Interesting but strange results Help needed!

[markbr]

Hello,

This Danelectro Honeytone guitar amp has a pre-amp (TL072) that can take
+/-18V. Originally it was designed to take +9V off a single battery with the -Vcc terminal grounded. However i've now added a DC-DC converter which roughly doubles the +9V to +16.6V and connect it to the +Vcc pin.

This is what I find strange:

1) When im supplying 16.6V using the dc dc converter, my sine wave output when the volume pot is all the way up is NOT symmetrical. At max volume I expect the sine wave to hit the supply rails and clip symmetrically(Im using a 1Khz test tone and measuring the output at pin 1 using an oscilloscope). But when im using a DC power adapter that gives +15V and connect it directly my sine output is symmetrical at max volume. Why is this?

I have attached the schematic and oscilloscope readings.

Thank you!

[Enzo]

SO the amp works as expected at 15v, but at 16.6v it does not?

The power amp IC is rated only to 15v from the data I saw. Possibly related.

[markbr]

Yes, it works as expected at 15V, there's sound at 16.6V but the oscilloscope reading isn't as expected.

Also, im measuring the output from the pre-amp output pin, not the power amp.

[jrfrond]

When operating a bipolar opamp from a single-ended supply with the -Vcc grounded (pin 4), bias must be applied to the active input to float it a 1/2Vcc. This i supplied in this circuit by R2 and R9. Raising the Vcc to 17V, almost doubling it, necessitates a change in the opamp bias to restore output symmetry. If my calculation is correct, your offset would be .5Vcc/2 = 8.5/2 = app. 4.25V. Is this correct?

Your other option would be to reconfigure the power supply as bipolar ±8.5 and remove the bias resistors. Then you don't have to think about it.

[markbr]

Im getting about 3.6V at pin 5 and about 3.3V at pin 3. When you say change in opamp bias is required when doubling the voltage, what do you mean by it? Is it change the resistor values?

On the other hand, i also have a voltage inverter which gives -15V output from the +9V rail. This can be connected to -Vcc (pin4) along with +16.6V to +Vcc pin, giving a total of 31.6V. If i add the inverter should i change the bias resistors? And if so what values? Thank you.

[Enzo]

Do you think the power amp IC which is rated for 15v will be OK with 31.6v across it?

[jrfrond]

Im getting about 3.6V at pin 5 and about 3.3V at pin 3. When you say change in opamp bias is required when doubling the voltage, what do you mean by it? Is it change the resistor values?

On the other hand, i also have a voltage inverter which gives -15V output from the +9V rail. This can be connected to -Vcc (pin4) along with +16.6V to +Vcc pin, giving a total of 31.6V. If i add the inverter should i change the bias resistors? And if so what values? Thank you.

You need to check your offset at pins 1 and 7, the opamp outputs. The bias system is there to offset the DC output voltage to zero. Again, this is not necessary when using a bipolar power supply.

Just curious as to WHY you are doing this in the first place.

[markbr]

Just curious as to WHY you are doing this in the first place.

I've uploaded my oscilloscope readings, they're measured at the output of C18. The first one is with just +9V to +Vcc with -Vcc grounded. Here, the signal clips at 7V with the volume pot all the way up.

The second is with +16V and -15V. Here the signal doesn't clip even when my volume pot is turned all the way up. The pk-pk is 19.4V. So a higher voltage to the op-amps yield a more amplified signal.

You need to check your offset at pins 1 and 7, the opamp outputs. The bias system is there to offset the DC output voltage to zero. Again, this is not necessary when using a bipolar power supply.

Therefore now im wondering if i should remove R2 and R9 with +16V to +Vcc and -15V to -Vcc?

Thank you.