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I have a pro flash gun (Toshiba ES-38L) which is powered by 6 AA batteries arranged in series.
The voltage across the positive and negative terminals is therefore 9 volts.

It eats up batteries and I would like to use it with an AC/DC transformer.

I have such a transformer, a variable voltage output that includes a 9 volt setting.
It gives a REGULATED supply, it is rated 1400mAh.

If I used this unit to supply DC power across the terminals that the batteries connect to (assuming the batteries are out!) would it cause any long term damage to the unit?

(I have tried it already and it powered the unit fine and recharged more quick, although the sound it made was different to when it powers up using the AA batteries - the sound was less smooth).

It is OK to use a regulated power supply instead of batteries. That is, if it really is a regulated supply.
If it is, then the difference in noise you notice is probably due to the internal resistance of the batteries (resulting in a voltage drop when loaded).
If it is not, the output voltage of the regulator may be higher than what is set, and it may be chopped. In both cases you may eventually damage the flash gun.

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I have a pro flash gun (Toshiba ES-38L) which is powered by 6 AA batteries arranged in series.
The voltage across the positive and negative terminals is therefore 9 volts.

It eats up batteries and I would like to use it with an AC/DC transformer.

I have such a transformer, a variable voltage output that includes a 9 volt setting.
It gives a REGULATED supply, it is rated 1400mAh.

If I used this unit to supply DC power across the terminals that the batteries connect to (assuming the batteries are out!) would it cause any long term damage to the unit?

(I have tried it already and it powered the unit fine and recharged quicker, although the sound it made was different to when it powers up using the AA batteries - the sound was less smooth).

Hi.
Sorry to disturb you. I have the same version of your flash. I'm new with it and I'd
Like to know any news to help me in using it.
In particular It's nice if you know any website (links) about this flash and his
Thecnical notes. Thanks a lot

It is OK to use a regulated power supply instead of batteries. That is, if it really is a regulated supply.
If it is, then the difference in noise you notice is probably due to the internal resistance of the batteries (resulting in a voltage drop when loaded).
If it is not, the output voltage of the regulator may be higher than what is set, and it may be chopped. In both cases you may eventually damage the flash gun.

The AVAILABLE current is 1.4 amps. This does not mean that your power supply will force your flash to over-indulge and damage itself. The strobe will only use what it "needs" given it's input resistance.