@toshko1
Hello, I found the thread and read it very carefully. Presently I am reverse engineering this type of block scheme of power amp
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Thanks for reading the thread carefully, however there is not enough information to calculate the transformer design.
Here is what I have found:
Damping factor for MC1200 and 2000 in 8 Ohms is about 40 which translates to 0.2 Ohm output resistance. In my experience with transformers, this translates to about 100 windings of 1.2 mm wire over a large cross section of E+I laminated steel. A little more for double C-core.
2 kW into 8 ohms is 126.5V and 15.8A
What is a large core? The double C results should not be any different than the EI. One needs to know the core area and lowest frequency to calculate the turns. The output impedance is due to many things, not just transformer winding resistance. Feedback makes it low.
MAC output devices are actually step up transformers. Why are they called autoformers is not clear to me.
An auto transformer has just one tapped winding where currents are shared. I wrote an explanation of how it is wound and how it differs from a conventional output transformer. Everything above the 2 ohm tap is step up in voltage. This allowed them to use low voltage transistors.
In the power supply 100V filtering capacitors are used, thus the rail voltage must be less than that voltage, lets say 90V or even less than +/- 80V for longevity. (typical commercial capacitors begin at 1200 hours at rated voltage and temperature and there are some rated at 13000 hours, but some people don’t turn off their gear which makes for a mere 77 weeks in the latter case... lowering the voltage and temperature multiplies the life multiple times)
126.5/80=1.575 is the transmission ratio of the transformer.
100/1.575
2000W and 80V translate to 3.2 Ohms load and 25 Amps current.
Note: output transformers are designed based on allot more complicated calculations.
This is very simple and vague calculation.
It is not unthinkable for the primary winding to be sub 2 ohms and the operating voltage of the supply rails to be under 50V with large currents.
The complication in the reverse engineering comes from the separate winding of the NFB loop, which loads both primary and secondary windings additionally and thus reducing their impedances.
The NFB winding does not load anything. It is there for stability under reactive loads.
From the photo of the autoformer in the McIntosh manual, the wires seem to be in the 0.8-1.2mm range. 1 mm wire can withstand 30 Amperes, which leads to primary resistance of about 2.2 Ohms.
Can I see this photo please?
In addition, aluminum or nickel wires are not unthinkable in regards of 22-30 meters of wire having 2-3 Ohm DCR.
I presume it is wound with copper, why anything else?
Why?
Everyone who have tried to lay down the design requirements of a powerful amplifier have come across the design limitations of high voltages.
Reducing the voltage allows for high-end audio grade components such as capacitors and transistors. At 140V there are no capacitors and a limited choice of transistors.
So the autoformer enables McIntosh to design a no compromise SS amp capable of large current due to the high transistor count and powerful power supply at the cost of some manageable complication.
P.S. Regarding the distortion, it is in octaves and in multiples of the base frequency, thus the 7th harmonic of 100 Hz is 14,800 Hz and the 7th harmonic of 1000 Hz is 148,000 Hz... That is why the H-Fi standard measures only to 3rd harmonic only up to 7 kHz.
