Using Transformers to Transform Audio

A transformer is a device that transfers electrical energy from one circuit to another via inductively coupled conductors – coils of insulated wire wound (most often) around a ferromagnetic core. Transformers are useful in audio recording, as they can block DC voltage and facilitate electrical isolation between gear, but they also shape the sound of signals sent through them by introducing subtle, and often very desirable “coloration”. This may be a result of any combination of frequency response, time response, distortion, or a number other factors for a particular transformer. These effects can be especially attractive when working in the digital realm, where sounds can be perceived as too “cold” or “digital”.
With all the talk of the sound of transformers, which reside in different pieces of audio gear, why not skip all the electronics and strap bare transformers into the audio path directly? The first time I heard this done was at a mastering session with Nick Blagona in Toronto, when he ran a pretty crispy sounding record through a pair of vintage Marinar transformers that he had lifted from a Neve module. The sound was just right for this record – fat lows, punchy mids and a silky top. I personally hadn’t had a chance to do much with this idea until I recently pillaged my repair guy’s stock bins, and now I’m hooked on trying all kinds of transformers in all kinds of situations.
What you’ll need are 600 ohm to 600 ohm “matching” transformers. These will generally work properly with the gear in our racks. Be careful of center tapped transformers (center tap to ground), If these are exposed to +48 volts (from phantom power) in your system they could easily be damaged. Simply wire your transformers up to cables that work with your gear or patchbay and you can insert them anywhere in your line-level signal path.
I’ve now tried transformers on all kinds of sources, and have fallen in love with many of the sounds I’m getting. My favorites so far have been a pair of vintage Triad HS-66s, as they add a mid-range thickness to the signal that really warms things up without obscuring the lows or the highs. On a full mix, they add a heft and three-dimensional quality to the sound that, while certainly not achieving the full effect, leans in the direction of a vintage console. On individual instruments they have a fattening affect that’s especially cool on kick drum and bass guitar. Pushing the signal can drive the transformer into distortion – sometimes a desirable lo-fi sound – though the need for an attenuator crops up pretty quickly. In one case I was tracking male/female duet vocals for the Brooklyn band The Loom, and found that the female singer had a strong midrange warmth that made the male voice sound a little thin in that area. By patching in one of the Triads after the compressor in his vocal chain, his sound warmed up just enough to match hers beautifully. Without disrupting the flow of the session, the transformer gave me just that little bit of harmonic complexity that I needed. It was like stirring a little more flour into the gravy, thickening to taste.
Another interesting pair of transformers were pulled from some odd quad encoder that the Japanese company Sansui was trying to develop in the ‘70s. As they were attempting to impress the pro audio market, they used some serious matching transformers. These add a harmonic richness to the midrange that can only be described as warm and fuzzy, and they also round off the highs well enough to function almost as a de-esser on certain vocals tracks. I wasn’t able to use them with any success on a mix, but on individual tracks they are able to add a strangely satisfying vibe I just couldn’t get with any other gear.
A UTC transformer that NYC engineer Matthew Agoglia liberated from a vintage compressor made a female vocal go from rich dark chocolate to three-dimensional licorice, adding a harmonic complexity that gave each syllable its own distinct vibe – way more interesting than it had been before. The same UTC on electric bass added a weight that seemed to get the signal out of the speakers and into the room, and that’s a big deal for me when working on my Pro Tools system.
With pairs of matching transformers slowly making their way to my studio, I’m beginning to see the possibility of putting them all in a (shielded) box, wiring them to DB-25 connectors and having them ready to be inserted between my converters and my Dangerous Audio summing box. Buying them in pairs allows me to send stereo stems out through matched left/right pairs, or to run full mixes through them. The cool thing about having multiple pairs is that you can have any number of different flavors on hand, rather than just multiples of the same sound as you would with a console. Then again, there is something to be said for the unifying effect of many of the same transformers. Experiment and see what works for you.
There are all kinds of transformers available, from vintage units by UTC, Triad, Marinar, CineMag and St Ives to brand new transformers from Carnhill, Jensen, CineMag, Lundhal, Sowter and others. While most of these companies are set up to supply gear manufacturers, I’ve found they’re all very approachable and willing to help a guy like me buy a pair. Most quality 600 ohm to 600 ohm transformers hover just above the $100 mark, so it’s a relatively inexpensive way to have some extra audio processing at hand. I highly recommend that anyone who is curious about this to try it out, and if you find some cool, obscure transformers with a vibe, do drop a line and let us know!
A Simple Transformer Explanation
A signal goes into one side of the transformer (usually the side driven is referred to as the primary), and the voltage that results on the output (secondary) is the input multiplied by the turns ratio (neglecting internal losses). The impedance ratio is the square of the turns ratio. Example: a 1:10 step-up transformer on a mic pre input would have a 1:100 impedance ratio. Meaning the voltage gain is x10, at a cost of increasing the output impedance by x100. A DI box is an example of going the other way, stepping the voltage down, to reduce the driving impedance. The lower the impedance a signal is being driven with, the less susceptible it is to noise

