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Post by jphoward on Mar 15, 2009 11:57:52 GMT
I gave my untweaked SCHA it's first test this evening, and was nearly entirely happy with it. There's just one problem - a very slight electrical hum in the right channel. I've tried different CD players, different headphones, and different cables, and still get the problem, so it's definitely the amp at fault.
This is my first electronics project, so I have no idea where to start debugging. Can anyone suggest what potential causes there might be, and what tests I could do to narrow it down? The hum is quiet enough that I can't hear it in my living room because the fridge is louder, but in my bedroom it's very clearly audible. It's not a hiss, but sounds more like the hum experiened when there's electrical interference.
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Post by Deleted on Mar 15, 2009 12:09:13 GMT
Jeremy What kind of case is it in , plastic or metal ? Are you able to post a photo showing the wiring etc. ? Alex
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robertkd
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Post by robertkd on Mar 15, 2009 12:09:11 GMT
I gave my untweaked SCHA it's first test this evening, and was nearly entirely happy with it. There's just one problem - a very slight electrical hum in the right channel. I've tried different CD players, different headphones, and different cables, and still get the problem, so it's definitely the amp at fault. This is my first electronics project, so I have no idea where to start debugging. Can anyone suggest what potential causes there might be, and what tests I could do to narrow it down? The hum is quiet enough that I can't hear it in my living room because the fridge is louder, but in my bedroom it's very clearly audible. It's not a hiss, but sounds more like the hum experiened when there's electrical interference. chance of any pictures I'm done with "using the force" as it were I take it the audio in bothe channels is about the same level but there is a hum in right channel when program audio level is low? Is the hum present with out any input leads connected? Robert
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Post by jphoward on Mar 15, 2009 13:34:57 GMT
I've pretty much got rid of the hum now - the problem was that the RHS RCA lead was too close to the transformer. I'll post a pic of the innards tomorrow - I'd be very interested to hear feedback then, since I couldn't find any good advice on the net or in my beginner's books about shielding and placement. I suspect I've done a pretty crappy job as a result!
Anyway, it's good enough now that I'm enjoying listening to it, so I'm going to spend a couple of hours with my music, then a few hours sleep - then I'll be back with pics...
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Post by jphoward on Mar 16, 2009 0:21:05 GMT
Although the hum is gone, there's still a slight "whisper" in each channel. It's like the sound of a very gentle breeze. It's a little louder in the right channel. Using 8 ohm sound isolating headphones makes it much more audible. When the CD player is off, it goes away. Changing the volume of the CD doesn't change the volume of the "whisper" noise. Here's a pic of my "quick n dirty" HA (click for larger) - any suggestions for obvious improvements? (I'm going to build a new one in a couple of weeks, with all the tweaks, so any advice on changes to the layout/case/etc would also be appreciated.)
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Post by jphoward on Mar 16, 2009 0:39:27 GMT
I forgot to mention: I tried picking up the amplifier PCB and moving it around, and it didn't make any difference to the noise.
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Post by Deleted on Mar 16, 2009 5:21:44 GMT
"Here's a pic of my "quick n dirty" HA (click for larger) - any suggestions for obvious improvements? (I'm going to build a new one in a couple of weeks, with all the tweaks, so any advice on changes to the layout/case/etc would also be appreciated."
jphoward O.K. , Ist things first. Let's see if you can tidy up the wiring a little ,before worrying about the other problem of hiss. In the absence of shielded cable for the inputs, try twisting together both wires of the left and right input sockets. Then run both lots of wires below the 3 pin power connection socket of the main PCB, then up the side of the PCB to the input socket locations. Next , disconnect the AC secondary voltage wiring from the PSU's 3 pin input block, and twist the 4 of them together while moving them over to the right hand side of the case, then reinsert the wires into the 3 pin terminal block. As you are using a plastic case , have you tried unsoldering the earth wire from the base plate to the pin marked earth on the main PCB ? This usaully isn't needed when using a plastic case. Only reconnect it if you have a hum problem. Try these suggestions, then let us know what the result is. We can then pay attention to the hiss problem with the CD player if it is still in evidence. Alex
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Post by jphoward on Mar 16, 2009 8:25:41 GMT
Many thanks for the tips Alex. I think I'll pick up some shielded cable tomorrow as well. Will let you know how it turns out.
Now that I've got a pretty good headphone and amp, I've discovered that nearly all of my CDs seem to be pretty poor recordings - most of them have a bit of hiss (much louder than the noise my amp is putting out), and the modern pop recordings that use a lot of dynamic compression are much more obviously flawed when using good equipment. I think I'll have to replace some of my CDs with better recordings soon...
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Spirit
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Post by Spirit on Mar 16, 2009 9:19:18 GMT
I think I'll have to replace some of my CDs with better recordings soon... I love how getting/making better gear brings to light the flaws inherent in your source material...
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robertkd
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Post by robertkd on Mar 16, 2009 11:35:30 GMT
Jeremy, can I make one vitally important recommendation given the unknown source you are driving the SC HA with remove R3 and R6 and replace with a wire link, you might find the "hiss" will disappear. Additionally get a volume control something minimal like an Alps or an attenuator. As Alex indicated some attention to the dress of the cabling, adding some screened audio cable to the input will help leverage improvements. Robert
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Post by Deleted on Mar 16, 2009 12:12:06 GMT
Jeremy If you don't intend using a potentiometer with the HA, the attached warning from Silicon Chip magazine should be followed : Important: When feeding your headphone amplifier from a preamplifier, or other low impedance source,Resistors R3 and R6 must be 0 ohms in value (use wire links.) ! As Robert says : "Additionally get a volume control something minimal like an Alps or an attenuator." If you can't afford either of those at the moment, a temporary solution could be to use a dual gang 50K logarithmic potentiometer such as the attached from Jaycar, however far better results will be obtained from the Alps dual potentiometer or a dual 50K attenuator.
