Metro fx loop

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Metro fx loop

Postby Intripped » 02 Jun 2017, 22:54

looking for infos and/or detailed pics of this little circuit that seems to be still not accurately traced.

I've looked deeply around and I've only found some blurry pics and two schematics (attached). These two schemes are substantially different, though. Main difference is the 470nf cap, that is connected to the drain in the first, and to the source of LND150 in the other.
Which one is correct?

Thanks
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Intripped
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Re: Metro fx loop

Postby Intripped » 07 Jun 2017, 00:37

I've watched and studied the available images on the net, and I'm 99% sure that the attached schematic of rev.3a is correct.
there's a little difference regarding the two zener diodes, but apart from that, it is identical to the second schematic previously posted.
i still have a doubt around the connections of the 330n cap, but i can't clearly see from the pics, and this seems to be the only way it could be.


it would be interesting to trace the last version of the circuit:
there is a +4 /-20db switch to set LINE or PEDAL level, where a 33k resistor is involved (and the 15pF cap is not present anymore), but i can't understand from the pics how it works.

I've also noticed some other differences:

- two zener(?) diodes have been added on the first LND150; i think that these two diodes are connected as on the second LND150.
- the 330n cap is directly connected to the gate of the LND150, but i cannot see/undestand where the junction between the 1,5k and the 1M resistors, is connected to.
- there is a 2.2M resistor not present before, and it's probably connected to ground.
- there is also a 3,3k resistor connected to ground, in place of the jumper fitted on rev.3a boards. i'm not sure at all, but i think that this resistor is for... elevating the ground reference of some components (the RET jack gnd, the 1M res and the 470E res); maybe this is for noise reduction?
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