High Current SS amps with input impedance = 200K

I'm looking to identify amps like this that would match optimally to an Audio Research sp16 tube pre-amp with output impedance of ~ 20K?
FWIW I find it hard to believe that an ARC pre amp has an output impedence of 20K. 260 perhaps, with a recommendation from ARC that the amp have an input impedence of 20K.

Just a WAG - I didn't bother to research it, I'll leave that up to you. :-)
NEwbee, you may be right.

Here are the specs from the arc site:

OUTPUT IMPEDANCE: 260 ohms Single Ended main; 20K ohms minimum load and 1000 pF maximum capacitance. Outputs (3): 2 main, 1 record (Single Ended RCA connectors).

I'm not sure how to interpret this though if anybody can clarify for certain.

Is 260 ohms the pre-amp output impedance and 20K OHMS the minimum recommended amp input impedance maybe?

BTW, I have seen at least 1 ARC SS amps out there have 150K ohm input impedance specifications.
Is 260 ohms the pre-amp output impedance and 20K OHMS the minimum recommended amp input impedance maybe?
Yes I think you are correct too, Mapman.
Well, that's very different then.

ARC's specs are the most detailed I've seen so far then in this regard, and my original question is a moot issue then I suppose..
But... You don't know if that 260 ohm output impedance is worst case or not (it probably is not; they don't specify a frequency). You don't how much the bass (where the output impedance is likely the highest) will roll off into a 20K ohm load. An acceptable amount of roll off to ARC may be quite different than an acceptable amount to you. The ARC specs are still insufficient to make an informed decision.
Bob_reynolds is right.
Mapman, Bob may be 'right' as Tvad sez, but the ratio is way beyond any nominal ratio you see bantered around like 10 to 1 for example. In this case it would be very close to 80 to 1.

Personally I'd tend to trust ARC when it sez a 20K imput impedence would be adequate (I actually did that without regret) and not get my panties in a knot over what the difference would be between say 20K and 47K or even 100K. I'd even take bets that in this instance even Bob couldn't tell the difference if he was listening with a stethoscope. In fact I'd take bets that it would be unlikely that he would hear any meaningful difference if the amp's input impedence was as low as 10K.

Personally I'd believe ARC. I'd even go so far as to think that their recommendation would be conservative and act accordingly, spending the copious amount of time necessary to solve these issues to insuring that the amps output impedence was a good match for the speakers to be used, to finding the right tubes to flesh out the pre-amp's tonal issues when matching it with any particular amp, or just sitting back and listening to some good music.

But hey, thats just me. I'm not very anal about all of these techie issues. :-)
11-06-09: Newbee
In fact I'd take bets that it would be unlikely that he would hear any meaningful difference if the amp's input impedence was as low as 10K.

If you heard the SP16 with two amps, one with a 10k ohm input impedance and the other with a 20k ohm impedance, I'd wager you'd hear a difference in the bass extension and clarity.

If you didn't, then I'd be very surprised.
Tvad, Note I said no difference at 20K or above, but I did add the words 'unlikely' and 'meaningful' to the reference to 10K. To me at least it probably wouldn't be 'meaningful'. I draw this assumption from my personal experience using ARC stuff and assorted SS and tubed amps with input impedances between 10K and 100K. I get far more wound up working out amp output and speaker impedence matching issues (as I know you do as well).

BTW, Tvad, have you actually heard the SP16 connected with a 10K amp AND an identical amp differing only in that the impedence value was increased to 20K. Im unaware of any such amps though. I think if we could find a techie who could set up such a demo by altering the output impedence of some amp in some way without otherewise changing the signal, we might then, and I think only then, know if there was a difference, meaningful or not.

But its fun thinking about all of the issues anyway. :-)
Newbee, I stand by my statement that I'd wager you'd hear a difference in bass performance between a SP16/10k ohm amp combo and a SP16/20k Ohm amp combo.

Beyond that, no it'd be impossible to compare identical amps except for differing input impedances, as no such device exists to my knowledge.

No, I have not heard an SP16 connected to a 10k Ohm amp and a 20K ohm amp...or any amp for that matter. However, I would take your bet that I could hear a difference in bass response.
It does make sense to me optimize the electronic interface between components in a system to whatever extent possible, all other factors aside, to maximize inherent performance.

The input impedance of my current Musical Fidelity A3CR amp is ~31K.

I've seen ARC SS power amps with 150K ohm input impedance. I'm thinking that it might make sense to go with an SS amp like this from a company that has tube pre-amps as a specialty.

