So my old CM Labs 920 Power Amp developed a problem. Not sure when it started, but only noticed recently.
The left channel produce a noticeable Hissing Noise with probably little scratch sound in it. I know it is normal that power amp has some base hiss noise but this is definitely a problem. Because it is only on left channel and loud enough to be noticed in normal listening position, the right channel has only noticeable hiss when I put my ear very close to the speakers. The hiss noise is also getting louder when I turn the input level control (on the power amp) up.
I opened the amp up, it shows that input level control is linked to the RCA input directly with a wire. The wire does pass the transformer but when I move the wire away from the transformer, it does not affect the hiss sound. That's all the diagnoses I've done. Any suggestions otherwise sending to a tech to fix which I will probably do? Thanks
The left channel produce a noticeable Hissing Noise with probably little scratch sound in it. I know it is normal that power amp has some base hiss noise but this is definitely a problem. Because it is only on left channel and loud enough to be noticed in normal listening position, the right channel has only noticeable hiss when I put my ear very close to the speakers. The hiss noise is also getting louder when I turn the input level control (on the power amp) up.
I opened the amp up, it shows that input level control is linked to the RCA input directly with a wire. The wire does pass the transformer but when I move the wire away from the transformer, it does not affect the hiss sound. That's all the diagnoses I've done. Any suggestions otherwise sending to a tech to fix which I will probably do? Thanks
My main suspects would be:
1. a leaky or damaged input transistor, causing excessive current noise (which in turn is converted into voltage noise by source impedance, which increases when turned up), or
2. input stage power supply, where a capacitor in parallel to a zener may have dried out over the years.
I would check the type of input transistor (-s, or FETs) used and whether that's known problematic by any chance. That said, the amp would have to be around 40 years old at this point, everything is possible.
1. a leaky or damaged input transistor, causing excessive current noise (which in turn is converted into voltage noise by source impedance, which increases when turned up), or
2. input stage power supply, where a capacitor in parallel to a zener may have dried out over the years.
I would check the type of input transistor (-s, or FETs) used and whether that's known problematic by any chance. That said, the amp would have to be around 40 years old at this point, everything is possible.
If the hiss changes with input level and the pot is directly to the input jack ... that noise could well be coming from your pre-amp or source.
Simple test.... switch the RCA plugs going into your amp... if the hiss changes channels, it is being fed into the amp from some previous piece of gear.
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