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Post by broovwa on Jan 22, 2010 22:10:30 GMT
Gday guys, first post here, but have been reading up here for a few months while i've been slowly building my SCHA. I've finally got it all together in its case and it's working quite well... Sounds fantastic as stock... but i have one small problem that i cant seem to iron out.
When the inputs from my soundcard (M-audio Audiophile 24/96) are plugged in, i can hear this very faint hum or whirring sound coming from both channels. When the inputs are removed, or when the computer is turned off, the whirring sound stops, leading me to believe that the whirring sound is coming from a fan or other component inside the computer. The thing that is throwing me is that when i listen to the soundcard through my rotel amplifier's headphone jack, there is no hum at all.
What should i do next?
Thanks for any help you can provide!
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Deleted
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Post by Deleted on Jan 22, 2010 22:25:55 GMT
broovwa Do you have the HA plugged into an adjacent socket on the same powerboard as the PC? Doing this sometimes helps .Do you have the earth pin on the PCB connected to mains earth on the transformer mounting plate ? If not, try that. Lastly, with an earlier version, I connected the mains earth on the transformer mounting plate,to a banana plug that I mounted on the rear panel of the HA. From there, I connected a lead under one of the top side panel mounting screws on the PC itself near it's PSU. All mine have been dead quiet,except for the one where I connected to the PC's metalwork .After that it was. Alex
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Post by broovwa on Jan 23, 2010 3:46:20 GMT
Gday Alex, i appreciate your help with this man!...
When i'm using the HA with the pc i've got it plugged into an adjacent socket on the same powerboard, so this isnt the issue. The earth pin on the PCB is connected to the chassis earth. For the chassis earth i pretty much did exactly what it said in the Jaycar instructions. This chassis earth is connected to the base of the chassis (old biscuit tin) and is connected to the mains earth, and the HA PCB ( i read somewhere the PSU doesnt need separate earthing). I've just sat down and resoldered the earth connections, the input leads and checked all the cabling connections.... turned it on and the same thing happens.
I tested it with an Iriver MP3 player, connected with a dual RCA to stereo jack and i get alot of noise while the player is off and just after turning it on... but once i hit play, the noise stops completely and the music is crystal clear. If i hit stop after playing for a while, the line remains clear. Once i turn it off, i get the noise again.
Does this info give you any other clues?
Cheers, Brad
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robertkd
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Post by robertkd on Jan 23, 2010 4:46:17 GMT
Silly question, did you put resistors in situ for R3 and R6?
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Post by broovwa on Jan 23, 2010 5:42:16 GMT
Hey Rob,
Nah, i just used wire links as i am using the volume control on the source (my pc)
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Post by Deleted on Jan 23, 2010 6:24:07 GMT
Brad Not really. Try connecting the earth of the SCHA to the metalwork of the PC as suggested. You could try this temporarily with a flying lead with one end under a cover mounting screw of the PC. Other than that, no other suggestions at the moment.
Alex
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robertkd
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Post by robertkd on Jan 23, 2010 9:38:00 GMT
Well to be honest, I recommend fitting a volume control, allows you to run the DAC and nominal full bit depth 16/24 as well as helps with a fairly untidy design of the SCHA input layout. 25KR to 50KR is fine and simple volume control or stepped attenuator what ever your choice. The manual says it puts out +2dbv as digital FSD It's quite normal to have digital hash from PC sound cards, doubly so when you have some gain attached with high input impedance! As a test you might try to listen to nothing, advance you gain on the control panel, listen carefully see if the sound changes when you move the mouse or scroll through text?? Remember to turn down the level before listening to any music
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Post by broovwa on Jan 23, 2010 21:36:23 GMT
yeah i'll be looking at getting a vol control for the HA but i wanted to make sure it worked 100% before i go and spend any more money on parts. The noise coming through the cans on my pc isnt affected by the output gain in the sound cards control panel. I can mute the card, but i still get this faint whirring sound. I've also done as Alex asked and used a fly lead to connect chassis earth to the pc's case, but this made the noise louder by around double. When i go to shut the pc down i can hear mechanical noises and the whirring flutters on and off for a few seconds before the pc shuts down.
