oscilloscope in audio tutorial

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#42
I don't know rha61, what I want to measure - I thought PS ripple was a measure of how good a PS was?

I tried again with batteries (4*12V 7aH in parallel + 10,000uF CAP) & I can distinctly see the waveform modulating with the music - this only happens at high volumes with the SMPS!! This suggests a bad PS source, I presume?
 
#43
with an ideal PS , ripple=0 ( internal resistance=0 )
for you , it depends on what you want , percentage of ripple vs PS voltage tolerable
you can download PSU Designer 2 simulation software at Duncan Amplification site to have a good idea of ripple percentage

i'm surprised with your battery result , have you tried to decouple the electrolytic cap with a little film cap ( 1uf MKP for example ) ?
 
#44
Jkeny, this is why I said right at the start that the WHOLE circuit diagram has to be available to look at. As to the topology of the power amps yes, there are different types of switching output stage and, myself and unclejed originally assumed a more conventional switching topology. Have you actually measured yet ? Is the rail voltage "processed" any further on. Any manufacturer specifications will be quoted under steady state i.e. sine wave testing. It would be a facinating exercise to see how supply voltage variations reflect in the output. Distortion figures under load and under transient conditions are a whole different ball game. Think you might have to do your measurements on the power amp boards as well. You will get more insight into this by using a sine wave source, and a dummy load on one of the main channels driven to perhaps 5 watts rms, and a further dummy load on the other main channel, this channel being fed with music. You should see no modulation of the sine wave by the music on the other channel. Or square wave test instead of music and see how this reflects on the other channel. This would also give an easily measurable and visible ripple so you could put some numbers into it all.
Regards Karl
 
#45
Hi rha61,
I'm surprised with the batteries also as I was when I connected them expecting better sonics. Could it be that the batteries are at the end of their life & not outputting much current? That is why I paralleled 4 of them but still see modulation.

I didn't try a film cap accross the 10,000uF because I thought that it would have no bearing on this modulation?

Hi karl,
I had hoped that this wouldn't be so complicated & that I could get to know a bit about oscilloscope use while looking into PS performance! I guess I under estimated what was involved - I don't have signal generators so I guess my experiments end here!
 
#46
There are always ways and means. A test C.D. with various test tones at different levels and any C.D. player will make a susbstitute signal generater. If you have 2 C.D. players (portables are best for this kind of thing) you can feed different signals into left and right channels. You also need a couple of 8 ohm or thereabouts, 15watt or higher resistors unless you can stand the noise from the speakers. Going a bit more off topic but relevant to your battery experience is this. Many many moons ago I designed a car alarm that communicated with the main alarm control unit by pulsing the 12volt car battery supply with a switched current source driven by 16 bit codes. All the modules talked to each other this way with no other wiring needed.It was easy to recover the codes directly on the terminals of the battery with nothing more than a 741 and a bit of ingenuity. You would have thought the impedance was so low here that you would see nothing but it was'nt so.
Regards Karl
 
#48
rha61,
You're refering to dwk's post where he finds batteries a major sonic upgrade over XR25 stock PS (and this is reckoned to be the best PS of the XR series!). I know! This and other posts from herehttp://www.diyaudio.com/forums/showthread.php?postid=285076#post285076
led me to believe that SLA batteries would be a huge improvement but I can only report what I hear & now am able to confirm with the scope (modulation of supply)

Edit: Of course there is always this :http://www.diyaudio.com/forums/showthread.php?postid=187556#post187556 . Reading that thread again , I notice that I get no sparking when I connect my batteries to the 10,000uF cap which feeds the o/p stage - does this signify low current?

Karl,
I'm currently downloading audio test CD from here: http://binkster.net/extras.shtml so should be good for doing some of these tests!
 
#49
no spark could signify low current, or that the cap is already charged. what's the current limit of the battery? what kind of battery is it? if it's a gel cell, chances are it may show fully charged on a voltmeter, but if it is a second-hand battery that spent most of it's life in a burgler alarm or a UPS, chances are that it's so badly sulfated that it's only capable of an ampere or two (or less). for a gel cell battery to stay healthy, it needs to be fully discharged a couple of times under enough load to heat the battery about 10 or 15 degrees above room temp and then recharged. i have "rescued" a few gel cell batteries over the years with this method. it doesn't always work, especially if the battery is so badly sulfated that it has open cells, or if the battery has shorted cells. also if the gel cell is bulging, dispose of it.

also since batteries are primarily designed as DC devices, not a lot of thought is given to AC characteristics, so even if the battery has a low internal resistance (such as 0.01 ohm) the AC impedance of the battery could be astronomically high.
 
#51
read my previous post. it may be possible if they're not too far gone. the trick is discharging with enough of a load to generate internal heat. i've had about 50-70% of the batteries i've cycled come back to life, not always to original specs, but much better than the condition i found them in. also, be careful, overcharging them can burst them. H2SO4 (sulphuric acid) is nasty stuff whether it's in gel form or liquid.
 
#52
I found a decent battery & powered the output stage with it -wow, sound is amazingly realistic even though the PS is still being modulated by the music to some extent (from what I can make out on the scope.)

I'm wondering if I parallel more batteries will this eliminate/reduce the modulation and how much better might the sound become?
 
#53
what gauge wire are you using to connect the battery? wiring is resistive. if you're using 18 ga or smaller, you will see the resistive effects in wire lengths over 6-8in..... 10 or 12ga wire would be good for the battery connection to the amp.
 
#55
Ok, I'm never sure about the guage measures so I checked it up:
From battery to cap there is the original wire that came off the UPS - it's 6 guage, linked though connector blocks by 10 guage wire to caps & 10 guage wire to output board - so high enough guage to not cause any resistance problems, I imagine, unless the connector blocks are introducing too much resistance!
 
#57
Just in case anyone is still interested in the Original Topic (not just Audio, General):

Part 1
http://www.youtube.com/watch?v=q_AFL5K1FQA&feature=user

Part 2
http://www.youtube.com/watch?v=i3qUTt9fK0o&feature=user

Part 3
http://www.youtube.com/watch?v=pcjmKs-o9Eo&feature=user

Part 4
http://www.youtube.com/watch?v=hIprEhx5BgU&feature=user

Part 5
http://www.youtube.com/watch?v=qrjOsuXpIw4&feature=user

Part ???
http://www.youtube.com/watch?v=zQDuZ3a6HAE

Bonus
http://www.youtube.com/watch?v=Y32suYIC-xs&feature=user

I hope other Newbies find this useful. There are many others available but you have to dig and sort through them. Good luck!

Regards//Keith
 
#58
i'll bookmark those, as there are people i know that i'm going to send them to.

btw Kieth, Wile E, since he was stupid enough to keep buying all of that junk from Acme, never thought of ordering food. he once caught the Roadrunner, then forgot WHY.....
 
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