Carver 350 Monoblocks

[paul_b]

I had the opportunity today to do some bench testing of the Carver 350 amplifiers. To start with, we pulled the cover off the output transformer to take a gander at what's in there:

This is a far larger transformer than I found in the 275. It's the same lamination size as the 280 watt Hammond 1650W, but half the stack height, so Hammond would rate something like this around 150W. There's also a nice ding on this one from being dropped, but the buyer would never have known that since it's covered up.

Here are the specs from the website (as of 7/31/2022):

Let's start with power, and I'll just leave the amp with high feedback on to keep the numbers as nice as they can be. At 1kHz into an 8 ohm load on the 8 ohm taps, the Carver 350 makes 7W at 0.5% THD. At 1% THD I see 11.28 and 24W at 2% THD. Moving up to 5kHz I am only able to get 2W at 0.5% THD. Back to 1kHz, I stopped measuring at 170W where distortion was up around 2.8%, which is where you see similar amps rated. Back at 5kHz and 2.8% THD, power is about 14.5W. The power bandwidth is claimed to go down to 24Hz, but at 0.5% THD and 24Hz I was unable to resolve any power. At 1% THD and 24Hz, 9W is available and at 153W THD is 5% and crossover distortion is very visible in the scope trace.

The specifications continue by claiming that the amp will make more power into 4 ohms, though the binding posts on the back for the 4 ohm connection are connected to the 8 ohm post...

At 1kHz, 4 ohms, and 0.5% THD, the 350 makes 4.5W. At 5kHz and 0.5% THD that drops to 2.25W. The amp hard clips at 6% THD and 272W, and 72W are available at 2.8% THD with 4 ohm loading.

Likewise the frequency response is nice, but not as advertised. To get a better stab at the extremes, I abandoned the FFT and just used an AWG and my scope. At 2.83V/1kHz switching the signal to 85kHz leaves me with 1.6V of signal, so about 5dB down at 85kHz. The actual -3dB point is 61kHz. At the bottom end starting at 100Hz (to give some adjustment for the swoopy overall frequency response), the amp is -3dB at 10Hz.

Oddly enough the source impedance from the 4/8 ohm tap is 1 ohm, so better than specified...

This pair of amps will be in the area for a while if there are other areas of interest.

 

[paul_b]

Yes, one amp was brought over for testing purposes. I'm sure I could have the second one brought by if that would be helpful.

 

[paul_b]

I measure a small output voltage with and without a load. The difference between loaded and unloaded output voltage and the known loading resistance is enough to calculate source impedance.

 

[prime minister]

Closed while I think what To do with this thread.

Edit: And no, Jim did not get the last word. He posted just I was locking the thread.

If anyone wants to continue this, please PM me.

 

[prime minister]

As this thread has become quite contentious, I've gone back through the thread and basically pared it down to the posts from Paul about his review, FOR THE TIME BEING. The deleted m posts are not gone, they are just not visible at the moment. There is a lot of great info and discussion here, and I'm trying to sort out a way to keep it all visible, while bringing the temperature down a bit.

 

[paul_b]

I took the other amp to Amir at ASR for testing and his measurements can be seen here:
Carver Raven 350 Review (Tube Amp)

From his review, 4 ohm power at 0.5% THD (-46dB on the graph) comes in at about 1.5W at 500Hz-1kHz and 250W before hard clipping, though he was able to get closer to the ratings with an 8 ohm load than I was.

Of particular note is the graph showing power vs. THD for many frequencies, and the 20Hz line never dips below the -45dB level, which is consistent with my original findings that there isn't available power down there at the specified THD level, nor is there any output below that THD above 10kHz in his findings.

 

[Audionut]

I took the other amp to Amir at ASR for testing and his measurements can be seen here:
Carver Raven 350 Review (Tube Amp)

From his review, 4 ohm power at 0.5% THD (-46dB on the graph) comes in at about 1.5W at 500Hz-1kHz and 250W before hard clipping, though he was able to get closer to the ratings with an 8 ohm load than I was.

Of particular note is the graph showing power vs. THD for many frequencies, and the 20Hz line never dips below the -45dB level, which is consistent with my original findings that there isn't available power down there at the specified THD level, nor is there any output below that THD above 10kHz in his findings.

Thanks for taking this on Paul and I am not surprised that Amir’s testing backed your findings (again).

 

[Ingenieur]

The amp clipped at 312 W at 8 Ohm
THD+N< 45 dB, 0.56%
<50 dB, up to 10 W, most of the listening window, 0.3%
Very low at 1 W, 55 dB, 0.2%

Response +/- 0,25 dB 20-20,000 Hz

Dynamic power , 4 Ohm, 530 W, 1% (peak)
Continuous 270 W (average)
As you would expect peak ~ 2x average

imo impressive for a tube amp.

