[rajacat]

That'll still work for almost any amp. You could go with a 2:1 output trafo rather than a cap but good ones are expensive.

Ah, I now think that I understand what you're saying. The output caps will keep the voltage 1.8V to all the amps instead of the voltage being dropped when it's split. Presently, with no buffer (reservoir) , the voltage drops below the minimum to prime the Crown. Or is it that the caps just supply a reserve of current and don't necessarily maintain the V level?

[DaveC113]

The output cap only removes DC offset that might be on the output due to the halves of the bipolar power supply not being perfectly equal. By feeding multiple amps in parallel your voltage remains the same but the load impedance drops, requiring more current, which is what the buffer provides. It's likely the 1.8V output of your DAC would be reduced to 1.3V or so though, it would be better if your source's output voltage was a bit higher. If your DAC has an output cap you can replace it with a 2:1 trafo. It's likely the trafo would work better than an output cap and if I remember right your DAC has a tube output stage so it probably does have an output cap. This would likely be an audible improvement and would double the output voltage of your DAC, the only downside is cost. Lundahl and Jensen likely make suitable trafos...

If you do go with a preamp with gain 6 dB is really all you need, that doubles voltage. The issue is most tube pres using common small signal tubes like 6SN7 will have 20 dB gain. Some DHT power triodes like 45, 2A3, etc. might have gain closer to ideal... that would be pretty cool, but would end up being pretty expensive too... plus is you'd never have to worry about current drive. 

[tortugaranger]

In case anybody's still interested , I've decided to use a tube buffer/active with the Tortuga for volume control.  My system is going to be active with separate amps for the tweeter, midwoofer and multiple subs. It's  too complicated to use a passive pre. because it severely limits my choice of amps and sources. I guess you can divide the voltage only so many times. I fooled around with the speaker level cable, resistor implant to the Crown with mixed results and since this is for the critical midwoofer, not a sub, I'd like a more straightforward implementation.

One issue I'd like to make sure I get right is the buffer vs. active choice. Do I need an active unit because of  splitting the voltage to multiple amps? The buffer adds to the current capacity but does nothing for the voltage. Definitely don't want to build a buffer and then find out I still can't power my midwoofer (AE TD15m) with the Crown or similar amp. I hope that the positive attributes of the Tortuga won't get lost with the addition of an active buffer.
It's unfortunate but I'll probably have to rebuild my nice enclosure so that there will be enough room to house the pre + the Tortuga. Fortunately, I have enough redwood to build an expanded faceplate. I'll reuse the side pieces but the top, bottom and rear panel will have to be replaced.
Now I'll have to decide which line stage to use. Dave, the Aikido kits look interesting. I'm open to suggestions as to which kit would be the most suitable for my application. The 12Vac PCB and parts kit http://glass-ware.stores.yahoo.net/aikido12vac.html might be the simplest to implement and I could use my Astron power supply. Of course, if Tortuga comes up with a buffer I'd be interested in that too.

The key issue with trying to drive multiple amps off of a resistive passive preamp is that the combined effective impedance of all those devices connected in parallel becomes so low that it overwhelms the ability of the source to deliver the sonic goods. It's not a matter of spreading the source output voltage across multiple devices. Said differently, having a higher source output voltage will not solve this problem. All the amps see the same voltage coming out of the preamp.

The output stage of your source (any source) is there to deliver the audio signal. The audio signal is contained in the AC voltage waveform. Most source output stages are not designed to deliver much current, hence they need to "see" a large impedance (high resistance) downstream. We use the term impedance here because the audio voltage waveform is AC and not DC. If that downstream impedance gets too low, the current demand on the source output stage gets too high and the voltage may even begin to sag a little. Sonic quality suffers; flabby bass, lack of dynamics etc.

So yes, a buffer downstream of the passive takes care of this. All the source sees is the preamp/buffer impedance which will be plenty high enough. The source never "sees" the amps. The output stage of the buffer on the other hand should have a very low output impedance and be robust enough (high enough current drive) to deliver its output voltage into the combined impedance of the multiple amps which will still probably be on the low side. A high quality buffer will have these  characteristics.

Where I differ with DaveC113's view's on preamps and buffers is he's convinced of the sonic superiority of active preamps (attenuator with output buffer) in all cases whereas the evidence overwhelmingly affirms that our LDR preamps achieve excellent sonic performance without a buffer provided the impedance bridging ratio between amp and source is high enough and the source output stage is robust enough. We've found that in practice most systems are good to go with out LDR passive in this regard although there are rare exceptions.

Your example of driving multiple amps is very likely going to one of those exceptions where a buffer is indeed needed.

[rajacat]

Thanks to all for your advice.
Well.... buffer it is. I'm weak on theory but I do follow instructions with care so one of the buffer kits probably won't be beyond my capabilities. Time to get busy.

Here's the case I built that I'll have to rebuild to make room for the buffer.

ED. Actually, after a little thinking, I could just build a new box for the buffer and connect with very short interconnects. My other amp is connected to the tweeter and it works fine with the Tortuga so no buffer required.

[tortugaranger]

ED. Actually, after a little thinking, I could just build a new box for the buffer and connect with very short interconnects. My other amp is connected to the tweeter and it works fine with the Tortuga so no buffer required.

I like that solution!