These are my research notes gathered in preparation for assembling a few different step-up transformers to be used in my system. In that light this article does not presume to be an authority on the subject, but rather a beginning of a learning process that should take me into the world of tube audio electronics. Those interested are welcome to join me.
Outline Notes On Moving Coil Step-Up Transformers Part 1
First, the playback chain exposed
From the groove to your ears. The physical imprint of an acoustic waveform is stamped into soft vinyl, an Lp. The platter spins the Lp. The stylus traces the terrain within the groove. The cantilever vibrates attached coil windings within the magnetic field of a strong magnet. There is an Electro-Magnetic induction causing a tiny current of electricity to be generated. This current, made of electrons arranged in an identical waveform to the groove modulations just tracked, flow through the tiny gage copper or silver wire up into a phono stage.* At the phono stage the tiny signal is multiplied in size. The frequency of the signal is equalized according to the RIAA spec standard. The signal flows out from the phono stage and into a preamplifier designed to input line-level signals from many different source devices. At the preamplifier, the signal is further processed and sent to the power amplifier and then out to the speakers. From the groove to your ears.
Some notes on phonograph cartridge types.
The phonograph cartridge converts the mechanical motion of its vibrating cantilever into electrical energy. As such it is a transducer. The other transducers in our playback system are the speakers which convert electrical energy back into mechanical energy. But that's another subject.
If one were to take a survey of the most commonly used phonograph pick-ups, and then stipulate a time period, say from that period just a few years after stereophonic sound became available, and beyond that to current day , then three basic types of phonograph cartridges would be included as being in common use.
(capacitance measure in pF = pico farads).
(mV = milli-volts)
Generally, low output moving coil cartridges have a reputation to be the best sounding of their type. But the main disadvantage is that the output voltage is so low it requires a step-up device in line between itself and the phono-preamplifier. Another critical factor is the introduction of noise and hum into this very fragile and vulnerable signal. Great care must be taken to transform and deliver the signal -cleanly- into the phono-preamplifier.
Medium output moving coil cartridges also require a step-up but of a lesser ratio.
The main benefit of the high output (HOMC) moving coil cartridge is that it can input directly into a phono-preamplifier designed to accept the signal from a moving magnet cartridge. It is a 'no-fuss option'. Another benefit is a comparatively lower price compared to many (but not all) medium and low output moving coil cartridges. It's disadvantage is poorer tracking ability and generally lesser quality of sonic output.
On the surface it would be easy to take the position that the moving magnet cartridge is the much more practical design and should be the way to go. This was the general take back in the late 1960's and 1970's. During that time most folks interested in high quality Lp playback tended to purchase moving magnet phono cartridges. The manufacturers of component audio gear designed and built their products to accept the signal from a moving magnet cartridge and to this day, most phonograph preamps are intended to function by means of moving magnet.
Even though moving coil cartridges have been around since the early sixties, the early examples tended to be used in professional broadcast environments or put to use on very high-end systems of their day. It wasn't until the late 1970's and 1980's that wider varieties of moving coils became generally available.
About phono preamps:
If one is to use an MC cartridge, except for the homc type, a voltage step up becomes necessary. The step up will increase the output voltage from the MC cartridge to a level suitable for input at the phono-preamplifier.
There are a couple of types of step up devices available to us. There is a powered device known as a "moving coil head amplifier". Sometimes these can be called pre-preamplifiers. Or line amplifiers. These are powered devices with their own power supplies that you need to plug it into the wall socket.
The other type, the subject of this article, is a moving coil transformer. These devices are passive and only need a source signal to operate and, as such, are placed in-line between the phono output and the mm phono-stage input. Moving Coil step-up transformers have a reputation for sounding good when used with tube phono-stages. If your phono-stage is solid state, you might want to look into a head-amp. That could be a more suitable alternative. It's not the law, but a common suggestion.
Above photo: An example of step up transformers in use.
About Step-up Transformers
Before making any choices lets examine a few facts about step-up transformers. The step up transformer architecture is essentially composed of two coils of magnet wire wound about an multi-layered iron core frame. The two coils are arranged within close proximity of one another. These coils of wound wire do not touch one another. In operation there is a transfer of electrical energy between the two coils by means of induction. The current is always AC. Transformers do not work with DC. The coil winding who's source signal is the output from the MC cartridge is called the primary and the other is called the secondary.
The MC step up transformer is considered to be a passive device because it has no external power supply. The tiny signal being generated at the MC phono cartridge is all that powers it. On a step up transformer the secondary coil will have a greater number of winding turns than does the primary. This difference in windings between the primary and secondary is called the turns ratio.