Other options I know of to consider are Bel Canto Ref 100m (150K ohm unbalanced input impedance, 300K ohm balanced)and Wyred 4 Sound (~60K ohm input impedance).

Stock ICE power class D amps appear to have only about 20K input impedance which would seem to make them a less desired match with many tube pre-amps.

Any other SS amps with high input impedance to consider? I doubt I would seriously consider a tube power amp for my application at this point with the big current hungry OHM 5s in my system.
Tvad, I'm sure I could hear the difference. That part would be a walk in the park. But I'm not so sure that I could connect the difference to the difference in impedence values as opposed to the amps design or how it interfaces with the speakers.

For Mapman, whose post follows. FWIW IMHO, the differences which may occur between the small differences, or deviation from optimum values, between amp and pre-amp that we are talking about (I'm NOT talking about gross mismatches, or even medium mis-matches) are absolutely dwarfed by the output capabilities as well as the tonal signature of the amp and the impedence curve of the speakers. But you have probably already figured out that that is my opinion. :-)

BTW, if you are looking for high power, high quality power ss amps, I'd recommend that you look at some vintage stuff, especially stuff made by Nelson Pass when he ran Threshold, especially the SA or S series stuff. Even if you had to have some recapped because of age they would still be a hell of a deal. If I ever went back to SS that is the first place I would go.
Newbee, your viewpoint on the impedance matching issue that you illustrate in this thread is just flat out wrong.

The possibility that you may not be able to connect what you hear to an impedance matching issue does not mean the issue does not exist, or that is of little importance.

The difference between an amplifier's input impedance of 10k Ohms and an amplifier's input impedance of 20k Ohms is not small. It's significant, and it'll be magnified by a preamp that has high output impedance somewhere along the frequency spectrum (which is typical of many tube preamps).

Spend just an hour reading John Atkinson's comments in his test measurements of a few tube preamps and a couple amps and you will discover the significance of the preamp/amp impedance matching issue.
My strategy will be to play it safe and go with a high current SS amp that doubles output into 4 and eve 2 ohms as much as possible and has a higher input impedance, along the lines of those I mention above. That will optimize electronic interfaces both in from pre-amp and out to speakers.

From there, whatever amp used will be in an optimal position to synergize with pre-amp and speakers and deliver its best performance as a result, whatever that may be, I would say.

Any SS Amp recommendations given all this?
Mapman, what's your budget (stretched...) and what's your power requirement?
This is primarily for the OHM 5s which have a somewhat difficult load, are only 87db efficient and the more juice, the merrier.

Power requirement: 250w/ch minimum doubling into 4, high current.

I'd like to stay under $2500 if possible for now.

Also, logistically, I would prefer to stay away from large heavy, hot running monster amps if possible. I know that limits my options....
Without commenting on sound, which is a subjective thing, I'd say the amps that best fit your criteria, but would have to be purchased used, would be Bel Canto Ref1000 MK I monoblocks, Belles 350A Reference stereo, or Belles 150A Reference monoblocks. You should know that the Belles amps perform best running balanced, IMO, but the 350A can be run single ended. The 150A Reference monos must be run balanced.

All these amps will drive your speakers extremely well, and they have input impedances of 50k Ohms (Belles 350A Reference) or higher, so they will work well will tube preamps.

The Belles amps run only slightly warm to the touch. The Bel Canto are stone cold.

I'd also recommend used Pass Labs X-250 or Plinius SA-250, but these run warm and are monster amps, so they don't really fit your requirements.

As far as new amps in your price range, I can't think of any at this time.
Newbee, the sonic differences between a 10k Ohm and a 20k Ohm amp when used with most tube preamps would be applicable to a real live person in a real room provided the person was an audiophile with an audiophile quality system. No question about it. Not just theory. Read Atkinson. He lays it out in plain English. No technical knowledge required.

Although you just tossed out the 10k Ohm impedance spec, it is not an unrealistic number for solid state amps. McCormack amps have an input impedance of 10k Ohms, and in my experience using them (DNA-2, DNA-500), they don't work well with most tube preamps due to the input impedance issue.
"I don't know why Mapman is reluctant to consider ARC recommendations "

Newbee, what gave you that idea?

I'm not. I would assume ARC to be a most reliable source for recommendations regarding their equipment given their considerable history, experience and reputation, and the fact that their unit has been a home run to-date in my system.

Tvad has been around with a lot of different equipment I believe and has a good attention to sound quality and technical details so I do not discount his observations in this area either.

