Luxman 350 Preamp sounds like crap...

[premiumplus]

I had been driving the DQ-10's with my rebuilt SX-1250, and yesterday I finished my PL400 dc protect mod, so I disconnected the 1250 and put in my Luxman C350 pre and the PL400. SOUNDED LIKE CRAP. Dull, lifeless, flat, compressed...so I reconnected the SX-1250 and used it as a preamp instead. WAY BETTER. I mean night and day better. What gives? I got the Luxman last winter, and changed out all the electrolytics except for the power supply ones. Cleaned and faderlubed all the switches. Sounded fine for a while, and this happened once before, the sound went to hell. For lack of a better description, it sounded like the tone controls had been turned down, removing bass and treble definition. It's very strange because it sounded great in my basement lab bench system.
What do you think? Power supply caps were not changed, so I guess the first thing is to ensure that the power supply is up to snuff. But after that, how does one find out where the sound degrades in a preamp? I have a couple of nice function generators, digital and analog Tek scopes, 3 nice Fluke 87 DMM's, signal tracers, cap analyzers, transistor and tube testers, solder station etc., but I find it really tough to zero in on a slightly degraded signal vs a completely dead unit. I guess this is where the technician earns his stripes, and although I've been doing this for years, this is the type of problem that I dread.
So I'll start by checking the power supply...hopefully that's where the problem lies. It does sound compressed and lifeless, so it makes sense that the trouble may be there.

 

[speakerman1]

Sort of like mono out of both speakers?

 

[Rex Everything]

Not that it is causing your issue but power supply lytics should always be the first ones replaced as they are the ones that can cause catastrophic damage like loosing a PT.

 

[premiumplus]

Not that it is causing your issue but power supply lytics should always be the first ones replaced as they are the ones that can cause catastrophic damage like loosing a PT.

Actually, I opened it up and found that I had indeed replaced all of the power supply caps too. Memory like a steel trap, man...everything that goes in gets mangled!!!
I injected a 1 KHZ sine wave and started probing, and I found that at the headphone jack I've got a distorted signal. Not sure what that means, becasue at the output jacks all is well. But that can't be helping things. Just getting started, and had to come upstairs to answer the mail man...WooHoo, my Dahlquist active crossover is here!
I'll check in later, thanks guys!

 

[premiumplus]

Sort of like mono out of both speakers?

Not really, just compressed kind of. It just is lifeless and flat sounding.

 

[Rex Everything]

Actually, I opened it up and found that I had indeed replaced all of the power supply caps too. Memory like a steel trap, man...everything that goes in gets mangled!!!
I injected a 1 KHZ sine wave and started probing, and I found that at the headphone jack I've got a distorted signal. Not sure what that means, becasue at the output jacks all is well. But that can't be helping things. Just getting started, and had to come upstairs to answer the mail man...WooHoo, my Dahlquist active crossover is here!
I'll check in later, thanks guys!

Thats anice thing to come in the mail...

If that Luxman keeps giving you trouble get it out of your hair and send it to me

 

[orange]

Now, I'm starting to get confused...is the sound flat and dual mono at the speaker outputs or the headphone jack (which is fed by it's own amp)?

I'd consider what is wrong at the headphone amp and work from that point backward.

 

[orange]

Another thing I should ask offhand is:

Have you unloaded the pre and tried one source at a time to find an overloaded input?

 

[premiumplus]

Another thing I should ask offhand is:

Have you unloaded the pre and tried one source at a time to find an overloaded input?

That's what I was doing earlier, and I can't find anything wrong with it. But it sounds lo-fi at best.
Tomorrow I think I'll try driving it a little harder with the function generator and see if I can locate any overloading inputs like you suggest, Orange.
The headphone amp magically began working after I powered it down and back up. And it won't fail again, but the thing still sounds bad...
I'm done for tonight. Thanks guys!

 

[premiumplus]

Went round and round with this, finally found a microfractured/cold solder joint in the power supply...
Gotta love it when things finally get right!

 

[laatsch55]

Next to ground loops and hum, a cold solder joint is the most frustrating.

 

[8991XJ]

So this thing sounds good again? More info is needed. How good? Really glad you got it fixed as we don't like to read about a Luxman that sounds bad.

 

[premiumplus]

So this thing sounds good again? More info is needed. How good? Really glad you got it fixed as we don't like to read about a Luxman that sounds bad.

Yeah, it's great. It is amazing what a microfracture or cold solder joint in the power supply can do to fidelity.

 

[Northwinds]

I have a Carver C1 that the tone does not work right, I normally don't use any tone controls but it just points to a needed overhaul when I can get it done. I am thinking of the BillD mod noted on the Carver forum but he is now deceased so I am hoping someone here may do the needed work in the future. I am not using it now, back to the Carver CT-24

Glad you got your preamp back up and running right!

 

[Gepetto]

Yeah, it's great. It is amazing what a microfracture or cold solder joint in the power supply can do to fidelity.

It all starts with power...

 

[premiumplus]

It all starts with power...

Absolutely. The sound that gets to the speakers originates in the power supply, and if that's not stable and clean, everything after it has no foundation.
That preamp sounds wonderful now.

 

[laatsch55]

It all starts with power...

Gadzooks and Forsooth!! Me thinks I've heard that before, Right after rule #1