A common step-up turns ratio is 1:10. In a 1:10 step up transformer the number of windings at the secondary is ten times greater than those of the primary. In a 1:10 step up transformer voltage seen at the primary will be approximately ten times greater at the secondary output. The transformer does more than just transform the voltage. It also adjusts the resistive load seen by the phono cartridge. The resistive load is transformed by the square of the turns ratio divided into the value of the MM input resistor. So, instead of seeing 47000 ohms, the cartridge sees 470 ohms of load through the system. Here are the numbers: 47K/10^2=470 Easy enough to figure.
Above illustration: A split primary step up transformer. One trannie per channel when it is stereo. This is one channel. On the left upper corner is J1. This is the RCA jack input from the phono cartridge. On the right upper corner is J2. That is the output. SW1 allows selection between 37.5 and 150 ohms on the primary.
Often times step up transformers are labeled according to their impedance ratio rather than their step up ratio. To convert back to voltage ratio take the square root of the impedance ratio. Example, the above illustration shows an impedance ratio of 37.5/47K. Divide 37.5 into 47K then square root the dividend. 47K/37.5=1253.3, sqrt 1253.3=35.4 volts The turns ratio is 1:35.4
Now lets look at an example of matching an MC phono cartridge with an appropriate MC step up transformer. The cartridge is the Denon DL-103. Its output voltage is rated at .3mV. We will partner this with a Cinemag CMQEE-3440A step-up transformer. The one in the above illustration. This transformer has a split primary coil that offers 2 possible turns-ratio settings. For our DL-103 we can choose the 1:35.4 turns ratio. When we do we end up with a resistive load of 37.5 ohms. Here's the math: 47Kohms / 37.5^2= 37.5ohms. This seems very near to a match between the resistive load of the system and the internal impedance of the Denon. All should be peachy....right? Well, lets slow down a little and take a look at the output voltage with this cartridge and transformer combination. Output voltage times turns ratio equals voltage seen at the MM input. .3 * 35.4 = 10.6mV Seems rather high doesn't it? It may seem like we will be overloading the MM input with this system but in actuality we won't. Here's why: When the resistive load is equal to the internal impedance of the cartridge, the cartridge output voltage is reduced by 1/2.**
That seems like a joker in the deck. For now let's accept this rule on face value.
When selecting an appropriate step up transformer for a given cartridge, it is necessary to remember that the resulting output needs to meet the needs of the input side of the phono-stage. Voltage requirements at the phono-stage range from a low of 2.5mv and a high of 10 mv. Too low and the signal is noisy. Too high and you overload the circuitry in the phono-stage.
Here's how to calculate voltage output with a given combination of step up transformer and MC cartridge.
voltage output as a function of the load resistor
It can be figured like this: (Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))
The above noted Denon DL-103 has a 'Vcart' value of .3mV and an 'Rcart' value of 40 ohms. The step-up transformer can be the Cinemag CMQEE 3440A with a step-up ratio of 1:35.4
R(Load_effective) = (47000 ohms / 35.4^2) = 37.5 ohms
(Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))
37.5/(37.5 + 40)=.4839
then multiply output voltage at the cartridge by .4839 to get voltage output as a function of the load resistor and then again by the turns ratio to see the voltage at the MM input. Or:
The system resistive load and the internal cartridge resistance is a near match and the output voltage is in the middle of the acceptable range for phono-stage input. This cartridge and transformer should be a very good match-up.
Now let's try matching this 1:35.4 transformer to a different MC Cartridge. A Shelter 501 type II.
The Shelter's output voltage is .4mV. Its internal resistance is 10 ohms.
Now lets calculate output voltage as a function of the load resistor:
(Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))
Output voltage is too high for the phono-stage input and the load resistance isn't really a match for the internal resistance of the cartridge. We can't use this combination as it is.
In order to resolve this mismatch lets look at some things:
Firstly, consider the following about step-up transformers and mm inputs:
If the primary concern was only about the voltage seen at mm input, then it would be easy to find a suitable transformer for the Shelter. Example, a 1:10 step-up would multiply the Shelter's .4mV output to 4mV. However the 1:10 transformer also transforms the load impedance seen by the Shelter. Resistance at mm input divided by the square of the turns ratio. 47K/10^2 = 470 ohms.
My experience with the Shelter tells me that 470 ohms of load isn't quite right for this cartridge. Sure, the sound is full bodied and enjoyable. Frequency extension is just fine at both highs and lows, and the mids are very nice, yet there is also a sense of listener fatigue after a few hours of listening. Since the manufacturer does recommend 100 ohms/active and 10 ohms transformer, perhaps I should at least try to get this figure somewhere in that ball park while, at the same time, transforming the voltage to an acceptable value for mm input.
I think it will be difficult, if not impossible to find a transformer that, by its turns ratio alone, will deliver something close to 10 ohms of resistive load while at the same time produce a voltage output within the range needed for mm input. So why not shoot for 100 ohms and then, if that doesn't sound good enough, do some resistor tuning. First let us find a transformer with a suitable step up ratio for 100 ohms and with between 2.5 - 10 mV output.
