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Line out schematic
Hi, just looking to validate if the following line out design checks out.

With the values in place I expect this to be usable starting at just under a watt of speaker output.

Are the line level side grounding and center taps looking alright?



Attached Files Thumbnail(s)

Driving a transformer from a pot is questionable, even though everyone has done it at some point. The transformer DCR radically changes the effective taper of the pot. Even if we ignore that completely and just assume the ACR, say 10k, then see how that works against your pad. You might find it more effective to have thee transformer first and then the pot on the XLR side.This will depend on the signal capability of the transformer, but the Hammond 109-series is capable here. You can use fixed Rs of low-ish value on the primary side to provide protection for lesser-rated line TXs.

The XLR jack ideally has pin-1 tied to chassis. Some units provide this connection through the mounting holes. The ground tie to circuit ground should be close to the XLR. Add reverse-parallel diodes to the RC network to have a safe ground-link-isolator.

have fun

Happy new year and thanks for the detailed response!

Something like this (10W amp)?


Maybe some definitions are required:

circuit ground: In your drawing this would be label as "ground speaker out". All of that ground wiring floats and is tied to chassis via the ground link isolator, which you have drawn correctly but connected backwards

chassis ground: This is labelled as "power ground" in your schemo but it should NOT be the same tie point as used for the AC safety ground, which is also chassis.

The XLR pin-1 should tie directly to chassis. I use XLRs that make this connection through one of the mounting bolts. Other XLRs require a separate connection.

So, to correct your drawing, move the "GNDPWR" symbol to the other end of the ground-link-isolator, then tie the freed end to the speaker ground (electronically, not literally). The tie point should be as described here and in TUT3.

Happy New Year

Thanks for you clarifications. changed the naming on the schematic reflect proper definition.

A clarification, the "Speaker circuit ground" in connected to a London Power PA66, which it connected to a LP PSU connected to chassis VIA a ground-link isolator. If the the speaker side circuit ground is connected in such a way, following the galactic grounding principles, should the speaker circuit ground be connected to this ground-link-isolator? Wouldn't the connection highlighted in blue possibly introduce ground loop noise?

Let me know if I ask too many questions, I learn fast but sometimes it takes awhile. Rolleyes 

Hi Guys

As from the first post, whether you add this second GLI depends on what XLR jack you use..

One with pin-1 tied to the mounting screw does not need the GLO. Just float the TX output to the XLr pins 2+3, with or without the pot in between

If the XLR has a floating pin-1, then tie it to the chassis directly near the XLR.

The GLI is only added if you want a soft ground at the amp end and this is only for the XLR.

The amp itself has a chassis tie point via its own GLI as usual.

The Hammond TX should only have four connections used, those for the ends of the windings. The CTs are left open.

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