First thing to do is to calculate the desired impedance for your MM input before calculating the value of the resistors you’ll need. Use this formula:
Wanted_IMP = (xgain)^2*(wanted_load_imp)
Now to calculate the value of the two resistors you need in our case:
Rload = 1/(1/10000-1/47000) = 12.7 Kohms
Generally: Rload = 1/(1/R1 – 1/R2)
Where: R1 is the impedance you want for your MM input and R2 is the actual impedance of your MM input
Let’s go back to our Ortofon MC30 supreme. With its output of 0.5 mV, I say to myself: I would like to obtain 5 mV in my MM input, so I need a 1:10 transformer.
Now, the load impedance. I would like to have a load impedance of say 40 ohms (here, the brainy guy will try different load impedances between 20 and 70 ohms, will buy several pairs of resistors to test those values and will only keep the pair that suit him the best).
With a gain of 1:10, I need an impedance of 4000 ohms on my MM input (10^2*40). The resistors I should buy must have a value equal to:
Rload = 1/(1/4000-1/47000) = 4372 ohms
For that cartridge, resistor tuning is very useful: you can obtain the desired load impedance with a 1:10 gain step-up. And now you remember that, the higher the gain, the higher the probability of having distortion (or the higher the price you’ll have to pay for your transformer): a transformer with a gain of 1:10, it’s not a high gain transformer (and it’s quiet cheap! Hooray!)
I know, you’ve just started saying to yourself “come on, with that cartridge, I can use a step-up with a lower gain than that 1:10 gain – that we’ll be better”. Yes, but the answer to this question will be a function of the gain of you MM input (and of its sensibility – remember, you need a minimum of say (but take a look at the specs of your MM input) 2.5 mV).
Imagine that you can use a 1:5 gain step-up (Lundhal, I have one, Jensen is proposing a 1:4 – it’s one of the most expensive of its products...). With the Lundhal, 0.5 mV*5 will give you 2.5 mV. In that case, to obtain the 40 ohms load impedance, with a gain of 1:5, the impedance of your MM input must be 5^2*40 = 1000 and the value of resistors you should buy is:
Rload=1/(1/1000-1/47000) = 1021 ohms
WARNING: BUY ONLY HIGH QUALITY RESISTORS!!!
More resistor tuning?
You now understand that, with resistor tuning, you can lower the load impedance. But what if you need to raise that load impedance?
It’s possible but not much fun: you understand that you need to modify the load impedance of your MM input. Open you MM stage and, somewhere (things are more complicated for me at this level), you’ll see resistors, close to the RCA sockets that determine the impedance of your MM entry (47kohms resistors), remove those resistors and solder higher value resistors.
Now, if your intention is to do that kind of thing, I deeply recommend you find yourself a real expert! There are certainly some limits to this kind of operation.
One last! For the road!
Remember the two hares you’re running after? Let’s add another one: your cables (and their inductance). The cable you’re using between the step-up and the MM input must be as short as possible and its inductance must be really low! Inductance problems can modify the linearity of your frequency response and it’s a very important factor with MC cartridges.
For more details on that subject, see: http://www.hagtech.com/loading.html
« I’m exhausted reading you, come on, give me a break! » Not yet and it’s not time for a conclusion!
Where is that fourth hare we must run after?
There is a little problem with resistor tuning. I insisted that you buy high quality resistors but remember that the best resistor is the one that remains in the store. So, be aware that using bad quality resistors may destroy your sound and that using high value resistors can do the same thing.
So, as a pre conclusion, let’s present the four hares you’re running after:
- you must have the correct load impedance
- you must deal correctly with the output of your cartridge
- use minimal resistor tuning (+ high quality resistors are necessary)
- use short / low inductance cables
Not that easy!
A final table, for the road!
To simplify things a little and to determinate more easily the kind of step-up you’ll need for your beloved cartridge, let’s try to understand this last table.
Table 3: output, load impedance, minimum and maximum gain
|x factor min||50,0||25,0||16,7||12,5||10,0||8,3||7,1||6,3||5,6||5,0|
Let explain the table: you have a MC cartridge with an output of 0.25 mV. You need a step-up with a minimum gain of 1:10 (to obtain 2.5 mV in your MM input)
The load impedance you would like to try will give you the maximum gain of your step-up. Suppose, you would like to try different values between 10 and 50 ohms, take the higher value and from the table, you’ll see that the maximum gain you could use is 1:30 (I consider here 7.5 mV as the maximum possible input to the MM stage – 10mV seems to me rather high…). The star, at the right of the 30, indicates that with a higher gain transformer, you will overload your MM entry. To modify the load impedance here (and reduce it), use resistor tuning.
If you want to load that cartridge at 375 ohms, the maximum gain must be equal in that case to 1:11.2. The ‘E’ at the right of the 11.2 indicates that if you choose a transformer with a higher gain, the load impedance will be necessarily lower than the desired value.
“Damned, at the crossing of the column that corresponds to the output of my cartridge and of the line that correspond to my desired load impedance, there is nothing! »
Sorry folks, if your MM input is a standard one (with no selectable gain, for instance), no step-up transformer for you! Use an active device (and you read the whole page for nothing – sorry).
provides help to choose the correct product you need for your cartridge
provides a lot of information too!
Bent Audio proposes some very interesting explanations
used to produce transformers for Sansui: they’re proposing a very interesting step-up transformer for MC cartridges, theHM-3 (and the guy is very nice!)