project 1 pre-amp

8/12/2019 Project 1 Pre-AMP

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Project

No. 1

PREAMP

Deflnltton: A preamp increases he level ot a signal.

For exampJe, preamp with a gain ot 10 can take a IV

input signal and turn it into a 10V output signa/.

Figure 5-8

Here is the preamp, mounted in a Vector-Pak card

module prior to installation in the pedalboard. The

switch on the left is the clean/dirty switch; the one

on the right is the in/out or bypass switch (see

Project No. 23 for details on how to wire this in-

to the preamp). The input jack is on the front, but

ali other connectors mount on the rear. In order to

conserve panel space, the VU meter has been

omitted [rom this module.

Background:

Some signals that emanate from devices ale very low in

level, such as the weak signals com ng out of the guitar

pickups or microphones. On the other hond. some de-

vices (tape recorders, hi-fi power amps, studio consoles,

and others) want to receive ine-level, or high-level signals.

A preamp can .take a low-level signal and convert it into

a high-level signal that is more suitable for interfacing

with professional equipment. Advantages include a "ful-

ler" sound and better noise performance compared to

using a nonpreamped signal.

. Choice of three outputs: low-impedanceunbal-

anced, nverting or noninverting, or use with gui-

tar arnps,same tape recorders,hi-fi power amps,

etc.; and low-impedancebalanced, or use willi

professional tudio equipment

. Built-in VU meter amp

. Separate gain control and master volume con-

troI to optimize signal-to-noiseatio

. Low-noiseand ow-distortion characteristics

. Operatesrom

:9 to tl8V OC

. Drives ong cableswithout signalloss or degraded

frequency esponse

. ReplaceshassIe-prane tudio direct boxes when

recordingdirect nto studio mixing boards

. Buffered, ow-Ieveloutput available t pad A

Features

. High-input mpedance etains he fidelity of instru-

ments with high-output mpedances uchasguitar

. Choice of "clean" (standard preamp) or "dirty"

modes.The dirty mode simulates he sound of an

overdrivenamplifier

Level of Difficulty: Beginner to intermediate.

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ConstructionsTips

Becausehis circuit is capableof high gain, certain eads

must be shielded,and alI wiring should be kepi asshort

and direct as possible. nput and output leadsshouldbe

physically separated rom each other by at least 1 cm

(3/8 ) to preventstray signalcoupling.

Useshielded able or the following connections:

JI to pad I; pad C to R6; SI to pad E.

LEDs Dl and D2 should not light up. They ale used

onlv to createdistortion in the dirty mode.

Using the Preamp

studio patch bar patching points; professionaI ape

recorder nputs; SOfiestudio effect inputs; and profes-

sionaI-quaIitymixers.

UseJ2 for applications imilar o J3, but wherephase

is unimportant. Using his output will giveslight1ybetter

noiseperformance ompared o J3.

. Third, decide whether you want a clean or dirty

sound. f clean, set SI to clean.With the mastervolume

about one-fourth of the war up, feedyour loudest nput

signaI nto the preamp.Tum the Kain control clockwise

until you reach he maximum evel that still givesa dis-

tortionless sound.Trim the mastervolume o the desired

output level (usuaIlyO dB when eedingsomethingike a

tape recorder).

If dirty, set SI to dirty. Adjust the gain control for

the desired amount of distortion (clockwise=more is-

tortion), then trim mastervolume or the desiredoutput

level.

Another war to obtain a dirty sound,evenwith SI in

the clean position, is to overloada low-level nput (such

as a typicaI guitar amp nput) with the high-levelpreamp

output. For this application, adjust the gain and master

volume controls for whatever you feel gives the best

soundquaIity.

. First, plug your instrument or microphone into Jl.

The preamp input accepts unbalanced output devices,

which usually terminate in 1/4 phone plugs. To connect

balanced line devices, which usually terminate in a three-

wire XLR connector or more rarely in a 1/4 stereo

phone plug, you will need to add an additional matching

transformer. See Figure 5.9.

. Second, determine which output jack to use for

patching the preamp into your setup.

Use J3 for feeding devices with unbalanced, high-lev-

el inputs. These include power amps such as BGW,

Clown and McIntosh; most consumer and semipro tape

recording equipment; budget PA mixers; line level studio

effects; most synthesizer modules; and synthesizer

extemal input jacks.

Use J4 for feeding devices willi balanced, high-level

inputs. These include studio mic preamp inputs; most

Figure 5-9

How to adapt a balanced, low-impedance micro-

phone to the preamp input using a matching trans-

former. The cara connecting the microphone to

the adapter is usually a three-conductor type with

XLR connectors at each end; the cord from adap-

ter to preamp is a standard cord with 1/4 phone

plugs at each end. The transformer is called a

high-impedance unbalanced to low-impedance

balanced type, and is available at mony musie

stores and pro audio shops or around $15 to $20.

Hooking up the VU meter amp: By monitoring the out-

pot of the preampwith the VU meter, you may adjust

the master volume so that your output signal s consis-

tently around the O-dBpoint. For max.imum ersatility,

the meter amplifier input is brought out separately o

pad M so that you may monitor different points along

the signalchain f desired.

To monitor the output of the preamp,connect pad

Mto pad P.

To monitor several ffects, efer to Figure5-10.

If you don't wish to use he meter, simply gnore er-

minalsM and N. The meter doesnot need o be used or

the preamp o work correctly.

