This IS an interesting study in how to change a +24VDC device to work with +/-16VDC, and cool to see a version of this circuit again.
Happy to see polarity on the output side.
The Z switch is for input, changes series/parallel connections on input transformer after the H pad. This is also a +/- gain switch, and changes the steps of the H pad a bit too.
Slenderchap said:
It's a pseudo balanced bridged T attenuator
It's a bridged H pad. Textbook definition of one.
I'll speak without benefit of any measurement analysis you may have taken in approving this approach. Simple signal generator source would not be an adequate test, bearing little resemblance to a wide array of potential mic sources.
No one uses an H or T on a mic input for very good reason, pretty much any designer would advise against, it violates bridging principle. Great volumes of commentary have been written over decades against using a constant impedance pad with microphones. With an H pad, this is set up as a line amp that happens to amplify mic signals. You have the same load/source Z presented to the input transformer and the mic. Heavier mic loading than standard, light transformer loading (which tends to come with diminished bandwidth). Neither mic nor transformer want to have this constant impedance between them. Textbook commentary, loading any dynamic/ribbon like this introduces a LPF, and tends to increase distortion in condenser mics along with reduced headroom.
Take it to -5dB (45dB) and it's 1Kish R load to each.
Take it to -10dB (40dB) and it's 645ish R load to each.
Take it to -15dB (35dB) to -50dB (0dB) and it's 600ish R load to each.
At no place other than full gain, no pad, is the mic subjected to a typical fully bridging input.
This all works out sorta OK for the input transformer when set to 600 input, less so for 150. The mic load from -10 on is ideal for the Shure SM57, but not a lot else. The source Z from a mic is always increased by the series resistance, also true with a U pad yet the U fixes the source Z to the input transformer unlike the H.
The U pad has been the universally recognized method for padding mic inputs for decades, and works equally well with the vast majority of line sources. I would have built out a U pad with multiple steps, starting at -15 or -20.
But hey, if no one complains, does it make any difference? Load Z closer to matching is a ‘feature’ of at least one other product, yet it still uses the U pad.
I went down the road of hacking this layout to a U pad, it could be done but it's fairly complex with at least 20 trace cuts and probably a wired path restoration or two. Given the 150/600 Z switching, I'd make the U shunt for the 150 since errors should be lower to the 600 than if you shunted for 600 and switched to 150.
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