2. amps-preamps
  3. 126066-498

How important is Impedance matching between solid state preamp and tube power amp

I've been away from these forums for quite awhile and, in the interim, have made some changes which seem to be working well for me. Two local guys, though, expressed concerns regarding impedance issues and how I would be better off all solid state, all tubes or, if I insisted on mixing the technologies, put the tubes in front. I understand the validity/logic for each of these combinations as well as what I have chosen in the current setup. In regard to impedance matching it's something that I've paid little attention to in the past thinking it was not an important enough issue but, in reading articles on the matter, several experts seem to have strong opinions regarding impedance ratios.  What I'm asking is, who's right?

Here's where I am now. Not based on an overall plan, I've gradually reverted, piece by piece, to a bunch of older components, even abandoning my 30 plus year devotion to Martin Logan speakers which freed up a some latitude in power amp choices, especially regarding amperage considerations. For general everyday listening we're using lossless music files through first generation Applle TV which I have been quite happy with. For single CD listening I'm now using the very old Theta Data Basic transport and Theta dsPro DAC. For multiple CD play we have the Sony cx777es with the Escient manager. Everything except Apple TV currently routes through the Theta DAC. Ok, here's where the question arises; everything routes through the Peachtree Nova hybrid integrated amp, used as a preamp, to the Cary Rocket 88R amp and, finally, to the Serie Reference 3R speakers.  Like I said, old. 

I know that there are newer components that, from a technical standpoint, do a superior job but this combination of components sounds as good or better to me as anything I've ever had in the house and I know that should be reason enough to accept what I have and leave well enough alone.  Anyway, most of us, seemingly, no matter how far we come toward putting together the perfect system, are open to advice from those who are more knowledgeable.  Any comments would be welcomed and appreciated. 




128x128broadstone
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almarg
9,670 posts
 
Hi Jim,

Nice to see you back here.

The input impedance of the Rocket 88R is 150K (150,000 ohms), which is easily high enough to make impedance compatibility a non-issue with virtually all preamps.
Two local guys, though, expressed ... [that] if I insisted on mixing the technologies, put the tubes in front.
This is incorrect. While there are many exceptions, in general an impedance compatibility issue has the greatest likelihood of occurring if the output of a tube-based component is driving the input of a solid state component. That is because more often than not tube-based components have higher output impedances than solid state components, and more often than not solid state components have lower input impedances than tube-based components.

Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result, depending mainly on how the output impedance of the preamp varies as a function of frequency.

If, as is often the case, the highest output impedance of the preamp across the audible frequency range cannot be determined, and only a nominal output impedance is specified (which is often based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. As Erik alluded to, many tube preamps and some solid state preamps use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.

The same principles apply, btw, to assuring impedance compatibility between source components and preamps. Although the issue seems to arise less often in those cases.

Best regards,
-- Al

broadstone's avatar
broadstone OP
310 posts
 
Thanks for the welcome back, Al. Until your post I didn't know the input impedance of the Cary but it looks like the 2 values way more than satisfy the ratios you suggest. If I had one tiny complaint it would be that with some recordings there's slightly irritating reproduction in the upper mid frequencies.  I realize that this may be a manifestation of my hearing issues or quality of the recordings but I'm always looking to blame something that can be fixed. I also think I expect way too much magic from DAC's. 

Eric, I do have an unfortunate listening room and acoustics was an issue that I tried to resolve using the Behringer DEQ2496. The automatic balance adjustment works great and even I could hear the difference when it finished the setup. Because of recent modifications to my system, though, I haven't yet decided  if I will reincorporate it.  Either way I'll look into ASRC; maybe jitter is a factor in my upper mid frequency issue. Thanks

Best regards,  

Jim
bpoletti
1,553 posts
 
In most cases, the impedance matching issue is between a tube preamp with their typical high output impedance and a solid state amp which could have a [even very] low impedance.  Some tube preamps may not be able to drive low impedance ss amps. 

IMO, placing more devices in a circuit just provides a greater opportunity to introduce signal distortion and sonic degradation. 
broadstone's avatar
broadstone OP
310 posts
 
Thanks. I completely agree that any component added to the stream has the potential of degrading content.  I'm also open to the idea that some of my equipment, because of deficiencies related to age, design or unfortunate pairing, could use some help. The bottom line is that I'm mostly quite happy with the current setup and if I listened only to solo quitar, which I mostly do, I'd be totally happy with the sound.  It's just that I find that some of the upper middle frequencies are irritating and if I could fix that problem my listening choices could be expanded. 
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