Idea: active headphone protecting circuit - Page 2

I didn't think about the reset port, this would limit the 8 pin chips to 5 IO lines. I want to avoid HV programming if possible - an STK500 board costs about 220$ here in switzerland, or 80$ + 35$ shipping + ??$ customs from digikey.
Olimex has an ISP programmer for a mere 9.95$. Another option would be this: http://elm-chan.org/works/avrx/avrxp8.png.

Yes, the ATtiny24/44/84 looks good - but I can't find a place where I could buy it (and remember, 35$ shipping fee from digikey...). Guess it's either attiny45 or attiny26 for me - the first can most likely also be substituted by a n attiny25, the second should be compatible with the upcoming attiny261 (26 is 'not recommended for new designs', but not yet marked as 'obsolete'. looks like full scale production for 261 is just starting)

So, how to use the pins?
3 Vcc, GND, Reset
2 ADC inputs
1 LED indicator
1 power on sense
1 relay trigger (we don't really need to adress the two relays separately)
This gives 8 pins. The power on sense could be replaced by a trimpot if the chip is powered by the same power supply as the amp.
With a 14 or 20 pin uC, there would be the additional possibility for:
2 io for pushbutton on/off
1 for trimpot
1 for serial out (not for debugging the code, but more for seeing the actual values the ADC generates. in the 8pin version, they could also be stored in the eeprom and then read out by the programming adapter)
2 additional LEDs, or 3 without serial out

or maybe not - connect it to the normal power supply, and use a transistor to turn on the following amp circuit. I also don't know if a power-sense port is really needed - if the protecting circuit is on the same psu it's not needed, and if the pushbutton function is used it's also not needed. It would be needed if you want to install it into an existing amp with minimal changes necessary (just hook the power-sense line anywhere where there's power if amp is turned on)

But I think people get confused if there are 15 different versions of a design of which they have to choose one. So an all-in-one solution, which would also be reusable in another amp without having to reprogram it would be the best thing in my oppinion. And I think that you can cram all of this on a double sided 1x1 inch board - even with a 20 pin soic (which are wider than 8 and 14 pin soic). With a 14/20 pin uC you can also have a start-up time trimpot. Pretty much first thing that happened to AMBs circuit was that someone modded startup time to 30s.

ok let's quickly forget about the attenuator idea for now



Yes, but I pretty much did until now . The circuit, how I described it in my posting above, would only work for positive DC currents... it would work for negative voltages if you refer the inverting input of the opamp to a negative voltage source. But where to get this negative voltage source? either 'borrowing' from the power supply, or using a voltage inverter to create it from the +5V supply. but then you'd loose the ability to apply any gain - it would overload the ADC. (the output would basically be DC offset +5v). Another idea: something like a rectifier bridge - but this would have a voltage drop, not a good idea if you measure in the mV range. Anybody got an idea how to solve this?

you sure? I thought they did.

Congratulations. The situation looks awfully familiar to me

I think I found a (partial) solution to the negative voltage problem.
First: the board will require at least 12v. Digital power supply is from the upper 6v, using a LDO regulator (or normal reg if you have at least 14v). opamp is +-6V.
You configure it as a differential amplifier, with a negative bias current of 100mV on the inverting input. Gain of 5.5. This yields an output of 0-1.1v if input is between -100 - 100mV.
You must of course add some clamp diodes to it. I just dont know yet where. I also dont know yet how to generate the -100mV.

Oh, error401: how are you going to programm a directly soldered on SOIC chip? Looks like theres a need for an ICSP connector on the 1x1" board...

Board looks good so far - I won't comment on every detail because of its PoC status. I would maybe go for a single sided board (at least for the first prototype). I really want to home etch something

About the negative voltage problem: I have not really found an easy solution - best idea so far is a precision full wave rectifier. like a diode but without any voltage drop (or only insignificant drop). Being able to include a gain stage and an active filter would be good, but most likely quite complex. This would require a quad opamp. Six opamps if I can't figure out how to integrate filtering and gain stage into this existing circuit .....

another idea: forget that precision rectifier, use the differential ADC input - one pin with -100mV, and then reference the two other pins to this input. But I have to check if the attiny45 will allow that electrically

um yeah. I would never have thought of such an easy idea.
I'm not sure about the type of power supply. a lm317/337 would of course work - but might be overkill. zener regulation could also work if the current draw isn't to big - this would mostly depend on the current needed for the transistors

Well, at least no need for 4 or more opamps...

Well, why not? if it's inside an amp, it most likely has a +- supply

I'm not sure if I understand you right - do you mean the 2.5v relative to GND in the +-2.5v scheme proposed by you? If you add a properly dimensioned voltage gain stage in front of it, you could get a full +-2.5v swing from the about +-100mV DC offset to be expected

edit: Sijosae used a LM2664 as voltage inverter in some of his amps. output current is 40mA - might be enough. But I think that you can take a dual psu more or less for granted

It should not be a problem if you wire up the opamp correctly. Ground should be the headphone signal ground AKA the 2.5v offset voltage at the ADC. Then with a gain of 25, a signal of -100mV becomes -2.5v, meaning 0v at the ADC. The same on the other side. Maybe the opa can be powered with the +/-2.5v (AD823? and many CMOS opamps), or directly from the supply rails.

Calibration should also be trivial - measure the value on the ADC when the headphone and source is unconnected. This can then be used as the value for 0v. on the tiny26 you could use a separate io port for going into calibration mode (and save calibration value in eeprom). for the tiny44, I'm sure that there is a way to somehow signal this state - maybe reuse another port. could the led output port misused as a temporary input port? should maybe work if the led port on startup is treated as an input port. then the enduser could do it with a wirebridge from led_out to gnd or vcc.

yeah, it doesn't have to be used with every single port. but combining binary input and led output should be possible.

Somebody in a uC forum has sent me an alternate, very simple circuit for measuring the +- voltages - have to look at it, maybe it's an alternative.

About the stability: I think the LM337/317 have quite a good stability - they shouldnt drift very far. And remember, at the uC there will be offset voltage times 20 or so. the opamp could be affected by noise in the power supply, but even cheapies have a PSRR of 100dB (tl072)

Yes, this could be done. With a timer, this could also be done just every 30 secs or so. But thinking of it, why not every sample, there's plenty of time we have. The additional ADC pin would in turn free up a calibration-enable pin. And knowing there's no drift is better than just assuming it.

The ATtiny44 would have an internal temperature sensor - this could be used to either detect a shortcircuit in or near the uC, or a general overheating in the amp case. This leaves the question what would be still an "acceptable" temperature - this is also highly dependent on the amp topology.