I agree with your point about validity of apples and apples comparisons between amps based solely on input impedance differences as well, but I do not doubt that higher amp input impedances relative to pre-amp output impedance is one (of several) technically sound measurements that one can attempt to optimize along with but not at the expense of others necessarily as well.
Mapman, Note that I deleted the post you are responding to. Sorry about that. I was really responding to Tvads observations and was not at all pleased about what I was saying especially as it applied to you. I had already communicated all I could really offer to you in previous posts. And I saw no reason to continue the correspondence with Tvad although he is so serious I must admit that I do, sometimes, enjoy rocking his boat. He is a good guy and means well.

But, FWIW, if you can accept the accuracy of ARC's recomendations about amp matching with that pre amp, restricting your search for an amp based upon a desire for higher input impedence will not add to the performance of the amp/pre-amp combo. If the car takes regular gas and the tank is full, that is as good as it gets.

Hope you find the right amp for your Ohms.
Here are Stereophile's output impedance measurements for the SP16L line-stage preamp, for which the ARC specs are identical to what Mapman quoted:


The measurements indicate 262 ohms at mid and hi frequencies, and 549 ohms at 20Hz.

Into a 600 ohm load (sic), they measured a loss at 20Hz of only 2.5db (see Figure 1). By my calculations, into a 10K load the 549 ohm output impedance would result in a loss at 20Hz of less than 0.4db.

So the output circuit, and particularly the output coupling capacitor, of the SP16 appears to be designed more conservatively than in the case of most tube preamps. And the recommendation of a 20K minimum load appears to be conservative as well.

-- Al
Glad you found that, Almarg. I spent a bit of time Googling it myself without success.

Atkinson states the SP16 should not be used with amplifiers having input impedance "much below 10k Ohm".

So, yes the SP16 appears to work well with most SS amps.

If that's the case, Mapman, you might consider one of the McCormack amps...like a DNA-2 Deluxe or a DNA-500. Check with Foster_9 about his experience using a DNA-500 with his Ohm loudspeakers. I believe he's using one with a VTL linestage.
Thanks for the details.

I can say with confidence that the sp16 sounds very balanced, natural and lively with the MF a3CR I'm using (31K input impedance spec).

There is a larger (>200 w/ch)MF A308CR amp up for sale by a local Agoner. I think the impedance specs are the same as the A3CR but need to confirm. Since I like the current sound an would be happy with more of it, that might be something for me to consider again now that I have a handle on the impedance issues.
Looks like a good call on the A308CR, Mapman.
New Krell EVO amps do have input impedance of 200 Kohm on balanced inputs. So if that is what you are looking for ...
Several newer class D amps have imput impedances in the 40K Ohms, including my JRDG 312. . . but the highest may be the Bel canto Ref 1000 Mk.2. It has raised its input impedance to 200K Ohms from the previous version, which was a respectable 20K Ohms already. R1000 Mk.2 handle my 6Ohms wabbly load Mahlers without any difficulties. Guido
Thanks Al for finding the Stereophile measurements. I looked in the tubed preamp section before I posted, but didn't see it. So, if worst case output impedance is 600 ohms, then yes a 20K ohm input impedance will be fine. So Bryston and Pass Labs amps would work.

I think in general my comments are relevant and I'm pleased that ARC has done a good job with the SP16. As Tvad stated there are numerous examples in the Stereophile archives where the output impedance of a tubed preamp gets very high in the bass region. Without JA's measurements the consumer is truly in the dark about which amps will mate well with such preamps.

As much as we sweat the details of cables and electrical outlets, to not consider proper impedance matching between components just doesn't seem rational to me. I'm glad to see that Mapman is attempting to get a good fit.
11-07-09: Bob_reynolds
...there are numerous examples in the Stereophile archives where the output impedance of a tubed preamp gets very high in the bass region. Without JA's measurements the consumer is truly in the dark about which amps will mate well with such preamps.
Amen to that, Bob. It's a point I've attempted to drill home to whoever will take the suggestion.
As much as we sweat the details of cables and electrical outlets, to not consider proper impedance matching between components just doesn't seem rational to me. I'm glad to see that Mapman is attempting to get a good fit.
Bob_reynolds (Threads | Answers)
Can I get another Amen!
How about a few Hallelujahs?

