Winding a toroidal output transformer for the Pleiades BD139 one transistor power amplifier

This is the second attempt. The first one failed as the core was small and became easily saturated by the DC quiescent current. It's diameter is about an inch.


This time a 50VA core is used. It's outer diameter is 3in. It is a power transformer made by Giatras. The 9mH (secondary for 9V) is about 100 turns.


Another nice thing about toroidal transformers is that auditioning while making changes can take place in almost real time as there is no time waste in every time removing EI laminations and then putting them back.


A balsa wood stick was made up which now carries about 100 turns of 0.25 diameter wire. This balsa wood is holding the wire to be wound so that it can be passed through the toroid hole.


While listening the secondary is connected to the 12Ω DEW Zeiss Ikon baffle speaker. The primary to be wound is connected to the collector circuit of the Pleiades BD139 one transistor amplifier operating in class A (electrons flowing all the time).


Out of fun and curiosity it was decided to listen to music as soon as the primary is start to be wound. This is done by stoping winding and connecting speaker and Pleiadss BD139 amplifier with small crocodile cable clips.


First audition is on current EI very big transformer. Sound is remarkably loud for just a 250mW? max output at operating point of 10V 50mA. Excellent quality. A bit or more distortion by overdriving the amp. Nice distortion by just one active device.

10 turns in toroid primary.
Sound loudness is very low. Distortion. But it sounds nice. Kind of smoothness too. Promising as as mentioned before the EI power transformer laminations may give a harsher sound.


At 20 turns. Better, smoother. Still distorted. Mid range coming in.


40 turns. Just a bit bass coming in. Louder sound. Distortion is somewhat less.


80 turns. Louder sound. A bit more bass. Inductance is 8mH. Distortion somewhat less.


Connecting the 12Ω speaker directly to collector. Much much better sound. Deep bass. Extended treble.


It is expected that so far (at 80 turns) sound leaves much to be desired. Reflected impedance is less than 12Ω. Inductance is still low almost short circuiting at bass frequency. Target is an inductance such as reflected (transformed) speaker impedance is say 70Ω.


More turns need to be wound. Will the big magnetic core prevent DC saturation?


What primary inductance would give the loudest and better sound? Impedance matching (of load) to transistor output impedance?


Continued next day:


Primary turns now 86 inductance of primary and secondary the same ie about 8mH indicating that the secondary is about 86 turns.


Primary turns now 100. Inductance is now 11.7mH. For first time with this core the 12Ω voice coil is transformed to a greater than 12Ω load as felt or seen by transistor. Listening to Carlos Jobim and Asturd Gilberto. Better sound. Great extended treble. Her bright recorded voice (by Phil Ramone at his A&R recording studios?) makes you cry. How would a similar to Neve microphone preamplifier sound with a BD139 instead of 2N3055, (at your risk). Bass is still deficient due to low primary inductance. Piano sounds like honky tong. Flute sounds amazingly bright and easy with euphoric or euphonic distortion. Connected also Tesla 4Ω speaker in Pleiades linoleum Auratone type 5C monitoring box. Much less loud due to low impedance, this speaker will need a higher turns ratio. Nice treble. This power transformer core looks very promising. On both speakers there is a velvety quality to the sound reminding good electron tube amplifiers.


Primary winding turns now 150. Inductance is about 25mH. Sound gets louder. Amazing treble. Bass coming stronger. Theoretically one octave down. Less Jon linear distortion. Piano sound gets body. Still more bass is needed.


More comments or thoughtσ: Starts getting loud. Nice or cute maximum power transfer theorem at play, less energy reflected back from speaker to amplifier. At the moment the transformer is 25mH:8mH so the 12Ω speaker is approximately transformed to 36Ω.


Now primary winding turns are 200. Primary inductance is 47mH. Louder sound. Clear. Great treble. Bass still could be better. 4Ω speaker sounds now with less nonlinear distortion. Although there sis still some deficiency in bass sound is so real as if there is no reproduction system playing. Sound becomes comparable to connecting the voice coil direct to collector and is louder with less non linear distortion. Better bass needed.


12Ω speaker was connected directly to collector circuit for comparison. Great sound but less volume. While listening the primary inductance was connected. It was expected to hear loss of bass. But for some reason the treble sparkle was a bit reduced.


Good news. Battery had gone flat to 7.5V. Quiescent current is just 40mA on above comment. A great world class sounding power amplifier in class A (electrons flowing all the time) is certainly possible operating with just a 9V battery in series with a fuse for safety. It can sound loud, dynamic, real like nature.


Further news, strange news but somewhat expected news: A fresh charged small 12V battery just came from the Solarex 19W solar panel. It is about 13.3V while amp operates. Quiescent current is 70mA and the sound is louder and shrill with no bass. Changing back to 7.3V bass and depth comes back. This indicates that core is saturating at 70mH at such number of turns. Other scenario is that at larger quiescent current the input impedance of transistor changes and Walkman CD player in series with variable resistor can't drive it properly ie bass in lost at input of transistor ie output of driving circuit.


A tap at 200 primary turns was created and then 50 more turns were wound. Ie total of 250 turns. Primary inductance is 80mH. Sound is worse especially no bass possibly confirming DC saturation. The much bigger EI core sounds much better and bug. A possible step is winding on a toroidal core of similar volume.