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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Doug Self Preamp from Linear Audio #5
Doug Self Preamp from Linear Audio #5
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Old 30th September 2017, 03:10 PM   #831
tER299 is offline tER299  Netherlands
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Quote:
Originally Posted by orreborre View Post
I have finally finished the project but have encountered a problem with the balance of the pre amp.

Any ideas of where to start to look?

Thanks
/Oskar
I'd guess there is some significant manufacturing error in your volume pot that the design can't compensate for. Maybe you can replace it by another 5K linear pot?
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Old 30th September 2017, 03:12 PM   #832
scottjoplin is offline scottjoplin  Wales
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A number of people (me included) had a similar problem. The circuit is sensitive to slight imbalances in the volume pot at the start of the track. See post 722 and onwards for advice
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Old 30th September 2017, 03:59 PM   #833
diyralf is offline diyralf
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As far as I remember, Douglas Self is preferring low impedance designs in Active Volume Controls. This is a problem in volume controls. The resistance is very very small. Somewhere I have read that the gain error should be less than 1.5 dB. They would hardly be heard. Your circuit seems to have more.
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Old 30th September 2017, 04:11 PM   #834
orreborre is offline orreborre  Sweden
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I tried scottjoplins method to lower the input signal to the preamp, and compensate by increasing the volume pot. This solves the problem with the gain imbalance. The problem seems to only appear when the pot is in its minimum range, regardless of what the output voltage of the pre amp is. Guess the questions comes down to if to switch the pot, lower the gain of the pre amp or lower the gain of the amp itself (honey badger)
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Old 30th September 2017, 04:23 PM   #835
scottjoplin is offline scottjoplin  Wales
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I think the pot would have to track very accurately in its lower settings to be satisfactory. I attenuated after the preamp to keep the level high for as long as possible, main volume control is before my active crossover
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Old 30th September 2017, 04:35 PM   #836
diyralf is offline diyralf
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I would not make the pot smaller than 10k. There is also a good off-gain.

Last edited by diyralf; 30th September 2017 at 04:37 PM.
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Old 1st October 2017, 05:57 AM   #837
ammel68 is offline ammel68  United States
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Originally Posted by scottjoplin View Post
A number of people (me included) had a similar problem. The circuit is sensitive to slight imbalances in the volume pot at the start of the track. See post 722 and onwards for advice
Regular volume pots that use carbon impregnated plastic(or polymers) are a waste of time and money if one is looking for reduced channel imbalance issues at lower(volume) levels.
That's why there are stepped attenuators and LDR attenuators.
Problem solved!

By the way Scott, I like your avatar of Boy George.
Sure brings back memories of those good ole' Culture Club songs from the 1980s.
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Old 1st October 2017, 10:25 AM   #838
scottjoplin is offline scottjoplin  Wales
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It's Kate Bush, a still from the Army Dreamers video
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Old 1st October 2017, 11:19 AM   #839
AndrewT is offline AndrewT  Scotland
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dual gang log vol pots always have some imbalance in resistances.
Anything you can do to swamp these imbalances will reduce the range of volume available from the vol pot.
The only solutions are switched, or digital controls.
LED/LDR does not offer a solution to channel imbalances..
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Old 1st October 2017, 04:59 PM   #840
georgebrooke is offline georgebrooke  United Kingdom
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Hi Carl,
I love the idea of the tone control section being used as an upgrade to the many bare-bones "audiophile" preamps that are out there. The preamp design has brought me to a stop.....I can't see spending more money & time on more builds. But modules...now there is an idea I could go with...as long as the modules can share PS, etc. Sounds a great idea

George
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