Whether it is optimum or not is ultimately determined by your ears, your expectations, and your speakers (not listed). I agree it sounds like an adequate match on paper but there is much we do not know.
I would forego the pursuit of impedence matching devices and wait until you have done a lot of serious listening to the basic set up with your speakers. If it ain't optimum to you then consider if the sound's inadequacy is caused by an impedence mis-match or something else. It could be corrected by something as simple as a tube selection in the pre-amp (isn't that why we like tube amps/pre-amps? :-), and not need impedence matching devices to get you where you want to go.
Hi Foster -
In terms of impedance matching, the VTL 2.5 will work very well with the DNA-500. Subjective quality is something else entirely, but I suspect that you will be pleased with the result.
You'll need to ask VTL the worst case output impedance of the 2.5 preamp before you can say all is well. According to the the Stereophile measurements of the VTL 5.5:
while the unbalanced output impedance was 184 ohms across most of the audioband, rising to a still low 409 ohms at 20Hz.
There should be no concern of bass roll off if the 2.5's output impedance follows a curve similar to the 5.5.
My concern is the effect of impedance on HF or low end roll off. Newbee, my speakers are the Ohm Walsh 5000. I also have 2 powered subs that would be connected to the 2nd set of pre outs on the VTL so that plays into impedance as well.
Foster 9, To add to your burden - some pre-amps with 2 outputs have them configured so that both outputs cannot be used at the same time to drive two different amps at the same time. I have a pre-amp configured as such and when you add a 2d system you load down the other output and change the impedence relationship.
I have another pre-amp with two outputs which will correctly drive 2 amps etc at the same time with out loading each other down. With the former, I compensate by using one set of outputs and using a Y connector splitting the signal between the two amps. If you are experiencing a roll off when you are driving two amps, etc, off two outputs, check with the pre-amp manufacturer to see how the outputs are configured, don't make an assumpion that something that you can fix is a problem.
While I agree with Bob's comments, this falls under the "but there is other information we do not know", I wouldn't be overly concerned with the actual impedence curve unless you are trying to solve a problem. I have a pre-amp with 600 ohm output which sounds fine with an amp with 10K input even though the manufacturer recommends 20K, and conversely I have a pre-amp with a 3500ohm output which doesn't interface well even though the amp has a 47K input. "On paper" they both should be fine but the mismatch (or lack of a mismatch) shows up on listening. Also, SS amp output impedence curves matching speaker impedence curves are not usually a problem for a high quality SS amp unless you are driving a substantial sub 4ohm load at certain frequencies in the speaker, a problem usually solved by getting amps with high current (and possibly power out put as well depending on the speakers efficiency) and the concurrent ability to drive 2 ohm loads.
Emerson -- Apart from the issue of the powered subs, I think that your original question has been well answered (and we are very fortunate to have Steve participating here at Audiogon).
Re the powered subs, there are three key questions:
1)What is their input impedance?
2)Does the VTL 2.5 have separate buffer stages on the two sets of outputs, or are the two sets of jacks simply wired together inside the rear panel (which is probably how the majority of preamps with two sets of outputs provide them)?
3)How long will the interconnect cables be, and if you are intent on using specific interconnects, what is their capacitance?
Thanks Newbee and Al. Al,I'm using 20ft runs of Blue Jeans LC1 cables which I learned from you are low capacitance. I believe the input impedance on the sub amps are 6k ohms.
Forgot to say the subwoofer cables are Blue Jeans LC-1.
Emerson -- Yes, thanks for reminding me of our conversation about the Blue Jeans cables, which have an extremely low capacitance of 12.2 pf/ft. So I don't think you've got any problem in terms of capacitive loading. Even if you were to have three twenty foot cables loading the same preamp output, their combined capacitive impedance ("reactance") at 20KHz (the worst case frequency) would be around 11,000 ohms, according to my calculations.
However, if the subs really have a 6,000 ohm input impedance, and especially if the VTL 2.5 doesn't separately buffer the two outputs, you've definitely got a problem there.
If there are not separate buffers, the preamp would see a load impedance consisting of 6K, 6K, and 10K in parallel, which is about 2.3K (the reciprocal of the sum of the reciprocals of the three numbers). If there are separate buffers, and if you had one of them driving the two subs, that output would see a load of 3K. Or if you had one of the subs connected to the same output that drives the DNA-500, that output would see a load of 3.75K. All of these numbers are most likely too low to be acceptable.
Let us know the make and model number of the sub, and perhaps one of us will be able to find a definitive indication of its input impedance. And perhaps check with VTL to find out if the two outputs are separately buffered (or if convenient, remove a cover which would allow you to see if the two output jacks for each channel are simply wired together, or if they appear to be coming from separate circuit points).
Oops! I wasn't thinking too clearly when I wrote the last post. I assume the two subs are on separate channels, so if the two sets of preamp outputs are not separately buffered, they would see 6K in parallel with 10K, or 3.75K on each channel. Still too low for comfort.
If there are separate buffers, each channel of the pair of outputs driving the subs would see 6K, which may also be too low if the preamp output impedance rises significantly above the specified 200 ohms at low frequencies, which it probably does.
Subs are Earthquake Supervova Mk12IV. I'll see what I can learn on the preamp outputs.
I did some looking, but couldn't find an input impedance spec on the low level inputs of the MKIV's. I think I did see a reference to the MKV's being 6K. So all I can suggest at this point would be to contact Earthquake Sound and ask them, and also to ask VTL if they can tell you what the preamp's output impedance is at the frequency for which it is highest.
Al, Thanks for everything. I'm now using an ATI MM100 Match Maker between amp and preamp as a work-around of impedance issues and the added subwoofer's impedance. It sounds good so far in the signal chain but I haven't turned on the subs yet.
Emerson -- I'm not sure that is a good solution. According to the MatchMaker specs shown here
, the output impedance of its unbalanced outputs is 1500 ohms!
The output impedance of its balanced outputs is very low, but it looks like the transformer it uses to generate the balanced outputs is optimized for a professional-type low impedance load ("600/150 ohms"), and it may not perform at its best if you were to run it into the DNA-500's balanced inputs, which I presume are much higher impedance.
Also, the unbalanced input impedance of the MatchMaker is 10K, no higher than (and in fact the same as) the DNA-500's!
Sorry to be the bearer of bad tidings!
Hello Foster_9, I have a problem with impedance between preamp and amp and was wondering how the ati mm100 worked for you. Danny
Acman3, the ATI is no longer in the chain. My gear is still settling in and first I want to hear where things are headed. I'll try to let you know.