[Predator88]

So I decided that I'm going to build a switch so i can go back and forth between my t-amp and my pimeta. Then I got to thinking of just making a passive preamp while I'm at it. Now here's my question, how could I add controls like bass and treble boost to a preamp? I assume it would then have to become an active preamp right? I have some electrical experience (built a cmoy as far as audio goes) so I'm not a complete n00b but still a little confused. If anyone has built one these would you mind pointing me in the right direction or to a guide that might exist?

 

[amb]

Yes, if you want proper bass and treble controls, then it needs to be active. See the following thread for my post with a schematic:
http://www6.head-fi.org/forums/showthread.php?t=107568

 

[nikongod]

passive bass&treble boost can be done too.

you may have to increase either the voltage from your source (if possible) or the gain on your main amp, but its totally doable.

 

[rickcr42]

Quote:

passive bass&treble boost can be done too

I use front end buffered passive LCR filters followed by a +30dB "makeup gain" stage .Mine has a comparator circuit that monitors the input/output level and three LEDs marked "low/unity/high" so that once the controls are set I can bring the entire EQ section back to the level of the original signal avoiding any volume changes (an idea ripped off from a very well known and well designed octave band EQ from the late seventies called level match

) that in an extreme case can overdrive the next stage if there is too much boost .Tonal changes only without the clipping.

That is for a system that has both a preamp stage AND an outboard EQ stage otherwise there would be no need for another gain stage the eq amp being the gain stage and how it was mostly done before tone controls went bye bye in home audio gear.Less is more and if you can lose redundant stages always better for SQ-add what you MUST to get the job done and no more

**

 

[jnewman]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif passive bass&treble boost can be done too. you may have to increase either the voltage from your source (if possible) or the gain on your main amp, but its totally doable.

If it's passive, wouldn't it actually have to be bass and treble cut, not boost?

 

[amb]

Quote:

Originally Posted by jnewman /img/forum/go_quote.gif If it's passive, wouldn't it actually have to be bass and treble cut, not boost?

Yes. Or, you could make the passive network to attenuate the signal by a bunch in the "flat" mode, so that in "boost" mode you merely restore some of that loss (e.g., if bass and treble are both at max position, your frequency response is actually a midrange droop).

To achieve actual boost, an active gain stage is needed (except if a step-up signal transformer is used instead, but let's assume that's not the case). The gain stage could be standalone, or built-in as part of the preamp or headphone amp.

 

[nikongod]

when dealing with a passive system weather we have bass boost or "trebble cut" is a question of perspective really.

if you look at a first order +6db bass boost starting at say 100hz and compare it to a first order -6db low-pass shelving filter with the -6db point being 100hz they are the same curves. the only difference is how the circuitry to make them happen works.

it is not at all difficult to make a -6db shelving filter as described above using entirely passive components. the voltage of the signal will need to be compensated for elsewhere, but that is not usually hard.

in the case of an active circuit, we are unquestionably adding to the bass, but im talking about a passive solution.

the real question here is do you like active solutions, or passive ones? after that is answered the "thing" wont be too terrible to build in either case.

 

[rickcr42]

Quote:

If it's passive, wouldn't it actually have to be bass and treble cut, not boost?

ALL tone controls not in a feedback loop are "passive".Using Resistor/capacitor/inductor gyrators are no more than filters that let through some frequencies while attenuating others just like a typical R/C Baxandal Tone Control so you need to FOLLOW the stage with "makeup gain" if you want to maintain the same levels that entered the network.

do a google on "Pultec" and "passive equalization" if you want the details on operation.

BTW-you also want to hit the input of a passive network with an input that has good drive and low impedance.Not gain but a low Z buffer unless the previous stage has good drive already at which time you can use a 600 ohm/600 ohm bridging transformer as the input

 

[rickcr42]

the baxandal tone control that is likely the most common means to change "tone" ever used for both home systems and guitar work

http://www.schmarder.com/radios/tech/tone.htm

the Pultec

http://www.gyraf.dk/gy_pd/pultec/pultech.gif

both 100% pssive networks

active EQ design

http://www.forsselltech.com/Evolutio...%20Design2.pdf

http://www.forsselltech.com/schematics/EQ1A.PDF

http://www.forsselltech.com/schematics/Gyrator1.htm

active tone controls are all over the place so left out