How to split pre out to connect power amp and sub

Not sure if a typical hifi shop would have a suitable resistor or equivalent device. They commonly have accessories to convert speaker level outputs to line-level signals, but those are usually designed for use with solid state amps, and present a high impedance (rather than an 8 ohm or so impedance) to the amp output.

An electronic components distributor, such as digikey.com or mouser.com, would likely have something suitable.

The Scandyna specs you quoted do not indicate input impedance, just input sensitivity. An impedance rating would be specified in ohms (or "K," denoting thousands of ohms).

How do normal people like me do, who have no knowledge of physics ?? I mean, I read the manual, and it was clearly specified that the pre-out was to welcome a power amp if needed. How should I know that it will fry the Shanling if I really do so ! It's mad..

Good question! Reminds me of computers, about which I happen to be very expert. And sometimes when even I am struggling with a difficult problem, I wonder to myself how a typical non-technical computer user is supposed to be able to deal with such problems. :)

Again, it's possible that the design has some special provision to minimize the likelihood of damage, perhaps helped by the fact that the power output is so low, but I definitely would not want to take that chance. And attaching the resistors will be very simple, as long as you can obtain them in your area or by purchasing via the internet.

Good luck!
-- Al

Thanks Al, I'll come back to tell you how it goes, need to find these bloody resistances first ! :)

Just to make sure, 8 Ohms should do the trick or should I go higher like 12, 16 or so ?

You're welcome!

Probably anything between 4 and 12 would be fine. Not sure about 16. Too high is what can be harmful.

From this link:

The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, & acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer & short the windings. I would not go above double the rated load on any tap. & NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

It's almost never low impedance that kills an OT, it's too high an impedance. The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary & between both half- primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, & the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, & eventually erode the insulation to make a conductive path between layers. The sound goes south, & with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, & now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal.

Best,
-- Al

Hello Almarg,

If you ever pass by this thread again.. I've emailed Charisma Audio about wether I would need resistances or not, or rather what kind of resistances they would advise, and they said I should put :

5 watts resistors with value between 20-30 Ohms

which seems to me much different than what we talked about here :

10 watt ceramic audio power resistors around 8 Ohms

Do you know why they would advise such a high impedance ? Does it actually change anything ?

Actually the one I suggested was 20 watts, 8 ohms.

The wattage rating is the MAXIMUM power that the resistor is rated to handle. A margin of at least 2 to 1, and preferably more, should be allowed between the wattage rating of the resistor and the maximum amount of power that would be put into it. You have a 3W amplifier, but into 20 or 30 ohms (rather than 8 ohms) the amplifier would only be able to deliver considerably less than 3W, which is why they are suggesting only a 5W resistor.

But the more important question is whether to choose 8 ohms or 20 to 30 ohms. If you have reason to have confidence in their statement, then perhaps 20 to 30 ohms is ok, but keep in mind (as indicated in the long writeup I quoted) that too high a resistance is what can be harmful, not too low a resistance.

The advantage of using the higher value 20 to 30 ohm resistor (and it is a minor advantage at most, in this situation) would be that it will draw less power from the amp, and everything will run a little cooler.

Regards,
-- Al