Very nice to see a good discussion showing a sensible approach...of course, many audiophiles will continue to blame their interconnects and keep spending when their problem is simply a matter of a poor impedance match and choice of components...around here cables and interconnects often get blamed for what are really equipment design/selection issues.
...of course, many audiophiles will continue to blame their interconnects and keep spending when their problem is simply a matter of a poor impedance match and choice of components
Shadorne (System | Reviews | Threads | Answers)
Tell it! Tell it!

Amen from me too, of course.

I found the review very quickly because I was too dumb to think of looking in the component category listings. It is in the Power Amp category, because it was reviewed in conjunction with an ARC VS110 power amp. I just entered "Audio Research SP16" into their search function.

Best regards,
-- Al
Thanks everyone for sharing your expertise and attention on this. Much appreciated. Impedance matching is one of those things that I know makes senes to look at for optimal "synergy" between components, but I learned a lot regarding the details of what might go right or wrong here.

Just because the IC connectors are physically standardized to connect things doesn't mean any two things all do it together equally well electronically for sure!
Search function? So that's what that's for! I always click on the category, then click sort by title, then guess which page the manufacturer's name will fall, then use binary search to improve my first guess... You get the idea -- it takes a while. You'd think a computer scientist would have a better algorithm.
Just because the IC connectors are physically standardized to connect things doesn't mean any two things all do it together equally well electronically for sure!
Almarg sez "Amen from me too, of course"

To the profusion of Amens I would add that Almarg should get a few hozzanah's and more than one Amen for fleshing out the actual detail and providing a concise answer to the OP's concerns regarding the use of the SP16 with SS amps and not joining the crowd wandering thru the desert firing at phantom snakes from the hip.

Almarg, you are a model for folks who want to help others asking questions here. You have intelligence, knowledge, and a generous interest in helping others that makes you stand out high above the crowd. The folks here are in good hands when you have the opportunity to join in threads.

You are an AGon treasure. !!!!!!
Newbee, regarding Al I could not agree more. Those snakes were phantoms this time, but there was no wandering through the desert. The path is well demarcated by Ohm's Law.

Also, even though Al's calculation, which I trust, of 0.4dB seems quite small, anything greater than 0.1dB is accepted to be audible and not interpreted as a level difference. Read through JA's reviews where he does A/B comparisons, you'll see that he matches levels to within 0.1dB.

Best regards,
Newbee (& Bob), thanks very much for the kind comments; much appreciated. I've learned a good deal since I've been here from your posts, too, as well as those of all of the other regulars who've participated in this thread.

My statement of "less than 0.4db" loss at 20Hz with a 10K load was a quick rough swag, and the actual number is probably close to 0.1db (and almost certainly less than 0.2db). The exact calculation is a little complex, because the preamp's output impedance is partly capacitive and partly resistive, and resistive and capacitive impedances don't sum linearly.

But with the numbers being that small at 20Hz, and undoubtedly totally negligible at say 30Hz and above, there's no need to be more precise. 0.1db is the commonly accepted rule-of-thumb for conservative level-matching, but that figures to be driven mainly by mid-range frequencies, where the ear is vastly more sensitive.

Thanks very much again for the nice words.

Best regards,
-- Al
Hi Bob, Please consider my posts in this context - I was not trying to do anything more than provide a 'simple' answer to the OP's question without getting technical nor doing his research for him. That is a methodology I have used professionally as well as in these forums. In retrospect I probably didn't do it well here. It didn't work out well did it! :-)

As Al pointed out Stereophile had reviews which specifically addressed this particular unit, information that was available to all who cared to read it. I knew about them as well and had read them when they were published. In the future I will try to emulate Al more and simply refer folks to the source and avoid adding an interpertation and describe how it would actually effect his situation (unless he were to ask of course). That will be more helpful to the poster's understanding, perhaps stimulate his curiosity and encourge further research into the issue, but as importantly, avoid potential unpleasantry between the responding posters.

Hope you guys all have a nice day...............
Here are the leading contenders for the position of amp in my main system:

1) Wyred 4 Sound st-1000 or sx1000, best value, 500w/ch Class D offers potentiall best overall dynamics in my price range, 60K ohm input impedance, reported best of class results by OHM guru and fellow agoner Mamboni

2) Musical Fidelity A308CR, similar to what I have, 31K ohm input impedance, 250w/ch local seller, can audition in my system, 60+ amps current deliver (total for both channels I'm assuming

3) Musical FIdelity A5CR, similar to A5CR but significantly higher current delvery based on user guide specs, none available currently but I am not in a rush and could wait.

Is anyone familiar with A308CR versus A5CR that can offer insight on what all is different?

