Steve Anderson wrote:Here's a modification of the previous inverter circuit where the load is of a low(ish) impedance. The previous circuit is OK with loads greater than 10k, but there will be a bit of attenuation in the 'inverted' position with loads of around 10k. (Around 30%).
Adding the second transistor provides a lower output impedance restoring the circuit to (almost) unity gain but with inversion. This will drive loads as low as 1k but C2 will need to be increased in value to retain the low frequency -3db point at around 2Hz. With a 1k load a value of 100uF will be needed.
As before Vcc can be anything from 12 to 24V, it doesn't need to be regulated, but it should be clean and ripple free, otherwise some of the noise/ripple/spikes will be added to the signal. At the higher volages of above 18V R5 should be a 1W resistor as it is dissipating just over half a Watt at 24V. TR2 will run warm, but it's within its ratings.
The circuit consumes about 36mA at 24V, half that at 12V. The transistors can almost be any general-purpose NPN device, 2N2222, BC107/8/9. I hope this is useful. I removed the switch just to make the circuit simpler.
Steve A.
Tonight I put this circuit together and gave it a trial run. No luck, but after a bit of checking pinouts on the transistor alternate, I realised I had the transistors in backwards -- fixed that, and voila!! Yes, this circuit works as advertised -- teriffic! I used BC549C as the transistors.
I'd been putting this task off for ages -- and it was necessary to allow me to use my 'you beaut' CD portable originally loaned to me. This one is the one I wanted to use because not only is it shiny (unlike the one I've been using, which is 'beat up'), it also handles MP3. I've very much wanted to do some testing with MP3 source, but haven't been able to because the shiny new CD player also has negative video.
So, the circuit built, now I could test it out.
Frankly, I'm appalled. There are several reasons, but firstly though the shiny new CD player has a "Line Out" I discovered it also has a volume dial associated with it, and there's no headphone out. In other words, it's not really a line out after all. So, I'll be using "headphone out" for my testing with this. Not a problem, the old beat up CD player had a headphone out which I had tested and which seemed to produce good pictures.
So, I switched the circuit over to "negative video", played a few sample clips (WAV) -- most particularly the test pattern #36 on CD1. This looked OK on first glance, but I noticed that whole chunks of the inner circle black surround are simply... missing. The signal coming from the CD is obviously so horrible that it's cutting out large and important bits of information from the picture. I don't think it is the "negative video-er" doing this.
Putting that to one side, now I could at least test MP3 tracks. I really am appalled. It's like going from perfect TV reception to something barely watchable with snow and interference all over it. It's terrible terrible, not just slightly degraded. MP3 video is most definitely out, at least when viewed using this "shiny new" (useless for NBTV) CD player. Ah well -- I wouldn't have been satisfied had I not confirmed this to my own satisfaction.
The conclusions (assuming the "negative video" corrector isnt' the fault)...
* MP3 video is bad, but it's terrible when played via a headphone out.
* there are CD players, and there are CD players. The same thing on similar players can produce a totally different result. On my original player, headphone out is fine. On the "new" one it's very far from fine.