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Sep 04, 2016, 01:52 AM
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Optical power amplifier success

It came together manely as expected. The mane difficulty was getting the STA540 to work. For the STBY pin, the datasheet says MAX V for play is 1.5V, MIN V for standby is 3.5V. In reality, STBY needs to be pulled up for play & grounded for standby. Trap for young players.

The CS4227 needed a substitute microcontroller to initialize it, as expected. Quite a few more registers had to be set in a certain order to initialize it, which weren't sniffed by the oscilloscope. The YM3436 needed a few pins biased, as expected. Traces had to be cut. The mane challenge was both chips needed reset to be low for a certain time after powering up. The YM3436 wouldn't initialize if reset started out high.

The complete DAC took a lot of power. The +25V rail wouldn't initialize when connected to the 10V regulator. The 10V came from a 5V regulator with a 5V virtual ground, but the +25V rail started below 5V. The regulator seemed to detect the +25V rail starting below its virtual ground of 5V & shut down by some kind of SCR latchup. +25V ended up connected to the raw 12V. The 5V virtual ground initialized properly.

The total quiescent current with the power amplifier & DAC was an insane 0.35A. The 2 5V regulators got quite hot. The STA540 needed a large heatsink. The mane problem when playing sound was the DAC needed its ground connected directly to the ground on the STA540. Connecting it anywhere else on either the signal or power ground caused distortion. Connecting to either signal or power ground on the STA540 worked, so the DAC being a DAC went to the signal ground.

This circuit definitely made less 60Hz hum & noise than the HTR-5230. The Baxandall from Elliot sound projects worked much better than the EQ in the HTR-5230. Despite doubts, the decision was made to max out both treble & bass. Put in a single turn pot for the bass to make it easier, but left treble as a multiturn since it would never change.

The distortion experienced on 8/21 ended up being from the op-amp. The LM324 isn't suitable for any audio applications. Whacked in an LF353 on a carrier board & it worked perfectly. The slew rate & GBW really do matter. Would recommended the best possible op-amps for audio, like the MAX412. For heaven's sake, don't use the LM358 in $parkfun's STA540 kit.

Having gone through all that, it was definitely a waste of time but a rewarding waste of time in reusing old parts for something which would probably last longer than a commercial product. It could have been done with a $25 car amplifier & a $10 DAC with $20 of shipping. The newer components would be more compact but the car amplifier wouldn't last very long.
Last edited by Jack Crossfire; Sep 04, 2016 at 02:00 PM.
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Sep 04, 2016, 03:09 AM
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saucerguy's Avatar
I don't know what you are saying specifically, but can pick up through osmosis with enough time. What I can say is that you are brilliant and a complete cut above what I normally encounter in this world. Keep it up and keep sharing!
Sep 06, 2016, 10:30 AM
Registered User
What am I missing? This is clearly an audio application, not optical. Sounds like you are using linear regulators to get to the CS4227 working voltages. This is highly inefficient and will burn a lot of power (hot regulators). Why are you using such ancient technology (CS4227)? FWIW - I have used LM324s in many audio applications. Why do you say slew rate is an issue and they won't work?

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