Dumb question: What is HEADROOM ?

[6l6gc]

Hi, Everyone! :lol:
I making JCM800 emul(by electrictabs).

I heard....headroom is related to voltage.

What is headroom?

[aron]

Here's a really short definition. The range of "clean" before the amp starts clipping (i.e. distorting obviously).

So when you start distorting, you have exhausted (or reached the limits) of your available headroom.

[6l6gc]

thanks aron. :wink:

Well then...

Q1.
Small headroom means "early-started-distortion" ?
Has it nothing but this?
Then, smaller headroom means -> it has more gain?

Q2.
In the JCM800 emul circuit...What can I do for increasing the "HEADROOM" ??

Q3.
What voltage recommended for JCM800 emul? (power supply)


Sorry for my poor English..
English is not spoken in my country.
I seek an understanding with you about my poor Eng. sorry.

[aaronkessman]

Q1
a. yes
b. not sure what you mean
c. by gain you mean drive/distortion right? not necessarily, just that whatever distortion you get happens sooner. A 5W amp will generally have less headroom than a 100W amp because you have so much more room to go in the 100W amp before the circuit starts clipping.

Q2.
not sure. headroom is really the result of many factors, some of which involve tweaking your gain-producing circuitry right. If you're making an emulator, they're probably FET's, so you might be able to supply them with more voltage to give them a little more headroom.

Q3.
no idea. it totally depends on the circuit you're working off of.

But really, the JCM800 is a high gain distortion circuit, not really intended to be played clean. why do you want to increase the headroom of this amp/preamp/pedal?

Aaron

[aron]

The times I have heard "headroom" have been "more" or "less" not bigger or smaller.

In any case, there are many ways to improve headroom on an amp, but it does require a knowledge of tube circuitry and amp design.

You might want to peruse the ax84.com forums or AMPAGE.

[toneman]

try googleing

i thought it was a place where everybody rolled their own(?)

[petemoore]

JCM800 emul...that uses Jfets>?
 Just check the ratings on the caps used and if necessary the voltage rating from the data sheets on the transistors used...be certain they are rated above 25v...if the measured supply voltage is less than that by say 10% or better, you should be fine at 18V supply...I think, I didn't look at a schematic so I might be leading you in circles here and have no clue what I'm talking about.
 If the tranny's and caps can handle it, put 18v on the circuit and see if any 'cleanery-ness' can be detected from the increased headroom.

[vanhansen]

As long as the JETS and caps choosen are rated more than 18V (25V preferred), you can run that emulation at 12-18 volts to get more headroom.

[Mark Hammer]

Here is a simple way of thinking about it.

Do you know EXACTLY what the level of your guitar signal into the amp will be?  No.  You have some approximate ideas, because you know it will probably be greater than, say 10mv, and it will probably be lower than 10v.  Just exactly WHERE in that range it will be will depend on how you pick, where your guitar volume is set, the pickups, any pedals in between the guitar and amp, and a million other things.  

Do you know what the GAIN of the amp is?  Yes, approximately.  I say approximately because the gain can be changed.

Do you know what the supply voltage of the amp is?  Yes,

If I consider how much the signal can "swing" at the amp (in terms of maximum voltage), and I think about how much gain I have asked the amp to provide, and THEN take into consideration the amplitude of the signal I am feeding it, then I will have a rough (VERY rough) idea of whether the signal will be amplified by a factor that is possible.

If my signal is +/-50mv, and I never ask the amp to apply more than x100 gain, and my supply voltage is +/-25v, then there is plenty of "room to move" between the signal level I am producing in the amp (100 * .050 = 5v) and what is potentially possible.

That is not a strict academic explanation, and there are many other factors as well, besides those mentioned, but it comes close enough.

In the most general, basic, common-sense view, an amp has more "headroom" if it is still capable of amplifying more when you feed it the hottest signal you would expect to feed it.

Pedals that run off 9v batteries often have insufficient headroom.  This is one of the reasons why you can take any old distortion pedal and make it over-the-top fuzzy just by clean-boosting the signal you feed it.  The distortion is designed to amplify the input by a certain amount, but based on an expected input signal.  If you feed it an input much greater than what it expects, then you will be asking it to produce an output MUCH greater than what it can do with a simple 9v supply.  Of course, if you were challenging the headroom of the pedal before, you are REALLY challenging it now.

Make sense?

