Diode Rectifier

Thread Starter

philwalker

Joined Aug 15, 2010
41
Would I need a power amplifier if I was going into another opamp?

eg, would this circuit work? If its not clear what I am trying to do let me know, thanks.

Phil.

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Audioguru

Joined Dec 20, 2007
11,248
Your opamps that are used as comparators are similar to the LM3915 VU meter IC so why not use it?
The LM3915 also has a resistor divider ladder but it is connected correctly. Yours simply lights the bottom LED all the time and the other LEDs some of the time even with no signal.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
What About this circuit?

Do I need a diode and ripple capacitor inbetween the amp and the comparators?

Im using these as I have a lot of them, cheers, Phil.

 

Audioguru

Joined Dec 20, 2007
11,248
I recommend changing the preamp so that it is an active "precision rectifier" that uses the extremely high internal gain of the opamp to reduce the forward voltage of the diodes to nearly nothing because the diodes are inside the negative feedback loop.

Your circuit passes the DC at the output of the preamp opamp to the comparators so that most of the LEDs are lighted all the time even when there is no signal.

The LM3915 VU meter IC has an input that is at 0V and when the signal goes positive then the LEDs light according to the signal voltage levels. So it doesn't need a rectifier.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
I will probably end up using said chip but I would be interested in learning why the comparators will making the LED's glow? Surely if the preamps input is 0v, the output will be 0v, and the comparator will prefer the inverting input and output 0v?

Is this down to the fact that the comparator can not output the rail voltages and usualy output a little under?

Thanks, Phil.
 

Audioguru

Joined Dec 20, 2007
11,248
The input and output of the preamp is +6VDC. The signal causes the output to vary from 1.5V to 10.5V. Then most of your comparators will be turned on all the time.

If the preamp has a dual polarity supply then its input and output will be 0VDC.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
The input and output of the preamp is +6VDC. The signal causes the output to vary from 1.5V to 10.5V. Then most of your comparators will be turned on all the time.

If the preamp has a dual polarity supply then its input and output will be 0VDC.
How about if i Put 2 diodes on the op amp output? Will this provide adequate voltage drop?
 

Audioguru

Joined Dec 20, 2007
11,248
How about if i Put 2 diodes on the op amp output? Will this provide adequate voltage drop?
No, it will reduce the signal's peak level too much.
Capacitor-couple the output of the preamp so it is at 0VDC with a positive-going part and a negative-going part. Then use an active precision rectifier circuit to convert its amplitude to DC to feed the comparators.
The preamp can be converted to an active precision rectifier with the same gain as your existing preamp.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
I dont understand why I am hitting problem after problem lol. When I was in college I did something similar usin 741 op amps and that was so simple.

Why don't I make a cirtual earth at 6v them. So the op amp works at +\- 6v. Would this make things easier?

Cheers, Phil.
 

Audioguru

Joined Dec 20, 2007
11,248
An LM3915 is inexpensive and does everything you need except it must be fed AC from an external preamp that is at 0VDC. It has 10 outputs that have current regulation. It uses a single polarity supply voltage.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
How wold I need to adjust my design to use the LM3915 instead then, I assume I would just connect the output from the op amp, to the input of the LM3915, but what value resisters etc. would I need to use?

Do I need to recify the op amps output at all?

The voltage out of the op amp will be 0-10V, is this a problem?

Cheers, Phil.
 

Audioguru

Joined Dec 20, 2007
11,248
How would I need to adjust my design to use the LM3915 instead then, I assume I would just connect the output from the op amp, to the input of the LM3915, but what value resisters etc. would I need to use?
The datasheet for the LM3915 shows how to use it. Its input needs a resistor to ground (100k ohms) so that it is 0VDC with no signal. The opamp has its output at +6VDC so it will need a coupling capacitor (100nf). Two resistors set the max signal peak voltage that lights the 10th LED and also sets the current in the LEDs. The LM3915 needs a supply bypass capacitor of at least 2.2uF so use 10uF.

Do I need to recify the op amps output at all?
No. The LM3915 rectifies the signal. I made my audio VU meter with a peak detector circuit (described in the datasheet of the LM3915) so that it stretches the duration of brief peaks long enough (30ms) so that they appear bright and clear. The 50ms fading appears quick enough.
without the peak detector the LEDs are turned on for only half the time (because of the half-wave rectification) and will appear a little dim. The brief peaks will not be seen.
 

Thread Starter

philwalker

Joined Aug 15, 2010
41
Ahh they are transistors sorry, do they act as a push pull amp of sorts?

Also Why do you have the 5V reg?

Also what are the power rail voltages for the First op amp as they are not stated.

Cheers, Phil.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,248
Ahh they are transistors sorry, do they act as a push pull amp of sorts?
No.
Q2 and Q3 are active rectifiers (peak detectors) with a fast attack and a slower release so that you can see the LEDs light for very short duration peaks (. Q1 matches the voltage of Q2 and Q3 to opamp IC1b.

Why do you have the 5V reg?
The regulator keeps the microphone's biasing, the biasing for the first opamp and the biasing of the RHI pin of the LM3915 (when signal levels are low) at a steady voltage as the battery voltage runs down.


Also what are the power rail voltages for the First op amp as they are not stated.
The first opamp IC1a is half of a dual opamp that includes IC1b. IC1b shows the power connections. The MC33172 has two opamps inside.
I used it because it works with a supply voltage as low as 3V, has inputs that work at 0V (pin 5) and has outputs that go down to almost 0V like an LM358 dual opamp but its bandwidth is much higher and it has no crossover distortion.
 
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