4) Larger ARC SS amp, D400 or 300.2, 150Kohm input impedance, 200 w/ch or so , I would have few reservations trying out other ARC gear that can fit in nicely with my pre-amp and the OHM 5s.

All thoughts welcome!

Interesting reading. Several of us made this mistake when starting. This is an excellent lessons learned for new audiophiles. Why don't A'gon create a lessons learned folder entitled "Audiophile 101". I learnt the expensive way.
I once thought I was in love with ARC D400, at least on specs. . . until I heard it. . . and my ears started to 'bleed'. Am I ever glad I did not buy it without auditioning!G.
I'm currently using a Burson AB-160 buffer between my McCormack DNA-500 amp and VTL preamp. I'm waiting for it and associated cables to break in and sonics to stabilize. I'll report back with developments after it gets some significant hours on it.

Yes, it will be interesting what that device might do in your case. On paper, the VTL and McCormack appear to be a pretty good match. The DNA 500 is the only other (non Class D) 500w/ch amp that I have considered to-date.
I think there is more than just a calculated drop of output in the bass frequencies. 0.4db would be difficult to hear especially at 20Hz.

My SS amps have input imped of 67K ohms. I have used a few tube preamp that has specified output imped of 100ohm to 600ohm (may be higher in the bass freq.) Subjectively, there is tighter coupling betw the pre with the lowerest output imped and the amp. I hear better transient attack in the mids and tighter bass. In some ways, it is similar to changing the load imped on cartridges.

I suspect that there is also interaction in the time domain and/or dampening.

I am pretty confuse myself. Any help is appreciated.
I'm currently using a Burson AB-160 buffer between my McCormack DNA-500 amp and VTL preamp. I'm waiting for it and associated cables to break in and sonics to stabilize. I'll report back with developments after it gets some significant hours on it.
Emerson, as I recall it was not readily possible to determine the low frequency output impedance rise of your VTL 2.5. However, Stereophile has numbers on the 5.5, which it describes as being descended from the 2.5, and the two models have nominal output impedances which are very close to one another (195 and 200 ohms). For the 5.5 they measured 184 ohms at mid/hi frequencies, and 409 ohms at 20Hz, which is a very minimal rise that would have negligible frequency response impact into 10K.

They also have numbers on a couple of other VTL models, the 6.5 (150 ohms at 20 Hz for the unbalanced outputs) and 7.5 (69 ohms at 20 Hz for the unbalanced outputs).

So it would seem that VTL's design approaches reflect sensitivity to this issue.

-- Al
Glai, it's only possible to offer speculation in response to your question, without having more complete information on the particular designs (which may be unobtainable). But I wouldn't be surprised if a tube preamp with a nominal output impedance spec of 600 ohms were to rise to several thousand ohms at deep bass frequencies.

And the 600 ohm impedance itself could have consequences at high frequencies as well, if your interconnect cables were long and/or had high capacitance per unit length. That could very conceivably account for the degraded transient attack you mentioned, by rolling off the upper treble.

Also, perhaps in the case of the particular designs you were using, there was some degree of correlation between lower output impedance and better quality parts in the output stage. As well as the possibility that variables elsewhere in the designs, unrelated to the output stage and impedance matching effects, might have accounted for the differences in those particular cases.

-- Al

Thanks for responding. Th preamp in questions:

Einstein the tube, 100ohms
Allnic L400, 250ohms (supposely flat with transformer coupled design)
ARC Ref 3, 600ohms (1400ohms at 20Hz)

I use interconnects 1 meter in length Nordost and Stealth.

Many thanks for your insight
Al, thanks for the feedback as usual. I have a bit to add on the Burson AB-160 buffer. It's been in my setup for 3 days with signal going through (72 hours). What they say about it is true. It will take a budget player and lift its performance. The Burson is not connected to any of my players; it's between my amp and preamp. I have 3 different players connected to my VTL preamp, a Raysonic CD128, a Jolida player and an Onkyo. The low fi Onkyo is playing like a more expensive player now. It surpasses the other 2 players in extension, dynamics, low end, openness, you name it. And it has some inexpensive Tributaries IC's connected to it. Budget stuff in all. For the past 2 years I've only been using the Onkyo player for sending signal to facilitate break in with new gear and new cables. This is my experience with the Burson Buffer in my system 3 days. As others have said, the Burson seems not to have as significant of an affect on the performance of better players. At this point, I would say it is a detriment to the other players I have. Listening to this budget player with the Burson in place has also revealed what my speakers are capable of in the lower bass region. I had not heard these speakers go as low before.