designing a stereo Booster using TL072 - help needed

[half_smith]

Hello, I have been tinkering with making a basic signal booster for my van. The AUX input is an RCA (red/white) in the back seat, so the 1/8" cable runs 10' under the carpet to the dashboard where I plug in my iPhone. The level is anemic and I figure a boost will help.
1st question, will a boost help?

2nd, I need stereo so I tried to rework the circuit to use a TL072. can anyone take a look and let know if my design is legit?

-mostly, I am unsure about how to route the 9+, does it need to cross both inputs (L/R) with the 2M2 resistors on each channel?

here's the basic single op amp circuit I used as a starting point: (http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/)


here's my best attempt at drawing my stereo TL072 layout in PhotoShop:



MUCH THANKS!

[dschwartz]

It will work as drawn, but it can be improved.
You can use the car's 12V supply, adding a 100uF cap between 12v and ground, close to the opamp.

Also, the resistors values are too high and will introduce some noise. Consider changing the gain by placing the pots instead of the 470k feedback resistors.

As drawn, if the input level is too high,  it will clip the opamp and no matter how you set the level, it will keep clipping.
If you change the gain by varying the feedback resistance, you can lower the gain and avoid clipping.

[half_smith]

dschwartz, thank you for your response!

I like the idea of moving the pots to be the R in the FB,
I've redrawn the circuit,
removed the pulldown Resistors 1/6 on the inputs,
removed the C2/C5 Caps in the FB,
changed the R2/3, R9/10 to 1M instead of 2M2 (because I don't have any more 2M2 so f it?
and lowered R4/7 to only 1K

I'm still going "single-supply" and just connecting -Vb of the IC to GND, is this ok or would I want to build a dual-supply?

Does this improve it or have I overcorrected?

[antonis]

You can reduce bias resistors value down to 10k and connect their junction to non-inverting input via a 1M resistor..

[amptramp]

You can reduce bias resistors value down to 10k and connect their junction to non-inverting input via a 1M resistor..

It's quite common to use a couple of 10 K resistors like this and have a capacitor to ground at their junction just to keep power supply noise from adding to the signal.  Since you have decent power supply rejection in an op amp, you might as well get the benefit of it.  This is called "noiseless biasing".  You have a 10 K resistor to ground in parallel with a 10 µF capacitor connected to another 10 K and a 1 meg resistor.  The other 10 K goes to the power supply and the 1 meg goes to the non-inverting input of the op amp.

[Mark Hammer]

A booster may well be called for.  It is a tried-and-true method for improving signal-to-noise, and making the most out of the available headroom.

HOWEVER...

We don't really know the source of the "anemia".  I take it you are disappointed with the volume levels you feel should be there.  That can be due to speaker inefficiency or frequency response, speaker placement that detracts from desired listening levels or tone, low amp power, and other things we haven't addressed, in single-minded pursuit of a hotter signal.

Again, nudging the signal level up a bit may be EXACTLY what's required, but I think a little more info might lead to a better solution.  But if the source of the anemia is elsewhere, boosting the signal may simply drive the input of the amp in a way that distorts.

[half_smith]

Thank you for the replies!

Antonis, Amptramp, have I implemented your suggestion correctly here? (changes highlighted in Blue and Green)




Mark, I appreciate your reply to Q#1 lol.
I haven't done much measuring, but the car stereo on AM/FM or CD is super loud compared to the AUX at full volume...I suppose there could be an issue with that input, but this booster idea has been bouncing around in my head for a year so at least it's an opportunity to branch out and do some proper circuit building.
Is there another approach I could try?

[antonis]

 

[half_smith]

ANTONIS! thanks man.
It looks like you made the 10uf Cap in parallel with the 10K R yes?
And wrapped the signal path around the bias resistors completely?
otherwise, good to build?

[antonis]

It looks like you made the 10uf Cap in parallel with the 10K R yes?

Yes..
It stabilizes bias voltage and also provides an AC ground path to feedback/gain 10k pot/1k resistor(s)

And wrapped the signal path around the bias resistors completely?

Yes..
Signal only needs to come into non-inverting input(s) where bias resistor(s) must be in shunt with signal path..

otherwise, good to build?

Let's find out..

[m4268588]

1<->2
6<->7

[antonis]

1<->2
6<->7

It's a bit weird draw but No..

[Steben]

Do I see two times the 10k/10k bias split?

You could go for this as well and connect the two 1M to the Vref point:

[antonis]

And what about Left/Right signals isolation..??

edit: My bad Steven..
You said TWO 1M bias resistors..

[half_smith]

guys! I'm learning the hard way again, but hopefully not too far off.

signal is clean, but not amplified. ??

The POT(s) act funny too. Nothing (silent) up to a point and then ON, but no change from there up to max.

I am hoping someone can see what I've done wrong and advise?
I'm guessing I routed the 1M signal shunt at the wrong place...

pics of my build:
(sorry for the 1st grade diagram, I believe this is accurate to what I assembled)

[antonis]

Try the following configuration..



Gain from unity(*) up to X11 ( 0 - 20.8 dB)

(*) For Pot zero ohms for FCCW setting..

[half_smith]

It works!
nice increase in volume, I'll have to see how it handles the noise of the road and wind.
The tone is full and open, and I haven't noticed any clipping, it seems very good.
Next I will see about running it from a Cigarette Lighter near the tailgate, so at 12v even more headroom yeah?

Pretty fun to try "point-to-point" soldering, and I think I will eventually understand the "noiseless biasing" and voltage divider section.
Plus, except for the fact that antonis designed the whole thing, I designed it from scratch!


Thanks once again DIYSBF

[antonis]

except for the fact that antonis designed the whole thing,

Antonis designed nothing by himself..!!
(he just implemented "good design practice" on other long in the tooth people's designs..)

[half_smith]

well, I can smash together the first two chords of Hey Jude, the second half of the bridge of A Train, and the turn around from Crazy and call it a new composition right?

Anyway, have you ever seen this exact circuit before? I think it could be worth keeping.
The Antonis Amp, sounds famous already! Just gotta goop it.

[antonis]

Let's mod it a bit and call it "General purpose high impedance stereo pre-amp"..



P.S.1
You can maintain 1M bias resistors for TL072 (or other FET input op-amp) for even higher input impedance..
100k above are intended mostly for bipolar input op-amps of input bias current to be reckoned with..

P.S.2
Next project will be a further modification of the above with an input "guard ring" (for huge input impedance) and some kind of Tone control (for a self respecting pre-amp)..