Step Up & Step Down Transformers



This is the design methodology of 100V line step down transformer. Calculations will be made to investigate relationship of maximum power handling of transformer’s maximum number of speakers parallelability and etc.



Take example: 100watts amplifier into 8ohms output. From the amplifier the output current is 3.54A and the output voltage is 28.28V. The required step up voltage is 100V. Therefore the number of primary to number of secondary turns ratio is . This is the step up transformer.


then                  Maximum power allowed into each speaker.                            

Wattage            20w                 15w                 10w                 5w                   20w

Ohms               8W                   8W                   8W                   8W                   8W

Current                   1.58A                      1.37A                      1.12A                      0.79A                      1.58A

Voltage                  12.65V                    10.95V                    8.94V                      6.32V                      12.65V



The 100V supply line sees individual speaker impedance as:

            500W               667W               1000W             2000W             500W

The amplifier sees individual speaker impedance with transformer as:

            40W                 53.3W              80W                 160W               40W

Number of speaker to parallel when 100w amp divides by max power for each speaker:

                        5_speaker        6.7_speaker     10_speaker      20_speaker      10 instead 5

Therefore when maximum number of speaker is parallel, the amplifier sees an impedance of:

                        8W                   8W                   8W                   8W                   4W wrong


This is matching transformers. The required turns ratio of the transformer is found by Vs/Vp.


Problem Solving

On the job for sensored, sensored sensored sensored on 7 April 2000 both primary & secondary stage was parallel to be 5watts as instructed by sensored sensored from sensored. But is it correct?


When coils are paralleled all number of turns multiply by two and then when making a ratio count it will get back to the same figure before parallel. What happens then? What effect does it have?


When coils are paralleled, the total impedance is reduced, for this case the coils value is same on primary and secondary respectively therefore the impedance is halved. When impedance halves, current doubles, voltage always remains constant.


Since value of turns ratio Vs/Vp is constant together with voltage. We can work out the other values by using the doubled current value from above. Take example from 5 watts working backwards from Vs/Vp on next page.


                        Original            Parallel Series  


Voltage             6.32V              6.32V              6.32V

Current             0.79A              1.58A              0.395



Ohms               8W                   4W                   16W

Wattage            5w                   10w                2.5w


New Findings

The step down transformer is now actually for a 4W 10w playback scenario and not for 8W 5w playback scenario.


One would think if it is connected to an 8W speaker, the DEMAND current will halve back to 0.79A and hence playback power reduces to 5watts as required.


For the series coil, the step down transformer is now for a 16W 2.5w speaker. Again if an 8W speaker is used, then the DEMAND current doubles back to 0.79A and hence gives 5watts. Either way is correct.


Correct Usage by Current Matching

It does not matter which way it is used, the only difference is the voltage and current capability of the coil must be designed to be able to withstand the correct current when used with the specified speaker.


It would not be good to manufacture the coil 6.32V @ 0.395A and use a 8W or 4W speaker because the DEMAND current over exceed it’s capabilities. Ideally the 6.32V @ 1.58A should be manufacture to save guard against any possibility of error, but at a higher cost.


When using series method the current is limited and hence this is the choice to use on a high power transformer to reduce to low power. Other wise for a low power transformer to be high power it needs to be parallel.


Problem Solving continue

For the job sensored, sensored sensored sensored on 7 April 2000, all the transformers are parallel. This is wrong for it needs to reduce it’s power handling and not increase it.


Previously a same transformer paralleling was done for a customer and that the SPL level were low. This contradicts to the above findings. The only other explanation that can be provided from these findings is that the transformer has a Vs/Vp turns ratio higher in Vp.


Meaning the transformers allows more speakers to be parallel at once. And if not enough speaker is used then the total impedance seen at the amplifier is high and not low enough and hence unable to DEMAND current from the amplifier and finally result in low level SPL.


Future Modification

Assuming dual coil transformer is to be used again. There are a number of combinations when wired differently. If only the exact turns ratio Vs/Vp is known, all possibilities can be estimated.


As of this moment it is assumed that the transformer should normally be wired single coil at both primary and secondary. That would be the normal usage. When higher power is needed then parallel the coils respectively. When lower power is needed then series the coils respectively.


If the primary are parallel or series, then Vs/Vp is assumed higher in Vp. This means more speakers can be paralleled. This assume only single secondary coil is used.


If the secondary are parallel or series, then Vs/Vp is assumed lower in Vp. This means fewer speakers can be paralleled. This assume only single primary coil is used.


Real Justification

By right the terms ”less or more speaker can be paralleled” is only correct when assuming one particular step up transformer is used. When other step up transformer is used, then the “less” will be either less “less” or more “less”, the figure moves. And the “more” will be either less “more” or more “more”, again the figure moves strongly related to step up transformer.


Because step up transformer are usually integrated with the supplied amplifier, it is always assumed to be correct. Hence all analysis should be based on the specified amplifier with step up transformer and changes where needed made to the step down transformer.



It is now confirmed that when a step down transformer is designed for A watts. Then it will work with any B number of speaker unlimited. The criteria now is:

(1) The power amplifier must be able to deliver A multiply by B wattage.

(2) The step up transformer is critical in this respect and must not be wrong.

(3) The step down transformer must have the correct current values to be able to deliver the current for a 4W speaker to safe guard against overloading.




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