How to split pre out to connect power amp and sub

Hello Almarg,

Thanks for the reply !

Gosh ! Though I knew tube amps should never be run without speakers, I must say I didn't even think about this issue !! Damn..

Is it not enough to run the sub or is it not considered as a speaker ? If that's the case, how do you normally use the preout then ?

I'm even more confused now..

Yes, my sub has separate left and right channel inputs.

I'll look into the non-inductive resistor you mentionned.

Resistor :

I googled your advice and I came upon a picture of the resistor. I would never thought it was that complicated just to give more punch to the Shanling...

How do you plug these things ? Do you need wire and does it fit in just like that ?

Sorry if my questions sound dumb, but it's getting way above my head !

Is it not enough to run the sub or is it not considered as a speaker ? If that's the case, how do you normally use the preout then ?

What is important to avoid the possibility of damage that I described is that the speaker outputs of the MC-30 (which are the outputs of its power amplifier section) should be connected to a load that is in the range of typical speaker impedances, such as 8 ohms. Whether or not a sub or anything else is connected to the outputs of the preamp section of the MC-30 is not relevant to that issue.

I suppose it's possible that the MC-30 has some special provisions in its design to avoid the possibility of damage if its pre-outs are connected to a separate power amplifier and its own power amplifier outputs are left unconnected. But I definitely would not count on it without seeing some clear indication in some literature that that is the case. Add the resistors, and you'll have no problem!

As far as the sonic effects of sending the pre-out signals to both the sub and the separate power amp are concerned, unfortunately the MC-30 has a fairly high output impedance of between 1,000 and 1,300 ohms (1K to 1.3K), as indicated in this review:

http://stereophile.com/integratedamps/308shan/index4.html

But fortunately the Prima Luna Prologue Five has a very high input impedance of 100K, which is a negligible load in relation to the 1K output impedance of the pre-outs. The input impedance of the sub doesn't appear to be specified, looking at its description on the ScanDyna website, but I suspect that you'll be fine because the very high input impedance of the Prima Luna means that from an impedance standpoint the pre-outs are essentially just driving the sub.

You might want to email ScanDyna, though, and ask them what the sub's input impedance is. I would say that if the answer is 20K or more, you'll be fine, and even 10K might be acceptable sonically.

In view of the high output impedance of the MC-30, though, do try to minimize cable length and cable capacitance, or upper treble rolloff may result. That applies to both connections, the one to the sub and the one to the power amp. High capacitance on the interconnects to the sub will affect the signals to the main power amp just as much as high capacitance on the interconnects to the main power amp.

Based on a quick calculation I just did, I would say that if your interconnect cables are of average capacitance (say 30 to 50 picofarads per foot), try to keep the total length of the runs to the power amp and the sub to less than about 15 or 20 feet (for example, 6 feet to power amp, 10 feet to sub, for each channel). If the runs will be longer than that, get interconnect cables that have especially low capacitance, such as these from Blue Jeans:

http://www.bluejeanscable.com/store/audio/index.htm

Hope that helps,
-- Al

Re the resistor, this is the particular one I suggested, which is probably what you've just looked at:

http://www.radioshack.com/product/index.jsp?productId=2062288&tab=summary

The photo appears to not include the full length of the wires coming out of each end. Those wires are undoubtedly much longer than what is shown in the photo, which should allow you to simply connect the wire from one end directly to the red speaker terminal, and the wire from the other end directly to the black terminal (using one resistor for one channel and another resistor for the other channel).

Regards,
-- Al

Again, I must thank you very much for this detailled answer. You are a person of great advice, and though I'm really new to the field of Hifi, I managed to understand all of what you said (at least I think so..)

Cable lengh won't be a problem, I have kept it between 6 and 10 feet actually.

I've emailed Scandyna about the input impedance of the sub, I'll see what they say about it.

Fortunately, the shanling and the primaluna don't seem to be a mismatch, at least !!

I understand now that I can't take the risk of not connecting speakers (or a resistance) to the Shanling. But the fact that there is a pre-out still leaves me wondering. 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.. There might be a built in systeme as you say, but who would want to take the risk ?!