I'm thinking ill go through the PCB with a fine tooth comb (and multimeter) and look for any mistakes (for a 3-4 th time).
I appreciate your input on this guys!
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rickcr42
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Post by rickcr42 on Jan 23, 2010 23:14:18 GMT
....until or unless you add a line level isolation transformer between the digital source and the analog playback path.ALL digital grounds casrry crud so why pass it along as if you have no other choice than to accept it ?
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robertkd
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Post by robertkd on Jan 24, 2010 1:18:03 GMT
Sure, adding the additional earth(chassis) connection making the noise louder indicates some noise on the ground system. Given most computer PSU's simply use the ground on the PSU thus allowing ground currents to generate noise between the system ground (zero volt return) and the mains earth (chassis).
It's been a long time since decent isolation transformers were available at reasonable prices and seriously, cheap units are not likely to be good in performance with regard to ringing and distortion. In addition you would also have to pay attention to matching impedances for best signal performance.
Whilst I agree in principle ultimately an external DAC might well be a better option, in any case adding a volume control and running the on-board DAC at 0 dbFS may well put the hum below or near the noise floor, as well as other options such as leaving the transformer earthed and removing the earth reference to the amplifier PSU.
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Post by broovwa on Jan 24, 2010 1:44:59 GMT
Sorry Robert, you've lost me. I'm really new to the DIY electronics world, so i have to clarify. Are you saying to disconnect the earth pin on the HeadAmp PCB? if i were to do that, how do i then earth the transformer or PSU PCB?
ALso, just out of interest, what would happen if i put the R3 and R6 resistors back in without inserting a pot in the input line?
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Post by Deleted on Jan 24, 2010 3:08:44 GMT
Brad Seeing that you made the hum worse by grounding to the PC. I agree with the recommendation from Robert. Alex
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Post by broovwa on Jan 24, 2010 8:56:56 GMT
OK Guys,
Been testing it out again in different conditions.... seems the noise only occurs when i run different programs. Like if i'm just at the desktop, the signal is clean... but if i open chrome browser, it makes a high pitched whirrrrrr.... then when i close it, it stops. Now i have my TV tuner software open and its making a much lower pitched, but slightly louder whirrrr.
So, tomorrow i'm going to order a pot to go in the input line, as Robert suggested. So, any suggestions on a reasonably good value pot to go in the line?
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Post by Deleted on Jan 24, 2010 9:43:52 GMT
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Post by Deleted on Jan 24, 2010 11:56:06 GMT
Brad Did you try it without the earth connected from the PCB to chassis earth ? Mine was sitting right on top of the PC and didn't seem to need that. You could also try fitting a 2.2 to 4.7 ohm resistor in series with that lead. Alex
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Post by broovwa on Jan 24, 2010 19:33:56 GMT
Alex, any aussie suppliers for that pot? I'm in Newcastle!
If i remove the earth from the PCB to chassis, do i need to earth the PSU board?
Also, another question... i bought a led light to hook up to the +5 and it worked for about 5 minutes when i first connected the unit up.. but now its useless.... is there any particular type of led that i need to use?
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Deleted
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Post by Deleted on Jan 24, 2010 20:45:37 GMT
Alex, any aussie suppliers for that pot? I'm in Newcastle! If i remove the earth from the PCB to chassis, do i need to earth the PSU board? Brad Sorry about assuming you were another lover of luke warm beer . Your email address didn't give much of a clue. Unfortunately, I had to get my original Alps Blue pots from overseas. Mike may also still have one left. You could ask (Mike's postage is much more reasonable too) While you are waiting for arrival from overseas, you could use a 50K dual gang pot from Jaycar as a temporary expediency. The Alps pots have a better sound and better level matching between L and R channels. Just try unsoldering the earth lead at the PCB pin and see if it improves matters. IF it does, leave it off or try fitting that resistor in series. No other earth is needed except the safety earth for the transformer, which should also be connected to the metalwork of the potentiometer. You do not need a +5V supply here, so my suggestion would be to remove both of the big PW5 resistors and the 7805 voltage regulator. Fit a 10K resistor where the 100 ohm PW5 resistor was. There is a full diagram in the "Tweaks" thread. LEDs need current limiting resistors. If a RED LED is too dull, decrease the 10K resistor value to 2.2K or a little higher. rockgrotto.proboards.com/index.cgi?board=chitchat&action=display&thread=3622&page=1www.jaycar.com.au/productView.asp?ID=RP3760&CATID=33&form=CAT&SUBCATID=776Alex
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Post by broovwa on Jan 25, 2010 0:14:51 GMT
I feel really dumb for asking this question, but where is the safety earth meant to be? Is it the earth lead going back to the power plug? I've looked at countless diagrams and pictures of these HAs and other amps and i have never noticed an earth going from the toroidal transformer to the chassis earth point. The toroidal only has the 6 wires - brown + blue power inputs - and the orange, red, black and white wires, which i have connected to the 15+/- inputs of the PSU PCB (black and white connected together to the middle)
Is this wiring correct?
