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Old 19th November 2012, 06:24 PM   #741
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Quote:
Originally Posted by bcmbob View Post
Jac,

Dario Is trying to help with some unexpected voltages on my builds - possibly leakage or some defective parts. This is what I'm getting:

Neg probe to PGND
Pos probe to safety/earth Gnd.

RC Build
Left AC= 45.9V-----Right AC= 27V
Left DC= +200mV --Right = +158mV (these readings keep going up - stoped at 300)

Beta Build
Left AC= 88V ------Right AC = 88V
Left DC= -58mV ----Right DC = -47mV

These are both on boards. Both play music perfectly if correct source is used. I don't think it is a no-metal-chassis problem.

?????????
NOW I UNDERSTAND!

This is a weird suggestion. Try the interconnects and measure the output of whatever is before the amp (pot I am assuming?). If I understand this correctly, the path to ground is the input cable shield. I high resistance or open circuit could leave the amp floating very far away from the safety ground.

Dario should comment first on whether this is a good idea, but I might try a fused connection between the input shield and safety ground. I'm guessing this is almost the same as connecting PGND and will introduce hum, but if it blows the fuse, then it suggests more power than "floating".

Man, I hope you figure this out.

Jac
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Old 19th November 2012, 07:03 PM   #742
bcmbob is offline bcmbob  United States
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I'm going to try a different VOM later today just in case.
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Old 19th November 2012, 07:22 PM   #743
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Quote:
Originally Posted by lehmanhill View Post
Just for clarification, I do not have any hum and I have the FEs mounted on a wooden board. That means, no connection between safety ground and PGND.
Fine

Quote:
Originally Posted by lehmanhill View Post
But it was still interesting to measure, so...... with the amps on and connected to the DAC + bare pot, I measured 0.4 mV DC, then switched to the AC channel and measured about 10.4 mV AC. This was the same for both channels and stayed the same with music playing or not.
I've similar readings.

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Originally Posted by lehmanhill View Post
so connecting PGND to safety earth might be expected to generate a hum.
Mmh, actually it's not so...

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Originally Posted by lehmanhill View Post
If we don't connect the FE amp to safety ground, but the chassis is properly grounded, then safety should be served as long as the box stays closed. I'm not sure this is true with Class 1 transformers. I need a better understanding.
I do agree but I'm not an expert here.

Quote:
Originally Posted by lehmanhill View Post
It looks like the FE and other MyRefs refer to ground through the shield of the input signal via R11, so the DAC or other source provides the reference to ground. That assumes isolated single end connectors at the chassis.
Exactly

Quote:
Originally Posted by lehmanhill View Post
If you follow Andrew's suggestion, then the input shield connects to chassis ground and, if you have a high quality star ground near that point for the safety ground, then you should minimize the hum risk and reference earth within the amp. This approach is recommended by Douglas Self. In neither case would PGND be connected, although it might be interesting to see what happens if you ran a wire from PGND to the star ground.
I'm not sure this is what Andrew said...

From what I know input shield connected to chassis (and chassis to safety earth) would imply a jumper in place of R11...

Quote:
Originally Posted by lehmanhill View Post
As I say above, I am just trying to better understand what is happening so that I can build a boxed version that stays quiet for hum.
For my brother build (and when I'll finally assemble mine) I've used a simple grounding scheme:

chassis to safety earth, input shield insulated from chasssi, PGND left floating.

When I'll have my chassis ready I'll try to connect PGND to safety earth via a ground breaker.

Quote:
Originally Posted by lehmanhill View Post
Dario should comment first on whether this is a good idea, but I might try a fused connection between the input shield and safety ground. I'm guessing this is almost the same as connecting PGND and will introduce hum, but if it blows the fuse, then it suggests more power than "floating".
I don't know for sure but it sound potentially dangerous to me...

I would focus my attention to transformers wiring...
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Old 19th November 2012, 08:23 PM   #744
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by ClaveFremen View Post
I don't know for sure but it sound potentially dangerous to me...

I would focus my attention to transformers wiring...
Last week I took the risk and reversed the power leads on one board. They normally are as in the photo - Green closest to M-Caps. Made no difference on hum and amp worked correctly.

Just now checked transformer while not connected to amp - 27.1 & 27.2.

I did blow a non-slow-blo 3 Amp fuse at first attempt at this, so I'm going to remove the transformers and check for internal short/leakage in each one. These are the Antek 300VA with extra shield. I did power the RCs with the ApexJr 120 VA toroids last week when I removed the sockets. Hum was still there.

Sunday I reheated everything except the heat sensitive pieces.

If anyone other than Dario, Jac and me has access to the measurements as shown above, please read and post when it's convienient.

Thanks All
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Old 19th November 2012, 09:45 PM   #745
bcmbob is offline bcmbob  United States
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Don't laugh - I'm just learning

My first attempt to use some diagnostic software - RMAA -with the MyRefs. I settled on a firewire Mackie Sarellite with high quality ONYX preamp section. It was the quietest of several tested. I don't know how helpful this is cause as I read the charts - I should put the sockets back in

Just used one channel and adjusted to 2.3 db on all tests. The Visual Analyzer is much more sophisticated but has a high learning curve and no English documentation. Here is a link to a tutorial if anyone is interested. The thread here is mostly inactive but I'm pushing -as usual. When I learn what I'm doing VA should be able to show a lot of voltage and scope info.

Not a lot of great or useful information here but - "Every Journey Starts With The First Step"

Edit: These were performed on the beta builds.
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Last edited by bcmbob; 19th November 2012 at 09:51 PM.
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Old 20th November 2012, 07:02 AM   #746
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hi Bob,

i'll take the measurements tomorrow and post them.
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Old 20th November 2012, 07:29 AM   #747
atupi is offline atupi  Romania
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Guys, it hapened second time to me to go for a speaker listening with my MYREF and to trigger SPIKE (i suppose).
First time happened with Spendor SA1 and now with XTZ 99.25MKII.
Things are the following: when i play to high sound levels but not so high i can hear some cracks in the tweeters. If i decrease a little bit (3db) sound level from preamp this cracks are gone.
Spike is about limiting voltage/current on output or is about chip temperature?
Should i decrease a little bit the transformers voltages?
I'm running 2 toroids 2x24v, 250VA.
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Old 20th November 2012, 07:39 AM   #748
bcmbob is offline bcmbob  United States
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What are you using for heat sinks? Can you post a picture?
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Old 20th November 2012, 07:44 AM   #749
atupi is offline atupi  Romania
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Heat Sink A4291-60 :: Heat Sinks :: Electromechanical :: Electronic Parts :: Banzai Music
I can post a picture from my amplifier tomorrow.

The sink does not become very hot after even long listening sessions, this happens with rather insesitive or power hungry speakers and from here i conlude is not related to heatsinking.

Last edited by atupi; 20th November 2012 at 07:49 AM.
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Old 20th November 2012, 07:51 AM   #750
bcmbob is offline bcmbob  United States
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That looks big enough. Did you use good thermal paste on the chip? You should also check that the LM3886 is truly flat against the heat sink. If the chip touches the board a gap can exist near the bottom. This is the FE, correct?
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