Impedance matching.

[Skeith]

Ive built a push pull 6V6 amplifier for 15watt output power. My problem is the audio impedance matching transformer. I cannot find one sutable, and dont have the money to spend on buying one. The transformer I need has to have a 8000-ohm primary impedance and 8-ohm secondary impedance. I was wondering, since this transformer is a 1000:1 ratio, couldent I find another one that has the same ratio (ex. 1000-ohm prim. 1-ohm sec) and use that? or would I risk over powering the tubes and burning out the amp?

[Staigen]

Hi there Skeith, your transformers voltage ratio is only slight over 31 to 1. You can try to use a 230V mains transformer, one that have a CT  for 115 V. There is also transformer with 2 primaries, each 115V. The secondary shall be about 7 Volt( 6 Volt vill do). A transformer with a rating of 25 to 30 Watt should be used. You can also wind your own OPT ( OutPut Transformer). Good Luck :)!

//Staigen

[binkabir]

hi i'm binkabir, i think we can makes thing better by use a common collect configuration of a transistor insteaded  of the bulky transformer, all you need to do is to find the transistor that  can work with that current and voltage at the output , this is easier cheaper and less bulky.  ;D

[MP]

Ive built a push pull 6V6 amplifier for 15watt output power. My problem is the audio impedance matching transformer. I cannot find one sutable, and dont have the money to spend on buying one. The transformer I need has to have a 8000-ohm primary impedance and 8-ohm secondary impedance. I was wondering, since this transformer is a 1000:1 ratio, couldent I find another one that has the same ratio (ex. 1000-ohm prim. 1-ohm sec) and use that? or would I risk over powering the tubes and burning out the amp?

Your output must be 8 ohms for the speaker load, thus you will need to use the same or very close.

MP

[audioguru]

A 1000:1 impedance ratio of a transformer designed for a lower impedance would reflect the 8 ohms of the speaker as 8000 ohms to the tubes at low frequencies very well, but would probably be a poor match at high frequencies because it is not designed for the low capacitance which is needed at high impedance at high frequencies.
Likewise, a transformer designed for a higher impedance would reflect the impedance of the speaker as 8000 ohms to the tubes, but would probably have a high loss because the resistance of its windings would be too high and its core might saturate by the increased current.

Transformers don't have an impedance, they reflect an impedance. ;D

[Kevin Weddle]

There is an easy way to remember how to calculate the reflected impedance. I know it's the turns ratio squared times RL but I can never remember if it's the primary over the secondary or the other way around. If you step down the voltage on the load, you decrease the current. Whenever you decrease the current you are increasing the resistance. Now you know that your reflected impedance will be higher than RL. Just put the numbers into the equation so that you get a higher impedance.

Be careful though with this because when you step down the voltage you are increasing the current when compared to the primary current. But whenever you reduce the voltage on the load you are lowering the load current.