Hmmm interesting...But how do you go about measuring DC offset? Do you measure it with a multimeter attached to the speaker output binding posts on the amp?
BTW: Sorry, I didn't mean to hijack this thread (I read the rules, new forum user here...) BUT it seems our thread author has resolved his initial questions about the speakers! hehe
BTW: Sorry, I didn't mean to hijack this thread (I read the rules, new forum user here...) BUT it seems our thread author has resolved his initial questions about the speakers! hehe
Yes, set it to mV range and measure across the speaker outputs with nothing attached (i.e. no speakers). If the number is over 50mV, (but below 500mV) then attach speakers and check again. if it drops below 50mV you are ok if not, it needs to be looked at.
If working well it should read within +/- 2mV.
If working well it should read within +/- 2mV.
Hi Arif,
The average person works with partial knowledge and therefore something that may sound reasonable to that person may be dangerous in reality.
The most common time for a technician to blow up a freshly repaired amplifier is when they are measuring bias or offset. The deed is straight forward, the mechanics of the operation are not. Remember that you operate with a store house of knowledge that you view as second nature and common sense. Your average person does not have that benefit.
-Chris
Not really. Given the in and out of warranty service we see the advice is proper. The way you look at things and a normal layman is not the same thing. mjraudio, heed the advice.
Some are quite unimaginable! I had a guy make his own 18 dB / Oct low pass crossover. We found out what he did the second time the amp came in for warranty service. Yup, he started with a 220 uF bipolar capacitor!!!!! He was only following web directions and refused to pay the "rip off" prices for a properly designed crossover. He also could not understand that warranty is not insurance ('cause he didn't know).
The average person works with partial knowledge and therefore something that may sound reasonable to that person may be dangerous in reality.
The most common time for a technician to blow up a freshly repaired amplifier is when they are measuring bias or offset. The deed is straight forward, the mechanics of the operation are not. Remember that you operate with a store house of knowledge that you view as second nature and common sense. Your average person does not have that benefit.
-Chris
I think I will heed the advice. I don't feel like replacing the fuses in my amp. And unnecessarily stressing the electronics.
On another note, check out this site, it discusses capacitors and mentions how and under what conditions they may blow up: http://www.repairfaq.org/sam/captest.htm#ctsdc
This is making me worried about a 20-year old ADCOM GFA-565 300watt monobloc amp with 120V 36,000uF capacitors....those things could kill me if i'm not careful!
On another note, check out this site, it discusses capacitors and mentions how and under what conditions they may blow up: http://www.repairfaq.org/sam/captest.htm#ctsdc
This is making me worried about a 20-year old ADCOM GFA-565 300watt monobloc amp with 120V 36,000uF capacitors....those things could kill me if i'm not careful!
mjraudio: You can measure DC offset safely, just set the multimeter to the correct (millivolt) range and connect it to the amplifiers speaker terminals with the amp's power off. make sure there is no danger of a short circuit, and remove connections to it's input. Then switch the amp on, whilst watching the multimeter.
The offset wil probably start momentarily high (around +/- 0.2V) but quickly, within seconds, drop to the tens of millivolts. As the amp warms up, if all is well, the offset should settle in the single digit millivolt range. This is typical behaviour of my 585.
Don't be afraid about capacitor explosions The PSU caps are safe (unless you touch their terminals 
). However it is electrolytic capacitors on the input PCB that can fail and leak electrolyte, causing the amp to lose control of the output, with massive DC offset as a result. The problem is known in the 565 and 585 models
This happened to the previous owner of my amp, and cost him the drivers of some expensive B&W speakers. (and the amp, which he subsequently gave to me )
Cheers,
Ed
The offset wil probably start momentarily high (around +/- 0.2V) but quickly, within seconds, drop to the tens of millivolts. As the amp warms up, if all is well, the offset should settle in the single digit millivolt range. This is typical behaviour of my 585.
Don't be afraid about capacitor explosions
The PSU caps are safe (unless you touch their terminals ). However it is electrolytic capacitors on the input PCB that can fail and leak electrolyte, causing the amp to lose control of the output, with massive DC offset as a result. The problem is known in the 565 and 585 models This happened to the previous owner of my amp, and cost him the drivers of some expensive B&W speakers. (and the amp, which he subsequently gave to me
)Cheers,
Ed
anatech said:
I couldn't agree more
I was really careful working around my 585, as it's the most powerful amp I have worked on - some parts would look quite at home in an arc welding rig... As you earlier mentioned, establishing a safe connection to the output stage bias test points takes some real care.
Ed
mjraudio said:
This wikipedia article should help : http://en.wikipedia.org/wiki/Electronic_amplifier
The output stage of many typical power amplifiers consists of a transistor pair, or multiple pairs (used in higher power amps for increased current capacity). One half of each pairing controls current flow from the positive power supply, during the positive part of the music signal, the other half of the transistors control current on the negative part of the signal. This is called push-pull or class B configuration.
Ideally, when the signal passes through zero, neither of the transistor sets would turn on. However it is very hard to achieve this without the amplifier missing a portion of the signal for regions close to zero, which introduced a distortion known as crossover distortion. To avoid this, amplifiers are designed so that at zero signal, both sets of transistors pass a small current, so that there is never a region where both + and - sections of the amplifier are inactive. This current is called the bias current or quiescent current. The design is known as class AB configuration (see the article I linked). It is important that this bias current is set to a suitable level (according to the individual amplifier design) so many amps provide test points that enable it to be measured - usually there is a resistor in the output circuit through which the bias current flows. Bias is checked by monitoring the voltage across this resistor. Service manuals usually contain bias specifications and directions for its measurement and adjustment.
Hope that helps,
Ed
Hi Arif,
Use a variac. Short out any surge limiting resistors before doing anything so you don't damage them. Be aware of the amplifier type in case there are other factors you need to be aware of (Carver PM 2.0 as an example)
There is no one blanket guarantied way of safely starting all amplifiers.
Sorry Arif, I'm not trying to pick on you. You have to remember that many people do not have your knowledge.
-Chris
Good thought, but not always. Some surge limiter primary circuits will spite you if you do that. Other amps will misbehave terribly when there is nothing wrong.
Use a variac. Short out any surge limiting resistors before doing anything so you don't damage them. Be aware of the amplifier type in case there are other factors you need to be aware of (Carver PM 2.0 as an example)
There is no one blanket guarantied way of safely starting all amplifiers.
Sorry Arif, I'm not trying to pick on you. You have to remember that many people do not have your knowledge.
-Chris
Hey Chris..
Weird issue... got a working 555ii. changed the power caps. Now I see a 4v dc offset... I didn't mess with the inputs... any quick ideas before i rip it apart?
Granted i did not check the offset before changing the caps just played some music.
clue: both channels are off by 4v. ??
supplies are a stable +/- 81.7 on both channels
Weird issue... got a working 555ii. changed the power caps. Now I see a 4v dc offset... I didn't mess with the inputs... any quick ideas before i rip it apart?
Granted i did not check the offset before changing the caps just played some music.
clue: both channels are off by 4v. ??
supplies are a stable +/- 81.7 on both channels
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