My friend, who visited me last Sunday, asking for a pair of MC step up transformer for his turntable set. I start to search up and low for good quality permalloy core for this project. I wanted to wind another pair for my own system too.
I noticed that permalloy cores always sounds crisp and clear, but sounds thin and lack of body. I finally ended up with small M4 grade c core.
The winding method of MC step up transformers are totally different with output transformers. I need to calculate carefully and choose the most suitable material for this projects. I believe that paper bobbin and all-paper isolation will sounds far better than film type of isolation material too.
The impedance in mind will be around 30 ohm to 50 ohm of primary impedance, and the ratio will be 1:10. I use multiple parallel-series windings for primary to reduce leakage inductance. The DC resistance can be reduced to about 2.7 ohm. for the secondary, the DC resistance will ended up at around 350 ohm. The finished transformer measured 530mH primary, and 42H at secondary. The leakage that "see" from the primary is 0.044mH. The low frequency roll off will be 30/(6.28X0.53)= 9Hz. by calculation.
The copper wires are too thin for the auto winder so I wind it manually. I use copper foils to shield it at final windings for the best S/N ratio. The finished transformers are fixed in a small box with gold plated RCA sockets. Below are some photos for the MC transformer project.
Primary windings on my old manual winder.
Finished coils for one side
Response of 20Hz
Finished MC step up trans.
The frequency response, it measured 7Hz-48Khz at -0.2dB, and the frequency range from 20Hz to 20Khz will measured less than -0.05dB. I turn my signal generator all way up and found that it response up to several hundreds of Khz. The S/N ratio are minimum compare to normal transistors or tubes MC gain stage, I'm shielding it with thin shielding material from military equipments.
For the listening test, I test it with my old Technics SL-10 with original 310MC cartridge, The build-in 10X MC head amp sounds really BAD compare to step-up transformer.
I'm really amazed with its performance, especially within its mid range / vocal, its really sounds natural and lively. Good and tight bass and crisp highs too.
I also tested it in Gyro SE with Ortofon MC 30 Super II on it, with Michell DINO MM / MC Phono Preamplifier and sounds a lot of improvement. I think I will wind another few pairs at different ratios for my others MC cartridges.