K&K Audio / Lundahl Transformers

RE: use of stepup in 2 stage DHT amp

93.218.42.96

In Reply to: RE: use of stepup in 2 stage DHT amp posted by KevinC on January 28, 2010 at 19:28:40

Hi!

I share Kevin's view on this.

The 7903 is an awesome transformer, but it requires a preamp which can drive it. Even if it's unloaded on the secondary, it's inductance needs to be driven. The reflected miller cap of the 801 shouldn't be a problem though.

best solution is a preamp with a Zout of 600 Ohms or less. But it should really be that impedance and not a theoretical number, limited by the size of the output coupling cap. Ideally the preamp is transformer coupled.

This can be eased a bit if the 7903 is not wired 1:8, but 1:4

Also important: the preamp should have zero DC offset at it's output.

You can check if your preamp can handle the 7903 if you load it with a 600 Ohm resistor. Check the freq. response. Output will drop, but if freq response is ok, it can drive the 7903 too.

A power amp like this deserves a decent preamp, othersie it is better to design a different input stage, either 3 stage or a IDHT driver with a higher mu.

You can contact me if you have any further questions

Best regards

Thomas

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