This is a bit confusing, line at 1% THD+N
Should have done it 8 Ohm
45 kHz BW for tubes?
But still should supply adequate power for any real world listening scenario.

 

[House de Kris]

Thanks paul_b for bringing this thread back to life. Just wondering if you'd care to share your equipment setup and methodology with us.

I was wondering about this comment in the OP: "The specifications continue by claiming that the amp will make more power into 4 ohms, though the binding posts on the back for the 4 ohm connection are connected to the 8 ohm post..." Most tube amps have the 8 ohm terminal connected to an 8 ohm transformer tap, with the 4 ohm terminal connected to the 4 ohm tap. Thus, when an 8 ohm load is attached to the 8 ohm terminal, you get the same power as when a 4 ohm load is connected to the 4 ohm tap. Doing what output transformers do, keep the reflected impedance at the primary the same, no matter dramatic changes in the actual load impedance. But, because the specs indicate you get more power with a 4 ohm load than an 8 ohm load, it is implying that there is no different tap for the 4 ohm terminal, yet you seem to express surprise at this.

 

[Shelby1420]

Can anyone out there tell me how the amp sounds??? Have worked on and tested several hundred pieces of gear, some tested bang on( Mac and Krell and a few others) that sounded absolutely dreadful, and then some-others that tested horribly ( Vector Research and a home made EL34 amp) that sounded simply sublime!!!!! Did Carver not state his gear would need to be tested under real world parameters?? Has he not stated that for his 50 or so years in the business?? Just curious…….

 

[Ingenieur]

Some additional thoughts:

20 Hz
How many speakers can produce this?
Typically most get down to 40-50, good ones 30 Hz, but at those levels THD/N is double digits, dwarfing the amps contribution. Plus the ear is not sensitive to them. It would have been nice to see 50 Hz.

> 5 kHz
Based on pink noise and typical music energy spectrum very little power there.
If we assume 100 W at 40 Hz, power at each is ~
5 kHz <2 W
10 KHz <0.5 W
20 kHz <0.25 W

I'm no amp designer but it would seem logical to design to optimize power performance in the 40-5,000 Hz range where most of the content is. If power rolls off above that so what, you'll never need 100 W at 20 kHz.

If your average listening level is 75 dB with 87 dB eff speakers (2 each, 3 meters, room gain). You will need ~ 0.05 W average.
With peaks of 100 dB ~ 16 W average, 32 peak.

I'm an engineer. Specs mean something, but so does context and correct interpretation.

This is not commentary on advertising etc., only on what the data tells us about application of the object of the data source for it's intended function.

 

[Ingenieur]

Thanks paul_b for bringing this thread back to life. Just wondering if you'd care to share your equipment setup and methodology with us.

I was wondering about this comment in the OP: "The specifications continue by claiming that the amp will make more power into 4 ohms, though the binding posts on the back for the 4 ohm connection are connected to the 8 ohm post..." Most tube amps have the 8 ohm terminal connected to an 8 ohm transformer tap, with the 4 ohm terminal connected to the 4 ohm tap. Thus, when an 8 ohm load is attached to the 8 ohm terminal, you get the same power as when a 4 ohm load is connected to the 4 ohm tap. Doing what output transformers do, keep the reflected impedance at the primary the same, no matter dramatic changes in the actual load impedance. But, because the specs indicate you get more power with a 4 ohm load than an 8 ohm load, it is implying that there is no different tap for the 4 ohm terminal, yet you seem to express surprise at this.

Exactly
There is only 1 secondary for 4/8 Ohm
You can look at it as you did, reflected Z, or V.
Assuming 1:1
The load will be 4 or 8.
The V will be the same for either.
The 4 Ohm load (obviously) draws more current, hence more power.

I assume one reason taps are used is to keep the apparent primary load constant. That way current is regulated and the amp can be optimized for a single load point, not a range which would be a compromise, the magnitude of the compromise depending on the load swing.

The xfmr also serves as a DC filter.

Considering this, and the normal Z swing of a speaker, 2-16+ Ohm, a 1:1 single tap xfmr seems like a reasonable design choice.

 

[Audionut]

Can anyone out there tell me how the amp sounds??? Have worked on and tested several hundred pieces of gear, some tested bang on( Mac and Krell and a few others) that sounded absolutely dreadful, and then some-others ( Vector Research and a home made EL34 amp) that sounded simply sublime!!!!! Did Carver not state his gear would need to be tested under real world parameters?? Has he not stated that for his 50 or so years in the business?? Just curious…….

I hear what you are saying, but what came into question is not that the company’s advertising “it sounds amazing!” And Paul found otherwise. Rather, it is their specs that have come under scrutiny and for good reason.
Two different amps, tested by two different persons and you get the same results. Oh, and one company rep that gets defensive/offensive and doubles down on half truths. I’m not sure what else is left.

