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Old 1st November 2009, 01:43 PM   #1
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Join Date: Nov 2009
Default Studio Series Pre-Amp

Hi guys

I've built the full Studio Series Stereo Pre-Amp (issue 214, July 06 - Feb 08 EPE). Not sure if anyone here would have built these, still less actually have the original magazines ... but any ideas would be much appreciated.

The only problem I have is with the CON7 Headphone amp switch.

Audio is never routed from CON14 to CON6 and into the headphone amp.

The preamp works well on it's own inc remote channel switching, volume (I have the powered ALPs pot). Audio is very good quality.

Also ... if I plug RCA connectors from a source directly into Headphone amp, that too works very well.

I just can't get the preamp CON14 to headphone amp switching to happen.
Regardless of whether the headphone is plugged into the first socket (I've tried both), audio is always available at CON 14 and never at CON 6.

I think it maybe to do with the PN100 (purchased from Jaycar).

What I've checked so far :

Removing JackSw wire from Headphone amp ->
With or without headphone jack in, I get 0v at the Con7 Jack switch (i.e. 5v into 1.5k resistor that goes to Switch ... dropping to 3v at switch).

When Jackwire connected, that 3v is earthed when no plug is in pulling voltage to 0v. This 0v obviously exists all the way up to the base of Q6.

When plug in, voltage returns to 3v. This 3v reduces to 2.57v as it goes through D6 amd then to 729mv after it has gone through the second 1.5k resistor therefore providing that 729mv to the base of Q6 (the PN100).

Now ... looking at Q6 (PN100), with the plug in and 729mv at the base, the emitter has 3v. I don't believe this is enough to energise the coil ... and thus it won't switch the relays - RLY6 and 7, diverting the audio to CON6.

Without the plug in, the base of Q6 is virtually zero (20mv). Interestingly, the emitter in this state is 5v .... doesn't this mean it should energise the coils and switch the relays ? It clearly isn't though - and without the headphone plug in it shouldn't be energising the coils at all.

With or with the plug in, the collector voltage of Q6 is 80-200mv. Not sure if that would be right or not.

What I do know is that the channel switching has pin1 of the relay at 0v regardless of whether it was the source or not. If it wasn't the source, voltage at pin 10 is 0v and if it is the source, pin 10 is 5v. This would suggest that RLY6 and RLY7 should be 0v at pin1 and 5v at pin10 when the headphone plug is in. In practise, with no headphone plug in, pin1 is 5v and pin 10 is 5v. With the plug in, pin 1 is 3v ! and ping 10 is 5v. This seems very wrong.

I definitely have the PN100 in the correct way around. I've checked the Fairchild datasheet. It also confirmed a max hfe when the right way around in my DMM ...

Any ideas would be hugely appreciated. It's a shame to be so close to a completely finished project ... but not quite there.

Best regards

Paul.
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