Simple signal generator

[rankot]

Hi everyone! I needed a simple signal generator for pedal testing, so I decided to build Wien Bridge oscillator. It is a small and simple project, but someone can also find it useful, so I post it here. It is built on a small PCB (36x49mm) and contains two pots - one is for frequency setting (double pot marked FREQ-A/B), another one is for volume. It is powered with usual stomp box 9V DC adaptor (- inside, + outside), and contains 6.3mm jack so you can connect it to pedals.

Hope you'll like this.

Ranko

[anotherjim]

That's very neat.

However, it looks like you couldn't use the same power supply for the pedal under test, because you have the supply negative "floating" down to -4.5v below 0v compared to the negative tied to 0v in a normal pedal circuit.

As drawn you have the DC socket with + inside, which I don't think you intended?

It looks like it should work well with a 9v battery with decent battery life, I think I'd find it more useful that way.

[rankot]

Right. I have a separate supply for this.

[rankot]

But it is not impossible to use it on the same supply as the pedal - just connect output jack to -4.5V (which is actually supply/pedal ground) instead of circuit ground. Adding output decoupling capacitor is also OK (but not necessary since all pedals have input decoup caps). I have also noticed that my DC adaptor was with reversed polarity, so here is fixed schematics for usual stomp box supply.

[rankot]

PCB images (300 dpi) for those who don't like PDF:




Updated schematics!

[rankot]

I wanted to add a simple 1W amplifier, so I can use that together with signal generator for pedal testing. So i did it this way:

Unfortunately, when I connect a speaker to TDA7052A output, only humming can be heard and V+ rail drops to some 2.5V. Without a speaker, rail is on 9.2V. Does anybody have an idea what is wrong?

[anotherjim]

Make sure the speaker (-) from pin8 is not grounded at the speaker.

Also, the A version of the 7052 does not appear to like resistance to ground at pin 2, so try without your R4.
http://www.diystompboxes.com/smfforum/index.php?topic=100868.msg888987#msg888987

C4 shown wrong polarity. Negative toward the pot please.

Input connects DC to ground via R6 and volume. This can upset the circuit under test (C.U.T). Another cap in the input will stop that. Since the DC at the C.U.T could be negative, a polarised cap type is not a good idea. Use the largest non-polarised type you can get. With R4 gone, 100nF input cap might be fine.

[rankot]

Make sure the speaker (-) from pin8 is not grounded at the speaker.

Also, the A version of the 7052 does not appear to like resistance to ground at pin 2, so try without your R4.
http://www.diystompboxes.com/smfforum/index.php?topic=100868.msg888987#msg888987

Speaker is definitely connected to + and - of IC, not to ground. I will try to remove R4.

C4 shown wrong polarity. Negative toward the pot please.

I actually use 470n for input cap, accidentally left polarized in schematic.

[rankot]

Thanks, Jim, removing R4 solved the problem! IC is TDA7052A, and I used datasheet schematics, but it seems that they are wrong! Now everything works fine! Here are the files if one wants to build this - I have also added decoupling cap at the input:





Schematic:

[anotherjim]

Nice
Thanks for posting.

[PRR]

> TDA7052A, and I used datasheet schematics, but it seems that they are wrong!

? ? ?

http://www.nxp.com/documents/data_sheet/TDA7052A_AT.pdf

Compare with your "dont work" arrangement.

[rankot]

You are right, but I wanted to have input level control, to protect IC input from too strong signal. So putting C4 behind R4 would make potentiometer and R4 work in parallel, and I wanted to avoid that, and also wanted to avoid adding one more cap at the end of chain. Things work fine without R4, so I presume I can leave it like that, or I can add one more capacitor between R4 and C6.

[anotherjim]

In this chip, pin2 may well have some DC voltage via internal bias that must not be interfered with.

I don't think you need the resistor at all as the pot is doing the same job, which is to reference the probe input to ground.

The Manufacturers Rs is "some source resistance" and may have been chosen at 5k as typical of an electret or MEMS microphone. Also, 5k input may have been in the test set-up used to provide performance data.

[rankot]

I have redesigned PCB so it can fit into small plastic box: