Conrad Johnson ART amps - Watts at 8 ohms vs 4 ohms

The idea with most loudspeakers (but by no means all) is to drive them with something called a 'voltage source'. A voltage source simply is able to make the same voltage regardless of load.
Tubes can do this, but only up to their power limit, which in this case is 150 watts or 300 watts depending on the amp above. To get a better match, an output transformer is needed. To maintain the optimal load on the tubes, the transformer has taps for 4,8 and 16 ohm speakers. So regardless of the speaker impedance, the most you will get out of these amps is 150 watts for the 150 watt amp or 300 watts with the bigger one.

Now here's something to keep in mind. It does not matter what kind of amplifier you have, it will make more distortion into a lower impedance (like 4 ohms). This distortion is audible as increased harshness and loss of detail. So your amplifier investment dollar is best served by a higher impedance speaker if high end audio reproduction is your goal. If sound pressure is your goal you have a 3 dB reason to go with 4 ohms, but only if you have a transistor amp that can drive 4 ohms with double the power it can do into 8 ohms.

In addition, speaker cables are far more critical when used with 4 ohm speakers.

In a nutshell, its not a good idea to make any amp work hard- the harder it has to work, the more distortion (harshness) its going to make.

The transformer is part of the issue. It boils down to effective output impedance.


Think of it as a resistor in series with the speaker. As the speaker impedance drops, more of the amp's voltage appears at the amp output stage, instead of at the speaker.


Solid state amps tend to be better by using lower output impedance devices, more of them, and feedback at the output stage.


Tube amps use output transformers because of the excessively high output voltages. In the hundreds of volts, vs. what's needed at the speaker.


I did read of a very interesting design, which used tube amps directly coupled to an ESL,which love high voltages.

It's all about Ohm's Law on the secondary of the output transformer. For the 300 watt amp:

The 4 ohm tap is wound to 35 Vrms -->  9 amps at speaker load 4 ohms; 
The 8 ohm tap is wound 49 Vrms --> 6 amps at speaker load 8 ohms;
The 16 ohm tap is wound 69 Vrms --> 4 amps at speaker load 16 ohms. 

All are 300 watts.

Can anyone shed some light on what these amps put out at 8 ohms? Would be appreciated. Plan on asking CJ directly as well and will certainly report back what they say, if it’s of interest to anyone.

It appears in this case that the particular amps only provide a single output tap, which is normally wired to provide the rated 150 or 300 watts into 4 ohms. (Per the statement quoted by the OP from the CJ website they can optionally be wired such that they can presumably provide those power capabilities into 16 ohms).

Given that, and given the lack of an 8 ohm specification and relevant measurements, I would not want to speculate as to how much power these amps can provide into an 8 ohm load. Or, for that matter, how optimal a match an 8 ohm load would be in terms of sonics. Let’s see what CJ has to say when they respond to the OP’s inquiry.

Regards,

-- Al

For everyone's benefit here are some specifications for the ART-150:
Power output into 4 ohms is 150 watts. Power output into 8 ohm is 101 watts.

With the above information, which comes from CJ, I would suggest that the ART300s (which put 300 watts into 4) will put about 200 into 8 ohms.

Not sure how they are getting those power ratings with only 2  kt 150 per channel   Seems dreamy.   

@batman1971 I've heard before that CJ extracts every ounce out of the tubes in a way that no one else does. I listened to the amps a couple times before and I do not doubt the 100wpc rating based on those experiences.
The VAC Signature 200iQ puts out 100wpc with 4 tubes as well, 2 per channel, and that is using KT88s (versus the KT150s found in the CJs).
My dream amp would be a 200 wpc stereoblock from CJ, wish they would.