2. amps-preamps
  3. 193768-203308

Did I just cook my preamp?

I have a Simaudio Moon 110LP phone preamp amplifying a Dynavector 20X2L cartridge on a VPI Classic. It feeds in to an Outlaw Audio RR2160 amp which drives Magnepan LRS speakers.
 

I recently moved and two months in I realized my speaker placement wasn’t quite right, so today I reorganized my listening room. This involved unplugging some power cables but I kept most of the interconnects in place. I did have to disconnect the phone stage from the amplifier.

 

After getting things back into place, I listened to some music using coaxial input before reconnecting the interconnects of the phono stage. When I tried to, I actually got some electric current that burned my hand slightly. This came from the back of the amplifier. I made sure everything was unplugged and tried again - this time a spark and smoke from the interconnect making contact to the back of the amplifier.

 

I’m so confused why this would happen, but eventually I did get everything connected. Now the output from the phono stage is just a bump every 1 second. It doesn’t amplify the signal from the TT.

 

My amplifier has a built in phono stage and using this I was able to verify that the turntable is still producing a signal. The built in phono stage sounds terrible, however, as thin and flat as paper. It is music, however.

 

When I connect the phono stage to the power, the blue light on the front illuminates for a moment and then goes dark.

 

Incredibly, when I was unplugging the phono preamp, I actually got some current from simply touching the exterior of the box. Something is seriously wrong and dangerous with my setup, and this box was grounded to the turntable with a ground cable, which was connected to the outlet with a three prong cable with ground.

 

Has anyone experienced anything like this before? I will email Simaudio and see if they’ll repair it. I’m also taking recommendations for replacements. I liked the 110LP and maybe will just replace with the 110LPV2.

obarrett
obarrett OP
28 posts
 

There is continuity from the ground prong of the TT to the tone arm ground lug. No continuity from either of the other two prongs of the TT.

I attached one three prong power cord to the outlet where the amp/preamp/CDP are connected and then the other to the outlet where the TT is plugged in. I turned mm to V/LoZ, attached one lead to the ground of one, the other to the ground of the other. 120Vac.

so the issue is clear: there is a 120V difference between the grounds of the two outlets, which explains every measurement I’ve made as well as explaining the spark event I observed.

doesnt this mean one of these grounds is hot? If so that seems super dangerous. I’m not entirely sure how to check which one is messed up, but this is probably related to the issue of reversed polarity I observed when testing the outlets where the amp is connected to. Maybe the polarity isn’t reversed, just the ground is hot. But when I put one lead of the probe into the ground there and the other on a piece of metal which is unplugged from anything, I see only 5V on the MM.

@devinplombier i don’t understand your criticism. The issue I just diagnosed wouldn’t be caught by your five-step recipe. The eventual issue is to me quite surprising but I’m not an electrician and I don’t know how common such an issue is.

I still don’t even understand what it means that the grounds of the two different outlets have a 120Vac difference.

thanks, Jim, for all your help. Without it, I wouldn’t have caught this issue, I would have bought a new preamp and I would’ve fried that one too.

jea48
4,671 posts
 

@devinplombier said:

This thread has become surreal.

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devinplombier post:

446 posts

In response @obarrett said:

isnt that it? 69Vac on LoZ is a major issue, right?

@devinplombier response:

It’s most likely a major issue with your meter. A good meter should accurately measure a hard voltage in either standard or LoZ modes.

"It’s most likely a major issue with your meter. A good meter should accurately measure a hard voltage in either standard or LoZ modes."

No, it’s not a problem with the brand-new multi-meter.

"isn’t that it? 69Vac on LoZ is a major issue, right?"

First a digital voltmeter with a LoZ circuit meter setting incorporates a load across voltmeter circuitry. That pretty much eliminates phantom, ghost, false, AC voltage measurements. I believe I read in the manual for the Klein multi-meter, (the OP bought), the load resistance is 3K ohms.

@obarrett posted this:

121Vac on V

68.9Vac on LoZ

There is a great example of how it works. The unloaded voltage reading measured 121Vac (Using the V setting on the multi-meter.)

The LoZ measurement of 68.9Vac reading indicates there in not a Bolted Hot ground fault. Bolted? A solid Hot to ground electrical fault. The LoZ 68.9VAC tell me the is a VD, (Voltage Drop), in the Hot to ground fault. Exactly what is causing VD, series resistance, I have no idea. I just know from @obarrett posted his voltage measurements it is not a Bolted Hot to ground fault.

FWIW, for troubleshooting lose or open electrical wiring circuits in the electrical field a regular multi-meter is worthless, unless it has a LoZ setting.

A regular digital voltmeter internal resistance is in the megohms.

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devinplombier
467 posts
 

@obarrett 

Not criticizing, just outlining a simple, safe protocol. 

A couple things though: 

- Stop using LoZ mode. It is dysfunctional in your particular meter, and in any event it is not appropriate in this situation.

- Do you still measure 120V between your two grounds when your DMM is set to standard AC voltage mode?

- If so, you may be looking at a bootleg ground AND reverse polarity at one of the outlets. Unusual but lethal.

- If that were the case though, it's likely been there for much longer and would have manifested in the past.

But if that is in fact the issue, it is super dangerous.

obarrett OP
28 posts
 

Do you still measure 120V between your two grounds when your DMM is set to standard AC voltage mode?

Yes I measure 120Vac across the grounds of the two outlets in V AC mode on the MM. well, to be clear, both outlets have 2-to-6 outlet converters sitting on top of them and I’ve connected a three prong power cord to each. So there are other things to check.

jea48
4,671 posts
 

@obarrett Said:

There is continuity from the ground prong of the TT to the tone arm ground lug. No continuity from either of the other two prongs of the TT.

OK

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I attached one three prong power cord to the outlet where the amp/preamp/CDP are connected and then the other to the outlet where the TT is plugged in. I turned mm to V/LoZ, attached one lead to the ground of one, the other to the ground of the other. 120Vac.

IF true that indicates the wall outlet the TT is plugged into has a Hot equipment ground contact at the receptacle. (At this point there is not an internal Hot fault in the TT)

Before I go any farther... Test the receptacle as you have the others for the correct AC hot and neutral connections polarity on the receptacle outlet and from the Hot to the equipment ground contact. EDIT. Use both the V and LoZ mm settings.

Is the Electrical panel for your outlets in your apartment? If not do you have access to where it is located?

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