Electronics > Repair
Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)
Fried Chicken:
I’ve gone deeper into audio than is healthy; I know I’m not the only person who’s been afflicted by this.
I recapped an old amplifier (Yamaha M-65) and removed some corrosive glues. I want to go in and actually test this work: channel separation, signal clarity, stuff like that.
My equipment to achieve this is limited: analog 2-channel scope, analog function generator, Fluke DMM, a smattering of coax adapters, and a couple power resistors.
The power resistors, I’m not sure can handle the full power of the amp.., I think I have two 4 ohm resistors, but the speakers I run are 8 ohm. They are expensive!
Anyway, I’m just fishing for advice here, open to suggestions on what to do, what not to do.
I’ve heard claims that capacitors need to “break-in”; I’m not sure I believe that. I can set the bias voltages (to center the A vs AB amplification I think this is? What the signal rectified around?). That and calibrating the VU meters is about all that’s offered in the service manual.
I think it’s here:
https://elektrotanya.com/yamaha_m-65_sm.pdf/download.html
Anyway, this is a fun project for me. If anyone has any especially wild tricks, like figuring out the THD with all analog stuff, that’ll perk my ears!
Thanks in advance
CaptDon:
Be aware of the subjective bullshit observations of 'Audiophools'!!! Remember that in a very real double blind test 12-2 NM house wire beat every other test sample wire including idiotic $30.00 per foot stuff you could use as a mooring line. And then the audiophools still insisted there must be a flaw in the testing because their expensive stuff MUST be better!!! Stop and think, how would you measure channel separation???? Drive one channel to maybe 80% of full output and look for residual output on the other channel. Then calculate how many dB from the voltage ratio. THD is nearly impossible to measure with the gear you have. Audio THD meters are a specialized instrument. Look up the Potomac Instruments 'AA' audio analyzer or the older HP audio analyzers. Measure the THD of your generator and then measure the output of the amplifier. Cheap function generators and audio oscillators have terrible THD to begin with. As for 'A' vs. 'AB' vs. 'AB1' vs. 'AB2' setting of the bias, a lot of it comes down to "What sounds best to you at your listening level". So much bad information among tube amp guitar players!!!! How many amps I have seen burned up because some jackass said "set the bias to where the plates just begin to glow red". W.T.F., the whole idea of class B push pull is that the outputs really don't need to idle more than about 10% up the conduction curve. These red plate chuckle heads are around the class A push pull point of operation!! Audiophools think little wooden wire supports help the sound and expensive wall receptacles "Give a more open sound stage". Idiots!!
Fried Chicken:
I appreciate it…
I was really really hoping not to encounter pro- or anti- audiophile bullshit here, and is a reason I asked here and not on audio forums.
Yamaha gives specific bias voltages in the manual, that’s what I intend to set them to.
How do I even know what sort of signals to apply to the amp? Should I use the front meters?
I can find the difference between two signals on a scope by inverting one and adding them, right?
CaptDon:
Many of the '50dB down' artifacts may be approaching the noise floor of your scope and test equipment. Many 'purpose built' audio analyzers can do THD, intermod IMD and other tests but often require the companion audio generator. Tektronix in their TM5000 (not the 500) series of modules had some audio distortion modules with matching generator. They still fetch big bucks on Ebay and NEVER buy from a seller who sells test equipment and lists something as 'untested'!!! That means they already tested it (after all, they are a test equipment dealer) and it is blown to bloody f--king hell!! I hate those crooks!!! May as well buy one laying in a bucket of rain water at a hamfest where it rained all day!! Anyway, best wishes in reaching your goals! Invest in good test gear as it will hold its resale value. Cheap junk won't!!
Fried Chicken:
Surely something can be done with a normal scope? I mean the scope samples at several orders of magnitude above audio signals?
Are there no analog tricks or similar?
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