[morganc]

Hi Guys,
     I have an older Modwright 9.0 Pre that was upgraded one year ago by the previous owner to the current Signature edition with all mods.   I used it happily for three months.   Now in the last week, the following has happened.

1.  A hum started from the pre on the left side and lasted for about two seconds on power up. 
2.   Then also maybe a few popping sounds coming from the right side for a second or two.

I changed all the tubes, put new ones in, and nothing changed, it even got worse:
1.  After a few days, I noticed all of a sudden a loss of 80% of my volume in my right channel.   I isolated it to my DAC, replaced the DAC, and then 2 hours later, guess what, blown channnel again in the second DAC. 
2.   I took out the DAC, added a CDP, and the channel came back to life.

Now all is good for a day or two until,
1.   I install new Coincident Frankenstein 300's and get a very loud, oscillating
hum through one channel (this time the left). It comes and goes and is not dependent on volume.   I change power cables, sockets, interconnects, etc. And no change.   Brand new tubes in the 300.  Change the tubes, and no change in the hum.  Change PC, outlet, etc. And no change.
2.   I install an Uber Buss and no change.
3.   I exchange the RCA input going into the amplifier (reverse left to right) from the Modwright, and the hum changes sides. 
4.    I them remove the modwright and connect the amp directly to the cdp and the hum is gone.

So what do I do now? 

All the problems started with a hum coming from the pre on power up followed by some popping noises that sometimes were loud and sometimes not so loud. Then the DAC blew x 2 , and now the loud hum that sounds like a ground loop hum......

Can anyone help?   I have asked many their opinion and no one knows. 
Thanks,
Morgan

[woodsyi]

I would send it immediately to Dan to have it checked out.  Since you got it used, you may have to pay for the repairs but I wouldn't trust the unit until it's been checked out by Modwright.  That's what I would do.

[morganc]

I would send it immediately to Dan to have it checked out.  Since you got it used, you may have to pay for the repairs but I wouldn't trust the unit until it's been checked out by Modwright.  That's what I would do.

Yes that is what I decided to do today and sent it in........

[modwright]

We will go over it thoroughly.  I do not see how a source fed into the INPUT of our preamp could damage the source, but no matter what, we will check the unit out and service it as required.

We service ALL of our equipment.  If out of warranty, their will be parts and labor charge, but we will service all of our equipment as long as they are in use.

Thank you,

Dan W.

[morganc]

Thanks Dan,
    I appreciate your excellent service and it doesn't make any sense to me either, but I thought it wiser to send it to you rather than find a local guy take a look at my entire system.  I look forward to getting your excellent pre-amp back home.  We all miss her already
Cheers,
Morgan

[morganc]

Just wanted to update everyone.......I took the Modwright out and Dan has given is a clean bill of health.......

In the Meantime,
i received a new Tranquility DAC that is working flawlessly
i have a Dodd Tube Buffer that is so nice

Yet, guess what, I plug in the amps that I plugged in when all this started, and i have a high frequncy oscillation out of both speakers.  This is a very low level but highly irritating screech.  This is nothing like the loud hum that I was having before.   The amp is the highly regarded Coincident Frankenstein 300 Monoblocks.

I change amps, and the hiss and high frequency oscillation goes away. 

Please tell me, is there any way this is anything but the amps?

The owner from Coincident tells me there is no way that both amps Would have a problem, but what else could it be?   I have had 15 amps and never a problem in my system until this one was bought....so please help:). 

[modwright]

It sounds like an oscillation at the tubes.  This could be an oscillation in the source that is feeding the amps, but I believe the buffer in between would cause it first.

I would first try to run the DAC straight to the amp, without buffer, provided you have a volume control somewhere in the loop.  It really sounds like tube oscillation and could simply be a bad tube in the buffer or amp.

I would first isolate the buffer and amp.

I agree that it is unlikely that both amps would do this, unless they are both being fed a signal that is causing them to oscillate - namely because they are being fed an oscillating signal.

Please let me be clear.  I am not implying in any way that the amp, buffer or DAC manufacturer are to blame.  I am simply trying to help you resolve this.

The preamp hum that you were experiencing (our SWL 9.0SE) had to do with the fact that the chassis cover was not making good electrical-mechanical contact with the rest of the chassis and was hence not grounded properly.  Also, the 5687 tubes that were sent with the unit were EXTREMELY microphonic and could have also further augmented the problem.  It is running now with a new pair of 5687's from our stock and is dead-quiet in terms of hum and microphonics.

In the end, I found nothing wrong with the preamp, other than microphonic tubes and improper chassis grounding.

I hope this helps.

Thanks Morgan,

Dan