Unlimited Headroom EQ

[Bill Mountain]

Unlimited headroom.  What does that mean to you and how would you achieve it?

I'm brainstorming another simple active and or buffered passive EQ and I want to make sure the power supply is a non-issue under all conditions.

The easy answer is to use a voltage doubler IC but why stop there?  Why not 50 or 100V or 300V?

Or maybe I throw away volume at the input so the EQ has room to work before hitting the rails.

I've lamented on this board in the past about my super high output basses.  I want to build an new EQ for them and standard 9V circuits just don't cut it.

Before I go ahead and make another 24V Graphic EQ (the first one was noisy so I don't use it much) I wanted to solicit some ideas.

[antonis]

Maybe some 1kV working voltage transistors or ICs should help you... 

I can't understand your need for more headroom as long as your EQ is pluged in some other circuit with much lower headroom..

[Bill Mountain]

Maybe some 1kV working voltage transistors or ICs should help you... 

I can't understand your need for more headroom as long as your EQ is pluged in some other circuit with much lower headroom..

I don't get what you're saying but I want to build a EQ pedal that does not run out of headroom (under reasonable-ish circumstances).

[ashcat_lt]

Yeah.  Most line gear runs on +/- 18V rails at best.  Consider that a typical ADC sets hits all on at +22dbu signals, and most analog gear before it will be distorting some by then anyway.  That's about 27.6V.  If you want to be sure that your device won't clip before other stuff, let it swing 30V.

Course, if this is for guitar, everything else has 9V rails and probably less that 8V of actual swing.  Common tube amps fall apart (not just start to distort) with inputs much greater than 4V....

[digi2t]

The only stompbox (in my limited experience) that I've seen that has the available headroom capacity of up to 32v is the TCE Integrated preamp. Volume, bass, and treble controls, do the job for me. I run my clone at 24v as a buffer, which doesn't even come close to running out of headroom in this application, but it does make for a great booster if need be. Would easily do the job at 9v, but I had the 24v adapter kicking around.

And I wanted to stay true to the original specs. 

[Bill Mountain]

The only stompbox (in my limited experience) that I've seen that has the available headroom capacity of up to 32v is the TCE Integrated preamp. Volume, bass, and treble controls, do the job for me. I run my clone at 24v as a buffer, which doesn't even come close to running out of headroom in this application, but it does make for a great booster if need be. Would easily do the job at 9v, but I had the 24v adapter kicking around.

And I wanted to stay true to the original specs. 

I just googled that.  It looks nice.  I would prefer the fixed gain after the EQ but it does give me some ideas.

In -> buffer-> passive EQ & volume-> gain recovery.

None of this is new but I've been going at it the other way just like the TCE (boost on the front end).

Maybe instead of a mega high supply voltage, I can try and keep it at 9-18V supply but put in a peak indicator so I know when I'm about to hit the rails.

[Bill Mountain]

Also to be clear I don't want a 30V swing on the output.  This is for bass so I don't need to push a big tube amp.

I just want an EQ that can handle the envelope (is that the right word) of the low notes and give me the option to tweak them and bring them to a reasonable volume after the fact.

[thermionix]

This is for bass so I don't need to push a big tube amp.

Basses have the biggest tube amps.  Don't they?

[JebemMajke]

Tube eq?

[bool]

Unlimited headroom.  What does that mean to you and how would you achieve it?

I'm brainstorming another simple active and or buffered passive EQ and I want to make sure the power supply is a non-issue under all conditions.

Flying rails? (i.e. bootstrapped)

If your basses are so hot, then buffer(with possible medium gain adjust)->passiveEQ->makeup->output would make the most sense to me. (which is about the most standard config anyway)

Perhaps look here: http://www.ti.com/lit/pdf/tidu887

[Gus]

First thing to do is look at the output of the basses with a scope. A DSO would be good so you could store the waveforms and look at them later.
Measure the max peak to peak voltage of the highest output bass

Are the basses active or passive?
If an active bass a simple voltage divider(fixed volume control) into a buffer with an limiter after it will reduce the signal into the EQ you might want limiting at the output of the EQ
If a passive bass a soft limiter before the EQ to limit the max peak to peak input the EQ will take without clipping AND if the EQ is active a limiter at the output set not the cause clipping of what follows

Noise in the EQ can be a design issue. If you have an external power supply and the EQ uses Rs and Cs  scale the resistors down in value and the caps up in value make sure the opamps have good output drive

What have you built and/or tried that did not work out?

EDIT it reads like you will want more cut then boost to get the EQ you want

[Johan]

http://www.aronnelson.com/gallery/main.php/v/Johan_001/limiter2.gif.html?g2_imageViewsIndex=1
perhaps a limiter is what you need to take the most dynamic peaks. Bass signal can be pretty wild.
I've been mening to build this for a long time but never got around to it.
For bass, i think gain in first stage can be reduced to unity. R9 and R10 represents a 10k pot
j

[blackieNYC]

Is gain recovery going to be noisier than a front end boost, when you have a passive EQ in the middle?
If I was playing that bass everyone would want me to use a compressor, but I'm not particularly good at bass.

[tubegeek]

A question that comes to my mind is - are you running out of headroom at the input to the EQ or within the EQ after a boost has been applied to some frequency? When I hear "EQ" that makes me suspicious - you can apply an awful lot of boost with some EQ designs and that would mean that your headroom would NOT solely be dependent on the bass's output.

I'd look at Rod Elliot's EQ designs, they seem quite well thought out. The page is awesome in any case, well worth a look, lots to learn there.

Seems like he has a new URL: http://sound.whsites.net/index2.html

[Ben N]

FA-1 also ought to be doable in higher voltage for headroom.

[ashcat_lt]

Is gain recovery going to be noisier than a front end boost, when you have a passive EQ in the middle?

Probably yes.  You take a signal that already has noise, turn it down, add more noise, and then turn all of that back up.  With gain first, that "second layer" of noise doesn't get amplified.

Gain first wants more headroom.  If you're turning it down and back up you still ultimately only need to pass the original signal level.  If you turn it up first, that signal is obviously bigger.

[snap]

Just found a "floating rail" principle headroom expander - http://www.diystompboxes.com/smfforum/index.php?topic=100281.0 - Quite simple - but does it work?

[Johan]

With bass, unwanted clipping in a 9 volt circuit is a bigger problem than background noise.
j

[vigilante397]

My big question would be why do you need super high output? Compressors and limiters have been mentioned, those are a great option, but if you have super high output why not try a passive EQ? You'll lose some gain, but if you're already super high output do you really need it?

[Bill Mountain]

Thanks everyone for chiming in.

I was distracted this week and didn't really have time to comment.

I don't have a scope sadly.  I'm not adverse to doing any of the things mentioned in this thread.  I just want to be able to plug in my bass and not have to obsess over gain staging when adjusting the EQ.

I just always like to solicit opinions before I do brute force engineering (what I call common sense stuff that just works regardless of how bulky or clumsy).

Sometimes you folks have some elegant stuff I've never thought of.

Thanks!