JRC 4558D - Guitar preamp

[Ben Martin]

Hi all,

I have this TDI4 passive DI box that needs a simple guitar preamp to be able to excite my TASCAM 16X08 - line-in TSR ports 11 through 14.
This TASCAM device offers no preamps associated to the ports from 11 up to 16, hence this ports are pure "line-in" channels.

I found a few guitar preamp schematics options.
Some preamps are replicas of guitar stomp pedals that add overdrive and other FX to the signal.

Since I have a few IC "NJM 4885D" around, I noticed that I can use this op-amp to assemble simple electric guitar preamps.

So, I designed a simple solution that allows me preamp 4 electric guitars using 2 IC "NJM 4885D" in the same PCB board.
This should allow me to connect 4 individual electric guitars to the 4 line-in ports of the TDI4 passive DI box (at the same time).

Do you guys have any idea if this is going to work based on my schematics?
What would you do different?
Keeping in mind that distortion is not welcome.
The aim is to get a pre-amplified clear and transparent sound signal able to reach the "line-in" record device with a good signal ratio.

Any sound FX (Distortion, Chorus, Delay, Fuzz, Scream-tube) will be added after record the clean electric guitar signal.
Please care to comment on the schematics.
Many thanks.

Ben

 

[rayma]

any idea if this is going to work based on my schematics? What would you do different?

You'll have some output DC offset (depending on the gain) without output coupling capacitors.
Also normally you would install decoupling capacitors at each op amp rail to ground. In this case
probably a 10uF 25V would be ok. The highs are rolled off at 3400 Hz by the 470pF capacitors.
You can always try other dual op amps as well.

 

[Ben Martin]

Hi rayma,

Many thanks for comment.
About the he highs being rolled off at 3400 Hz by the 470pF capacitors, I guess it is not bad for electric guitars. The golden range for guitars in a mix is from 80Hz up to 1500Hz. The guitar may go up to 5000Hz (maximum) but from 1500 up to 5000Hz it is already considered a "red zone" for guitars and should be avoid into a mix but if you have any other suggestion for this 470pF capacitors, please let me know what values you propose. .

About the electrolytic 10uF capacitors.
Well, I'm planning to use the following schematic for Power Supply (see attached please).

I guess the decoupling it was already been taken care off here, right?
Please, let me know what you think.

 

[rayma]

if you have any other suggestion for this 470pF capacitors
I guess the decoupling it was already been taken care off here

The particular capacitor type will affect the sound also.
Use whatever pF value you like for the 470pF capacitor,
but choose preferably a polypropylene type, or at least a
COG/NPO ceramic, for the least distortion. No Mylars.

At least a 0.1uF capacitor should be at each IC package supply pin,
even if there is already one by the regulator. The local capacitors bypass
the inductance of the pcb traces between the ICs and the power supply.

 

[Ben Martin]

The particular capacitor type will affect the sound also.
Use whatever pF value you prefer for the 470pF capacitor,
but choose preferably a polypropylene type, or at least a
COG/NPO ceramic, for the least distortion.

Alright, will go for a polypropylene type, then.

At least a 0.1uF capacitor should be at each IC package supply pin,
even if there is already one by the regulator.

Ok, going to add that to the schematic and then I'll post it again when it's done.
By the way, ceramic or something else?


Thanks for helping me.

Ben

 

[Ben Martin]

Hi,

I'm back... so it is something like this you are suggesting (capacitors 0.1uF) see the new attached image.

I noticed in the schematics of the Power Supply regulator that, besides the 10uF polarized capacitors there are also 0.1uF capacitors included (before the final points +12v 0 -12v stage).

Despite that, you still recommend to add extra 0.1uF capacitor at each 4 and 8 pins of the ICs 4588D, correct? (like it is now in the schematics).

Ben

 

[rayma]

you still recommend to add extra 0.1uF capacitor at each 4 and 8 pins
of the ICs 4588D, correct? (like it is now in the schematics).

Yes, exactly. This is required universally for most all ICs, linear and digital.
Often an additional 10uF is needed per pin, along with the 0.1uF.
Depends on the loading, etc.

 

[Ben Martin]

Oh, I just noticed that I put the 0.1uF capacitors in "series" with the 4 and 8 pin of the IC, and that is wrong.
I should have put them from the pins to the GND.
Already corrected in this new schematic.


Anyone else has some extra tips to make this 4 channels guitar preamp using only 2 ICs 4558D? By the way, I'm using this IC because I have 8 spare units around from a previous project.

 

[Guerilla]

You may have to make it pretty linear. To make it sound right you will need to use a cabinet-simulator in your mixingsoftware. That would probably not sound right if the input signal is allready rolling off.
I think what you need, would be called a 4 channel di-box -hardly a guitarpreamp You need a gain button for each channel to raise signal from microphone level to a clean line-level, and then use 100 wet signal in the mix
I dont read scematics, so maybe thats what you have already

 

[JMFahey]

1) as is you can ground pin 2 and 6 at some pot settings and get infinite gain, not good.
Add, say, 4k7 or 10k between pot wiper and pins 2 or 6 , that will limit max gain to 20X or 10X tops.

2) also add a 1uF to 10uF electrolytic in series with that gain limiting resistor, currently your preamp amplifies DC and that again is not good.