What are audio transformers?

Audio transformers are recognized as “wide band” transformers. In real meaning a “transformer” is two or more windings attached by a common magnetic field. It is this magnetic field, which offers the means to pass the voltages and currents from any primary winding to the accessible secondary winding when alternating current that flows. Basically the chief purpose of an inter stage audio transformer is to isolate the DC and attached the signal, with minimal loss. The transformer windings looks like short circuit to DC, still are seen as complex impedances to the AC signal.

Much that follows on the topic of audio transformers is of need rather over cut down to give a universal overview. Audio transformers were actually used in valve or tube amplifiers as inter stage and in any output transformers. Early transistor amplifiers likewise used audio transformers for pairing and output stages.

There is nothing specifically spiritual about audio transformers even though their use is not for the most part widespread today. It is expected the most interest comes from people paying attention in restoring or construction tube audio amplifiers. Basic requirements should be considered first.

We should assume loyalty is primary in the minds of most restorers. Therefore the prime need have to be a comparatively huge bandwidth; this is in the audio sense. Resistance coupled amplifiers basically create undistorted output signal voltages regardless of plate load resistance value. Audio transformer coupling is basically applicable to power stages for impedance matching.

Maximum output power is created when the impedance of the load matches the plate impedance of the tube. Using triodes, to some extent less output power but considerably less distortion results when the load impedance is among two or three times the plate impedance.

However tetrode or pentode tubes have extremely high plate impedances and the first impedance of the combination transformer is often among 1/10 to 1/5 of the tube plate impedance as it not totally possible to make audio transformers with primary impedances beyond 20K. As Audio transformer mixture is on the whole appropriate to power stages for impedance matching

Audio Transformer

Amazed by the electronic music of your favourite band? Mesmerized by the way suddenly an electric guitar goes as loud as the singer? Like most of know, there is an expert recording engineer present in the studio to make sure that music instruments and the voices make a perfect blend. A critical device that helps in creating this blend is an audio transformer. Works just like any other conventional transformer; audio transformers are designed to work at audio frequencies, i.e. between 20Hz to 20 KHz. Interestingly, the initial need to design to such transformer was to enhance long distancevoice communication. Now serving essentially for professional musicians, audio transformers stand as one electronic-audio component. This article will cover technical and applicative features of audio transformers explaining how they are different from the conventional counterparts.

Typical specifications of an audio transformer (42TM028) are

• Maximum output: 200mW

• Frequency response: ±2dB 300HZ-3, .4KHZ @ 1 kHz 0dB

• Insulation resistance: >10MW @ 100VDC

• Impedance ratio: Pri 600WCT, Sec1 600W, Sec2 600W

• Impedance variation: ±10% @ 1 kHz, .25V

• Primary wire: ø.08mm x 2 turns 320 ref.

• Secondary wire: ø.08mm x 2 turns 640 ref.

• DC resistance: Pri 78W ±15%, Sec1 (6-7) 65W ±15%, Sec2 (4-5) 60W ±15%

• Core type: EI-19

• Heat tolerance: +80°~+90°C

• Wave solderable

• Waxing: vacuum treatment

AUDIO TRANSFORMERS- FUNCTIONS

Like any other transformer, audio transformer comprises of a primary winding, secondary winding and the core and the primary function of an audio transformer is to couple the energy from the primary windings to the secondary without introducing any distortion. Audio transformers are designed for audio range of frequencies, i.e., for frequencies ranging from 20 Hz to 20 KHz and are used primarily in audio circuits.

The important functions of an audio transformer are:

1.      Impedance Matching

The impedance of the output stages of the amplifier circuits is often different from the impedance of the audio speakers. For maximum power transfer, impedance of the output amplifier should be same as that of impedance of the audio speakers. For lossless power transfer, impedance matching is done with the help of reactive elements, i.e., inductors and capacitors.

