Amplifier Distortion, DC-Offset, and You!

Oops...sry.

The 555ii has a servo control for DC offset, and should measure about 1mV or less on both channels. Might take a minute or two to get there, but unless both channels ultimately drop to a very low reading, there's an issue.

Heatsinks getting hot is a bias issue.
 
Oops...sry.

The 555ii has a servo control for DC offset, and should measure about 1mV or less on both channels. Might take a minute or two to get there, but unless both channels ultimately drop to a very low reading, there's an issue.

Heatsinks getting hot is a bias issue.
Any cause for concern for voice coil damage? Will definitely not be using it until its fixed, but with measurements of .5mv in one channel and 10mv in the other, i would i think it at least wasn't a danger at the time of using it.
 
If you heard something untoward when connecting speakers, then offset isn't the problem. Set your meter to read AC, or connect a scope to the output and see WTF.

If you want to get deeper, this isn't the thread to do it. Start your own in Solid State.
 
hi EW, this thread is awesome!

I have a couple of questions:

I have recently acquired a Yamaha B-1 and a pair of BX-1's....I know I am still wetting my pants of excitement when I think about it...(and to be honest, of some fear as well)

I started looking at the B-1 first. Very virgin, plenty of dust and according to the seller, it has been at the doctor recently for check-up.

As far as the offset, this is what I found (and pleas educate me on this)
Two meters set to mV, connected positive to positive, negative to negative at the speaker terminals.

Both meters displayed about (-)2.8mV at the beginning, as if the polarity was reversed. Gradually, the voltage went down. After about 15 minutes, one channel went down to zero, then started ascending to positive 0.4mV where the other remained in the negative territory at (-)1.9mV

I realize that the numbers are damn low, but I am confused about polarity. I know I had the leads connected correctly.


As far as the BX-1's, not as virgin...it looks like they have been worked on with possibly new caps and some re-floated solder joints through. Old trimmers still in place. The work looks professional but not a complete rebuild.
Both BX-1s started in negative territories but transitioned rapidly to positive and settled at 24mV and 34mV, respectively. Nothing to write home about but not too bad.

Side note: The BX-1's sound amazing! The could B-1 will benefit from some work. Both will be going to the storage for a while until I have time or will find someone to work on them. No, I am not pitching, I know you won't touch them :)

I will be posting some additional details in the Yamaha forum. My only curiosity for now is about the polarity difference in the B-1. These are both DC amplifiers, correct?
 
The polarity of the measurement is not important as the offset pair can drift in either direction + or -. Your offset readings are just fine.
 
Krell KAV 300i 0,1mV on both channels
Sony TA-F830ES 0,5-1mV on both channels

both of them are designed w/o dc servo
dc offset should be allways lowest possible, lets say 1-5mV will be acceptable
 
Recapped my NAD 2100 last night and then did the amplifier adjustments.

1. Center Voltage Check: Connect DMM between ground and a leg of a specified resistor for left and right channels. Spec is 0V +/- 100mVDC. I'm getting -60mV on L channel and +22mV on R channel.
2. Idle Current Adjustment: Spec is 15mV +/- 2.5mVDC. Adjusted two pots for bang-on 15mV.
3. Final Adjustment: Leave unit on for a minimum 5 minutes, repeat center voltage check and idle current adjustment.

Unit warmed up for a little while and I dialed in the idle current to near 15mV, but the center voltage check remained the same. I'm within spec, but just wondering why there is such a difference between L & R channels?
 
Hi everyone! First post here from a gearhead/musician looking for some advice on DC offset with regards to instrument solid state amplifiers.

I have some older guitar/bass amplifiers, and some don't test so well according to the chart here for DC offset. Now we ARE talking about guitar instrument amplifiers (and bass guitar), but do the same rules apply? As for the ones that test around 100mV; do they basically have bad transistors/drivers/capacitors/other failure?

Carvin B3000 (250W solid state bass amp) #1
130mV

Carvin B3000 #2
105-110mV

Yamaha B100 (100W solid state bass amp)
100mV

Crate GC-130XL (stereo solid state guitar amplifier)
Left: 39mV
Right:40mV


I also want to disclose that I've never heard any of these amps played through speakers, so I'm not sure how good or bad they sound, but I want to be sure they are in good working order before I trust them with a speaker cab!
 
Sansui AU-517

Right: 16.5 mV
Left:445 mV

I guess there was a reason it was at the thrift. I don't know anything about that switch on the back. Off to get the manual.
 
517 has offset adjust.
Can you fill me in on that switch on the back? One position is direct coupled and the other is cap coupled. Does it matter which position the switch is in when testing the offset? What is the benefit to each position in regular usage?
 
...talking about guitar instrument amplifiers (and bass guitar), but do the same rules apply? As for the ones that test around 100mV; do they basically have bad transistors/drivers/capacitors/other failure?
Does anyone have any insight on this? I'm not sure how (if any) SS home stereo amps and instruments amps relate as far as this test goes.

Carvin B3000 (250W solid state bass amp) #1
130mV

Carvin B3000 #2
105-110mV

Yamaha B100 (100W solid state bass amp)
100mV
Also, are these safe DC voltages to run a speaker on?
 
Absolutely.

It takes many volts (not the tenths of volts you are seeing) to do any real physical harm. For high-fidelity use, we want the DC voltage as close to zero as possible. For a guitar amp, not nearly so critical, but with any amplifier it's always nice to have the offset as low as possible. But don't obsess over it.
 
Echo, thanks for your reply and clarification!

When you say many volts, how many DC volts do you think it would take to harm a typical 25W-50W guitar speaker?

Lastly, how about when intitially turning on the amp, or after shutting off the amp; some of them show 3V, or even 4V on one of them. Is this still far below the threshold?
When turning on some of the older ones, it usually hangs around 500mV-1V and settles down in a matter of minutes, while all of the above amps were putting out around 2-3V for quite some time (5-10 minutes or more). Is the 2-3VDC when shutting off the equivalent of the "pop" noise the speaker makes when shutting it off? If a speaker were attached instead of a multimeter, does it dissipate into the speaker instantly (thus "popping" the speaker) when doing so, or does it hang out at around 2-3V for a 5-10 minutes?

Thanks again!
 
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