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Old 26th January 2012, 05:51 PM   #1
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Default Loss of low frequency in one channel


I have a Samson Servo 170 amp that I've owned for a good few years. I've recently replaced the power-on relay and all seemed well. I then noticed that the right channel had a dodgy connection for the speaker cable. Took it apart again and found a dry solder joint on the connector. Now all sorted.

However, I have just noticed that the bass frequency response from the left channel is substantially weak compared to the right channel. The mid-range and top end seem fine. I've never noticed this before but can't be sure if it has occurred during my fettling or has been like that for a while.

One thing to note is that on the second time I put it back together, when I switched it on I got a spark between one of the transistor casings and a flying lead which is mounted just above it. I switched it off, moved the flying lead connector away and all seemed well when I powered on again.

The pic shows where the short occured:
Click the image to open in full size.

Could that have killed anything that would lead to the loss of the low frequency range in the left channel?

I'm not that good on circuits but know which end of a soldering iron to hold!


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Old 28th January 2012, 06:37 PM   #2
Mooly is offline Mooly  United Kingdom
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That sounds a strange problem. I can see where you indicated and can see the slight burn mark too on the transistor metal. Depending on the design of output stage and the configuration that could be either the positive rail or the speaker output. Dead easy to check, just see if supply is present.

That single component on the heatsink could be a thermistor or diode and will be used for temperature monitoring and used to reduce or even shutdown the amp if neded. Any damage done would be in that part of the circuit but that wouldn't effect just one channel and wouldn't reduce bass.

Lack of bass... if there really is problem then it's normally capacitor related. Speaker OK ?

This probably sounds a strange statement so hope it makes sense,

Amps are ultimately all DC coupled and so respond down to DC, and that applies even to single ended AC designs. It's the coupling caps/feedback return caps etc that define the low frequency response and return the gain to 0 (zero) at DC.
Too much of a coincidence this happening when you zapped something for it to be a normal fault.

You need to measure the output with known test signals, and preferably scope it to confirm what is happening.
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Old 28th January 2012, 07:02 PM   #3
DF96 is offline DF96  England
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Wild guess: the electrolytic in the ground leg of the feedback network has been zapped so you are just relying on a smaller parallel bypass cap. This assumes normal power amp topology (likely) and a bypassed cap (less likely?).
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Old 29th January 2012, 11:48 AM   #4
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Right, firstly thanks for the replies. Secondly, i'm an idiot! After swapping speakers over, cables over, trying different sources I then swapped the outputs so that the right output of the amp was going to the left speaker and vice versa. The same problem was apparent. I then decided to move the left speaker (with the lack of bass) to the same location as the right speaker and hey presto the bass was back. So the only thing I can conclude is that it's the shape of the room.

The right speaker is in a corner so I guess the low-end coming out of the back of the speaker is reflecting back towards me whereas the left speaker has an open cupboard behind and to the left of it where I guess the low-frequencies out of the back are being absorbed slightly.

I've just moved in to this room and as you can probably tell, have no sound absorbing material anywhere. Maybe that should be the next job.

So all in all, although I feel a tad stupid , at least it looks as if my amp is ok.

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Old 29th January 2012, 06:36 PM   #5
Mooly is offline Mooly  United Kingdom
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Room acoustics and image shift can be a problem. Not always an easy answer to that one.

Pleased you have found the cause though.
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Old 29th January 2012, 08:48 PM   #6
sregor is offline sregor  United States
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Spark was probably from mounting of diode or thermistor which is connected through the screw to the heatsink, and the case of the output, which means circuit should be fine, if any damage was done, it would be to supplies, usually a blown fuse, and common to both channels - so it looks like you are fine.
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