Conrad Johnson LP series monoblocks and ground

[Another Johnson]

I had a pair of conradjohnson LP275Ms. These were the CJ flagship 15 or 16 years ago. They’re based on a linear pentode design, single ended, and CJ upgraded them to KT120s when the KT120 became to new “go to” big power tube. There were 8 KT120s and three 6922 driver tubes per amp.

These amps had to both be plugged into the same power strip to avoid hum. The first room had two dedicated 20 amp circuits, both on the same ground bar in the breaker box and had run a pair of big Class A Krells without hum for years. I had the same hum issues with the CJ gear in four different buildings.

When Stereophile reviewed the LP125Ms, they ran into the exact same problems that I had with the bigger brothers. Here’s a link to their review which describes their frustrating experience

Conrad-Johnson LP125M monoblock power amplifier

For anyone who's been around the audiophile block a few times, Conrad-Johnson Design is a brand that needs no introduction. My first acquaintance with Conrad-Johnson was before I began writing for Stereophile (more than two decades agotime sure flies fast when you're having fun!). I was in the...

www.stereophile.com

I had ALL of the same problems, including failure of driver tubes. The solution to the hum was stated as plugging both amps into the same power strip. Obviously this merges the ground planes and kills the differential emf.

Can anyone explain why this ground loop issue seems to be inherent in the CJ LP monoblock design?

 

[XV-1]

I have Premier 8A monoblocks. How to deal with any grounding issues is in the manual and back of the amplifier. From memory when I had to separate dedicated AC lines - one for front end, one for power amp I had some noise. I just removed the shorting link per manual and zero noise. haven't thought about it since till now

Is this not on the later models? BTW - never any issues with driver tubes.

 

[Another Johnson]

That’s not what I asked.
I was impressed by the knowledge shared by some posting on the general power issue. I hoped maybe someone would know about the design issue that causes the problem I described and that is perfectly described in the Stereophile review of the LP125Ms.

 

[XV-1]

Bob Deutsch reports on the problem he had with ground loops with the LP125M. I didn't find the amplifier particularly susceptible to grounding problems,

If you read the measurements section by JA , he had no issues with ground loops.

I had no issues with grounding. You don't answer my question re the shorting links.

Your statement that cj amps have some sort of inherent design issue is false.

Have a great day

 

[Another Johnson]

There are no shorting links on the LP series.
Their existence on the P8s is more “quirky charm.” In the days of the ancient P8s, CJ was still suggesting cheater plugs too.

JA had hum issues. The solution was to lift ground on one amp, or put both on one power strip.

There are guys here who could make a legitimate stab at my question.

I’ve had my interactions with Jeff Fischel on this issue. He says they get questions about this “every day.” The solution is to put everything on one power strip. Ask him yourself.

I don’t want a solution to the hum. I want to know why CJ exhibits this behavior while Marantz, ARC, Krell, Burmester, McIntosh, Cambridge Audio, and Prima Luna (all tested in same systems) do not. There is something about how the LP series handles ground that makes it necessary. And that something is NOT superior sound, except over about half of those marques.

Please let someone deal with the technical question of why the LP Mono design has exhibited hum issues in known applications until plugged into the same power strip (I know of at least two other cases, so I don’t think it is rare with the LPs. My own problem persisted in four different buildings).

Start another thread for the shorting links on the P8 if you want. They’re not relevant to my question about the LPMs.