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mrarroyo
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Post by mrarroyo on Jan 25, 2010 0:54:47 GMT
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Post by Deleted on Jan 25, 2010 2:19:37 GMT
I feel really dumb for asking this question, but where is the safety earth meant to be? Is it the earth lead going back to the power plug? I've looked at countless diagrams and pictures of these HAs and other amps and i have never noticed an earth going from the toroidal transformer to the chassis earth point. The toroidal only has the 6 wires - brown + blue power inputs - and the orange, red, black and white wires, which i have connected to the 15+/- inputs of the PSU PCB (black and white connected together to the middle) Is this wiring correct? Brad The earth from the IEC socket is connected to the metalwork that the toroidal transformer is mounted on. The large mounting bolt that secures the transformer goes through this earthed metalwork. In the case of 1 unit rack cases etc. it was necessary to scrape away the powder coating around the mounting hole for the toroidal transformer to ensure good contact. Alex
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Post by broovwa on Jan 25, 2010 6:50:36 GMT
ALright, cool bananas! I am still meant to have the rubber divider between the toroid and the metalwork though right? If thats the case, then i'm sweet.
I'll try remove the PCBs earth tonight when i get home from work and see if that helps. I tested the HA on my media centre computer (with a Bluegears b-enspirer soundcard) and it has no problems whatsoever, crystal clear.
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Post by Deleted on Jan 25, 2010 6:56:17 GMT
Brad Yes, the toroid sits on the rubber. Alex
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Post by binary01 on Feb 5, 2010 8:49:18 GMT
Hi gents. I've just completed my first SCHA build too and have a similar query to broovwa, however a slightly different situation.
I have fitted an Alps Blue volume pot (100k was all I could source quickly, not sure if this is a problem), and can hear a hum of increasing volume from about 65% to 95% turn on the pot. When I reach 100% (full volume) the hum disappears completely. It occurs regardless of whether I have an input connected. I have the transformer mounted on an aluminium plate and the aluminium plate connected to the IEC earth, trying to emulate Alex's picture on page1 of the tweaks section. I also have the HA input and pot earthed to the same point. I've tried moving it off from a powerboard, and connected to an MP3 player rather than soundcard.
Any suggestions on what would cause the hum/how to trouble shoot it would be greatly appreciated.
Cheers, Liam.
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Post by Deleted on Feb 5, 2010 9:02:09 GMT
Liam I am not sure what you mean about the HA input being earthed to the same point. Only the case of the potentiometer, and perhaps the Earth PCB pin on the HA PCB should be connected to earth on the aluminium plate. (Which is connected to the earth tag of the I.E.C. mains socket.)
If the case is metal, or the rear panel is metal , the L and R RCA sockets should be insulated from the metal panel. Are you able to post a photo ? Alex
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Post by binary01 on Feb 5, 2010 9:05:51 GMT
Sorry, yeap the earth point on the HA PCB, not the inputs, are earthed to the IEC earth/alum sheet. Case is the very beige looking jaycar plastic instrument case (looking for something more appealing now I'll grab a photo of the internals now. Edit: Photo added. Sorry if its not clear enough, let me know any other details you need me to capture. IEC plug is a fused and switched unit. This is my first project so go easy on my handy work
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