Alex
P.S. Remember to cut the shaft to an appropiate size before fitting to the front panel, if using the cheap alternative for now.
CAT. NO. RP3760 RRP $4.45 50K 24MM POTENTIOMETER LOG DOUBLE GANG (A) With imperial standard shaft & bushings made to our rigid engineering specifications. The potentiometers are full size - 24mm diameter types .
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Post by jphoward on Mar 16, 2009 14:22:14 GMT
given the unknown source you are driving the SC HA with remove R3 and R6 and replace with a wire link, you might find the "hiss" will disappear. After trying a few sources I've settled on my Squeezebox Duet as the best sounding. Actually, my Squeezebox 3 sounds maybe a little better, but only if I turn off the display entirely - otherwise I get some electrical noise - and it's annoying to turn the display on and off every time I want to change music. I've now removed R3 and R6 and put in wire links, and I can't say I hear a difference. Mind you, it's past 1am and I've had a couple of drinks, so perhaps I'm not very with it. I don't need a volume control on the amp, since I have a volume control on the Squeezeboxen. I had assumed that putting a pot in the amp would result in lower quality sound, and that I'd be better off using the volume control on my source. Is this incorrect?
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Post by jphoward on Mar 16, 2009 14:29:23 GMT
I love how getting/making better gear brings to light the flaws inherent in your source material... I'm not sure whether to be pleased, or horrified... Just a week or so ago I decided to try my hand at building a little electronics project for the 1st time, and picked this project since my amp doesn't have a headphone output (and nor does my Squeezebox Duet). Now I seem to be on a never-ending quest, having for the first time discovered that my music can sound an awful lot better than I previously realised. [This is also partly the fault of me buying myself a present of some new headphones a couple of weeks ago: the Audio Technica ATH-AD700.] I guess everyone on this forum has a similar problem, so I can't expect any sympathy for my new affliction.
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Post by jphoward on Mar 16, 2009 16:00:36 GMT
I've checked, and the impedance of the Squeezebox is 220ohms. Does that count as 'low'?
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Post by Deleted on Mar 16, 2009 19:58:26 GMT
I've checked, and the impedance of the Squeezebox is 220ohms. Does that count as 'low'? YES. No. As long as you will only be using this source with it, and nothing else that doesn't have inbuilt volume level control. Alex
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Post by st on Mar 16, 2009 20:19:37 GMT
Jeremy, can I make one vitally important recommendation given the unknown source you are driving the SC HA with remove R3 and R6 and replace with a wire link, you might find the "hiss" will disappear. Additionally get a volume control something minimal like an Alps or an attenuator. As Alex indicated some attention to the dress of the cabling, adding some screened audio cable to the input will help leverage improvements. Robert can someone explain why these links need to be removed? and if I was to run my scha from the preout of my arcam p85 would i be best to remove these?
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Post by Deleted on Mar 16, 2009 21:50:03 GMT
st It has to do with impedance matching, and obtaining the lowest possible distortion from the I.C. at a typical volume control setting. The improvement is only minor, and they may be left out. If not used as stated in the SC article, there is the possibility of the opamp oscillating , which may manifest itself as added grain to the signal, or in Robert's temporary initial lashup, a far worse scenario ! The suggested values are a compromise only, and are referenced to a typical listening level with a 50K potentiometer.This was apparently worked out by the article's author after measuring the resistance from the volume control's wiper terminal, to it's earth terminal at a typical,most frequently used, listening volume. Put simply, if you aren't using the 50K volume control, replace these resistors with wire links. Alex
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Post by jphoward on Mar 17, 2009 4:10:12 GMT
Using shielded cable helped a lot. There's still the slightest bit of hiss, but it's equal in both channels, and quiet enough that I'm not sure if it's from my source or my amp. After reading about the Squeezebox Transporter in Stereophile (http://www.stereophile.com/mediaservers/207slim/) I decided to order one today, so when it arrives tomorrow I'll have a really good source to test with. Regardless, I'm not going to work any more on this 'untweaked' HA, since I've nearly got all the bits I need to build the tweaked version, so I'm going to start on that now and will focus all my attention on that. So, here's where this 1st attempt finished up:
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Post by Deleted on Mar 17, 2009 4:17:59 GMT
jphoward Why not unsolder and remove the 2 PW5 resistors on the power supply PCB ? They aren't necessary with this application, and you may even notice an improvement due to less load on the power supply. It will also run a little cooler. Alex
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Post by jphoward on Mar 17, 2009 9:49:46 GMT
Why not unsolder and remove the 2 PW5 resistors on the power supply PCB ? Thanks for the brilliant suggestion Alex. That has now completely fixed the problem! I now have a much better reference point to test the tweaked version against when it's done. PS: I did the power supply for the tweaked version this arvo, and it sure is easier the 2nd time! It took an hour to put it together, whereas the 1st time it took 3 hours.
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