Amp vs preamp question

Perhaps the term “high” needs some definition… My vk5i total output imp was 800 ohms, 400 per phase… the vk32 has an output imp total of 600 and the Rex is 200 per phase or 400. .. I’m figuring the VK 51 is in there somewhere between 400 & 800 ohms total..

Unless I read an article reviewing the CJ 350 Premiere solid state amp in error, one of the two mainstays, either Mr. Conrad, or Mr. Johnson said a ratio of 10:1 is sufficient for preamp to amp matching. So if the preamp in question has an output imp of say 1000 ohms total, then the amps input imp needs to be 10,000 ohms or better…. Many amps, even the modest input impedances of Spectral amps, would meet that criteria.

My single ended thor output impedance is I think 400. it’s driven every amp I’ve connected to it with eease. 60wpc tube amps, 250wpc SS amps, 100 wpc SS etc… in to speakers with low impedances of 1.8 ohms, 4, 6, and 8ohms. With eff ratings from as low as 85 db!

So what’s a high impedance output for an active preamp??? 1100 ohms? 1500?

All the BAT amps I just looked at have input impedances of 90K +. Tube or SS.

I doubt seriously you’ll run into trouble in that preamp > amp, regard.

BTW none of the BAT amps double down in their power outputs… they just have enormous power reserves. Other’s use negative feedback to ehlp out there.

…and let’s face it… exactly when will one need to have an amp which produces 500 wpc… or 600, or 1000 wpc?

90% of the music is made in the first 20wpc anyways.

It will all come down to what your aim is for driving your loudspeakers. Wanna peel paint? Produce live rock and roll shows?

I’ve got a pair of 87db, 4 ohm speakers which drop to 2 ohms… I can run them easily with a pair of 120 wpc tube amps… very well….. or with a 150 wpc SS stereo amp too! Even with my SS HT receiver which questionably put out 110wpc!

Kevin Hayes of VAC told me once he runs many of his amps on a pair of Thiel 2.6s…. even his 30wpc SET amp! Check the specs on some of the VAC amps which are very well regarded, and you will notice they don’t double power with halved impedances.

This deal ain’t that hard. A quality amp will do well…. And here’s a thought…. Call thiel and ask them what amps on your short list they would recommend.

Usually, its the bass at lower to moderate volumes that suffers driving speakers with difficult loads with amps that are not up to the task.

IF you're willing to live with that for some other aspect of a particular amps sound, then I guess you will be fine, but you may not be getting everything that your speakers are capable of delivering at lower to moderate volume.

In my case, I replaced a 360w/ch Carver amp that did not come close to doubling output with impedance with a ~100w.ch Musical Fidelity that does.

The Carver went louder and sounded fine at higher volumes, but the bass and dynamics were noticeably missing at lower volumes. The Musical Fidelity does not go as loud, and the sound is dynamic and balanced now at the volumes I more typically listen at.

09-27-08: Blindjim
...one of the two mainstays, either Mr. Conrad, or Mr. Johnson said a ratio of 10:1 is sufficient for preamp to amp matching. So if the preamp in question has an output imp of say 1000 ohms total, then the amps input imp needs to be 10,000 ohms or better…

My experience trying several tube preamps with Pass Labs solid state amps tells me the ratio rule-of-thumb is a only useful as a general guideline, and may not apply in the real world.

Recently, I tried an ARC REf 3 preamp (600 ohms) with Pass Labs XA-60.5 (30kOhms)...a ratio of 50:1. The match was not ideal. Not bad, but not ideal.

If one reads Stereopile's measurements section of their preamp reviews, one learns that output impedance changes with load. For example, the Audio Research Reference 3 has an output impedance spec of 600 ohms balanced, but in J. Atkinson's test of the preamp it's output impedance rose to 1437 ohms at 20Hz. This would indicate an early bass roll-off when used with some solid state amplifiers, and indeed when I tried the Ref3 in my system with the Pass Labs amps the bass was lacking some depth and control.

Atkinson wrote about the Reference 3:
"The output impedance was also to spec., at 635 ohms balanced and 326 ohms unbalanced in the treble and midrange, but rose to 1437 ohms and 625 ohms, respectively, at 20Hz."

So, the moral of the story is that output impedance specifications are only part of the tale. Test measurements are another part of the tale. Listening to the actual combination completes the tale.

Regarding VAC amps, it may be true that Kevin runs some of them on Thiel 2.6 speakers, but my experience with the 160wpc VAC Musicbloc monoblocks on Silverline Sonata III (5 ohm minimum impedance) was that the VAC amps produced an unbalanced sound lacking bass depth and definition, leading to an overall "tipped-up" presentation due to the lower power output into the lower impedance frequency region (bass) relative to the higher impedance frequency regions.

I absolutely agree that total WPC is not the most important criteria. However, the impedance issue is crucial, IMO.

Just curious about these impedence matching observations - BTW Tvad, I think your observations are good - how often do folks feel that impedence matching issues, which might be only marginal, if extant, are exacerbated by improper selection of long IC runs from pre-amp to amp.

For instance, using the same components and either 1, 2, or 5 mter IC's the sound doesn't differ much, perhaps a tad with the 5 meter cable, but not enuf to matter (to me). But put in an 8 meter IC of the same brand and type and the roll off is marked and not just in the bass.

Is it possible that much of which is charged to the consequences of impedence mis-matching might be laid at the foot of IC mis-matching as well? Seperaately and concurrently?

Just a thought............

I have to (mostly) agree with Tvad, here. The 10 to 1 ratio is only a guideline and there are many respected voices that disagree. Though a bit off topic, other considerations re: pre to power amp matching include output and input sensitivity, the potential for DC leakage and how it is tolerated, and single ended vs. balanced operation. And let's not forget sonic synergy.
If one reads through the archives here on Audiogon, one will find that some are satified will fairly low powered amps on their Thiels, but more often than not, those who have tried various quality amplifiers, find that the higher powered ones work best. While the majority of music played doesn't usually require gobs of power, the balance can suck up huge amounts very rapidly. With the Thiel's, I think the cost of appropriate high quality ss amplification can be a bit dear and the cost of high powered tube amplification can quickly come cost prohibitive. SS amps seem to offer a better value with Thiels(!). IMHO, ss amps sound best when they are not run into their limits... with ss amps, having some headroom is advisable.
BTW, I certainly hope that Ledenhheim doesn't have an "18 X 18" room!