The waveforms don't appear to be oscillation or at least of the usual simple periodic type that we normally see. You have something low level and a little more complex there which suggests environmental, SMPS or digital processing noise.
Assuming you have or can get another linear amplifier from somewhere, take a reality check on the noise levels at similar circuit nodes wuithe same test set-up. You may just find the same noise problem which may have nothing to do with your amplifiers.
Regarding large foil caps for signal handling, it is possible to mitigate the aerial effect if the outer foil layer of the cap is identified and is the one connected to signal ground. Unfortunately, that isn't possible with a DC blocking cap. To solve this particular noise problem for several guys whose builds I've had to fix in the past, I either strapped the caps to the chassis remotely and used shielded wire to couple to the rest of the input stage or wrapped copper foil around the ^@#+!* things and then grounded that.
Yes, people do get attached to their favourite components and brands but I have fooled myself on occasion with some very different parts types too. It's also amazing how our hearing, perception and preferences change over the years and from individual to individual. So much so, that DIYs tend to treat it like choosing their consumer goods, even your favourite shirt that your friends think is ghastly.
Assuming you have or can get another linear amplifier from somewhere, take a reality check on the noise levels at similar circuit nodes wuithe same test set-up. You may just find the same noise problem which may have nothing to do with your amplifiers.
Regarding large foil caps for signal handling, it is possible to mitigate the aerial effect if the outer foil layer of the cap is identified and is the one connected to signal ground. Unfortunately, that isn't possible with a DC blocking cap. To solve this particular noise problem for several guys whose builds I've had to fix in the past, I either strapped the caps to the chassis remotely and used shielded wire to couple to the rest of the input stage or wrapped copper foil around the ^@#+!* things and then grounded that.
Yes, people do get attached to their favourite components and brands but I have fooled myself on occasion with some very different parts types too. It's also amazing how our hearing, perception and preferences change over the years and from individual to individual. So much so, that DIYs tend to treat it like choosing their consumer goods, even your favourite shirt that your friends think is ghastly.
Hi Gareth,
Well, if you prefer these capacitors for their sound, then that's it. I guess my only question to you would be, do they make these rated for a lower voltage that would reduce the physical size? If so, try those out. A smaller part that fit's on the PCB really would be helpful in supporting that part, and the leads for connection. I would definitely use some silicone adhesive that remains pliable to support the body of the capacitor on the PCB.
-Chris
Well, if you prefer these capacitors for their sound, then that's it. I guess my only question to you would be, do they make these rated for a lower voltage that would reduce the physical size? If so, try those out. A smaller part that fit's on the PCB really would be helpful in supporting that part, and the leads for connection. I would definitely use some silicone adhesive that remains pliable to support the body of the capacitor on the PCB.
-Chris
Badger testing. I have played about 3hrs of music and it sounds great. Before closing up the case I retested dc output thru r17 that was good. testing bias it was down at 8 mv when I adjust it up thru r30 I get to slightly over 15mv and it jumps rapidly into the +60mv range. It was doing that before I played music so I set it at 14mv and it sounds good. How much does bias affect sound? What could cause the jump? The case shows no heat when testing
That sounds like it might be oscillating. As Chris suggested you should connect a scope and see what's going on. Another tell tale sign is R50 might start getting hot. R 50 should be stone cold. It only gets warm when there is high frequency (50kHz +) signal at the output which should never be present in the amp.
Too low a bias setting might possibly make the amp sound a little harsh or cold but likely you won't heat a difference. An oscillating amp can sound harsh and metallic too.
Too low a bias setting might possibly make the amp sound a little harsh or cold but likely you won't heat a difference. An oscillating amp can sound harsh and metallic too.
If I'm interpreting the photos properly, there is about 1Vp-p noise of at least 4MHz. The scope is sampling at just 16MHz so this may not be accurate. Looks most likely outright oscillation due to the circuit itself.
Suspicious things. I can't see where the G1 ground (Zobel ground) is on the photo of the amplifier. There are two sets of smoothing caps? There are two caps in parallel with the two smoothers closest to the board. The speaker output wire and ground return wire are not closely bound. The output inductor on the PCB is not a great idea but probably not the root cause.
The input cap size in't the root cause.
I assume you have only powered one board up so far so we don't know if the other will show the same problem at this stage.
Suspicious things. I can't see where the G1 ground (Zobel ground) is on the photo of the amplifier. There are two sets of smoothing caps? There are two caps in parallel with the two smoothers closest to the board. The speaker output wire and ground return wire are not closely bound. The output inductor on the PCB is not a great idea but probably not the root cause.
The input cap size in't the root cause.
I assume you have only powered one board up so far so we don't know if the other will show the same problem at this stage.
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The resistor temperature I suggested is a pretty good indication of oscillation. There are cheap USB scope or sound card options that work. I've got a couple real scopes but keep a cheap Vellerman PCSGU250 on the work bench for quick initial operation testing. Something like that gives you a good indication if things are stable. PCSGU250: USB-PC SCOPE + GENERATOR (2CH.) – Velleman, Inc.
Hi rock12,
You can get a really nice used 'scope for $200 or less.
If you are going to continue working with electronics, you absolutely need an oscilloscope. Beside a meter, an oscilloscope is the most used piece of equipment. Get a good one.
Before you get all depressed, I started with a single trace, recurrent sweep (no triggering) 500 KHz tube instrument. That was in the 60's, but I was a young kid and was really well set up. An analogue scope is something I keep on the bench even though I have a really nice digital one. There are some things an analogue scope does better.
Anyway, it would be money well spent.
-Chris
You can get a really nice used 'scope for $200 or less.
If you are going to continue working with electronics, you absolutely need an oscilloscope. Beside a meter, an oscilloscope is the most used piece of equipment. Get a good one.
Before you get all depressed, I started with a single trace, recurrent sweep (no triggering) 500 KHz tube instrument. That was in the 60's, but I was a young kid and was really well set up. An analogue scope is something I keep on the bench even though I have a really nice digital one. There are some things an analogue scope does better.
Anyway, it would be money well spent.
-Chris
Set bias with no load connected to the output. I also had this problem withdraw wetting bias because of a decent strength oscillation noise I've got. As soon as you connect up the output to a load (resistor of speaker) it shows your bias setting as being higher. Also I've found bias to dramatically change from when I set it. Then once I turned it on again, and also once I get it hot. So test it many times and turn it fully off and on a few times to ensure it sits where you are happy with. I myself will aim 50-70mv myself.
I am still to find the cause of my noise but ive not had the time yet. I feel as if it's on my board and not an outside influence since I've built many amps and never had this issue connected to all the same power outlets in the same room etc. It is loud enough that the amp is unusable with my 101db speakers. It is not being introduced by my computer or my test equipment. It's present when nothing is connected except power and a speaker.
First double check your part selection and soldering. if you inadvertently changed the gain of the amp by installing an incorrect resistor it may oscillate. A bad solder joint on a small capacitor may also cause it. Different batches of transistors can require different compensation as well. Very experienced builders can tell you where to look by the frequency it oscillates at (I'm not that good yet). This is why Chris is suggesting you need a scope.
I have played about 3hrs of music and it sounds great. Before closing up the case I retested dc output thru r17 that was good. testing bias it was down at 8 mv when I adjust it up thru r30 I get to slightly over 15mv and it jumps rapidly into the +60mv range
Do not adjust as it sounds great.
Do not adjust as it sounds great.