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Peeking inside a Carver Crimson 275 Tube Amplifier

So, I just had to pop the hood on the Carver Crimson 275 tube amplifier. I was so curious as to how this little guy weighs so little and sounds so lovely.

  • The layout is simple and clean looking. Unlike the larger monoblocks (that cost $10k), this model uses a PCB.
  • The DC restorer circuit is nicely off to one side and out of the way. It doesn’t look all that complicated but I’m no electrical engineer. Why don’t more designers use this feature? It allows the power tubes to idle around 9.75w. Amazingly efficient.
  • The amp has very good planned out ventilation and spacing. No parts are on top of each other.
  • Most of the parts quality is good. There’s a host of Dale resistors, what look like Takmans, nice RCA jacks, heavy teflon hookup wire, and so on.
  • Some of the parts quality is questionable. There’s some cheap Suntan (Hong Kong mfr.) film caps coupled to the power tubes and some no name caps linked to the gain signal tubes. I was not happy to see those, but I very much understand building stuff to a price point.
Overall, this is a very tidy build and construction by the Wyred4Sound plant in California is A grade. I’m wondering a few things.

Does the sound quality of this amp bear a relationship to the fact that there’s not too much going on in the unit? There are very few caps--from what this humble hobbyist can tell--in the signal chain. And, none of these caps are even what many would consider decent quality--i.e. they aren’t WIMA level, just generic. This amplifier beat out a PrimaLuna Dialogue HP (in my room/to my ears...much love for what PrimaLuna does). When I explored the innards of the PrimaLuna, it was cramped, busy and had so much going on--a way more complicated design.

Is it possible that Bob Carver, who many regard as a wily electronics expert, is able to truly tweak the sound by adding a resistor here or there, etc.? Surely all designers are doing this, but is he just really adroit at this? I wonder this because while some parts quality is very good to excellent, I was shocked to see the Suntan caps. They might be cheaper than some of the Dale resistors in the unit. I should note that Carver reportedly designed this amp and others similar with Tim de Paravicini--no slouch indeed!

I have described the sound of this amp as delicious. It’s that musical and good. But, as our esteemed member jjss [ @jjss ] pointed out in his review, he wondered if the sound quality could be improved further still. He detected a tiny amount of sheen here and there [I cannot recall his exact words.] even though he loved it like I do.

I may extract the two .22uF caps that look to be dealing with signal related to the 12at7 gain tubes and do a quick listening test.
128x128jbhiller
jjss49's avatar
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jjss49
6,612 posts
 

quick report

i just had my amp in for a check (some tube noise and transformer hum) at wyred 4 sound - w4s is the exclusive contract manufacturer for these crimson amps

had a friendly chat with their senior tech who worked on my amp - i know the w4s guys reasonably well, like them, have several pieces of their gear (digital, power supply, class d amps), w4s is a good company, well designed well built, good sounding high value gear, good reputation, and straight shooters in my book

i was told that based on their bench experience, the crimson amp can make 75w driving one channel... (didn’t ask for frequency range spec, sorry), can’t in both channels though (’more like 60 thru both channels’), but this isn’t due to the output transformers as limiting factor, but rather, the power transformer, which can’t quite deliver the juice for 75 wpc

asked how such a small light tube amp makes that kind of power, was told that the reason is the kt120’s are pretty strong, plus ’the neat feedback circuit employed from the output, holds the distortion in check, other guys don’t do this, we were kinda surprised it can do that through those small output transformers...’

so that is what i learned... perhaps frank m, after consulting with bob carver himself will chime in again, say more

for me, i own the amp, bought it used on a lark to try, like it quite alot, drives my spendors and harbeths very nicely, sounds great, runs pretty cool, is small, light, easy to move around... so i’ll be keeping it in my stable of amps - yes, the power output spec may well be overstated by the seller ... oh well, i’ve done a lot worse :)

funky54
20 posts
 

Nobody so far thats independent of Carver affiliation, is getting even 60 watts per channel. Yes they should have used a bigger power supply and maybe gotten a realistic 20 wpc. That would have been possibly a decent marketable product. But hey, if you only run one channel without half the frequencies that matter, it could be a wonderful center channel home theater amp.

For potential customers that use efficient speakers I’m sure it does sound great. Lots have attested to that. But what about the other half who have in-efficient speakers who wanted to pair them with a well selected amp? Carver knew they’d lose half the potential buyers, so they lied. Now half the people who bought them are not seeing the full potential of their sound..

