IIC+ Switching / Grounding Help

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Jay Omega
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IIC+ Switching / Grounding Help

Post by Jay Omega » Mon Aug 01, 2022 12:47 pm

Hello - I've been trying to convert my IIB HRG to a IIC+ by making my own preamp boards and have failed twice so far. I've ended up with a ground loop / buzz that I can't tolerate so I've stripped the amp down again.

Can anyone help me understand the grounding on the switching please? I think the 1N5352 and the 470uF 16V cap are grounded back to the PT bolt or the power supply card ground, but it looks like the 6.2V zeners / pull lead switch are grounded to the signal ground pour on the preamp board? I can't follow the preamp ground pour as it's hidden under or obscured by components in the pictures I've seen.

Any help would be greatly appreciated!

Matador
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Re: IIC+ Switching / Grounding Help

Post by Matador » Wed Aug 03, 2022 3:21 pm

I take it it's a hum/buzz problem only when the lead mode is enabled?

I'm not sure "grounding" is the issue: there is no AC current flowing through the switching network: the LDR's are DC biased. The circuit works essentially like this: The 60VAC supply is half-wave rectified, and is regulated to about 13V with the 15V Zener and 470uF cap. This voltage goes to the cathode's of the LDR's through a 3.3K resistor. This 13V supply also flows through a 470ohm resistor to another Zener, this time at half the first Zener's voltage (around 6V).

When lead mode is disabled (off), you have 13V on the cathodes, and 6V on the anodes, so all of the LDR's are reverse biased, hence they are all off and the lead channel is bypassed. When you hit the lead footswitch, or pull the lead pot, the cathodes are grounded, setting the cathodes to 0V, but current for the LDR's drops across the 470ohm resistor and the 6V Zener turns off.

This current is limited to the bottom left portion of the board, and returns to the PSU board via the ground cable between the preamp board and the psu board. If you really wanted to eliminate AC bleed as a problem, you can disconnect the wire from the PSU that goes after the 1N4006 diode and 680R/3W resistor on the top corner of the PSU board, and power it directly from a 12V battery. This will provide a pure DC to the channel switching network and if you are still getting hum/buzz it can't be coming from the switching circuit(s).

Jay Omega
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Re: IIC+ Switching / Grounding Help

Post by Jay Omega » Wed Aug 03, 2022 7:38 pm

I appreciate the time you put into the explanation, agreed with your description of how the switching circuit works and that there is no AC current present. My theory is the buzz was coming from a ground loop. If the LDR supply is connected to the chassis at the PT or via the power supply board common, and the preamp board has one ground "plane", and the preamp board is also connected to the chassis by the flat wire braid, you have a big ole ground loop and could have the pulsed DC charging currents from the switching supply traveling to the chassis thru the input jack and back to the PT.

*I think* the twisted wires that bring the 60VAC and ground to the preamp board near V4 are only used for the half-wave rectifier and the 5W zener and 470 uF cap return currents. It looks to me that the 6V zeners are tied to the main preamp board ground which is used for signal. My instinct tells me best practice would be to keep the entire switching supply (6V zeners included) separate from signal ground to avoid a ground loop, but pictures are ambiguous and that's what I'm hoping somebody can help me to verify.

Matador
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Re: IIC+ Switching / Grounding Help

Post by Matador » Thu Aug 04, 2022 11:42 am

Is it true that you only get the noise when you are in lead mode? I inferred that was the situation based on your question but I realize you never said so explicitly...

Jay Omega
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Re: IIC+ Switching / Grounding Help

Post by Jay Omega » Thu Aug 04, 2022 1:47 pm

It's been a while since I broke it down unfortunately, but I only remember it bothering me on the lead channel. I might just build a standalone preamp similar to a Studio preamp and see if I can get that going without noise. I'm worried about rebuilding the IIB too many times as the tabs on the EQ sliders are fatiguing and Mesa won't sell me replacements.

Matador
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Re: IIC+ Switching / Grounding Help

Post by Matador » Thu Aug 04, 2022 4:51 pm

If signal follows audio 0V, then loops shouldn't be possible.

This is the ideal arrangement, with only two connections to the chassis: one, at the safety ground to the power cord, and another, to bridge analog 0V with chassis right at the input jack.

Image

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