3) reduce RF rolloff from 470pF to 220pF and even better 100pF which is the unwritten standard value.
a) there is some content "up there" , no need to kill it beforehand.
If you wish, you can do that later.
b) those are "slow" filters, so choosing 1500Hz as you did at the beginning, will start attenuating quite below that; no guitar will have sparkle.

4) for a smooth gain variation related to knob movement, try to get a "C" curve pot (reverse log) .
A Linear one will do very little over most of the range and jump a lot near the end, hard to adjust.

5) supply bypassing is fine, now don´t obsess with capacitor dielectrics, that makes more sense in high quality Audiophile stuff but for Guitar use, Electrolytics, Ceramics and "any" plastic film is way above normal needs.

In fact, although Electrolytics "should" be bipolar when no DC voltage is applied, here applied AC voltage is so small that you can safely use plain ones.
FWIW everybody does (Fender/Marshall/VOX/Laney/Peavey/etc.)

EDIT: for your intended use, you don´t even need a gain pot, just use fixed 5X gain (say, 100k+22k+1 to 5 uF) and your Guitars will nicely fit the "line level" inputs.

 

[Guerilla]

1)
EDIT: for your intended use, you don´t even need a gain pot, just use fixed 5X gain (say, 100k+22k+1 to 5 uF) and your Guitars will nicely fit the "line level" inputs.

Cool. I didnt know that. Makes it easier/cheaper

 

[mjf]

example..........
choose your preferred parts

 

[Ben Martin]

Hi JMFahey and mjf,

Many thanks you both for your valid input.

JMFahey, I'm curious about your propositions.
Any chance you can put that in a simple schematic so I can be sure about what you mean without any miss interpretation?
Specifically about thid sentence:

EDIT: for your intended use, you don´t even need a gain pot, just use fixed 5X gain (say, 100k+22k+1 to 5 uF) and your Guitars will nicely fit the "line level" inputs.

I'm not sure I understood what you mean with:

 

[Ben Martin]

example..........
choose your preferred parts

mjf, is this a DI BOX schematics?
It seems a regular op amp amplification circuitry based on the IC RC 4558D.
I see that the decoupling is done by using 2 capacitor (C3 + C4) in series between pin 4 and 8 ( +12v -12v ).

 

[Enzo]

Rather than thinking of C3, C4 in series, just look at them as to ground from either power rail.

 

[Ben Martin]

Rather than thinking of C3, C4 in series, just look at them as to ground from either power rail.

Hi Enzo,
Thank you for your input.
Yes, you are right!
I didn't noticed that in mjf schematic, both capacitors link to GND. Dhooo.
Well, actually, that's what I have in my scheme too.
See attached images;

 

[PRR]

A "DI box" goes to a microphone input.

If you only have Line inputs, all you need is hi-Z gain of 4 to 40 which a '4558 can do (though a TL072 may be better). Then sell your DI box.

 

[Guerilla]

A "DI box" goes to a microphone input.

If you only have Line inputs, all you need is hi-Z gain of 4 to 40 which a '4558 can do (though a TL072 may be better). Then sell your DI box.

I believe a di-box can have gain and even ballanced out, but ben martins doesnt, and I agree I cant see what use he has for it.


Originally Posted by JMFahey View Post
EDIT: for your intended use, you don´t even need a gain pot, just use fixed 5X gain (say, 100k+22k+1 to 5 uF) and your Guitars will nicely fit the "line level" inputs.

I'm not sure I understood what you mean with:

-You dont need a guitar preamp

You only need a surcuit that amplifies the guitars output 5 times (20db?) And change its output impedance to suit the line in on your recorder.

Useualy(cant spell) people use mics in front of guitarspeakers when recording. Nowadays many pedalboards and guitaramplifiers. Have build in guitaramp-(guitarspeaker really)simulation that allows you to record directly without a microphone, or play directly thruu a PA system-without its sounding strange.

You choose to record the clean signal from the guitar and proces it when mixing,which should work fine. Maybe even great.

 

[JMFahey]

Hi JMFahey and mjf,

Many thanks you both for your valid input.

JMFahey, I'm curious about your propositions.
Any chance you can put that in a simple schematic so I can be sure about what you mean without any miss interpretation?
Specifically about thid sentence:

I'm not sure I understood what you mean with:

MJF solved and drew it for you

Just follow his circuit which incorporates all the improvements, specially: ~5X gain and ruler flat from below 40Hz (so it also covers Bass ) to above 10kHz, and with such a degree of NFB it´s practically distortionless.

He was also kind enough to simulate it.

Treat it as a "Black Box" : Guitar/Bass In>Line level Out.

To boot, having very low output impedance, you plug your guitar into it with a not too long cable, output from mixer to Sound Module can use quite a long one, easily 20/30 meters, with no problem, so you can, say, play in the center of a stage and have Module (or a Mixer or a Powered cabinet) at some distance on Stage side with no problems.

Input impedance is perfect for a passive instrument while plugging straight into module would probably be 10k to 20k , too low.

 

[Ben Martin]

A "DI box" goes to a microphone input.

If you only have Line inputs, all you need is hi-Z gain of 4 to 40 which a '4558 can do (though a TL072 may be better). Then sell your DI box.

Ah... ah...
I still need my passive DI BOX (4 x instrument level inputs) to when musicians come to my place to record. Sometimes they bring their own preamps and stuff.