Perhaps we should take another look at the Cinemag CMQEE 3440A transformer. This is a well regarded transformer among vinyl aficionados. It was a near perfect match for the DL-103 when using its 1:35.4 turns ratio. But this transformer has a split primary with another turns ratio option. On the label it says either 37.5 or 150 ohms. So lets calculate to see if the 150 ohm setting will provide an acceptable turns ratio.
Example, the above illustration shows an impedance ratio of 150.5/47K. Divide 150 into 47K then square root the dividend. 47K/150=313.3, sqrt 313.3=17.7 volts The turns ratio is 1:17.7 We could round it off to 1:18
Shelter output is .4 * turns ratio = volts at mm input.
After factoring in the voltage output as a factor of load impedance it looks like voltage at mm input will be acceptable at 6.7mV. The 145 ohms of load at the cartridge is considerably higher than our target of 100 ohms but my intuitive sense says to give it a listen. Then, if the result isn't quite right, some resistor tuning could dial in the setting closer.
Beyer Dynamic TR/BV 351 215 006. It has a split primary. It can be wired 1:15 or 1:30. Impedance ratio is 200/47K and 50/47K. I bring this model up because I have a pair of these transformers. link to page
If I use the 1:15 tap with the Shelter it looks like this:
If I use the 1:30 tap:
Neither option is a perfect match to the Shelter. However I suspect the 1:15 tap may sound good at 209 ohms load and 5.7 mV of output. The way to find out is to try it. .
Perhaps if I find a transformer with a 1:20 turns ratio. Lets see:
It's beginning to look as if the Shelter isn't going to get a perfect match up without some resistor tuning.
This is the end of Part 1. We've covered enough basic ground to roughly understand what a step-up transformer does and how to select between different models. There is much more to know but that will be seen in Part 2. In Part 2 of this series we will see how to fine tune load impedance by adding resistors at either the primary or secondary side of the transformer and, just as importantly, how to figure what value of resistor to use. We will also discuss the various pitfalls associated with using resistors. It's not a perfect world. Also in Part 2, a pair of step-up transformers will be assembled into a case with rca jacks at input and IC's at output. Then we put it to use. That's all for now.
* The terms phono-stage and phono-preamplifier refer to slightly different devices that perform the same function of amplifying the small signal from the cartridge to a level that can be input into the main system amplifier. This device also equalizes the signal according to the RIAA specification. However the phono-preamp will offer a means of adjustment for the user to set the amount by which this signal is raised, known as 'gain'. The phono-stage does not offer this adjustability but otherwise performs the same task. For the purpose of this article either term implies the function of a phono stage.
** When the resistive load seen at the cartridge is equal to the internal resistance of the cartridge the voltage output is reduced by 1/2. See links below for supporting text on this
Thanks to Gary Bronn for help with some of this math.
These links were referred to during the research for this article.
Some greatly simplified definitions of the terminology used in this article:
Voltage: The potential difference in pressure between two ends of the same wire when there is an excess of electrons at one end and a deficiency of electrons at the other end. Also called EMF (electromotive force) It is the force behind the flow.
Current: A flow of electrons forced into motion by voltage. The amount of current is measured in amperes. (amps)
In other words the pressure behind the flow is called volts. The flow itself is called current. (like the current flow of a river)
Impedance: opposition to current flow in an AC circuit. Specified in Ohms
Resistance: opposition to current flow in a DC circuit. Specified in Ohms
In other words, opposition to current flow is called 'impedance' when the circuit is AC and it is called 'resistance' when the circuit is DC.
Capacitance: A capacitor has the ability to hold (store) a charge of electrons between two conducting bodies. The number of electrons it can hold under a given electrical pressure (voltage) is called its capacitance. The unit of measure is the Farad. This is a very large unit. Typical measurements in audio electronics are often expressed in pF (pico farads) It is important to note that any given length of conductive wire has the ability to hold a charge of electrons within its length. This becomes an important consideration when selecting suitable cabling for the tonearm. The amount of capacitance in the wire.
Magnetic Fields and Induction: A magnetic field exists around any conductor with a current flowing in it. The strength of this field is proportional to current flow. The magnetic field might be thought of as having invisible lines of force, called 'flux', and this flux is at a right angle to the wire through which the current flows. The 'flux' has a polarity (direction).
If an alternating current flows through the wire the intensity and polarity of the flux will vary at the same frequency and in proportion to the current.
If another conductor crosses into the above magnetic field a voltage will be induced into it. The induced voltage will be proportional to the 'rate of flux change'.
If we place two coils near each other an ac current in one coil will induce a voltage in the second coil. Like a transformer!
end part 1