Impedance matching device


I did some searching. There is no shortage of threads on impedance but I don’t find the answer to the following question.

Assuming you have an impedance mismatch between your pre and power amp is there a device which will correct or compensate for this? Can someone supply a link if this beast exists and doesn’t degrade the signal?

I just tried a new used amp (Sumo Andromida II) in my system and I immediately noticed how much more effective the volume control on my VTL 2.5 pre is than it was with my currently out for repair Aragon 8008ST. By the specs the VTL/Aragon should work but it is on the outer limit of what most consider acceptable impedance matching. VTL output impedance is 200 ohms. Aragon input impedance is about 23 K ohms.

It does work and I’ve lived with it for years but now that I’ve tried something else I am wondering if perhaps the performance of the Aragon was being affected by the borderline mismatch. I’m suspicious because I found the Sumo to sound as good if not better than the Aragon in my system.  I expected the opposite. I do realize there are many factors involved but this is a starting point.


energeezer
Hi there, you don't have a borderline mismatch or anything like that.  The VTL should drive an amp with 23 Kohm input impedance just fine.  Why do you think this is a problem?  What do you mean by much more effective volume control?  Perhaps it has something to do with the sensitivity of both the amps.
Yes, it’s not an impedance mismatch. Seems like a gain issue; do you know the sensitivity specs?
I agree with the others, that what you are describing sounds like a gain difference, not an impedance compatibility issue.

However, according to this datasheet the 2.5i version of the preamp has a specified output impedance of 200 ohms, the same value as you indicated for your 2.5, yet under the "Features" tab of this description it is stated that the 2.5i "drives amplifier loads down to 45kohm without audible bass roll-off." Which would seem to say that the output impedance rises to much higher values than 200 ohms in the deep bass region, no doubt as a result of its output being coupled via a capacitor. (The impedance of a capacitor increases as frequency decreases).

To assure that there won’t be an impedance compatibility issue, load impedance should be at least 10x greater than output impedance, at the audible frequency for which output impedance is highest (which is generally 20 Hz if the outputs are capacitively coupled). That doesn’t mean that there will necessarily be a problem if that criterion is not met, it means that there won’t be a problem if it is met. Whether or not there will be a problem depends on how the output impedance varies as a function of frequency.

If you have not sensed any deep bass rolloff with the Aragon/VTL combo, other than what might be expected to result from the speakers, I suspect that improving the impedance match in some manner is unlikely to be worthwhile. Despite the fact that the Aragon’s input impedance is much lower than 45K.

FWIW, though, another member here (Bifwynne), who uses ARC Reference series electronics and whom I know to be an astute and discerning listener, faced an impedance compatibility issue a few years ago involving the need for his preamp to drive a subwoofer having low input impedance as well as his power amp. He resolved the issue by having a gentleman named Tom Tutay, of Transition Audio Design in Florida, custom make a suitable buffer stage for him, at modest cost (hundreds of dollars, not thousands). I don’t think Tom has a website, but his contact info can be found via a Google search.

Regards,
-- Al

Thank you for the responses. It could very well be sensitivity related. Im really not sure.
Hi there, you don't have a borderline mismatch or anything like that.  The VTL should drive an amp with 23 Kohm input impedance just fine.  Why do you think this is a problem?  What do you mean by much more effective volume control?  Perhaps it has something to do with the sensitivity of both the amps.
When used with the Aragon amp the VTL preamp volume control is too sensitive. A tiny movement of the dial makes a huge difference.  It is usable and controllable but just barely.

If you have not sensed any deep bass rolloff with the Aragon/VTL combo, other than what might be expected to result from the speakers, I suspect that improving the impedance match in some manner is unlikely to be worthwhile. Despite the fact that the Aragon’s input impedance is much lower than 45K. 
The bass seems fine. In comparison to the Sumo the treble seems a bit rolled off and mids are dark.  From research I believe that could be a normal sonic characteristic of the Aragon.
There may be nothing wrong. I was just surprised to find the Aragon did not blow the Sumo away and was looking for reasons.
Much has been said of system synergy so when I noticed the volume control issue I figured I'd investigate. 
I'm not sure what you mean by the volume control being more effective? Does that mean you have greater volume range, or that the volume seems to increase sooner, or something else? Some volume controls actually work better as one uses more of its range. Sometimes the volume control works more like a brake than an accelerator.
The two amps are likely not that different in sound otherwise but the Sumo has more gain.

This means the preamp is not working as hard and that's likely the difference you are hearing.
energeezer Assuming you have an impedance mismatch between your pre and power amp is there a device which will correct or compensate for this?
There are no set rules:
I say with all the listening/measurements I’ve done if the output impedance of the source is 10 or more times lower than the device it’s driving into, your fine.
If it’s not then a buffer is needed.
I’m suspicious because I found the Sumo to sound as good if not better than the Aragon in my system.
Two different amps, you preferred one over the other, and there is a gain difference between the two.

Cheers George
Most tube buffers will help with impedance mismatches. They are buffers.  As Almarg was saying a buffer stage may help.  Most seem to think their purpose is to add tube flavor.  I don't think they do, my view is that a tube system excels at imaging.
energeezer

Could could go for what I suggested here, it has all the active flavours, all with low output impedance's.
https://forum.audiogon.com/discussions/specific-pre-amp-advice

Cheers George
phttp://schiit.com/products/freya

George
That does look nice but I am really happy with my VTL. Something to think about.