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Old 25th February 2009, 04:07 PM   #1
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Default Quad 405 experts needed.

Hi, I wish to reduce the gain on my Quad 405 - which I built in my youthful enthusiasm soon after they were first introduced and had a few cap changes since then! but still a workhorse with all the original transistors still working (op-amps have been swapped, zener mod for increased low load current a la 405-2 etc etc).

I have upgraded according to the Bernt Ludwig/Keith Snook type mods etc but his gain reduction was a factor of 3 for 1.5V sensitivity.

I wish to reduce the gain by a factor of 2 to make it the same as my Audiolab 8000Ps ie 1V rms for 100W into 8R or 29dB overall gain for easy comparisons (taking into account the phase inversion).

My electronics theory has not been used for MANY years so would be obliged if another brain could check my calcs.

The original was gain of 15 on the op-amp from 22K and R6 330K. This needs to be reduced to 7.5 for my new overall gain so I think 180K for R6 should be ok (160K is a tad low)?

The time constant for R6 and C4 needs to be constant, so C4 should then be 0.082uF?

And the feedback time constant through C2 needs to be changed to reflect the new gain so 47uF non polarised should give the same 20Hz?

I don't think any other changes are needed just for the gain... any help gratefully appreciated.
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Old 27th February 2009, 06:50 AM   #2
Mooly is offline Mooly  United Kingdom
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That's looks the hard way, the easy way is to alter R3 and C1.
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Old 27th February 2009, 11:00 AM   #3
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Hi, thanks, yes I see how 43K and 0.33uF gets the same gain while keeping the design high pass.

I wonder why Ludwig chose the other way? maybe to keep the noise low to try and get max benefit out of more modern op-amps?

thanks for your contribution
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Old 27th February 2009, 11:13 AM   #4
Mooly is offline Mooly  United Kingdom
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Will just add that sonics can change with different gains. A high gain and passive input attenuator... not the way to do it I know... can sound more "open". Don't know what Opamp your using but I find the OPA604 pretty decent in a lot of applications... my favourite tbh.
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Old 27th February 2009, 01:07 PM   #5
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I've got a OPA134s at the moment.

Funny that, for years I used a little input box with 30K series (switchable in or out) and a 10K dual gang pot (for tweaking balance) virtually as a preamp for the 405 when cd was introduced - since the cd player had its own volume control.

It will be interesting to see any difference without this passive attenuation! thanks.
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