Impedance Matching Question

iFi iTube2. A little miracle box.

A line transformer can be used. IME, not an elegant option, but it works. You are better off with a preamp that can drive the amp properly.

astelmaszek - thanks for your reply. That’s an interesting looking device and definitely appears that it would work, however probably more than I want to spend in this situation.


atmasphere - and thank you for your reply. I’m not concerned with elegance. I just want something that will work and sound decent. Just trying to use some extra gear to make some sound in a not so important location. Your reply did prompt me to do a little more "Googling" and I found this,

https://www.bhphotovideo.com/c/product/217366-REG/Rolls_MB15B_MB15b_Promatch_2_Way_Stereo.html

From looking at the specs for RCA, it appears that it should work, however I have no experience with Rolls products. As I said, not looking for super quality, but don’t want junk either. This line transformer sounds like a DIY project. So, is it just getting the proper transformer, 2 pair of female RCA’s, and a project box, then solder and assembly?

That'll work as long as it has the bandwidth.

So, is it just getting the proper transformer, 2 pair of female RCA’s, and a project box, then solder and assembly?

That would work too.

micro – iTube2  is good to go, but it is multiple function device. An audio transformer will be a great option.

Impedance Matching Question

Most know that the input impedance of an amp needs to be ~10X the output impedance of the preamp for proper matching. Is there an inexpensive device that can be placed between a higher impedance preamp and lower impedance amp to help solve the issue? This is not for critical listening, but I do want it to sound good.
tls49

"Most know that the input impedance of an amp needs to be ~10X the output impedance of the preamp for proper matching."

This is correct 10x is fine some even say 5 x is ok. If you can't do this ratio a unity gain low output impedance buffer will do the trick. Like this.            http://www.6moons.com/audioreviews/burson/buffer.html

Or a tube one, but some have a high output impedance as this one at 1kohm you may not be better off, and it could even be worse than 1kohm in the low bass because of the dc coupling capacitor that's needed on their outputs if it's not large enough.
http://www.decware.com/newsite/zbox.htm

Cheers George

The Burson Buffer has not been made for quite a while now, and are hard to find. However Burson has this ingenious RCA cable with a small solid state buffer built into it. It is powered by 5 Volts DC. It will help with most any impedance mismatches.

Specifics of the gear being connected, cable type and length would help. It may be possible to simply change an output or input resistor.
It is also possible that with short cable runs, worry is not warranted.

atmasphere - thanks for your additional info, made an inquiry to Rolls about the bandwidth, but have not heard back. The box, RCA's, solder, and assemble would be no problem, however getting the proper transformer is beyond my expertise.

George - the preamp is an old VTL Maximal with a 5K ohm output impedance. I have tried a few amps with it. One was a 75 watt Belles with a 25K ohm input impedance. The result was poor, soft, rolled off, and no dynamics. That amp sounded great with a SS preamp. Surprisingly, the best sound with the VTL has been with the 18 watt power amp section of an Advent 300 receiver, quite a bit better than the Belles on the same speakers. It's input impedance is 100K ohms, so that proved to me that proper impedance matching is important. Thanks for the links to the buffers, however as I said previously, more than I want to spend in the location.

koestner - thanks for the info, that Burson RCA cable looks intriguing. Appears that it would solve the problem and it might be entertaining trying it on some other systems I have. I sold some cables in the past year, so it's easy to justify spending some of that money on new cables. :-)

VTL Maximal with a 5K ohm output impedance.

Wow that’s hi, I can understand it would be a no go (1:5) with the 25kohm of the Belles. But with an amp of 50kohms or higher (>1:10 ratio) as you found out with the Advent it was fine.

Remember back in those days the "industry standard" for input impedance of amps and preamp’s was 47kohm and that VTL would have been happy then as it was made then, and sources were very low in output volts phono ect and also high in output impedance, and you needed preamp’s for the extra gain they gave. But today gain is not the issue, as Nelson Pass says "we have too much."

Where today source have output stages that are the equal sometimes better than preamps, with high output voltage more than >2v, with low output impedance less than <100 ohms output impeance, and many amps today only need 0.5v to 1v for them to give their full wattage before clipping, and we have >2v available from our sources, do the math.

Cheers George

koestner
The Burson Buffer has not been made for quite a while now, and are hard to find. However Burson has this ingenious RCA cable with a small solid state buffer built into it. It is powered by 5 Volts DC. It will help with most any impedance mismatches.

Don’t know how good that would be, as it runs off the +5v usb supply, and from what I’ve seen it looks to be an opamp based buffer, and for audio these +5v single ended supply buffers are not too good, and I think they also would have to be capacitor coupled as they have dc offset with switch on/off thumps. 
Cheers George

George, thank you very much for your additional comments. I was already leaning toward not getting the Burson cable. I have found that the Polk Audio RTi A3 is on closeout for $240/pr, so not much more than the cable. It got an excellent review in Stereophile and made the Recommended Components list. It also has a real wood veneer cabinet, so definitely seems to be worth the current price. I'm thinking it's 89-90db sensitivity is likely enough that I'll just use the Advent 300 with the VTL. It's a small room and I don't need extreme volume. The Advent is surprisingly strong for 18 watts. Guess it was designed to have good current output since it was used with the original Advent speaker. Thanks again for your help.

Polk Audio RTi A3 is on closeout for $240/pr, so not much more than the cable.

Yeah definitely the money shot is for the speakers, how come we don’t get those kind of deal down here in Oz, it really ****** me off. Enjoy!


Cheers George

I'm sure if you look around you find an iTube around $200 soon enough, 

Nice find, now that looks good I think, seems to be tube input with ss output great combo to get low output impedance and high input impedance.
100k input impedance and 1ohm!!!! output impedance, can be used as a unity gain buffer, or preamp with gain and volume control.

" 8 DIP switches that allow us to select the gain (0dB/6dB) and whether using it as a pre-amp or as a buffer with the help of a flat screwdriver (like the one included)."

https://www.head-fi.org/showcase/ifi-audio-itube.16539/reviews

https://www.audiostream.com/content/ifi-itube-active-tube-bufferpreamplifier

Cheers George

Thanks George for the iTube comments.

astelmaszek - initially I thought the iTube2 at $399 was more than I wanted to spend, however after finding a deal on the iTube for $239 shipping included, I’m thinking that’s just a little more than the Burson cable at $200 and the iTube has more capabilities than a cable. This same authorized dealer is also offering the iTube2 for $279 shipping included, so I ordered one. I’m thinking I’ll just have fun trying it in various places of my systems, and then always have a good way to use any amp with the VTL if I so desire. Thanks for your recommendation of the iTube.

I've had mine for ~2 months and still delighted. Hope you are too, but use good interconnects, not the included ~6" ones.

Also agree about the miracle box thing😀. It serves the music so well..

This maybe interesting for those with impedance matching questions, that I posted on another theard, I'll copy to here as well.

I’ve dealt with impedance matching for many years now, having the product that I have.

With impedance matching if one stays with the 1:10 or higher ratio you’ll have things covered. This goes for source to pre and from pre to amp/s

All you have to worry about then is if the "1" (source or pre) has an output coupling cap, (if they’re dc coupled no problems) and that cap is large enough to maintain that impedance 1:10 ratio flat from 20hz to 20khz you’ll have no problems

If not and it’s too small you may get a change in that ratio in the bass maybe down to 1:2, if this happens because of that coupling cap being too small then you’ll get a "voltage divider" scenario happening in the bass rolling it off too early, maybe at 50 or 100hz and giving a thinner sound with lack of perceived level in the bass.

Cheers George