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First of all, all's well that ends well - nothing is permanently damaged. Here's what happened:
I'm playing with a tube amp now, and up to a moderately high volume, it sounds wonderful - beats my resident, and very good, SS amp in 3D soundstage, and especially naturalness of human voice. The problem is that "moderately high" is not how I normally listen :-). At my preferred levels, on tracks with prominent bass, things fall apart - the distortion is readily audible.
No problem, I thought, because the tube amp was aquired with the intention of trying it in passive bi-amping configuration, in part inspired by review linked below, where reviewer bi-amped the speakers that are direct predecessors of mine. Of course, I'll need some means - high-quality volume pot, for instance - of matching the output voltage at speaker terminals at the crossover frequency (225 Hz). Not having those means right now, and the volume subjectively very close between the two amps at the same position of VC on the preamp, I decided to give it a try anyway. After all, there's protection circuitry in tube amp, and the SS one is chock-full of fuses - what could possibly go wrong?
Long story short - it worked, and sounded very good, with slight overabundance of lower frequencies (so, SS amp must be a bit louder). I listened several times during the day, leaving everything powered. Then, in the evening, decided to take the tube amp out of picture, switch speaker cables to SS amp, and compare to its sound standalone. Unlike the power-on sequence, where I turned tube amp ON first, waited for it to actually start (soft start - couple of minutes), and then turned ON SS amp, I turned OFF the tube amp first, then SS. After switching speaker cables to SS amp, and turning it on, there was no signal through the speakers - and investigation showed that it blew the 10A rail fuse. There was nothing audible (pop or anything) through the speakers when tube amp was turned off.
Now - obviously, the first recommendation is "don't do that anymore". Second is to reverse power-off sequence. However, I would like to understand what happened, and whether the idea of biamping in such fashion is viable at all.
Thoughts?
Follow Ups:
First of all passive (I assume you mean speaker-level) biamping is not really biamping. What you are doing is replacing the crossover with two parallel filters, a HP and LP. The signal rejected by the HP filter is no longer "crossover-ed" to the woofer and similarly for the LP and tweeter. In other words roughly 1/2 the signal is "wasted" to ground.
Secondly, it is critical to match the gain (not sensitivity) of your amps otherwise the balance between the two amps will change with every alternation of the preamp attenuation.
Next, biamping is a little more complicated than it might seem at first glance and matching amplifiers is not as easy as just grabbing two amps that happen to be available. While you can do biamp horizontally with two different amps, it is preferable to vertically biamp with identical amps so there is integration of the woofer and tweeter.
Bottom line what you want is line-level biamping (whether passive or active). Take a look at Rod Elliott's discussion .
I am not going to speculate on why your amp crapped out. Whatever you did, don't do it again :). From my own experience, I would suspect accidental shorting of the cables although you seem to have taken the proper precautions.
I've been going thru this same discussion over on the Emotiva Boards. With little traction.
They still 'insist' that splitting your crossover, each half fed by its own amp IS biamping. I keep pushing for either a passive OR active line-level crossover.
Lots of misinformation out there about this issue. Even the nose-bleeders are not attracted to the potential for an additional 3db of headroom possible with line level biamping.
And trying to explain the simplicity of the Passive version is pretty much a waste of time, even when comparing to UPGRADING the speaker level crossover which potentially costs a small fortune.
I've been open with the downsides, too, meaning you need to know input and output impedance ad simply can't run LONG leads from the crossover to the amp. That you can fit the crossover for each channel in an Altoids Tin doesn't seem to register.
Too much is never enough
... resulting in oscillation in one of them - as alluded to in one of the responses below.
In a case of one person who had the same issue, reversing the power-off sequence helped - but the proper solution seems to be isolating pre outs, using high quality transformers.
With a crossover of 225hz, you need MORE power ABOVE then BELOW if you intend to run out of gas about the SAME time.
Also, unless you are using a LINE LEVEL crossover, you are NOT gaining any power. BOTH amps are STILL required to reproduce a full bandwidth signal.
A line level crossover, either of the Passive or Active type will net you as much as 3db additional power.
Your example with nearly 8db of output difference between the 2 amps AND the crossover near the theoretical 50:50 point of 350hz, means the tube amp SHOULD run to cllipping first. Way First.
I LIKE the idea of disconnecting AC before doing cable swaps. Real workmanlike.
You MIGHT have, as someone suggested, simply run the fuse near death. A 25 watt amp trying to keep up with that Parasound is a losing proposition and I'm sure you ran the tube amp near redline much longer than you suspect. With nearly ZERO headroom.
Too much is never enough
The only things in common are make/model of the speakers (well, more or less - mine are newer iteration), and the fact that SS amp is on bass, and tube - on mids/highs.Also, the fuse blew up in SS amp, and I'm pretty sure it was coincidental with turning the tube amp off.
PS: Why it actually works:
Although, as you said, in the absence of a line-level X-over the amp still sees the full-range signal, it's not tasked with actually reproducing all of it - into impedance it's not comfortable with. If you look at the impedance curve of the speakers at the link in my OP, you will see that an amp must work much harder, to reproduce bass.
As long as you don't approach clipping with tube amp driving mids/highs - it should work, and I can confirm that it does. There's no distortion audible at very high volume, unlike the tube amp driving speakers full-range.
Edits: 01/13/15 01/13/15
Because the toobs clip most audibly in the Bass, the top and mid will just get dull and soft until you hit 20% thd, the SS does the opposite , exercising it's distress in the mid/high frequency. Very unlikely you are not clipping that 25watt/channel amp , even if you are enjoying it, use a 100 watt/ch one and see.As to original question:
When you powered down your tooby amp, you may have caused your SS amp to oscillate, popping the fuse, hence no pop from woofer that you noticed ..
Regards...
Edits: 01/13/15 01/13/15
Maybe the fuse was just on the ragged edge all along. One of my tube pres was sounding less than usual until it went out. The main fuse had blown. I swapped it out and it sounds fine again. I then remembered not leaving it in standby until it had first warmed up a couple of times.
I'd suggest you disconnect the inputs and speaker connections. Swap all fuses and power it back up. If it holds, shut it off, reconnect things and fire it all back up.
Do remember, the tweeter amp is always last on, and first off. Even though you are running with the internal x-over, it is a good practice.
I can't think of what might have happened. I've had a passive horizontal bi-amp for several years. My passive linestage is a Placette resistor ladder and I turn the amps on first and have no issues. I'm glad when it did work it sounded pretty good. My two amps have the same input impedance, sensitivity AND gain so are an ideal match. If you have much difference between the two one will be louder and that one will need inline attenuation after your linestage.
Kathryn made some good points too.
E
T
You have three boxes, correct? I would turn on the preamplifier, then mute it, turn on the SS power amp, then turn on the tube power amp. For off, mute pre, turn off both tube amp and SS amp, then preamplifier.
With no gain control, there is not much to do other than change the input sensitivity of the SS amp; you need to have technical skills to do that. Remember the older SS McIntosh, with the sensitivity selector? That is just a bit of added resistance to go from .5 volt to 2 volts, ohms law here. Any tech can do that and for less than adding a pot. You have to know the input sensitivity of both amplifiers.
The other problem, it is hard to say? Some speakers leave a change and you disconnect the cable and it might happen? You may have had some power in the caps somewhere? The best way is to shut it down overnight before changing anything. I remove the AC power before I take off any cable, just to avoid problems.
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