i need at least 30 db gain as a buffer...what do i do?

[casey]

i am trying to push a 7 band eq....what type of buffer
do i need?  it has to be at least 30 db, a simple build,
and minimal coloration......im using it for mostly an acoustic
guitar...so it's got to be clean.... any suggestions?

[gez]

Non-inverting op-amp would be my choice.  With something like a JFET there's no guarantee you'll get that much gain (the transconductance of the FET you happen to use might be on the low side - it'll vary from device to device).  Also, the output impedance would be higher than if you used an opamp (drain resistor would have to be fairly large value) unless you buffered the FETs output with a follower (would be wise considering what the buffer is driving).

[gez]

PS dual supply would be an idea with that amount of gain, just to give you some headroom.  I take it the EQ is a passive network, hence the need for gain, to compensate for losses thru it?

[casey]

yes, it's a passive eq.....7 band....
i was thinking about using a tl081 or using
both sides of a tl082.......

but, i'd like to keep it simple.

[Ed G.]

What about one of those high-performance opamps like the burr-brown OPA2604? Supposed to be high gain, low noise, high bandwidth, should be good for that sort of application.

[Jason Stout]

If my math is correct(for ~30 dB gain), you could use the standard non-inverting opamp configuration with the feedback resistor 1.5kΩ and a Rg of 51Ω.

Edit:

In this example R2 = 1.5kΩ and R1 = 51Ω
This config has very high input impedance ( >1MΩ )

use capacitors on the input and output.

[casey]

If my math is correct(for ~30 dB gain), you could use the standard non-inverting opamp configuration with the feedback resistor 1.5kΩ and a Rg of 51Ω.

o.k.... thanks...

yeah, ed g.  that would be the way to go....burr browns are really quiet.

[gez]

Haven't got round to using one of these yet, but there are purpose built 7 band graphic EQ chips (I dare say most modern commercial pedals use them) 'available' (not that easy to find) such as the LA3607.

Rapid electronics in the UK sell them, the attraction being that you can keep the parts count down (or so I should imagine - haven't checked the data sheet for it).  Would probably be a more efficient EQ than a passive network and I shouldn't think you'd need all that gain (which presents problems in itself - headroom, noise etc).

[casey]

that's super cool.....

ill check out that chip.

thanks.....

[Jason Stout]

gez wrote:

PS dual supply would be an idea with that amount of gain, just to give you some headroom. I take it the EQ is a passive network, hence the need for gain, to compensate for losses thru it?

Yes, good point!

+ and - 15 volts *should* be enough for minimal clipping.
On the other hand, I don't think + and - 9 volts would be enough.

How "hot" are acoustic pickups? I am assuming the output is similar to a standard single coil.

[casey]

i am using a l.r.baggs i-beam passive pickup....not very hot at all.

i was thinking about putting it through this little 7 bander i have
designed first, and then running it through my presonus tubepre.

right now, i havent coming through my tubepre...and it sounds really
good....but i would like to control the eq on it....

[gez]

Did a quick google search and found a couple of US suppliers.

http://www.matelectronics.com/acatalog/Mat_Electronics_LA3607_8032.html

(Minimum order of $25 though)

http://www.angelfire.com/biz/techsonic/page3.html

Didn't read thru all the blurb, so apologies if these links are useless.  Would be best to confirm that the chips are the right ones too (I think Sanyo make them) as the ones in the first link seem suspiciously low in price (second link is about right).

[puretube]

bootstrapped (hi Z-in) non-inverting!

[RDV]

RDV

[puretube]

L.R. Baggs says, the pickup wants high Z-in,
i.m.h.o., that`somewhere above 5M...

[Gilles C]

Maybe that you could take a look at the Boss GE-7 schematic to help you figure it out.

http://www.geocities.com/j4_student/schematics.htm

Or the preamp of this amplifier.

http://sound.westhost.com/project27.htm

[gez]

If my math is correct(for ~30 dB gain), you could use the standard non-inverting opamp configuration with the feedback resistor 1.5kΩ and a Rg of 51Ω.

Edit:

In this example R2 = 1.5kΩ and R1 = 51Ω
This config has very high input impedance ( >1MΩ )

use capacitors on the input and output.

You need a resistor from + input to ground (with dual supply) and this will determine input impedance.  Personally, I'd use larger value resistors than shown (though I'm probably splitting hairs)  

PT's advice is sound - bootstrapping will give you extremely high z in.

[Jason Stout]

Personally, I'd use larger value resistors than shown (though I'm probably splitting hairs)

No, your not splitting hairs, increasing R1, R2 will keep the output "unloaded". Try R1=500Ω R2=15kΩ

[Kleber AG]

I'd be really happy if someone else could try the RG Adjusticator and give some report, I've 3 of them and it's amazing!

I used 5532 opamp.
It has enough boost for using it as "nice" practice amp with my 4x12 marshall cab, I was scared when I first tried it as an amp.

Regards
Kleber AG

[toneman]

Hey Kleber,
i was going to suggest the 5532 also.
it's quieter than the TL062/72/82 stuff.
it drives headphones phine also since it has
a little more current out capibility than the others.
Paia uses it in their stereo headphone amp,
& their stereo headphone distribution box.
I added one headphone amp 2 my StackInABox.
it sounds great!!   Great little practice combo...
afn
tone