Both channels fluctuate. Channel B keeps fluctuating from 24.5mV to 11.9mV and Channel A from 1mV to 6.2mV.
What does that tell you?
Channel B might be a tad noisy, but other than that nothing to be concerned about. Everything within +/-100 mV is perfectly within reason, and unlikely to be dangerous below +/- 1 V. Amplifier protection circuitry tends to engage at around +/- 600 mV at most.
I push the dustcap and it moves "normally" I believe.
(I couldnt tell whether what I'm measuring is DC or AC resistance by reading my multimeter's manual, so I attached a pic)
Nevertheless, the auratone shows 007.0 Ohms on the multimeter (pic attached). I'm not sure how much ohms that is as I'm a math-illiterate, sorry about that. The multimeter manual says resistance measure ranges from 400ohms to 40M ohms.
I also got a very cheap crappy chinese multimeter and when I put the probes on the auratone connectors, it started to make crackling noises.
All of that would seem to indicate that the speaker is still very much alive.
The multimeter always measures DC resistance. There is no such thing as AC resistance, the AC extension of this concept is called
impedance (it's also complex-valued, but getting into complex numbers is probably going too far if you're math-challenged). Resistance is impedance at a frequency of 0 (a.k.a. DC).
And 007.0 ohms is - 7.0 ohms.
Sounds about right.
Are you saying that in my configuration here, turning a 250Watts amp on with its 2 knobs at max but without music wouldn't blow this speaker?
Exactly that.
The amplifier could deliver this much power
if asked to, but that doesn't mean it's doing that all the time. It's just like a power supply in that regard. That will output a (largely) constant voltage, and output power will depend on what kind of a load you're attaching.
Let's say the load is a resistor. Then Ohm's Law tells us that
R = V * I or
I = V / R.
So the current flowing is determined by the ratio of voltage and resistance.
The power dissipated in a resistor is
P = V * I
With the above, this becomes
P = V * V / R = V² / R
So if you see 20 mV of DC at the output and you've got a 7 ohm resistor (speaker voice coil) attached to it, a grand total of
57 microwatts (µW) is going to be dissipated in said resistor.
An amplifier just provides a certain amount of
voltage gain, i.e. it
multiplies its input voltage by a certain factor. Its circuitry provides enough output current to back that up into typical load impedances. If you multiply
a whole lotta nothing by a constant factor, it's still a whole lotta nothing.
According to P2080 specs, that constant factor (voltage gain) is 26.3 dB at full input volume. You have hopefully come across the unit
decibel before (1 dB = 1/10 Bel, remember that Alexander Graham chap with the telephone?), the logarithmic measure that makes large ratios that much easier to handle. In linear terms, that's a factor of 20.65.
According to the P2080's block diagram
(gotta love these old Yamaha manuals!), gain should actually double in mono (BTL) mode, so it would be approx. 41.3 or 32.32 dB (= 26.3 dB + 6.02 dB).
A factor of 2 is 6 dB when we're talking voltage or 3 dB in terms of power, that's a good one to remember. Another is a factor of 10: 20 dB (voltage) or 10 dB (power). Why they're different? Actually, these are the same dBs either way. But remember P = V² / R above? If R is kept constant, power increases with the square of voltage. So twice the voltage = 4 times the power, or twice the power = voltage times the square root of 2. In the world of logarithms, the square relation becomes a factor of 2.
Properly understanding a lot of things in electronics does not come without at least some of that dreaded
maff, I'm afraid.
Btw. I attached a pic showing how I made the bridge/mono connecting to show you.
Looks alright to me. That said, this little 25 W speaker needs a 250 W BTL output about as much as a hole in the head (or
diaphragm, for that matter
). You're just increasingly likely to encounter audible hiss. Connect it to Channel A and be done with it, a nominal 80 Wpc is still plenty.