Amps and pedals aren't the only things that have "headroom".  Speakers do too.  The wattage rating of a speaker indicates how much sustained power it can tolerate running through its voice coil.  The peak rating indicates how much power it can tolerate in its voice coil for a very brief instant without burning up.

Like amplifiers, though, simply knowing the amount of "headroom" does not say anything about the loudness, or many other things.  A speaker may have a high wattage rating, and a MUCH higher peak power rating because it is designed to be inefficient.  In other words, it takes a LOT of power to move the cone and the peak power (headroom) rating is high because the speaker almost "ignores" the amp output, not because it can be extremely loud.  Many of those car subwoofers you can find for $30 that boast of 300w ratings have ratings like that because the cones are thick, heavy and don't produce much output.

A guitar amp can have tons of headroom without being very loud.  The simplest case wold be plugging your guitar directly into the return jack of your amp (if it has an effects loop).  The power amp expects a signal much bigger than your guitar's.  You can slam away at the strings as much as you want, and the amp and speakers will still sound very clean and not very loud, because you are feeding the amp a signal much lower than it expects -> lots of headroom.

Steel guitar amps and amps for keyboards are generally built/designed to have lots of headroom.  Why?  Because the difference between the signal produced by tinklling two keys in the top octave, or playing two strings way up high, is MUCH lower than the signal level produced by pounding on  30 keys with your elbows and fists, or slamming 13 strings way down low.  The amp has to be able to expecta VERY wide range of signal levels without changing the tone much, so there is less gain buil;t into the amp, and the power supply is designed to be capable of respoinding to volume peaks without giving up.

[6l6gc]

Oh, thanks eveyone. Thanks very much.
then...
high-gain pedal don't needs a big headroom?

[chrishopkins]

No, you'd only want more headroom if you wanted to stop the circuit distorting too early.  For instance if you were building a Jazz Amplifier you'd want it to have as much headroom as possible as distortion is considered a "bad thing" by most Jazzers.

[Mark Hammer]

It occurred to me this morning on the way to the bus that I was wrong in something I said about pedals and pre-boosts. There ARE some headroom issues but not of the sort I suggested.  Ignore, please.

The stuff about amps, though, was largely accurate.

[6l6gc]

That's all right, Mark!
Your infomation is very helpful to me.

For the best drive sound in the JCM800 emulator, what voltage recommended??

[vanhansen]

For the best drive sound in the JCM800 emulator, what voltage recommended??

Start with 9 volts. If you like how it sounds, leave it at that.  You can try it at 12 volts but then you'll have to rebias all the FETS to 1/2 the supply voltage (or roughly 6 volts).  It may not make that much of a difference, if any at all.

[6l6gc]

Thanks vanhansen!
It's better use 9V. Biasing is so difficult to me :cry:

-p.s: Your web is so cool~

[vanhansen]

Thanks vanhansen!
It's better use 9V. Biasing is so difficult to me :cry:

-p.s: Your web is so cool~

Biasing is actually pretty easy.  Using your DMM (digital multi-meter), attach the negative to circuit ground, and touch the plus lead to the drain of the FET.  Then, using the trimpot, adjust it until your DMM reads 4.5 volts or so.  Bingo, your FET is now biased.  Sometimes it can be tricky though.  You may need to swap out the FET for a different one if it just won't bias or if the trimpot is bad, just swap that out.

For example, in my latest build with 2 FETS, if I swap them around, one will bias but the other won't.  If I swap them back, they both bias to 4.5 volts.  In another circuit, I had to use a 200k trimpot to get it to bias but it was really touchy but it worked.  These are just examples of what you may come across.  If you can, stick with 100k trimpots.  Sometimes just swapping out the FETS does the trick so have plenty on hand.

P.S.  Glad you like the site and thanks for the nice words.

[d95err]

...speaking of headroom, I just realized something that could help illustrate the concept.

The master volume control of an amp can be seen as a headroom control. At max, you get all the headroom the amp can produce. Lower master volume is the same as less headroom, enabling you to get more distortion at lower volume.

[Hal]

everyone's got it all wrong...

its that room the guitarist and groupie go after the show, sometimes also called the VIP room or dressing room...

(sorry, couldn't resist)

[chrishopkins]

Someone shoot him now!

lol

[6l6gc]

cool guys~
Thanks very much~~~ :wink:

-p.s:
vanhansen: thank you very much!