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Old 18th September 2011, 04:05 PM   #1
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Default output transformer power rating

Is there a way to know how much power can be put through a particular output transformer? I know that size and weight are good indicators. But is there a formula or a test that can be used to rate a particular transformer's power rating?

Thanks for any help you could provide.

Last edited by mr2racer; 18th September 2011 at 04:08 PM. Reason: spelling
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Old 18th September 2011, 06:36 PM   #2
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I presume you imply push-pull.
Becareful with the weight reasoning: large core area can imply extended LF range for a given size winding wire and modest power, the original 20W Mullard design stipulated a large core 50x150x125mm which bears little relationship with power throughput but can handle flat response below 10Hz. Generally, a High inductance implies good LF response for a given Bmax (excitation flux), however can also be achieved with large AE (core area) and /or lesser number of turns; this last bit is ideal for the HF response and the designers biggest headache is maintaining the upper response with minimum losses.
Taking the Mullard size example, an output transformer with LF cutoff at 40Hz for the same power would be designed perhaps 2/3rd smaller for the same power throughput.

A specifically sized output transformer designed for 20Hz full power cutoff would be half the size at 40Hz for the same power. It makes sense that E guitar string is around 39Hz and designing the transformer to this frequency makes everything more portable. YOu wanna to go to a lower frequency using tha same transformer, then reduce the power vs frequency to fit the classic formula:
Bmax = Vin/(4.44xAexFxN) If one knows the original design Freq, with both Ae & N fixed, simply substitute the classic equation to find F for a given core size. Ae as e-4 metre sqr'd.

In the pic, both two p-p output transformers from the same manufacturer have windings rated for the same power, both 150W. However the larger one has cutoff designed at 15Hz and the other 20Hz. The copper wire OD on the larger core (hence larger bobbin) has a larger diameter wire OD to reduce the DC resistance which is desirable to keep THD and winding heating losses down. Problem: the larger core with more copper area has a higher leakage inductance & capacitance hence, the smaller core can often out-perform on the HF response; 75Khz versus 50Khz. This the reason that output transformers are critical in design and a pain to optimise as they get bigger.
One will never get far with output transformer dynamics without stretching one's knowledge of physics and importantly magnetics.


richy
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Old 18th September 2011, 06:45 PM   #3
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Assuming push-pull. not single-ended.

First, you need to determine minimum cross-section area based on lowest frequency (almost always taken 20 Hz) and output power required. This is quite simple.

Flux Density in Gauss B = 10^8 * sqrt(Power * Primary Impedance) / (4.44 * Area * Stacking Factor * Primary # of Turns * Lowest Frequency)

Assuming maximum number of primary turns 3500, lowest frequency 20 Hz, stacking factor 0.92 and maximum flux density B = 12500G and B = 14000G for EI and double C core respectively, we will get:

Area in sm^2 = 10^8 * sqrt(Power * Primary Impedance) / (B * 4.44 * 0.92 * 3500 * 20)

Thus, for example, transformer 50W, 5000 Ohm primary impedance, you will need at least 13.99 sm^2 (EI core) or 12.49 sm^2 (double C core). However, make sure that core have enough window size to accommodate required turns of both primary and secondary.

Last edited by LinuksGuru; 18th September 2011 at 06:51 PM.
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Old 18th September 2011, 09:57 PM   #4
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Cool Awesome what we learn here! The lowest note on a standard tuned giitar is about 80Hz, (not 40). Which explains why the OT on Fender's 40W guitar amp is smaller than Hammond's 1620, a 20watt HiFi OT. The inductance on a Deluxe OT was only 4H, and it is used with 6L6, the turns ratio is 4k:8. Even so it sound great for it's intended use.
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Old 18th September 2011, 11:18 PM   #5
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Post #3 by LinuksGuru edited to correct core area values cited as example with information he subsequently provided.
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Old 19th September 2011, 03:03 AM   #6
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Quote:
Originally Posted by SemperFi View Post
Cool Awesome what we learn here! The lowest note on a standard tuned giitar is about 80Hz, (not 40). Which explains why the OT on Fender's 40W guitar amp is smaller than Hammond's 1620, a 20watt HiFi OT. The inductance on a Deluxe OT was only 4H, and it is used with 6L6, the turns ratio is 4k:8. Even so it sound great for it's intended use.
if you look at dynaco transformer spec sheets, you will find that there are several power ratings, always with a bandwidth attached to power spec, please look at the third row:

Click the image to open in full size.
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Old 19th September 2011, 07:11 AM   #7
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Good physics stuff this is:

Note:-Remember these aren't mains transformers so for min thd for HiFi the max core flux (Bmax) is much lower for the reasons outlined below.

Intermodulation and harmonic distortion results from the non-linearity between the magnetising current and the magnetic field in the transformer. Everyone who meddles with the physics should know that. These curve dysfunctions can be reduced by keeping
Bmax below a certain value around 7000G (0.7T) for output transformers using standard transformer sheet. So that's the reason one should NOT compare mains and output transformers, like for like sizes !


On my 4string You'll find me playing E at 41 + odd Hz and in the past all MI transformer designers calculated at that frequency.

richy

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Old 19th September 2011, 08:55 AM   #8
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Quote:
Originally Posted by richwalters View Post
Good physics stuff this is:


On my 4string You'll find me playing E at 41 + odd Hz and in the past all MI transformer designers calculated at that frequency.

richy
Ah, you were talking 'bout bass-guitar, I thought you meant guitar-guitar.

I think MI OTs for guitar-guitar calculate around 80Hz as the lowest usable frequency, at least from just looking at the size of a lot of 40-50watt OPT on guitar amps.
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Old 19th September 2011, 09:16 AM   #9
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Quote:
Originally Posted by richwalters View Post
Intermodulation and harmonic distortion results from the non-linearity between the magnetising current and the magnetic field in the transformer. Everyone who meddles with the physics should know that. These curve dysfunctions can be reduced by keeping
Bmax below a certain value around 7000G (0.7T) for output transformers using standard transformer sheet. So that's the reason one should NOT compare mains and output transformers, like for like sizes !
7000G is a way too low for CRGO/GOSS, 12500-:-14000 G will work fine with modern M4. Low or medium power (~25W) transformer with low flux is more or less OK, but 60-80W unit will be, simply put, too big, and suffer from another problems like high stray capacitance, or poor performance at low signal level.

Rich is right not to compare size of main 127/220V and output transformers, without precise math it doesn't make any sense.

Apart from minimum cross-section area, core must have window large enough to accommodate windings with much higher number of turns then those usually found on power line units.


PS. Once I have seen monster 6W output transformer weighting 40Kg (88 lbs) made in Russia, but IMHO this is some kind of schizophrenic design.
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