Since transformers can change the impedance (source sees the load with higher input impedance, while sink sees the source with lower output impedance), they are used for matching the impedance of the amplifiers and that of speakers.

The relationship between the impedances (primary and secondary) and turns ratio is as given below

Where,       

            Zp = Primary Impedance

            Zs = Secondary Impedance

            Np = No of turns in a primary

            Ns= no of turns in a secondary

Thus, if output amplifier stage has an input impedance of 1800 W and loudspeaker impedance is 8W, a transformer is needed having turns ratio of

With the transformer in between the amplifier and the speaker, amplifier will see an output impedance of 1800W, though actual load impedance is still the same, i.e., 8W. Similarly, loudspeaker will see an input impedance of 8 W though actual impedance is 1800W.

In a similar fashion (as explained above), audio transformers are used to change the impedance between the microphones and input amplifiers so that signal power is not reduced.

2.      DC Blocking

Typically the output amplifier’s AC signal is very low and is superimposed on the high voltage DC signal. It is often imperative to remove the DC signal from the composite signal before it enters the loudspeakers.

Since conventional transformers do not pass DC, audio transformers are used to prevent any DC signal from the amplifier entering the loudspeaker affecting the audio output.

UNDERSTANDING SPECIFICATIONS

a)      Power Levels

Transformer windings must be able to handle maximum current and voltage during the operations. Accordingly, wire of the windings must be chosen.

Maximum power level specified on the datasheet of an audio transformer refers to the maximum power that can be delivered to the load. Power levels are specified at particular frequency, typically at 1 KHz.

Power level depends upon the current handling capacity of the wire used for the windings-primary and secondary.

b)      Frequency Response

Frequency response indicates that voltage output levels at all the frequencies in this frequency range do not vary more than the given limits (typically ±3 dB, ±1 dB, ±and 0.5 dB).

The upper frequency falls off 3 dB when the normalised impedance equals leakage inductive reactance.

This suggests use of higher primary inductance for lower frequency operation. This translates to requirement of larger core. This will make difficult to obtain higher frequency limit.

As high frequency response is governed by the leakage inductances, it could be improved by reducing the number of turns (leakage inductance is directly proportional to the number of turns). But since number of turns affects lower frequency limit, only option to improve high frequency response is to interleave the primary and secondary windings.

c)      Total Harmonic Distortion

Total Harmonic Distortion is primarily a function of operating flux density (at the lowest operating frequency) in the core. Distortion due to this cause falls off rapidly with increase in frequency. Reducing the flux density reduces the distortion. This is a function of the magnetic material used for the core.

The distortion is higher at low levels due to magnetic hysteresis and at high levels due to magnetic saturation

Distortion is specified in the datasheets in terms of percentage at a specific frequency, typically 1 KHz, and at rated power levels.

d)     Insulation Resistance

Windings of the transformer are properly insulated to ensure that the current flow along the coiled conductors. The insulation is characterised by Insulation resistance. Typical value of insulation resistance is 10 MW.

e)      Impedance- primary & Secondary

Impedance ratio of primary and secondary is chosen as per the application. Primary and secondary impedances are separately mentioned in the datasheets. If CT is suffixed to the impedance value, it implies that that winding is center tapped.

f)       Rated DC Current

Rated DC current (IDC) is the amount of direct current that can be passed through the transformer winding without causing any damage. The DC current level is based upon the temperature rise at the maximum rated ambient temperature.  The rated current informs about the transformer's ability to minimize the power losses in the winding by having a low DC resistance. 

g)      Insertion Loss

It is the measure of the power available out of the secondary Vs power input to the transformer. It depends upon the losses in the transformer; loss due to DC resistance of the windings is the most obvious and hence should be kept to the minimum possible.

h)      3 dB Bandwidth

The range of frequencies over which the insertion loss is less than 3 dB with respect to mid-band insertion loss is called 3 dB bandwidth.

i)        DC Resistance

Direct Current resistance is the resistance of the transformer winding measured with the help of DC current. This number should be kept to the minimum possible.

Apart from these, datasheet of an audio transformer mentions about the core used, turns ratio, wire dimensions, weight, and temperature range and may be few more parameters.

