Amplifier Distortion, DC-Offset, and You!
- Thread starter EchoWars
- Start date
gonzothegreat
Super Member
Broken diff pair link
Does anybody have the info from the link mentioned in EW's first post in this thread? The link about the benefits of a balanced differential pair is 404ed.
Does anybody have the info from the link mentioned in EW's first post in this thread? The link about the benefits of a balanced differential pair is 404ed.
outlawmws
On the Run
OK, I read all 19 pages, (I agree, great thread!). I still have a question, and I don't think it was asked at least in this thread.
Do you have to make the measurements at the output terminals, or could you make it at the end of the speaker wire? My amp is pretty buried, (and I'm not even sure I have an unused input... Have to check.)
I would think it would affect readings, but I'm not sure...
TIA EW!
Again, a superlative thread! I learned more electronics in those 19 pages than in the past 10 years, And I'm pretty savvy in electronics, without being a tech, as I spent years designing Printed Circuit Boards, Analog, Digital, Mixed signal, as well as Microwave RF Thin film substrates, and the Thick film Bias circuits.
Do you have to make the measurements at the output terminals, or could you make it at the end of the speaker wire? My amp is pretty buried, (and I'm not even sure I have an unused input... Have to check.)
I would think it would affect readings, but I'm not sure...
TIA EW!
Again, a superlative thread! I learned more electronics in those 19 pages than in the past 10 years, And I'm pretty savvy in electronics, without being a tech, as I spent years designing Printed Circuit Boards, Analog, Digital, Mixed signal, as well as Microwave RF Thin film substrates, and the Thick film Bias circuits.
fropiler
Super Member
echowars, are you familiar with this site? (anyone?)
http://www.vintage-electronics.cc/
I'm considering sending my newly acquired Sansui AU-888 to this fellow for tweaking. I just came across this thread, so I can hardly wait to try this DC voltage check.
My problem is audible static and hum on the left channel, but only when phono 1 and 2 and MIC inputs are selected. Tuner and AUX inputs are silent and work well. Happens with nothing connected to inputs as well. Sounds great with a cd player hooked to AUX.
In any case, thank you for all of the great information presented already!
Ed
john_w
Miscreant
fropiler --
In "the Springs"?? Maybe I should try this guy.
Y'know, if you have an inclination to send it somewhere, echoWars sounds like he knows quite a bit more than the hacks I've been running into lately. That might be a good bet as well. Seems to me I recall him mentioning his own repair business contact info someplace else on this site.
Myself, I'm not often too keen on shipping stereo gear. I have a brother that works two jobs: UPS, AND a high-end stereo outfit. They ship via commercial freight and avoid consumer shipping whenever possible. But, I will do it if I don't find any decent local techs.
The Springs, however, is about 45 minutes away from me. My gear can ride shotgun on the way down, and I won't have much to worry about!
One of these days, I'm going to find time to try and tweek my own bias levels, now that I know half of them suck, but I'm spending too much time on things like audioKarma!!
- John
In "the Springs"?? Maybe I should try this guy.
Y'know, if you have an inclination to send it somewhere, echoWars sounds like he knows quite a bit more than the hacks I've been running into lately. That might be a good bet as well. Seems to me I recall him mentioning his own repair business contact info someplace else on this site.
Myself, I'm not often too keen on shipping stereo gear. I have a brother that works two jobs: UPS, AND a high-end stereo outfit. They ship via commercial freight and avoid consumer shipping whenever possible. But, I will do it if I don't find any decent local techs.
The Springs, however, is about 45 minutes away from me. My gear can ride shotgun on the way down, and I won't have much to worry about!
One of these days, I'm going to find time to try and tweek my own bias levels, now that I know half of them suck, but I'm spending too much time on things like audioKarma!!
- John
Unlikely...but consider that:
1) It's a simple fix for units that have an adjustment, and
2) Most non-tech types have no idea what it is or what it means.
As long as the protection doesn't kick in, most figure that all's well. Hard to blame the seller...hell, most shops don't even bother to check or set it on units that they service, so how can you blame the seller? Just get it set and call it a day.
Fluctuation is normal...transistors heat and cool, and gain changes depending on that. Matter of fact, watch the offset while you blow on the driver board.
Get the pic now about how touchy it can be and dependant on thermal issues?It might well sound pretty good. Get it zeroed out and you can hear what it's really supposed to sound like.
gonzothegreat
Super Member
Like EW said, if your amp doesn't have enough bias, you get crossover distortion.
So what is crossover distortion? Walk this way and find out...
This is a nice lab experiment for techs that explains the idea of crossover distortion in class B transistor amps.
http://www.allaboutcircuits.com/vol_6/chpt_6/10.html
Here is a link that describes crossover distortion in a tube amp.
http://www.aikenamps.com/CrossoverDistortion.htm
Didn't mean to step on your thread EW but CDfixer sounded like he needed a little more theory on amp bias.
So what is crossover distortion? Walk this way and find out...
This is a nice lab experiment for techs that explains the idea of crossover distortion in class B transistor amps.
http://www.allaboutcircuits.com/vol_6/chpt_6/10.html
Here is a link that describes crossover distortion in a tube amp.
http://www.aikenamps.com/CrossoverDistortion.htm
Didn't mean to step on your thread EW but CDfixer sounded like he needed a little more theory on amp bias.
