Preamp mod with a low cut shelf filter - diyAudio
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Old 9th December 2012, 12:07 AM   #1
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Default Preamp mod with a low cut shelf filter

Hi,
I've been looking for an easy, quick way of reducing bass amount when watching/listening late at night. I don't need an eq or full tone controls, just a switch that activates a fixed high pass filter. Problem is I want the signal completely untouched when the filter is bypassed (aka source direct on some systems).
I came up with this schematic for a simple circuit to build into my preamp. I'm worried that even when the filter is bypassed the R and L channels will still be connected to common ground through R2 and R4, thus "touched". Even if I cut the ground as well with a 3-pole switch, R and L are still connected to ground via the connectors in the output end. See red markings. Do I need to find a 4-pole switch to cut R and L in both the input and output ends? Or would you consider the bypassed signal "untouched" in this scematic?
I'm planning to put the filter after the amp section, directly before the output to powered speakers.
Click the image to open in full size.
BTW, I used CircuitLab, very nice: http://www.circuitlab.com/
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Old 9th December 2012, 12:05 PM   #2
AndrewT is offline AndrewT  Scotland
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please attach your pics.
Makes for a faster download of the page and allows rescaling of the pic so that the text wraps properly on to the available screen width.
If I "back" out of the page and then "forward" to reload the page the text will wrap for me, but attaching avoids us Members having to load a page twice.

The disadvantage you see is simply a 11k of extra loading on the source.

If the source has a low output impedance and the receiver has a highish input impedance then the extra 11k in parallel across the receiver should make no audible difference.

Check you source and receiver impedances.

Another unhelpful post?
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Last edited by AndrewT; 9th December 2012 at 12:09 PM.
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Old 9th December 2012, 01:26 PM   #3
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Sorry, I was looking for an attachment button but I couldn't find one, but now I see it clearly. Must have been tired...

I don't have any way to measure output / input impedance at this moment, but let's assume they are fairly matched. How can I design the filter to not increase the pre's input impedance when bypassed? (When activated it will of course, since it's passive.)
would a 4-pole switch do it? Like this (all SW1 switches are assumed the same physical panel mount switch)
hipass-2.jpg
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Old 9th December 2012, 01:50 PM   #4
DF96 is offline DF96  England
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Either use a 4-pole switch, or just switch the output side - assuming the source is happy to drive the load presented by the circuit.
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Old 9th December 2012, 03:05 PM   #5
Davey is offline Davey  United States
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Johan,

You're over-thinking this too much.

You really should measure (or find out) the input resistance of your power amp and output resistance of your preamp. Once you know the power amp input resistance you can get rid of R2(R4) and scale the values of C1/R1(C2/R3) to provide the shelf you're looking for. Then just put a switch across the combination to disable/enable the filter.

Cheers,

Dave.
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Old 9th December 2012, 04:17 PM   #6
DF96 is offline DF96  England
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Unless your power amp has quite a low input impedance then omitting R2 etc. may increase noise. Whether this would be noticeable depends on many factors.
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Old 9th December 2012, 04:55 PM   #7
Davey is offline Davey  United States
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Well, yeah. If the input resistance is 470k then probably not a good approach.
But what if the input resistance of the amp is 10k?

My point echoes Andrew's....you really need to know the amp/preamp parameters. Not being able to measure or find out just doesn't cut it.

Cheers,

Dave.
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Old 10th December 2012, 08:06 AM   #8
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Ok, so how do I measure this. Google shows me different ways of calculating input/output impedance which I have a hard time understanding... I have a multimeter. Can i put it in Ohms and measure across R/L and ground? With the power on?
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Old 10th December 2012, 09:35 AM   #9
AndrewT is offline AndrewT  Scotland
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start by putting a fixed test signal through the Source.
measure the source output with the output open.

Then attach your normal receiver
Now measure the source output with the receiver connected.

The difference in the two output measurments gives an indication of the "ratio" to the source output impedance to the receiver input impedance.

If the measurement drops by 1% then the receiver:source is ~ 100:1
If the measurement drops by 10% then the receiver:source is ~ 10:1

The actual calculations can be given if you know the actual resistance of one of them.

If you have a spare resistor somewhere in the range 10k to 200k, then you will have a receiver and know the receiver resistance.
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Last edited by AndrewT; 10th December 2012 at 09:37 AM.
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Old 10th December 2012, 05:53 PM   #10
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Hmm. This may be a little off topic, but, how do I know that my source, pre, and power (active speakers) aren't already completely unmatched for each other. I never measured them, didn't look at the specs, didn't knew about input/output impedance when I bought them (except at speaker level). They sound good though. The preamp is a diy project from China wich currently is disconnected. Argon DAC1 to active Audioengine 5. I just found out that the A5 has an input impedance spec of 10k. The DAC's specs doesn't say - isn't that strange, since it lists dac-chip, OP-amp, and even comes with a small torx driver in case you are curious to see how it looks inside...
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