RB-1080 HF Distortion

[Rod S]

Folks,

Quick check to see if anyone has ever experienced any sort of high frequency distortion with their RB-1080. After trouble free listing for almost 4 years, today I immediately noticed a distorted sound coming from the right channel. From listening to the speaker, the distortion is focussed in the tweeter; from swapping connections and sources the issue is definitely in the RB-1080.
Anyone ever had anything like this?

Cheers, Rod

[Glen B]

Assuming that your swapping of connections was thorough, ruling out a problem with the tweeter, two other common problems that can result in distortion are excessive DC offset, and insufficient bias.

[brodricj]

Wouldn't noise from DC offset be present in both channels of the amp? With noise from insufficient bias, are the heat sinks from one channel on the amp significantly warmer than the other?

[Rod S]

Thx very much for the replies.

Haven't had a chance to check the temperatures but will look at it tonight.
I'll open up the chasis and take a quick look for burning or obvious issues.

Hopefully it's not too serious.....


Cheers, Rod

[Glen B]

Wouldn't noise from DC offset be present in both channels of the amp?

No. DC offset level is an indication of the health of the differential stage, and each channel has its own such stage.

With noise from insufficient bias, are the heat sinks from one channel on the amp significantly warmer than the other?

Not necessarily IMO. It would depend of how much difference in bias there is between the two channels. If bias on the "good" sounding channel is also low but not enough to cause noticeable notch distortion, temperature difference between the heatsinks may go unnoticed.

[Glen B]

Thx very much for the replies.

Haven't had a chance to check the temperatures but will look at it tonight.
I'll open up the chasis and take a quick look for burning or obvious issues.

Hopefully it's not too serious.....


Cheers, Rod

Do you have a digital multimeter (DMM) ? Its easy to use one to check the DC offset of your amp. If your results are good, that would rule out high DC offset as the cause of your distortion. You can follow these steps to check offset:

• With the amplifier power off, disconnect all input and output connections.
• Plug the positive (red) and negative (black) leads into the "volts" and "common" input jacks respectively of the DMM.
• Attach the red and black probes to the positive and negative output posts of the amp's problem channel.
• Set the scale of the DMM to the "mVDC" (millivolts DC) scale. If the DMM has multiple mVDC scales, start with around 300-400mV.
• Turn on the DMM, then turn on the amp and let it settle for 5-10 minutes. DC offset may change over time as the amp warms up. Note the mV reading on the DMM's display. It is okay if the reading bounces around a little. It also does not matter if the reading is + or -mV.
• Repeat the above steps to measure offset in the other channel.

Any reading in the range of ± 0-5 mV is excellent. Between 6-50mV is less than optimum but there should be no audible effects. With offset greater than 100mV, there would be significant distortion.