CDFixer
Number 1 USC Trojan Fan
Thanks again, Here's the problem that I have, with my G-9000. At about 8khz I can see a small distortion forming just below the zero crossover on the positive transition of the output signal. As the frequency is increased the distortion becomes a sinewave of about 20Usec duration and moves up the positive transition until at about 14Khz it reaches the peak of the output signal and stops moving. Increasing the freq at this point causes the blip just to grow a little. If I increase the bias voltage (should be 16 millivolts) to about 85 millivolts then the distortion goes away. I think I may have a defective output transistor. Just worndering what you guys think about that.
Also the distortion goes away if I remove the load from that channel. The other channel works as advertised in the sales brochure.
Also the distortion goes away if I remove the load from that channel. The other channel works as advertised in the sales brochure.
Could be outputs, drivers, or the bias stage itself. In theory, could be anything in the entire amp section all the way back to the differential pair.
The bias stage generally has either a Vbe multiplier, or thermal compensation diodes. Either way, set the bias to factory specs and look at the DC voltages on the base of the drivers...compare with the voltages on the good channel. If that looks OK, then it's a driver/output stage problem. Shotgun it...too wierd to nail down with a meter...replace the drivers and outputs.
If you want to discuss this further, ya oughtta start a new thread in the Sansui forum. I'd be happy to contribute.
The bias stage generally has either a Vbe multiplier, or thermal compensation diodes. Either way, set the bias to factory specs and look at the DC voltages on the base of the drivers...compare with the voltages on the good channel. If that looks OK, then it's a driver/output stage problem. Shotgun it...too wierd to nail down with a meter...replace the drivers and outputs.
If you want to discuss this further, ya oughtta start a new thread in the Sansui forum. I'd be happy to contribute.
lorne
Sonic Lizard
QUESTION: RE: DC OFFSET
Concerning an AU-307, but perhaps apros pos for almost any amp — if my PS FILTER CAPS (bypassed by mylar caps @ 10% of value) were leaking too much DC, could this show up downstream as a meter reading for too much DC offset?
Some of you will immediately see that I am short of knowlege in regards to circuits, but I am trying to gain understanding. As a hazarded guess, I would say 'NO'.
I have to start somewhere — that is finding out from where I am getting too much DC on the taps, because there is NO DC offset adjustment on the AU-307. This is a fact according to the manual (in Japanese) and the horse's mouth — the Japanese tech who owns the manual.
The meterings were L= 129-139 mV; R= 100mV. The tech (nice guy) who does a lot of the local Sansui amp service says that this is within tolerance [Daijoubu desu. Shimpai nai, sore wa fudan desu yo] 'It's OK. Don't worry; it's normal.' Well, commensurate with our discussion here, I am still not very happy.
BTW, this same guy adjusted my big Denon PMA-1090 down to nearly zero DCV offset when he was setting up the bias after an output re&re.
I hope that an answer may benefit many of us in our common search for the cure for the 'the disease' or rather the eradication of the symptom. It stands to reason that the disease may vary. Cheers! And thanks in advance ...
Concerning an AU-307, but perhaps apros pos for almost any amp — if my PS FILTER CAPS (bypassed by mylar caps @ 10% of value) were leaking too much DC, could this show up downstream as a meter reading for too much DC offset?
Some of you will immediately see that I am short of knowlege in regards to circuits, but I am trying to gain understanding. As a hazarded guess, I would say 'NO'.
I have to start somewhere — that is finding out from where I am getting too much DC on the taps, because there is NO DC offset adjustment on the AU-307. This is a fact according to the manual (in Japanese) and the horse's mouth — the Japanese tech who owns the manual.
The meterings were L= 129-139 mV; R= 100mV. The tech (nice guy) who does a lot of the local Sansui amp service says that this is within tolerance [Daijoubu desu. Shimpai nai, sore wa fudan desu yo] 'It's OK. Don't worry; it's normal.' Well, commensurate with our discussion here, I am still not very happy.
BTW, this same guy adjusted my big Denon PMA-1090 down to nearly zero DCV offset when he was setting up the bias after an output re&re.
I hope that an answer may benefit many of us in our common search for the cure for the 'the disease' or rather the eradication of the symptom. It stands to reason that the disease may vary. Cheers! And thanks in advance ...
If the amp has a differential pair input, it's job is to cancel it out. A lot of low budget amps have no adjustment, and all that can be done is to replace the diff pair with a hand-matched pair (all this is covered on other pages of this thread, but at 20 pgs, I can understand the difficulty in locating it). A servo could also be built and added, but that's a whole new ball game.
Leaky PS caps? Unlikely. The amp is simply unbalanced and needs to be put right.
BTW, most manufacturers will spec DC offset as '<50mV', so most manufacturers would say this one is 'out of spec'.
Leaky PS caps? Unlikely. The amp is simply unbalanced and needs to be put right.
BTW, most manufacturers will spec DC offset as '<50mV', so most manufacturers would say this one is 'out of spec'.