AUDIO TRANSFORMER DESIGN FOR TUBE AMPLIFIER WITH IMPROVED QUALITY AND REDUCED COST

Project Team: Evgen Pichkalyov Meltem Sevim Evgen Dzyuba Submitted to: IEEE Standard Education Committee

Supervisor: Prof., D.Sc. Julia Yamnenko Faculty of Electronics, National Technical University of Ukraine “Kyiv Polytechnic Institute” (Ukraine, Kyiv)

Submitted date: May 23, 2012

INTRODUCTION In 21th century, electronically devices are everywhere. According to this, there is a huge

electromagnetic pollution that damages environment and human health. People can’t see electromagnetic field with eyes so it is not possible to understand how much damaging to our health without measurements with special devices. Particularly heart diseases are the most dangerous and frequent serious human diseases. For protecting environment and human health electromagnetic pollution should be reduced.

Audio and sound equipment widely used in many places and objects. Sound quality is the most important consideration in audio equipment. There are many producers (e.g. Marshall, Fender, ...) [1] make audio equipments and systems that can be provide clear professional sound. As a result of high quality cost increase proportionally. Therefore, only way to get a good sound is to pay essential sum because when sound is getting more quality, components have to be more quality.

Thus, problem of highest sound quality providing together with cost decreasing is very urgent for many people worldwide with appropriate standards.

Amplifiers are one of the main devices among specialized sound audio components. Tube amplifiers are most common amplifiers for professionals and sound experts. Tube amplifier’s cost mostly consists of the cost of output transformer and quality of tubes that is usually the factor which limits sound quality when used.

Audio transformer is main part of tube amplifier which generates electromagnetic field. That’s why goal of this project is develop output transformer for single ended tube amplifiers which defined and reduced magnetic fields to reduce negative effects of magnetic fields to environment and human health with adequate sound quality. Therefore these standards - IEEE C95.6-2002, National standard of Ukraine 4210-2003 and IEEE C95.3.1-2010 for electromagnetic fields and human exposure to these fields is very important for human health and we used them in our project.

STRUCTURE OF TUBE AMPLIFIER

Tube amplifier consists of 4 main parts (fig. 1): 1. Power System – generate stable DC voltage (250V) and AC heater voltage for tube lamps (6,3 V); 2. Preamplifier consists of tube 6n1p and connects with audio input; 3. Output amplifier consists of output tube 6p1p and output transformer (fig. 2); 4. Audio speakers.

Fig. 1. Block scheme of tube amplifier Fig. 2. Circuit of tube amplifier device

Output transformer is important part of amplifier, which requires the calculation of parameters. Creation of an amplifier is important for the experimental part of the project.

CALCULATION OF OUTPUT AUDIO TRANSFORMATOR An output transformer for single ended tube amplifier is a device which matches high input

impedance (impedance of tube) to low output impedance of speakers (4, 8 or 16 Ohm). For designing an output transformer we need to define output power according to tubes (6p1p Russian tetrode produce 3.5W), input and output impedances. Input impedances change according to tube characteristics. 6p1p Russian tetrode tube has 5 KOhm impedance. Output power selected as the 3.5 W.

Ratio between impedances will give us coefficient of transformer. Ratio of impedances calculated following with formula:

n=(Zp/Zs)1/2

In formula above where Zp and Zs are primary and secondary impedances of transformers. We selected input impedance as 4096 Ohm as recommending impedance for tube lamp 6p1p. Impedance of audio speakers is 4 Ohm.

According to formula above, it is possible to calculate turn numbers of windings. These turn numbers proportional with input and output impedances ratio:

n=Np/Ns=(Zp/Zs)1/2

Where Np and Ns denote primary and secondary turn numbers. n=(4096/4)1/2= 32;

After that it is possible to calculate equivalent impedance of transformer, (that we already knew tube impedance is equal 5 KOhm and Zp is input impedance equal to 4096 Ohm)

Req = (Rtube*Zp)/(Rtube+Zp), So: Req = 2251 Ohm

According to this information we can calculate inductance: L = Req/2*pi*f1

Where pi – 3,1415…, f1 – lower frequency of amplifier. Output transformers generally designed in between 20 Hz and 20 kHz (as a human ear can hear). We

chose f1 as 40 Hz. In experiments transformer observed, in between frequency ranges 40 Hz-18 kHz transformer works in best performance.

