Impedance matching transformer

[analog_sa]

Originally posted by nagard
..they work great in (after) my Atmasphere OTL' s and Paul Speltz is a nice guy...

An output transformer in an OTL is just hilarious. $450 for a pair of basically power supply quality transformers?

[coloradosound]

Paralled output stages would do a much better job for
the imp. ratio. Transformers are primarilly used for tube circuits
where there is a larger imp. ratio

[rhapsodee]

Ok, i rewired the sub today. Previously, i was driving each of the 2ohm voice coil (DVC sub) with one channel of the NAD.

Today, i series-ed the sub to make 4 ohm, and bridged the NAD. I then used a 240V/110V 200VA transformer to connect the sub to the bridged amp. 110V side to sub, 240V side to amp.

By my calculations, 240/110 = 2.182. Giving a impedance multiplication of 4.76. This presents a 19ohm load to the bridged NAD.

When i first hooked it up, i did a tap test of the woofer to see if it had better damping. One of my main gripes with the sub previously was boominess into the midbass frequencies, despite a 3rd low pass with 60hz corner freq. Anyway, it had greater control of the woofer, with a well damped thud.

When i played music, it was very soft!!! i turned up the sub output to max (+10dB) and it was still sounded like 10dB softer than the previous connection method.

I was stunned for a while, and obviously quite disappointed. Then it struck me that the small 200VA transformer had quite a bit of DC resistance. A quick check confirmed it: 10ohms. Is this the cause of the low output? How does it affect the power transfer between the amp and the sub?

1) Is the 19 ohm effective impedance after the matching transformer too high? As a result, the bridged NAD is not able to effectively drive the sub?

2) How does the 10ohm resistance in the transformer coils affect the whole equation? I'm thinking of getting a 500VA transformer, would it help? Will measure the DC resistance of the windings, how low should it be to be negligible? 1 ohm?

3) I'm in singapore, so the recommended transformers are not readily available. What would be my best option? Get a custom wound transformer? What would be the optimum turns ratio with the 4ohm series voice coil and the bridged amp?

4) Lastly, what is an F4? Sorry, for my ignorance.

Thanks for the time and comments!

I guess a picture speaks a thousand words...

[Nelson Pass]

You can find the F4 thread on this forum.

10 ohms sounds like a bit much for a 200 VA transformer.

[rhapsodee]

Ok, went to get a new transformer over the weekend.

Settled on a toroid for 50bucks, customised the turns ratio to 1.41, to give a impedance ratio of 2X.

So this gives a 8 0hm load from the 4 ohm series voice coils, and the amp is happily driving, with better control and less distortion.

Subjectively, there is much less boom in the bass, and it blends much better with my mains.

Thanks a lot for the help and advice, truly appreciate it!

Not sure of the VA rating, but it does not even feel warm after prolonged playing at high volumes. The amp runs cool too! Amazing!

Pic attached. Will prob install the toriodal trafo inside the box when i finally get round to building the proper transmission line box for the sub.

[rhapsodee]

Another pic..