peak program meter question

I am building a peak program meter for my tape machine.

The circuit consist of a precison rectifier and display circuits for LEDs etc.

I have a general question regarding the rectifier circuit, which has the program as an input and produces a varying DC output which is then displayed and used to record at the proper level. This circuit has the proper attack and decay times which makes the whole thing a peak program meter and not the regularly used VU meter.

Here is my problem:
I feed an audio signal at a certain voltage into the rectifier and measure the DC output the circuit produces. The output varies slightly with frequency.
At 60 Hz and 3 V rms I get a rectified signal of 4.01 volt (and not 4.24 as I would have expected, 3V/0.707). As I go up with the frequency the output decreases to about 3.7V at 20,000 Hz.

Is my assumption that for a pure sine wave the relation Vpeak=Vrms/0.707 holds regardless of frequency correct?

If that is true there must be something wrong with the circuit or the design.

Help anybody??

Uwe

Are you using an old 741 or LM324 opamp or rectifier diodes that have poor high frequency response?
Please attach your schematic with part numbers shown.

I hope this works, it is a page from the LM3915 data sheet...

Uwe

PS. I built the bottom circuit

If you have the Precision Full-Wave Peak Detector circuit built with fast LF353 opamps and its response is down at 20kHz then its resistor values are too high and cause the rolloff with stray capacitance.
Reduce the values of most resistors by 10 times. Keep R6 as 1k. Make C1 2.2uF. Then its high frequency response will be much better.

Thanks for your quick response.

I made the changes and did a measurement again.

Before I had the following outputs

20Hz 200Hz 2000Hz 10,000Hz 20,000HZ

3.43V 4.12V 4.06V 3.79V 3.57V

Now I get the following outputs

3.41V 3.78V 3.74V 3.71V 3.62V


Much more even!

But I still don't understand how a 3V rms signal turns into these values?
Since I am trying, in the most accurate way, to get a AC program signal recorded on tape I think it is important that this meter turns a 3V rms signal into a 4.24V peak signal.
Or do you think now that the response is flat this will be simply a matter of calibrating the finished meter.

Thank you much for your help sofar.

Uwe from Maine

If the resistors and your meter are accurate then if the output load resistance is very high the output should be 4.24V. If the output load is 10k ohms then the output voltage is loaded down to 3.85V.

After a little break I am still working on my precision rectifier for a peak program meter.

Since the range of my the finished meter should be 50- 60 dB I am getting down into the 1 or 2 millivolt range on the low end.
I had a hard time nulling the LF353 and changed over to a TL074 chip which didn't need any nulling, the offset was undetectable by my means (well under 1 millivolt). But I noticed that the linearity of the circuit was bad at the low end. I guess that has to do with the diodes but I do not understand the circuit enough to know what to change to end up with a very flat response (not only as a function of frequency, 20 to 20k Hz, but as a function of the input voltage range from millivolt to around 5 volts).

Can anyone point me in the right direction??

Uwe

Hi UWE,
With the resistor values 1/10th what is shown on the datasheet for a flatter high frequency response and a 2.2uF input capacitor, the response of the precision full-wave rectifier circuit is down 3dB (x 0.707 times) at 11 Hz so will be down about 1.5dB (x 0.85 times) at 20Hz. If a 22uF cap is used the low frequency response will be perfect.

Electrolytic caps are polarized and have non-linearity. See if you can get a non-polar electrolytic cap. Two polarized 47uF electrolytic caps can be connected in series and in opposite directions to make a non-polar 23.5uF cap.

The linearity is affected by the supply voltages. What are the positive and negative supply voltages? Does each supply voltage have a 100uF bypass capacitor across it?

You know the saying: If you give a hungry man a fish you feed him for the day, but if you teach him how to fish!!! etc. etc.

You mention the frequency response of the rectifier in your last post and I would love to understand how this can be determined.
Probably even more important would be to figure out the impedances of circuits, you said the rectifier circuit has low output impedance and would be bogged down by a relatively low input impedance of the next stage.

How/where can I learn to determine these things??

This will be very relevant for me. I got a mixing console from a radio station, a beautiful thing with great specs which I measured, but when connected to the rest of my equipment the sound is weak and tinny and since the specs by themselves are good it can only be a case of impedance mismatch.

My rectifier circuit, after changing the cap and the op amp the thing is working fairly well. I think a breadboard sometimes can give you trouble, these contacts are not always perfect and when you deal with 1 mV erratic things can happen. So I am optimistic, I just need a bit more theoretical knowledge.

So can I learn how to fish around here???

Uwe

When a low impedance source feeds a coupling capacitor that has a load resistance, then the frequency where the reactance of the capacitor is equal to the load resistance is down -3dB. Lower frequencies are down more.

Since you had a problem with missing bass frequencies then either the value of the coupling capacitor was too low or the value of the load resistor was too low. For the best bass, a low impedance source should feed a high resistance load.

I think a breadboard is a useless piece of intermittent and interference-prone junk. I solder my prototypes on stripboard with very short wires so that the circuit doesn't pickup interference and so it doesn't oscillate.