Amp circuit wanted

[guddler]

Does anyone have a suitable circuit for taking as input:

Audio 1

Audio Common (Gnd ?)

Audio 2

And producing a jamma compatible:

Speaker +

Speaker - (Gnd ?)

I used a Bally/Midway "Dual Amp w/Mixer" board for years but gave it back for use in a cab recently.

Failing that, how do other people rig up stuff like Atari boards (Missile Command for example) to their test rigs?

Ta.

 

[andyman]

I've used an lm385 in the past, fairly small output but only needs a couple of external components and is quite happy on 5v, not for arcade related though.

Andy.

 

[guddler]

Have you got a circuit, and would it handle the two audio inputs mixed down to 1?

I'm really curious to know what other people use for Atari boards. You can't all be using Atari power bricks and AR1/2/3's surely? Especially not for the raster based games.

 

[andyman]

Does left/right input down to 8 ohm spkr, gain is adjustable over 100db , component depending, will do something in paint later.

Must have been a 386 even, pic stolen from datasheet

Andy.

andyman2010-09-23 21:06:03

 

[guddler]

That's only a single input. Are you suggesting 2x that circuit? If so, how are you mixing it down to a single output speaker?

 

[andyman]

Sorry memory is a bit foggy, was a long while ago

Just match impedance of 2 signals and connect as one, not ideal but for test rig is ok. You can up the gain between pins 1 & 8 to compensate for losses at the input, if any.

Andy.

 

[guddler]

Ok, well I've knocked something up today...

Remarkably I didn't have anything suitable other than LM380's so I was kind of forced to run with that seeing as a trip to Maplin is a 3hr round trip for me!

This is the result:

It won't win any design awards and it's not the best of amps from an Audio point of view but it'll do perfectly well for testing boards.

Basically, the LM380N is a 2W amplifier. I've built two, one for each audio input. The bulk of the circuit is from here. Initially, I couldn't get much life out of it but after realising that the chips weren't even getting warm, and after doing a bit of digging I realised that the PCB (Missile Command) was kicking out a 5VPP signal and the operating range of the 380N is 0.5VPP (er, oops!?). So, I devised a quick resistor circuit to drop the voltage to around about that and it kicked into life. Good to know that the chips clip rather than blowing up!

So, the purple and red wires are the two inputs, yellow is +12v, black is GND. I then pinched what Atari do on the AR / AR2 boards and took each output to either side of a pot, then the speaker ground comes off of one side of the pot too with the speaker +ve coming off the middle pin. To quote Atari "giving a push-pull arrangement for the output".

Tada, it works. Should work fine for other boards too but might need a little value tweaking on the inputs so I'll leave it in the proto board thing. The pot is hideously out in terms of value. It's 90K and should be 50R but it's all I had that was up to the 2W output. And as we found out earlier, 50R is damn expensive so I'm not wasting one on this.

I tried implementing the adjustment on the input side as per the datasheet but I must have been doing something very wrong as the chips made a hell of a noise and got super, super hot. No matter it barely needs any adjustment anyway at just 2W

Job done I think.

 

[andyman]

Nice

Never seen that 'push/pull' thing before, will remember that one. Mind you, never looked at Atari stuff. Must find my breadboard, its in here somewhere.

Andy.

 

[guddler]

Holy crap!!

 

[Equites]

Man thats paradise!