Input Pad that changes impedance?

Potato Cakes said:

Hello,

I'm working on a couple of preamp/EQs and while doing initial testing I thought I was having an issue with the noise floor, but it turns out there was an impedance mismatch between the generator (line level) and the preamp. When I connect a microphone or my battery powered handheld generator I do not have this issue. I have selectable H-pad installed to allow the preamp to handle line level signals so I can use the EQ for mixing, but this won't do me much good currently as the noise level with the impedance mismatch is the same regardless of the pad being in or out. So I'm wondering is there a different type of pad that could be implemented  which would also address the impedance/noise issue? I'm working within the parameters of the existing board and design, so essentially I have to be able to do this with the existing DPDT switch that I am currently using for the pad. I know that this can be done at the input transformer and a number of other ways if I was building this out as a new project, but this would require more alterations to the board and metal work than I am willing to do for these particular preamps.

Thanks!

Paul

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user 37518 said:

Any resistive pad will include loss of signal and noise, the way to do it (RF Style) would be to use reactive elements, in audio thou, more frequently you can use a transformer, a 4:1 ratio  should suffice to attenuate the level. I still dont know whats the "mismatch" issue with your setup, audio is mostly impedance bridging instead of matching, as long as the output Z of the equipment is much lower that the input Z of the other equipment you shouldnt have any problems

You dont mention the output Z of your generator or the input Z of the preamp, when you say that you want to match them you mean that you want the reflected impedances to be the same? like 600 ohm to 600 ohm?

If you insist on using resistive pads instead of transformers, there are quite a few that you can use in the book "Handbook for sound engineers" by Ballou

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abbey road d enfer said:

H-pad is probably not the best option.
Post schematic of said H-pad.

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Potato Cakes said:

Attached is the H-pad in it's current configuration. It's on a DPDT toggle switch which is not shown on my crude drawing. It does give me the 30dB of reduction which ends up being unity gain with the gain knob all the way down and the pad engaged. As mentioned before, with a line level source (the Aurora I am using) the noise floor remains constant with the pad switched in/out but the signal is affected as intended. With a microphone or other similar impedance the noise floor is decreased dramatically to what I am expecting. Ideally when I engage the pad it would increase the impedance to match a line level source better in addition to attenuating. But as I'm thinking about the whole scenario related to this circuit I don't see how that would be possible without a second mechanical action.

Thanks!

Paul

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