Pream Building Question-

[QSQCaito]

Hi, I'm Andrés Cao from Argentina. I've recently build a project, and ive got the 2nd on my way.. Bit I was recently told, that if im going to have a several effects connected in series, it was necessary to have a preamp first, is this true?
Moving on to the next question, i somehow managed to get Craig Anderton's "Electronic project for musicians" book, which(dun think legal) i could give to you, privately(i got it in pdf file).. Well, there's a preamp to do, and i'd like to know if it's not too old, lol, and that it has 2 double opamps, which are quite old, 14 pins each... It's worth doing this preamp, with clean and dirty control, or is it too old?

name:         RC4739
function:     Op Amp
package:      14P DIP,14
manufacturer: Raytheon
added-by:     Steve Cowan
comment:      equivalent to XR4739, apparently no longer in production?

                +--()--+
amp A out | 1  14| Vcc +
            nc | 2  13| amp B out
            nc | 3  12| nc
            nc | 4  11| nc
amp A + in | 5  10| nc
amp A - in  | 6   9| amp B + in
     Vcc -     | 7   8| amp B - in
                 +------+


I'd like to know if i could substitute them with any regular (as tl072 ) double op-amp? Seems to have the same pin out, if we take awaylegs,2-4 and 10-12.(i think theyre used just to travel signal from one leg to other)

I'll leave the schem and pcb, maybe it helps to know if that IC can be replaced..
But first of all, please, i'd like to know if it will sound good, or if there are better preamps, nowadays, easy ones..


Thanks a lot!
Diego Andrés Cao

PS: forgot to add, it has a vu meter, how can it be ommited?

PCB LAYOUT PREAMP: http://img445.imageshack.us/img445/5039/pcblayprezt8.png
SCHEMATICS PREAMP: http://img391.imageshack.us/img391/639/schemlaypreqr6.png

[Mark Hammer]

Although that was a pretty good preamp for 1981, you can make something else just as good (maybe even better) these days with other op-amps and also with FETs.  The other advantage is that they can be powered with a 9v battery (or adaptor) instead of a +/- supply.

If you do want to build it, do not even try to use a 4739.  Finding one will take months and it will cost you twice as much as something much much better.  If you go here, you will see a long list of booster/preamp projects on the left: http://www.generalguitargadgets.com/index.php?option=content&task=view&id=30&Itemid=58
I like Craig Anderton's work and thinking a lot, but all of these projects are at least as good as the one in EPFM.

I almost forgot, WELCOME!!

[Cliff Schecht]

You can build a preamp into your guitar also. I've done it to my 2 main guitars because I run long cables into 10-12 effects and audio loss is not a problem. If you want to save a lot of hassle, just go buy the EMG PA-2, it's available with or without a bypass switch (with the switch it can be mounted in a guitar with the toggle out) and converts the output to low impedance, so that high frequency isn't lost when running through a lot of effects or longer cables.

[usbdevice]

Hi Andrés

I don't think that a preamp is needed. It could be beneficial to have a buffer in front of your chain if you're running a lot of true-bypass fx.
The preamp will probably also work fine as a buffer but it adds functionality that you'll probably never need.
As for replacing the RC4739, i found these 2 posts using the search function  : http://www.diystompboxes.com/smfforum/index.php?topic=14352.0 http://www.diystompboxes.com/smfforum/index.php?topic=14351.0 which seem to indicate that you can use any old dual opamp in which case you'll have to make you own pcb layout or the NTE725 which is a pin-for-pin replacement.
As to whether it'll sound good, you'll have to try.

Lenny

[QSQCaito]

OK! First of all thanks a lot for the welcome and for the answers.
There's seems to be difference of opinion, i was told preamp is needed, and that preamp is not needed. Is it needed?If it is, which one should be if my only choice(and will always be my only choice, i live in argentina) is to do it myself

and converts the output to low impedance, so that high frequency isn't lost when running through a lot of effects or longer cables.

I can't get this,  if i run through 6-7 effects, high frequencies will be lost?

And one last question, in this chain,

Marshall Blues Breaker - Parametric  Equalizer - Ross Compressor - PT2399 delay, will i need any preamp? is it in correct order? would you add somehitng diy-able?

And regarding pt2339 that is in geo's page, i was also told that there have been several failing reports, do you know any way to test if it will work, without connecting the ic, or some voltage values, because here that circuit is 16U$S and that's quite the value of a whole pedal.

Once again, thanks a lot!

Diego Andrés Cao

[QSQCaito]

I add a question, is it worth it doing a preamp, clean one, no tone control, no distort anything?

[billings]

Guitar cables have a small capacitance, on the order of tens of picofarads per foot.  Guitar pickups have a very high output impedance (meaning that they can't push current very easily).

The capacitance in the cable presents an impedance to the high frequencies in the signal.  If the signal can't push enough current, that impedance can be perceived as a high frequency rolloff.  All the buffer does is take that signal and make it beefier and more low impedance, so that resistances don't bog it down so much.  It may also present a higher impedance input to the guitar than the effects in the chain - which means that the guitar will have an easier time driving it, and thus an easier time driving those highs through.

Impedance is sort of like inertia.  High impedance things are light, low impedance things are heavy.  So when a signal drives a lower impedance input, it's going to struggle to get through;  when a signal drives a higher impedance input, it will cut through easily.

There are some exceptions to the above rule, e.g. the fuzz face - it sounds better taking the load from the pickups than it does taking the load from a buffer or another effect.  And almost all effects that the signal passes through will buffer it somewhat if the effect is on.

[QSQCaito]

That's ok! reallt hanks a lot, indeed. So, i think it would be good to make a preamp, so my signal does not struggle to get through .


I've changed my mind so many times, but finally i think, that it'd be better to make preamp with no distort..
Do you know which one should i make? Which one is better?

Thanks a lot!

Bye!!!

Diego Andrés Cao

[petemoore]

I can't get this,  if i run through 6-7 effects, high frequencies will be lost?

   One test is to take the pedals out of the chain and connect the guitar directly to the amp through a nice medium length cable.
  Another way is to put a buffer at the beginning of the chain and see if the highs aren't more sparkly with the buffer driving the cables. 6-7 effects is what I use and the highs ... the signal is degraded badly without something driving the cables.
  Many commercial effects have input/output buffers as part of the bypass switching..

[QSQCaito]

... the signal is degraded badly without something driving the cables.
  Many commercial effects have input/output buffers as part of the bypass switching..

So, as i dont have any commercial and effect, no to lose signal, it's better to have a preamp?


thanks a lot for all of your answers
Diego Andrés Cao