Series II 700 hiss

[ChopperCharles]

I have an unrestored, unmolested, constantly-in-use-since-bought-new Phase Linear Series II Model 700. This was my uncle's, and he owned it since new. He entered assisted living, and he knew I'd appreciate it, so he gave it to me. (Along with a Series II 3500 preamp and some Klipsch speakers). It was repaired once in the late 80s with new power transistors, and recently I replaced the main power switch, because it was burned internally and welded itself into the on position. I also de-oxited the volume pots at that time.

When powered on, there is a hiss from the speakers, even if no input is attached and the volume for each channel is at 0. The hiss gets slightly louder as the volume is turned up to halfway, and then quieter again as it goes past halfway, and is about the same loudness at max volume as at min volume (Weird that it's louder at half volume). Hiss is present in both channels. You can only hear it in quiet passages or when there's no music playing at all. Amp sounds perfect otherwise. There's also a POP from the speakers about 30 seconds after the amp is powered off. I figure that's normal, since there's no speaker protection relay in the 700.

I'm curious what could be wrong with my beloved 700, if anything. Seems a little odd to have more hiss coming from the amplifier than from my R2R deck... makes the noise reduction unit I have not all that useful, ya know? But it could just be the way the amp is for all I know. (Although my Series II 200s (both of them) are completely silent).

Thoughts?

Charles.

 

[oldphaser]

With regards to Phase Linear 700 series two amplifiers, I posted this on Phoenix a little while ago:

"Any twisted pair wiring that runs to the pc board from the volume pots IMHO should be replaced with shielded wire. To illustrate the difference..... listen to the noise floor of your amplifier with nothing plugged into the input. Carefully observe the noise floor while rotating the volume pot clockwise through your speakers. The twisted pair is noisiest when the volume pot is at 12 o'clock and then gets quieter as your rotate to the full clockwise position. NOTE: I also recognize that this is not a normal way to observe noise since you are not shorting the input (as would be the case with shorting jacks or the volume pots fully counter-clockwise). In this case, with twisted pair you are more than likely picking up any ambient RF."

Trial and error replacement of the I.C. (i.e. LF356 or LF351) may reduce the turn off thump. (NOTE: I would suggest installing some 8 pin gold IC sockets to minimize the chance of lifting the traces off the p.c. board). Otherwise Phase Linear considered the thump to be normal.

Ed

 

[ChopperCharles]

With regards to Phase Linear 700 series two amplifiers, I posted this on Phoenix a little while ago:

"Any twisted pair wiring that runs to the pc board from the volume pots IMHO should be replaced with shielded wire. To illustrate the difference..... listen to the noise floor of your amplifier with nothing plugged into the input. Carefully observe the noise floor while rotating the volume pot clockwise through your speakers. The twisted pair is noisiest when the volume pot is at 12 o'clock and then gets quieter as your rotate to the full clockwise position. NOTE: I also recognize that this is not a normal way to observe noise since you are not shorting the input (as would be the case with shorting jacks or the volume pots fully counter-clockwise). In this case, with twisted pair you are more than likely picking up any ambient RF."

Trail and error replacement of the I.C. (i.e. LF356 or LF351) may reduce the turn off thump. (NOTE: I would suggest installing some 8 pin gold IC sockets to minimize the chance of lifting the traces off the p.c. board). Otherwise Phase Linear considered the thump to be normal.

Ed

Oh excellent! That's good to know. I'll source some shielded cable and replace it post-haste.

Charles.

 

[oldphaser]

Oh excellent! That's good to know. I'll source some shielded cable and replace it post-haste.

Charles.

I almost forgot to mention.....

Also try reversing the (non-polarized) line cord and plug it into the wall again. You may notice approximately a 10dB+ difference in the noise floor.

Ed

 

[orange]

At least he didn't say anything about transistor noise.