After calculation: L = 9 H And then it is possible to calculate core size according to power level [2] used as a reference for

picking core of transformer. For this project preferred to use EI shape laminated iron core transformer because this kind of transformers is mostly using for single ended tube amplifiers output stage (fig. 3). Also ‘EI’ type cores give better output characteristics than ‘toroidal’ type for single ended tube amplifier, with easier winding methods.

Fig. 3. EI type of transformer core

According to output power (3.5 W) size of transformer has been chosen as EI 16*24 with 3.81 cm2 middle leg (or tongue) area [2].

After that, the number of windings was counted [3]: w1=1200*(L)1/2=3600;

w2=w1/n=112,5; The type and thickness of the wire: w1 – 0,125 mm, ПЭВ-1 (enamelled copper wire, type 1); w2 – 0,5 mm, ПЭВ-1 (enamelled copper wire, type 1). Audio output transformer is shown in fig. 4.

Fig. 4. Created output audio output transformer

Between part “E” and part “I” paper insulation is used.

DESCRIPTION OF STANDARDS For this project we used IEEE C95.3.1-2010, National standard of Ukraine 4210-2003 and C95.6-2002

standards. IEEE Standard C95.3.1-2010 [5]. First of standards as we mentioned in our abstract, C95.3.1-2010

IEEE Recommended Practice for Measurements and Computations of Electric, Magnetic, and Electromagnetic Fields with Respect to Human Exposure to Such Fields, 0 Hz to 100 kHz:

Scope of standard : This recommended practice describes 1) methods for measuring external electric and magnetic fields and contact currents to which persons may be exposed, 2) instrument characteristics and the methods for calibrating such instruments, and 3) methods for computation and the measurement of the resulting fields and currents that are induced in bodies of humans exposed to these fields. This recommended practice is applicable over the frequency range of 0 Hz to 100 kHz.

Purpose: The purpose of this recommended practice is to describe preferred measurement techniques and computational methods that can be used to ascertain compliance with contemporary standards for human exposure to electric and magnetic fields in the frequency range of 0 Hz to 100 kHz such as IEEE Std C95.1™-2005 [B55], IEEE Std C95.6™-2002 [B58], and similar standards.1 This document is intended primarily for use by engineers, biophysicists, and other specialists who are familiar with basic electromagnetic (EM) field theory and practice, and the potential hazards associated with exposure to EM fields. It will also be useful to bioeffects researchers, instrument developers and manufacturers, those

developing calibration systems and standards, and individuals involved in critical hazard assessments or surveys.

We used device (ELF Sensor [4]) corresponds by standard. As this standard includes measurement techniques, we measured magnetic fields with magnetic field meter.

National standard of Ukraine 4210-2003 [6]. This standard recommends provide measurements at distance not less 10 cm for frequency range from 45 Hz to 600 Hz. Temperature has to be between 15 ̊ C and 30 ̊ C. We used National standard of Ukraine 4210-2003, when measurements distance has been more than 10 cm.

IEEE Standard C95.6-2002 [7]. In our abstract we did not mention about C95.6-2002. But when search has begun, we needed to define exposure limits for measurements. And we decided to add standard C95.6-2002 which referenced as C95.3.1-2010. IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0–3 kHz:

From scope of Standard: This standard defines exposure levels to protect against adverse effects in humans from exposure to electric and magnetic fields at frequencies from 0–3 kHz. This standard was developed with respect to established mechanisms of biological effects in humans from electric and magnetic field exposures. It does not apply to exposures encountered during medical procedures. The defined exposure limits do not necessarily protect against interference of medical devices or problems involving metallic implants

Purpose of Standard: The IEEE has previously defined safety standards for human exposure to electromagnetic fields in the frequency regime from 3 kHz–300 GHz (IEEE [B46]). The purpose of this standard is to define exposure standards for the frequency regime 0–3 kHz. For pulsed or nonsinusoidal fields, it may be necessary to evaluate an acceptance criterion at frequencies outside this frequency regime as explained in 5.2.4.2.

