How to split pre out to connect power amp and sub

Thank Al, once again your explanation is crystal clear ! I'll keep you posted on how it all went..
Cheers,

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

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 ?

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

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 ?

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

Actually, Scandyna The Ball 2.1 input inpedance may be specified in this document : http://www.podspeakers.com/Files/Filer/specs/the_ball21.pdf

it reads :
L,R input sensitivity : Input @ 1kHz 450mVrms @ rate power output
Sub input sensitivity : Input @ 1kHz 270mVrms @ rate power output

Is it correct ?

About the resistance, do you get them in a normal Hifi shop ?

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 ?!

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

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

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 !

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.

Solst1ce -- As I indicated above, whether or not doing that results in sonic degradation would depend on whether the combined input impedances of the sub and the power amp (and the third device, if you choose to split three ways) remains much larger than the output impedance of the preamp.

Also, keep in mind that the cable capacitance which the preamp would have to drive is the sum of the capacitances of the two (or three) cables that would be connected to each channel. That could become a significant factor if preamp output impedance is high and cable capacitance per unit length is high (e.g., 100 picofarads per foot) and cable lengths are long (e.g. 20 feet each). So I would suggest using low capacitance cables (e.g., less than 40 or 50 pf/ft, especially if the run lengths are long). Otherwise, upper treble rolloff may result.

In your case, I suspect that you'll be ok in all of these respects. I found the following statement at the CJ website about the Premier 10:

The audio circuit of the Premier Ten consists of a single triode amplifier direct coupled to a triode cathode follower. The low output impedance of the cathode follower permits the use of the Premier Ten with highly capacitive amplifier interconnect cables without attenuation of the high frequency information.

BTW, to calculate the combined impedance of three loads in parallel, calculate the combined impedance of two of them as I described above (product divided by sum), then calculate the product divided by the sum of that result with the third impedance. Or, alternatively, the impedance of the parallel combination of three loads is the reciprocal of the sum of the reciprocals of the three input impedances.

Regards,
-- Al

I am going to be purchasing a preamp (CJ Premier 10) with one pair of outputs. If I have two active subwoofers that I can connect mono (one run to each), am I okay splitting the signal at the Premier 10 with y-adapters at each terminal, one in each pair (channel) going to amplifier, and the other going to a [powered] subwoofer? Will this result in a signal quality loss? Can I split again to make it a three way split? Thanks for your help!

A further thought: Since the Shanling has such a low power output, that would make it readily practicable to substitute a resistor for the speaker load which would no longer be connected to it. Doing so would completely eliminate the possibility of damage that I described!

Radio Shack part number 271-120, which is an 8 ohm 20 watt non-inductive resistor, costing $1.49 each according to an older catalog I have, would do the trick. Get two of these, and connect one between the Shanling's red and black left channel speaker terminals, and connect the other one between the red and black right channel speaker terminals.

Hope that helps,
-- Al

P.S. The comments in my previous post about the y-adapter and impedance matching are based on the assumption that your sub has separate left and right channel inputs, or alternatively that you have a separate sub for each channel. Upon re-reading your post I'm not sure that is the case -- please clarify and I'll answer further tomorrow, if necessary.

Regards,
-- Al

There is a more significant issue to what you are proposing to do, which is that a tube amp should NEVER be operated without a speaker load, at least while music is being played through it, or it is likely to suffer serious damage (such as a blown output transformer).

By running the Shanling's pre-outs into the Prima Luna power amp, with the speakers connected to the Prima Luna power amp, that is exactly what would happen to the Shanling. If you would like further technical explanation of that, let us know and I'll answer tomorrow if necessary -- I have to sign off shortly for the night.

Apart from that, the answer to your question is that you could split the pre-out's with a y-adapter and send those outputs into both the sub and the power amp, provided that the parallel combination of the input impedances of the sub and the power amp is substantially higher than the output impedance of the preamp outputs. An example of "substantially" would be a 200 ohm preamp output impedance working into a combined sub+power amp input impedance of say 10K (10,000 ohms) or more.

If the impedances don't meet those criteria, no damage will result (as far as that issue is concerned) but sound quality may suffer.

To calculate the combined input impedance of the sub and the power amp, divide the product of the two impedances by their sum. Example: If the sub's input impedance is 25,000 ohms (25K), and the power amp's input impedance is 50,000 ohms (50K), their parallel combination would be (25 x 50)/(25 + 50) = 16.67K.

But again, DON'T DO IT, or you will destroy the power amp section of the Shanling.

Regards,
-- Al