I listen on average around 76-82 db in a 26 x 40-ish room with 13’ ceilings. Even at those db’s, I can easily hear the difference in a 100 watt per channel from a 200 watt per channel. I can hear the difference between one power supply and two power supplies with the same wattage. The Carvers that I almost bought, would have been a disaster.

mulveling's avatar
mulveling
1,871 posts
 

Honestly if I were an owner right now, a solid 60 Watts x2 would be OK with me, even at THD a point or 2 above 1%. I’d certainly be disappointed at being sold misleading/erroneous specifications, but that would be "close enough" - 60 versus 75 is still usefully powerful. 17 - 20 is another story. Hopefully we'll get more clarity soon. 

I know W4S cited the PT, but I’ll be really surprised if those OPT’s push out 60 Watts x2 as recognizable music.

Anyways, my audio tech & now dealer has been Gordon Waters here in Marietta GA for many years. He’s designed speakers and restored countless vintage tube amps (including a couple of mine). I trust him implicitly. On Audiokarma he posted the following interesting analysis about the Carver 275 schematic (which to Carver’s credit was included in the manual) - no comment on the transformers, just the circuit:

I was just having a conversation with some really astute techs about the Carver 275 amp, regarding a feature I just noticed, in the schematic:

carver_crimson_275.jpg

Look at resistor R53 and R44, connected to the negative speaker lead. That’s basically the same type circuit as the original Fisher 55A "Z-Matic" output-impedance adjusting circuit, just without the adjustability that Fisher provided (including the ability to turn it off, in the Fisher!). Here, it’s set to a fixed level of current feedback, reducing the damping factor. This will act to "color" the bass of most speakers- in a not-so-predictable way (each speaker has different impedance characteristics, which will interact with the network in different ways). This is ostensibly the "listening to the speaker in the room" thing that Carver was talking about, I would think. It’s funny, though, that it’s something that most experienced techs recommend REMOVING from the Fisher amps, since it’s rarely needed (unless you have very early speakers, designed for use with for amps with very low damping factors, such as 1950s JBLs, Altecs and such), and it’s rarely beneficial...

Also, more than one tech mentioned that they were struck by the myriad compensation networks needed for stability and response modification in this amp. Low-frequency shelving on the input (though switchable), HF frequency response limiting on the output of the first gain stage, LF shelving between the phase inverter and the output tubes, HF snubbing/shelving on the primaries of the output transformers, a Zobel on the output, AND a two-stage shelving network in the feedback loop itself, as well as the "output impedance modification" circuit described above. It’s rare for ANY amp to need THAT MANY different networks. Maybe two or three, tops- for example, the Eico ST70 "Hot Rod" uses three (plate-to-grid HF compensation on the phase inverter, HF snubbing/shelving on the OPT primary, and a Zobel)- but almost never SEVEN different response-modifying networks in one amp. I wonder what square wave response looks like, under normal operation, and also with each of those networks disconnected one at a time, to see exactly what each one of them is doing...

Mind you, having all those networks doesn’t necessarily mean the amp would perform badly or sound bad- but with each network added, there’s always the chance of "throwing away the baby with the bath water", in terms of sound. Every compensation network represents a compromise of some sort...

Regards,
Gordon.
jbhiller's avatar
jbhiller OP
1,387 posts
 

After all this dust settles, let's not forget what the late great Ken Ishiwata said, 

"Unfortunately, specifications don’t tell you about sound quality. That’s not just for DACs, it’s for everything. Those specifications are all based on static measurements, but music is dynamic and there are many other parameters that influence performance."

 

 

I'll take Mr. Ishiwata's view over egghead testing alone.  This guy had a dedicated listening space where he tested and tweaked products on sound quality--albeit he did care significantly about specs too.  

mulveling's avatar
mulveling
1,871 posts
 

@jbhiller

Absolutely, I agree listening results come first. This is specifically about whether Carver needs to down-rate its amplifiers, not whether the amp deserves to exist. Likewise, it raises the question: do we have a problem in the high-end 2 channel amp industry right now? Do we need to look at other companies and how they’re rating amps? Maybe we’ve been too comfortable since the HT receiver "peak power" rating debacle in the 90s / 2000s - back then it was easy to say "haha, HT guys - not my Krell!". What about now?

I hate taking sides with the ASR guys because they’re measurement zealots and (mostly) outright anti-audiophile. The overall theme of that forum is listening tests come DEAD last - which to me, means their real hobby is measurements - not music.

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