bandagedhand
New Member
Apologies, will do. I was actually trying to be good in posting this here rather than starting a new thread. My mistake!
Amplifier Distortion, DC-Offset, and You!
- Thread starter EchoWars
- Start date
Apologies, will do. I was actually trying to be good in posting this here rather than starting a new thread. My mistake!
slpcorner
AudioPhan - On the Cheap
(copied and pasted from the first page)
0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong
16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow.
50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.
100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)
I thought the 1040 was capacitor coupled. Leading to my question, can I check dc offset on my capacitor coupled s.s. amps by checking voltage upwind of the capacitor?
OverLoad
High On Solder Fumes
You thought wrong. The output is not cap coupled. Measure the DC-offset at the speaker terminals as normal.
Thanks. I have some amps that are cap coupled and was wondering if there's value in checking for dc offset on the amp side of the caps?
OverLoad
High On Solder Fumes
There's not much point in measuring before the output coupling cap. There IS gonna be DC. What do you think the whole purpose of the coupling cap is? Measure the cap coupled output with a load.
No, the offset before the cap will usually be ca 1/2 total available rail voltage in a complementary totem pole output circuit. It is what it is by design.
Checked my CR-620 today and it was showing values in the 150mv range. Thought this was odd. Gave the unit a smack and the values jumped around like crazy. Poking the board with a chopstick revealed a dirty board to chassis ground. Cleaned and holding steady at 10mv l and 6mv r.
verde57
Check the 'Reset' box if
What do you mean?
OverLoad
High On Solder Fumes
In a word, YES.
Kahoona
Super Member
Thanks. Both sides low in the single digits now. This little beauty is working great. I wanted to say "who would have thought that this little beauty would be running this well so many years after it was made" Thing is though, when this was made they expected things to do just exactly that! Gotta love this stuff.
Rob
What are symptoms of bad output transistors?
I caused a short in 2 amps during dc offset/idle adjusting(don't ask, dumb mistakes) and afterwards I got ultra high dc offset readings(7-10 volts!)
I suspect the transistors overloaded since the problem would transition to the other channel as well(to a lesser degree).
I ordered new output transistors to replace but was wondering what other components can cause skyrocketing DC offset/idle values after a short?
I caused a short in 2 amps during dc offset/idle adjusting(don't ask, dumb mistakes) and afterwards I got ultra high dc offset readings(7-10 volts!)
I suspect the transistors overloaded since the problem would transition to the other channel as well(to a lesser degree).
I ordered new output transistors to replace but was wondering what other components can cause skyrocketing DC offset/idle values after a short?
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Rasputin47
Woad
I recently acquired a Marantz 2252
Right -6mv
Left -4mv
Right -6mv
Left -4mv
Bose 1801 DC Offset
Just got this and a 4401 preamp. Found 1 bias pot faulty and changed it out.
Tested dc offset. L 80mv R 120mv---no adjustment pots. I added a 10k pot
to each input op amp --mc1556. Adjusted bias to 90ma per spec.Adjusted dc
to around 1 mv each channel.Put on cover and let soak for a while. L 1.4 mv
R 14.2 mv both steady. Sounds much better, tighter bass, more transparent.
Just got this and a 4401 preamp. Found 1 bias pot faulty and changed it out.
Tested dc offset. L 80mv R 120mv---no adjustment pots. I added a 10k pot
to each input op amp --mc1556. Adjusted bias to 90ma per spec.Adjusted dc
to around 1 mv each channel.Put on cover and let soak for a while. L 1.4 mv
R 14.2 mv both steady. Sounds much better, tighter bass, more transparent.