Are there some good resources (internet and/or books) about amps you would recommend. (I'm actually started to consider taking an Electronics course in some online faculty)
Rod Eliott's pages
diyAudio forum (power amplifier stuff would generally go in
Solid State)
Douglas Self: Audio Power Amplifier Design Handbook
Bob Cordell: Designing Audio Power Amplifiers
RC zobel network, Protection relay, BTL-SE Switch.. This is first time I've heard about these components.
That's fantastic my friend... amp troubleshooting.
I really want to learn this stuff.
I could go through my YT subs for electronic troubleshooting related channels. Off the top of my head, there'd be:
12voltvids (consumer TV & audio repair)
The Guitologist (guitars, guitar amps & misc. - also see
Uncle Doug for vintage amps)
More towards the realm of microsoldering and board repair of mobile devices, there's e.g. Louis Rossmann (Macbooks & life advice) or Hugh Jeffreys (phone repair). Musical instruments - Markus Fuller. Old radios and TVs - shango066. Misc. amplifiers / DIY - JohnAudioTech. Teardown of mains-powered electronics - bigclivedotcom (regarding dodgy electronics, also see
DiodeGoneWild). I also like South Main Auto Repair (vehicle electrical troubleshooting), My Mate Vince (amateur repair enthusiast who'll take on toys, consumer electronics, watches, whatever) and Adamant IT (from building and fixing PCs to board repair). That'll do for now.
The BTL-SE switch is the switch labeled Mono / Stereo on the back of your amplifier.
SE = single-ended. Load is attached between amplifier output and amplifier ground.
BTL = Bridge-Tied Load. Load is attached between two amplifiers, one of which is driven with an inverted signal. BTL delivers
twice the power into twice the load impedance. That's how 2x 125 W into 4 ohms becomes 1x 250 W into 8 ohms. The load is basically split in the middle.
BTL is a technique commonly employed when you are limited by voltage. It's pretty much standard in power amplifier ICs for car radios, for example (a nominal 13.8 V isn't that much!).
The protection relay, also indicated in the amplifier block diagram on manual p.12, is an electromechanical switch that connects the amplifier to the output terminals if everything is normal (that's the click your should be hearing upon power-up). If an abnormal condition is detected, relay drive is disabled and the output is disconnected. This amplifier's protection circuitry checks for not only excessive DC, but also cuts out when overheating is detected.
The contacts in these relays can tarnish (they're usually Ag-Ni plated, and silver doesn't appreciate sulphur compounds in the air), and if they're opened or closed while substantial voltage is present, arcing can destroy part of the contact surface. While longevity may vary greatly depending on parts quality and operating environment, they are more or less to be considered consumables - mere tarnishing can be fixed with contact cleaner and some elbow grease, but severely burned contacts mean you need a new one. As it's just as much work either way, often they'll be replaced no matter what - generally a new one can at least be made to fit without too much hassle, only a few old types are problematic.
Diagnosing a bad relay usually involves briefly cranking up the volume to see whether that motivates it, or tapping the suspect with a screwdriver handle while listening for changes in output.
The so-called Zobel network consists of a resistor and capacitor connected in series across the amplifier output. In a nutshell, it improves the amplifier's stability (resistance to oscillation), which is always a concern once negative feedback is applied - at some frequency it invariably tends to become
positive feedback. The Zobel allows for better robustness when faced with capacitive loading and for generally higher levels of negative feedback, improving distortion. Like Bruno Putzeys says - there is no thing such as too much negative feedback, the crux is finding out how to apply all of it.
It turns out that the speaker wasnt toasted at all.
Just as I suspected, really. So now the question is, why won't your amplifier say anything? Does it turn on and click? If so, check the mono/stereo switch and relay as stated (try some
percussive maintenance on the case). Hold your ear to the speaker and listen for any hiss coming out - perhaps the signal gets lost somwhere in the input stage.
It can't hurt to double-check your wiring either - make sure that enough of the insulation has been stripped at both ends to make good contact, and the strands should also be twisted a bit to avoid any loose ones possibly shorting out.
Sorry, I'm a non-native English speaker. 