Op-amp mystery

Show us a pic of your breadboard!

Sometimes people note that a CMOS circuit works properly with no power supply, because some ESD diodes are rectifying a logic input.

I never use those dreadful Veroboard or plastic block plugin things.

Try this:

A cuppa Tea solves EVERYTHING!

John :-#)#

Stupid op amps!

RL

I find little tasks that can be done whilst waiting for the tea to cool. The problem is that I sometimes get so engrossed in them that I don't remember the tea until it is nearly cold.

I habitually top up the kettle straight after pouring from it. This is a habit I acquired when using a wood-fired Kelly Kettle (AKA Storm Kettle) to make tea for outdoor work parties. If you didn't top it up immediately, there was a risk of burning it out.

until all the inputs are low, then it only works until the bypass cap is discharged ;)

seem to remember seeing a boost converter made from just a cmos gate and an inductor, using the output as input and the supply pins as output via the ESD diodes, but I can't find it again, but I found this

This will drive a blue or whatever LED from a low voltage supply.

It is possible you were remembering my posting this in April 2016:

All power flows through the inductor. The ESD diodes are only involved during startup, once oscillator running the output stage n-fet is the boost switch and the p-fet is the synchronous rectifier. A version built using 74HC132 was in a successful product which needed micropower 5V from 2-3V input.

piglet

I've posted circuit diagrams and a picture at:

Thanks, interesting headphones volume setting.

I was originally just going to copy the circuit of the headphone amplifier in a Marantz CD player (why bother to re-invent the wheel?), then I spotted that they used a 100-ohm resistor in series with the headphone socket. This would remove the damping, so that headphones with any unwanted resonances wouldn't give as good sound quality as they would have been capable of if they had been fed from a low impedance.

To keep the output impedance low, I took the feedback from the output terminals.and put the 100-ohm resistors in the collector circuit, where they would still limit the output current to a safe value that wouldn't destroy the headphones.

The volume control circuit was derived from a Doug Self design in which he used a unity-gain buffer to minimise the loading on the pot wiper. I had a bagfull of 5k twin-gang pots, so I decided that I could omit the buffer as long as the feedback resistors around the op-amp were high compared with the pot itself. The pot is linear but the scale is approximately in dB, which works a lot better than trying to make two logarithmic pots track accurately together (or buying some exotic and expensive controls).

Originally I was aiming for similar gain characteristics to the line driver amplifier (pot half-way up = 0dBm output) but I found that was far too much gain and the volume control never needed to go beyond a quarter of a turn. The input signals going into all the output stages are around -10dBu with a cartridge giving around -45dBu on an 'average' record; this allows 30dB headroom before the volume control, which is necessary to avoid clipping the crackles from a damaged 78.

The circuit I finished up with actually generates a loss, rather than a gain, but that gives a more favourable pot setting for comfortable listening. Despite not workoing quite as Doug Self intended, it manages to produce a smooth transition across the middle of the scale, without 'crowding' near the top ot the bottom.