DESIGN OF AUDIO TRANSFORMERS

Most important elements in the design of an audio transformer are the selection of core type, size and material, turn ratio, windings and casing.

a)      Core Size & Material
Choice of core size and its material depends upon frequency response, power levels, and harmonic distortion. Audio transformers should preferably use both high-permeability cores and the largest number of coil turns to create high primary inductance.

The most commonly used audio transformer core materials are M6 steel (a steel alloy containing 6% silicon) and 49% nickel or 84% nickel (alloys containing 49% or 84% nickel plus iron and molybdenum). Nickel alloys are substantially more expensive than steel but being high permeability materials require lesser number of turns.

Size of the core is determined based on the power requirements. Standard lamination tables needs to be referred. Once core is chosen, turns ratio to provide required inductance for a given size and magnetic material is obtained using the lamination catalogs.

b)     Turns Ratio

Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

If the turns ratio and the input voltage are known, the output voltage can be determined as follows

Where, E₁ & E₂ are primary & secondary voltages, N₁ & N₂ are the number of turns in primary & secondary winding.

c)      Windings

Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.

The material used for the windings is application specific. Insulated solid copper wire is used for small power and signal transformers.

d)     Hum Reduction

When signal levels are low, it is often essential to keep external magnetic fields to the minimum. For this purpose, transformers are enclosed in a case of high permeability materials.

APPLICATIONS of transformer

Audio output transformers are used when load impedances are low, as in line drivers, while audio input transformers are used when load impedances are high, as in line receivers.

Output transformers have very low leakage inductance in order to maintain high-frequency bandwidth with capacitive loads. For low insertion loss, they use relatively few turns of large wire to decrease winding resistances. They use fewer turns and operate at relatively high signal levels.

On the other hand, input transformers directly drive the usually high-resistance, low capacitance input of amplifier circuitry. Many input transformers operate at relatively low signal levels, often have a Faraday shield, and are enclosed in magnetic shields.

 

1.      Microphone Input

A microphone input transformer is driven by the nominal 150 W source impedance of professional microphones. One of its most important functions is to transform this impedance to a generally higher one more suited to optimum noise performance. The optimum impedance may range from 500 W to over 15 kW, depending on the amplifier. For this reason, microphone input transformers are made withturns ratios ranging from 1:2 to 1:10 or higher.

High CMRR is desirable form an input transformer. To achieve this, they must have two attributes. First, the capacitances of its two inputs (to ground) must be very well matched and as low as possible. Second, it must have minimal capacitance between its primary and secondary windings.

 

2.      Line Input

Line input transformers are generally driven by balanced line and drives an unbalanced line. They also transform the impedance as microphone input transformers and have high CMRRs.

3.      Moving Coil Phono Input
Moving coil phonographs are very low impedance devices (~3W). Due to this, it is very difficult to achieve good noise performance in an amplifier. Hence, the transformer is used in step-up configuration so that amplifier sees an impedance of 600 W in order to achieve good noise performance.

4.      Line Output
Typically, audio transformer is driven by an amplifier and loaded by several thousand pF cable capacitance and high input impedance of line receiver.Therefore, a line output transformer should have low output impedance which remains low at high frequencies. This requires both low resistance windings and very low leakage inductance.

5.      Interstage and Power Output
Interstage coupling transformers used to be popular in vacuum tube based designs. They used 1:1 to 1:3 turns ratio and classical push pull power amplifier in the output stage.

 

6.      Microphone output

Ribbon microphones& Dynamic microphones use step up transformers whereas condenser microphones use step-down transformers. Ribbon elements have impedance of the order of 1 W and hence need a step up transformer with turns ratio of 1:12 or more to transform its impedance to somewhere around 150 W. Similarly, step-up is required for dynamic microphones having impedances of the order of 10-30W. On the other hand, condenser microphones have high impedance and use step down transformer so that amplifier sees smaller input impedances.

 

7.      Speaker Distribution

When number of low impedance speakers are located at a distance from the  power amplifiers, the speakers are connected with the help of technique used in grid electricity supply, i.e., use of step-up transformers at transmission end and step-down transformer at receiving end.

 

8.      Telephone directional coupling or “Hybrid”

Telephone “hybrid” circuits use bridge nulling principles to separate signals which may be transmitted and received simultaneously on a 2-wire line. This nulling depends critically on well-controlled impedances in all branches of the circuits. This nulling is what suppresses the transmit signal (your ownvoice) in the receiver of your phone while allowing you to hear the receivesignal (the other party).