The function of pad A: The output signalappearing t

pad A is a buffered equivalentof the signalappearing t

the input, and may be connectedup to a jack in a fash-

ion sirnilar to Jl. Use t for feedingeffects that ale de-

signed o wark only with 10w-Ievelnputs. This option

will not be required f you ale using he preampsolely n

conjunction willi other projects presentedn this book.

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Modifications

igure5-10

This diagram shows the preamp, compressor, nd

super tone control, with their inputs and OUfputs

brought out to separateacks. J've omitted other

connections-powersupplies,pots, tc.-forclarity.

Pad M connects o arotary switch which selects

the part of the signal chain we wish to monitor.

Position 1 monitors thepreampoutput, 2 the com-

pressor output, and 3 the super one control out-

puf.

. The preamp esponses t IdB from 40Hz to 10kHz

(1kHz = 1000Hz). You may remove CI and C2 to ex-

tend the responseo 20kHz if you've been carefuIwilli

your wiring (no Iong, sloppy Iead lengths, or inputs

drapedover outputs).

. If the preamp has too much gain for your needs,

changeRIO to 220k. If too little gain, changeRIO to

1M.

. If you will only be using he preampwith guitar,

and if that guitar does not include built-in electronics,

you may replaceC3 with a wiTe umper for improved

noiseperformance.

.

There may be instanceswhere you don't needall

the versatility of the present design.Let's say you're a

guitarist, and all you want is a preamp o overload our

guitar amp input-and that's all. In this case, imply plug

your guitar into Jl, your amp nto 12, and you're ready

to go. You don't need J3, J4, R3-R5, R8, Rll, C4, C7,

or C8; thesemay be removed rom the board,or designed

out altogether.The clean/dirty switch as well asR6 and

R7 will still be completely unctional.

In Case f Difficulty

. Distortion, even n clean mode: Tum down Kain

control until distortion goes away, and tum up master

volume o compensate.

. Intemal feedback, squeals,or other oscillations:

Check ead layout. Use shieldedwires where ndicated.

MakeSUTe and C2 are n place.

. "Uneven" soundingdistortion in dirty mode: Veri-

f y that Dl and D2 haven't been overheated r damaged

during the solderingprocess.

Specifications

Current consumption: S mA. @ 9V DC supplyvoltage

Frequency esponsecleanmode, CI & C2 = 5pF):

tl dB, 40Hz

-

10kHz

t2dB, 40Hz- 0kHz

Frequency esponsecleanmode,CI & C2 omitted):

tl dB, 40Hz- 0kHz

Output headroom,clean mode: greater han IOV pk-pk

Maximum input before clipping, clean mode, R6 fully

clockwise: OOmVpk-pk

Output headroom,dirty mode: 3V pk-pk

Maximum input, berom:cli~Riilg, dirty mode, R6 fully

clockwise:15'mWp*,"pJ(;

*mA = millamperes;:.1mA'=iJ:OOA.

tmv = millivolts; lmV= O.OOV.

m = one thousandth.

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How it Works

IC2A is a buffer stage hat loads he input signaldown as ittle aspossible;

capacitorCI promotes stabilityand helpsminimize radio frequency nter-

ference. From hele, the signalcoup1eshrough C5 into the gain control.

This varies the amount of signal hat goes o IC1A, which is set up as a

high-gainstage.With R6 up fulI and SI in the c/eanposition, the signal s

amplified 100 times (p1entyof gain,even or 10w-Ievel ignals).Willi SI in

the dirty position, LEDs Dl and D2 shunt across his high-gainstage o

clip the signal and create distortion. Under theseconditions, tuming up

R6 doesn't make the signallouder, but insteadmakes t mOle distorted.

The next step along the signal path is through C6 to R7, the master

vo/ume control. This signalgoesdirectly to J2, the inverting output. The

output of IC1A algo goes nto IC1B, which sirnp1ynverts he signalagain

to produce a noninverting output, and feeds his signal n o J3. Sincea

balancedoutput requiresboth inverting and noninverting signals, he in-

verting output of IC1A and the noninvertingoutput of IC1B feed he bal-

ancedoutput connector.

IC2B is the meter amp. It basical1y resentsa minimum of loading to

whatever signal connects o pad M, and then couples nto the VU meter

throui ;hCB.

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Project No. PARTSLIST

Resistors ali are 1/4W, 10% olerance,exceptasnoted)

Rl lk

R2 4.7k

R3-RS lOk

R6 lOk audio aper pot-controls gain

R7 lOk audio taper pot-controls mastervolume

R8 lOOk

R9, RIO 470k

Rll 1M

Capacitors rated at mate than 10V for -i9V, mate than 15Vfor t15V)

CI, C2 5pF ceramicdisc

C3 O.lilF (mylar preferred,disc acceptable)

C4 O.22ilF mylar preferred,disc acceptable)

C5 2ilF, electrolytic or tantlum

C6-C10 1OilF, electrolytic or tantalum

Semicond ctors

ICI, IC2

Dl, D2

D3.D4

RC4739 or XR4739 duallow-noiseop arnp

Red LED

lN4001 or equivalentsilicon diode

MechanicalParts

Jl.J3

J4

SI

VU

Misc.

Opencircuit 1/4 mono phonejack

Optiona1XLR connector

SPST witch-selectscleanjdirty mode

VU meter

Case, nobs, wo 14-pin C sockets, ircuit board

solder,wiTe,etc.

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project 1 pre-amp

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