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Old 3rd August 2003, 11:00 PM   #1
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Default EQ Bypass Question

This should be an easy one for the experts...I'm building an integrated amp with preamp, poweramp and active tube EQ built-in. I have a multi-contact switch to allow the EQ to be bypassed when pressed, but have a dilemma.

Is it permissible to wire the switch so that it effectively parallels the EQ's input and output, thus routing the audio signal from preamp to poweramp while shorting the EQ in/out? Or, should I wire it so that the power amp's input can switch between the EQ output and the Preamp's output (leaving the EQ output floating when not selected). Of course, the input of the EQ would still be fed regardless.

What do you think?
Thanks,
Cory
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Old 3rd August 2003, 11:09 PM   #2
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Hi,

Quote:
Is it permissible to wire the switch so that it effectively parallels the EQ's input and output, thus routing the audio signal from preamp to poweramp while shorting the EQ in/out?
A short is a short so how could it matter?

Neither one of your propositions should influence the circuit it any way.
In fact both of your propostions achieve exactly the same if I read you correctly...

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Old 4th August 2003, 09:10 PM   #3
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Default Re: EQ Bypass Question

Quote:
Originally posted by cadman
...active tube EQ built-in. I have a multi-contact switch to allow the EQ to be bypassed when pressed ... should I wire it so that the power amp's input can switch between the EQ output and the Preamp's output (leaving the EQ output floating when not selected)....
Let us assume this tube EQ is a single band for stereo, so a total of two channels. In the above quoted scenario, you want to use a DPDT switch to select either the original signal (the input to the EQ) or the EQ-modified signal both channels at the same time. No problem.

In the scenario that I didn't quote, you want to use a DPST switch to be able to short the input of the tube EQ directly to the output, or at least this is what you described when the grammar is correctly parsed! Since the tube EQ likely has overall unity gain, this would make a screamin' oscillator that would likely send the tubes into meltdown.

Hope this helps.
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Old 4th August 2003, 09:56 PM   #4
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Jeff, You have parsed correctly! The EQ circuit I'm using is the infamous Headwize Ckt

Oscillation was my biggest concern. Best for me to play it safe and DPDT from preamp output to EQ output.

Thanks-
Cory
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Old 4th August 2003, 10:11 PM   #5
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Huh. Interesting circuit - what is the parent article/project for it at Headwize?

Another thing to keep in mind is that if the overall gain of the EQ isn't unity, then the mismatch in gain between Bypass and EQ positions will make any comparisons unfair.
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Old 4th August 2003, 10:15 PM   #6
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Hi,

Quote:
Another thing to keep in mind is that if the overall gain of the EQ isn't unity, then the mismatch in gain between Bypass and EQ positions will make any comparisons unfair.
That's an opamp based EQ.

Cheers,
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Old 5th August 2003, 04:58 PM   #7
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Here's the link to the Original Article

Cory
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Old 5th August 2003, 05:16 PM   #8
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Quote:
Is it permissible to wire the switch so that it effectively parallels the EQ's input and output, thus routing the audio signal from preamp to poweramp while shorting the EQ in/out?
IMO: No.
I'm not clear which EQ system you'll use, but there are bound to be unpredictable results.

Another point is that the EQ circuit will have an insertion loss.
Ideally you should switch an attenuator of the same value in place. Alternatively switch the gain of another stage. Carefully chosen, that will compromise the S/N less.

Cheers,
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Old 7th August 2003, 04:46 AM   #9
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Not a bad idea John, I'll take that attenuation into consideration. I doubt I'll use the bypass button much, but considering the chassis is pretty much finished and I can't think of anything else useful to fill the switch hole with, I'll press forward

I'm practically on month 10 of this amp, all I need now are some bits and pieces..a second pair of EL509s, proper code to get my LCD display to interface with the microcontroller running the show, and some infrared remote connections left to make. Why is it that I get the bug to build one of these suckers, and each time I have to out-do the last

Cory
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Old 7th August 2003, 08:58 AM   #10
dhaen is offline dhaen  Europe
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Cory,

Interesting combination: LCD's, code & PL509's

Haven't I seen your moniker somewhere? Was it last time I changed a Tek CRT?

Cheers,
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