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Old 17th December 2010, 04:22 AM   #641
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Quote:
Originally Posted by madisonears View Post
... try Dario's recommeded switch of R1 and R4 from the Dale metal films supplied in the kit to KOA SPR3 carbon film.
I can confirm Tom's observation that CFRs at R1, R4 remove most of the edge off the upper mids.

I switched to 2W CFRs a while ago after experimentation. However, most values of KOA aren't available locally, so I went with the local Watts brand, which is fine - except that the resistor body is slightly magnetic. The leads and end-caps are non-magnetic, though, and it is nevertheless better than both metal-oxide and metal-film at this location.

I haven't tried them, but carbon compositions may be even better, if you can find 2W values that fit.
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Old 18th December 2010, 03:25 AM   #642
jordyyy is offline jordyyy  United States
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If I remove the large capacitors from the RevC board and add leads to them, to make them fit into my enclosure, will it have any noticeable effect on the sound?
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Old 18th December 2010, 09:30 AM   #643
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Quote:
Originally Posted by jordyyy View Post
If I remove the large capacitors from the RevC board and add leads to them, to make them fit into my enclosure, will it have any noticeable effect on the sound?
If you want to do that, it's recommended to move the bridge rectifier and snubber also off-board onto a separate PSU board on which the capacitors are mounted. That way, the large charge/discharge currents through the rectifier and caps can be isolated and restricted to a relatively short trace length on that PSU board, and the leads to the MyRef will only have to carry DC and (some of) the speaker-return current. You'll also have to move the rectifiers for the protection relay to the PSU board.
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Old 18th December 2010, 09:54 AM   #644
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Default Grounding

Some more questions to aid my understanding of the circuit/boards.

First I notice there are only two outputs for ground: OGND and PGND. I assume OGND is for the "cold" speaker terminal, whilst PGND is for attaching to the safety earth, either by directly attaching to the same mounting point for mains safety or via another moutning point that ensures a conductive path is present between PGND and mains safety. Are these assumptions correct?

Second, I notice there is no output to allow all signal grounds to be connected together for both of the amplifier boards. I am assuming that star grounding has been built into each board for all the signal grounds for that channel, but I can't see a way for bringing both channels together such that they have a common signal ground that can then be earthed to the chassis. How is this achieved?

Thanks
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Old 18th December 2010, 10:31 AM   #645
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Quote:
Originally Posted by westers151 View Post
First I notice there are only two outputs for ground: OGND and PGND. I assume OGND is for the "cold" speaker terminal, whilst PGND is for attaching to the safety earth, either by directly attaching to the same mounting point for mains safety or via another moutning point that ensures a conductive path is present between PGND and mains safety. Are these assumptions correct?
OGND is the Speaker return terminal.

PGND is actually the transformer 0 V terminal, not safety earth.

If you want to use a Safety Earth, connect a CL60 thermistor/isolator between PGND and the Chassis star ground.

Quote:
Second, I notice there is no output to allow all signal grounds to be connected together for both of the amplifier boards. I am assuming that star grounding has been built into each board for all the signal grounds for that channel, but I can't see a way for bringing both channels together such that they have a common signal ground that can then be earthed to the chassis.
There's no need to connect the signal grounds to the chassis - in fact, you might potentially create conditions for a ground loop if you do that.

Simply connect each signal ground to the shield of the shielded cable from the RCA connector. If there's only one ground terminal at the RCA connector block, this will short the signal ground from each board at the RCA connector block.

So far, I have not seen any problems with this arrangement, though there is still a ground-loop that can arise from the individual shields of the signal cable from the audio source to the RCA connectors. The two shields are in close proximity, so the stray AC frequency magnetic field lines cannot induce much voltage in the enclosed area between the two shields. A low-amplitude, barely-audible hum cannot be ruled out.

Last edited by linuxguru; 18th December 2010 at 10:34 AM.
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Old 18th December 2010, 02:17 PM   #646
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Ok, thanks. So is signal ground "floating"?
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Old 18th December 2010, 03:46 PM   #647
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Quote:
Originally Posted by westers151 View Post
So is signal ground "floating"?
The signal ground is "lifted" from PGND through a 1-ohm isolator on each board. If PGND is floating (not tied to the chassis), then the signal ground will also float.
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Old 18th December 2010, 06:33 PM   #648
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In my latest round of modest modifications, I added a CL60 between the PCB PGND and safety AC ground inside each monoblock chassis. Both amps hummed at a low level, barely audible from my listening position. I removed the CL10's and the hum disappeared from both amps.

I will not compromise the otherwise stone silent performance of these amps to add the safety feature of PGND tied to the AC safety ground. That is a choice each builder must make for himself. Electricity is dangerous, but it's not evil. I, personally, do not envision the likelihood of mains AC ever finding its way through the circuit onto the speaker posts or RCA jacks. I have no toddlers or pets in my household. The chassis itself is safely grounded, and that's good enough for me. In the remotely possible event that I die from electrocution by touching the input or outputs of a catastrophically failed, powered amplifier, I will die happy, knowing that I heard these amps at their best.

Still, I recommend including the CL10 or some other method of tying the amps to safety ground. If you can do so without inducing hum, you are fortunate and can feel secure knowing you have eliminated one unlikely but potentially fatal hazard in a world filled with them.

And why on earth (ha ha) would you want to tie signal ground to the chassis?

Peace,
Tom E

Last edited by madisonears; 18th December 2010 at 06:40 PM.
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Old 18th December 2010, 06:55 PM   #649
AndrewT is offline AndrewT  Scotland
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Hi M,
have you tried connecting Pgnd to Safety Earth with a direct wire connection?
It could be better than the NTC or worse or the same.
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Old 19th December 2010, 07:58 AM   #650
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I was intending to connect safety earth to chassis. P GND will be just the two secondaries of my trafo to create 0V. O GND will run to my (insulated) binding bost. Input GND from RCAs will be connected to a star point that will, in turn be connected to the safety earth with a single run. Notwithstanding the comments above about ground loops, I wish to replicate the absolute absence of hum when I have been testing the amps on my slab of wood testbed.

Cheers

Jon
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