Granted, tube amps generally don’t test well, so I’d not buy one for their specs (low watt application), but If I buy an amp claiming to make 75 or 375 watts (because I need them for my power hungry speakers), then I’d like to know if it does or doesn’t meet those specs. Now, if I am happy with the sound, then I ignore all of this and enjoy my purchase.

 

[Ingenieur]

The specs are vague.
What does the 0.5% mean?
What BW, how many harmonics?
Power or voltage? They are different.
It does not include N, that is a separate parameter.

Baseline 350 W/8 Ohm/52.9 V

Power based:
0.005 x 350 ~ 1.75 W, 3.74 V
3.74/52.9 ~ 7% relative to voltage
-23 dB

V based:
0.005 x 52.9 ~ 0.265 V, 0.009 W
0.009/350 ~ 0.0025% relative to power
-46 dB

This would seem to indicate that THD is at a fixed power and frequency, not rated power across the BW.

I (we?) need the mfgs test conditions before making an informed opinion. Also 20 Hz is outside spec, the low end is 24 Hz. Measure there. Much difference? Who knows.

 

[JoeThePop]

I hear what you are saying, but what came into question is not that the company’s advertising “it sounds amazing!” And Paul found otherwise. Rather, it is their specs that have come under scrutiny and for good reason.
Two different amps, tested by two different persons and you get the same results. Oh, and one company rep that gets defensive/offensive and doubles down on half truths. I’m not sure what else is left.

Granted, tube amps generally don’t test well, so I’d not buy one for their specs (low watt application), but If I buy an amp claiming to make 75 or 375 watts (because I need them for my power hungry speakers), then I’d like to know if it does or doesn’t meet those specs. Now, if I am happy with the sound, then I ignore all of this and enjoy my purchase.

I agree @Audionut. If manufacturers would just test to established standards it would help individuals like me that have very little access to brick-and-motor HiFi stores do some preliminary shopping to determine if an amp may be a good fit power wise (and other parameters) for my speakers. For that reason and as a means of keeping manufacturers honest, I am sorta a measurements guy.

The problem I have with some of the "measurements are everything folks" is on display in the ASR comments, and why I am a member on this forum and not ASR. Most of the people their seemingly dismiss Tube Amps offhand based on measurements most likely never having heard one. And then they deride and mock those that would even consider owning anything but the best measuring amplifiers. Too much for me.

 

[Audionut]

I agree @Audionut. If manufacturers would just test to established standards it would help individuals like me that have very little access to brick-and-motor HiFi stores do some preliminary shopping to determine if an amp may be a good fit power wise (and other parameters) for my speakers. For that reason and as a means of keeping manufacturers honest, I am sorta a measurements guy.

The problem I have with some of the "measurements are everything folks" is on display in the ASR comments, and why I am a member on this forum and not ASR. Most of the people their seemingly dismiss Tube Amps offhand based on measurements most likely never having heard one. And then they deride and mock those that would even consider owning anything but the best measuring amplifiers. Too much for me.

You definitely need to have thick skin if you like tube technology and are an ASR member. Like you, I too buy most things based on measurements (and advertised specs, along with consumer ratings). Sadly, more and more the “walking into a store” to try something is going away. All the more reason to have honest companies and third parties testing said specs.

I bought my Cambridge phononstage based on a recommendation from a havenite here. I then verified it with great reviews and a positive ASR write up.

How a company rep or dealer responds to critique, tells a lot about their character and/or the companies Moda Operandi.

 

[Jim Clark]

Bob Carver Amplifier Design Philosophy - What Sounds Good?


Since the early days, after earning my physics degrees, my approach to audio design has created controversy.

My unconventional approach has brought both criticism and accolades. World wide recognition for achieving musical excellence for my wonderful fans, while offering a more affordable product, compared to most other brands of comparable products, is a great pursuit.

My amplifiers have often been smaller, lighter and less costly than others, while remaining powerful, musical, and accurate. These designs and their musical performance, compared to others are quite successful.

What Makes An Amplifier Sound good?

Dynamic power, low distortion and wide frequency response. My tube amplifiers have high voltage (B+) and the power supplies have ability to ‘bounce' and increase voltage, closely tracking the musical load with very little distortion.This is an important key to a musical performance.


Do You Design Amplifiers Using Load Resistors or Speakers?

Both. On my bench I start out with resistors, then I use different speakers, with a scope and voltmeter connected, while playing music and measuring the amp and speakers reacting together. The back EMF that is present makes speakers slightly easier to drive. Power response, by design, tapers below 80Hz, yet frequency response goes below 20Hz.

My designs will drive difficult loudspeaker loads, playing music far better than the specifications listed, without clipping, and with lots of headroom available.

These long held design targets have served the industry well. The designs have delivered excellent performing, highly musical products that more people could afford, without sacrificing the powerful and musical performance when powering loudspeakers.


Stay tuned for more of my very latest designs and the on-line store coming soon.