Exposure limits and influence tables of magnetic fields are taken from Standard C95.6-2002. Low frequency measurement in between frequency range 45 Hz-600 Hz made with techniques which standard includes. Transformer analyzed in this frequency range. In this frequency range transformer’s magnetic field can be harmful to human brain and heart in appropriate distance. Transformer analyzed about these measurement.

Standards include limits of exposure (). The results from experimental records which made according to standard C95.3.1-2010 considered with exposure limits which C95.6-2002 includes.

Table 1.

Table 2.

According to tables above low frequency measurements determined. Table 1 includes frequency

ranges considered with Table 2 and found concrete magnetic field density.

MEASUREMENT OF ELECTROMAGNETIC FIELD We began with measurement of amplitude-frequency characteristics (fig. 5):

Fig. 5. Amplitude-frequency characteristics of audio output transformer

According to fig.5 we defined that in between frequency ranges 40 Hz-18 kHz transformer works in

best performance. We used ELF Sensor [4] to provide measurement of electromagnetic field. Test equipment consists of

following parts: 1. Sensor and Electronic Circuit; 2. Indicator. Parts are connected by cable (length = 0,9 m). Accuracy of device is 5% for measurement results and 5% for frequency range (45-600 Hz).

Measurements have been provided in three axis system (fig. 6). We began with measurement of magnetic induction of axis “X” (fig. 7). Distance increased from 15 mm to 200 mm between sensor and transformer.

Fig. 6. Three axis system for experiment

Fig. 7. “X” axis results of measurements

The same measurements have been provided in “Y” and “Z” axis (fig. 8-9).

Fig. 8. “Y” axis results of measurements

Fig. 9. “Z” axis results of measurements

According to Table 2 obtained measurement results didn’t exceed exposure limits (1 G = 0.0001 T).

CONCLUSION

An output transformer for single ended tube amplifier is designed according to IEEE standard C95.3.1-2010, National standard of Ukraine 4210-2003 and IEEE standard C95.6-2002. All standards are no charge.

The device represents a compromise between the choice of cost, quality and compliance standards. Chosen tube for output part of amplifier is Russian tubes (6p1p) with 3.5 Watt output power which been enough to create proper sound quality. Frequency range of transformer is between 40 Hz and 18 kHz.

Transformer’s magnetic field measurements were made used ELF Sensor device. This test equipment answers to IEEE standard C95.3.1-2010 and National standard of Ukraine 4210-2003.

Transformer analyzed in between 45 kHz-600 Hz in this measurements and exposure levels stayed in between exposure levels of brain and heart. Transformer designed not to exceed magnetic field exposure limits for brain and heart. Results of measurements induction level of magnetic fields answers to IEEE standard C95.6-2002 (Table 1 and Table 2).

REFERENCE

1. http://marshallamps.com/ 2. “Transformers-Native Apparatus of Radio Electronics” by I.N. Sidorov, S.V. Sikorniakov 3. Journal “Radio”, Russia, 1947 4. http://www.cylex-usa.com/company/expantest-incorporated-7893745.html 5. http://standards.ieee.org/findstds/standard/C95.3.1-2010.html 6. National standard of Ukraine 4210-2003.

Electromagnetic compatibility. Professional audio-, video- and audio-visual facilities and apparatus for controlling lighting facilities. (http://irbis-nbuv.gov.ua/cgi-bin/irbis_nbuv/cgiirbis_64.exe?Z21ID=&I21DBN=EC&P21DBN=EC&S21STN=1&S21REF=10&S21FMT=fullwebr&C21COM=S&S21CNR=20&S21P01=0&S21P02=0&S21P03=U=&S21COLORTERMS=0&S21STR=%D0%97940.1%20%D1%8F86)

7. http://standards.ieee.org/findstds/standard/C95.6-2002.html