Class eh buffered attenuator

[GV#27]

I'm in the process of converting this passive attenuator into an actively buffered one.I built it to allow for level adjustments of the woofer sections in my active 3way system as my Bryston 3B does not have level controls.

It consists of a Hammond case,Alps blue beauty pot,Connex RCA jacks,Panasonic film caps. The nice volume knob was a surplus store find,it's from a mid 90's Classe preamp.Im not unhappy with it but thought that adding a high current, low distortion,low noise buffer might improve bass/upper bass punch and dynamics as I have usually found passives to be lacking in these areas.

The buffer is a discrete BJT design(the little green PC boards in 3rd pic) somewhat similar to Burson's buffer.The raw power supply will be external as obviously there isnt much room, plus having the transformer separate will reduce the potential of inducing hum.The Panny caps will be retainted in the active version to eliminate the possibility of any DC reaching the woofers voicecoils.

[Face]

Very nice Fred. I wonder how that would sound compared to a B1?

[zingo]

Great project indeed! Are you just going to use a wall wort?

[megasat16]

Neat Stuff, Fred! How does it sound (I mean like still transparent or adds any color to the sound?) when using with 3B? Let us know when you are done testing.

[GV#27]

Very nice Fred.

Thanks Mike

I wonder how that would sound compared to a B1?

Im curious about that aswell and would like to do a comparo.I may build up another for use as a pre.If I do I could send it your way to compare with your B1.:D

Great project indeed! Are you just going to use a wall wort?

Thanks.No wall wart ,I have an outboard supply that feeds two other small components and it should have no problem powering this one aswell.

Neat Stuff, Fred! How does it sound (I mean like still transparent or adds any color to the sound?) when using with 3B? Let us know when you are done testing.

Thanks mega,having used this exact circuit in other projects,like the analog output stage in my CD/SACD player I know it to be a very transparent low distortion device with great dynamics.It will interesting to hear if there is any improvements while operating it over a limited bandwidth of 60-300hz.

[ben62670]

Fred please keep these ideas to your self. I allready have too much to do!
Just kidding. Do you know if you can replace the cheap Alps pots that come on the motorized remote units with the Alps you are using? Also is the phase going to invert?
Have fun.
Ben

[GV#27]

Fred please keep these ideas to your self. I allready have too much to do!

Sorry.....Not!:D

Do you know if you can replace the cheap Alps pots that come on the motorized remote units with the Alps you are using?

This pot is essentially the same as the blue motorized Alps sans the motor.They are decent pots.I haven't yet checked the tracking accuracy between the two sections but plan to do so soon.

Also is the phase going to invert?

No phase inversion.It is configured as a noninverting, unity gain opamp.

Have fun.

Always

[ben62670]

Sweet! I'll be paying attention of coarse.

[GV#27]

I tested the Alps pot for tracking accuracy between sections and was mildly disappointed.There is a full 1db disparity in the first half of it's rotation then between the center point and full rotation it improves to an acceptable( but just barely).5db.(Another good reason to use stepped attenuators instead of pots).

1db is generally considered the threshold at which sound level differences can normally be percieved but from a perfectionists veiw I would prefer the pot match to within a 1/4 of a db. Luckily how I'm using it in my setup it will be set at about the 3/4 point so the .5db should not be noticable.

Without out having several on hand to compare it's hard to say if this is typical for this pot or not.This particular one and it's motorized sibling are amongst the most widely used in higher end pre's and Integrated amps,but manufacturers buy them in large quantities and one hopes that they test them to weed out the poor performers.