Output impedance of Commodore 64 audio output

Schematic: http://personalpages.tds.net/~rcarlsen/cbm/c64/SCHEMATICS/250425/251469-1of2.gif

The transistor 2SC1815 is driving the audio output. What's its output impedance?

Thanks.

Is this a school test? "Insufficient data".

Do you need POWER? Then use a power amp.

Do you want to hook up to it? The 1K emitter resistor suggests it will drive a 10K load well, maybe something down to 2K, depending on output level and signal purity desired.

PRR said:

Is this a school test? "Insufficient data".

Click to expand...

life is a test

Do you need POWER? Then use a power amp.

Click to expand...

give me more power Scotty....

Do you want to hook up to it? The 1K emitter resistor suggests it will drive a 10K load well, maybe something down to 2K, depending on output level and signal purity desired.

Click to expand...

Plug it in, does it work?

Yes, not problem.

No, doesn't work.... problem!  what does it do wrong?

JR

.....
No, doesn't work.... problem!  what does it do wrong?

JR

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I bet, that after all the years the backup-battery will be empty..... 

Sorry for being light on data - I was smack middle of modifications and hacking -  converting C64 to be a dedicated analog synth using http://www.mssiah.com/

The unit is perfectly functioning and in great shape, I'm adding VGA output, potentiometers and proper audio I/O plugs.

Then I started worrying about galvanic isolation. This thing may run in potentially hostile live environment. I know the audio output will happily drive "line" levels. I'm not experienced enough with transistors to calculate the output impedance of that 2SC1815 connection. What I usually do is add opamp buffer to drive 600/600 ohm output transformer - it just works. But what can that 2SC1815 happily drive by itself to possibly save time?

Put on a 600:600 transformer. My gut (and noting the small output level) says it will be fine.

Seems to work fine. I can't detect a bass cut that I would care about, or increase in THD. Maybe I'll measure the low end later just to make sure.

Hey... i was trying to do something similar end of last year.  I finally just grabbed this
https://www.plogue.com/products/chipsounds/
its close enough for what i needed.  They have an emulation of the SID chip from the C64.

I have enough plugins already. This is what I got instead.





Some minor tune ups left. I attempted to steal 5VDC power for S-Video to VGA converter from the Commodore for convenience. It caused terrible ground noise (and other noise) leakage to audio I/O. I attempted to relocate its ground to dump the noise elsewhere - those yellow/green wires. Didn't work. Oh well, I'll just power it externally with its wall-wart. Minor inconvenience.

That is pretty bad ass.  Would love to hear it.

I did an experiment and it paid off. The SID audio chip ground is shared with everything. There's no ground plane - just a complete non-hierarchical mess of two-layer PCB board ground. The audio is as noisy as you can imagine. Video chroma/luma conversion crap leaks into it with a mess of 50hz and its multipliers. Looking at the overall design this was of course not a concern for a cheap consumer computer. Good enough they probably said.

But it can be fixed on the cheap.

1. Disconnect all the SID 5VDC, 12VDC and ground ie. cut all those traces.  Share absolutely nothing with the system.
2. Steal the systems unregulated ~20VDC into two additional regulator stages dedicated just for the SID. First drop it to 12VDC (audio), then further to 5VDC (control). It's only like 100mA total, no heatsinks required. I used LM317 and a jung regulator I had laying around, but any standard regulator works of course.
3. Connect these to the now floating and trace-cut SID. Make sure the only ground that gets to SID is the one coming from the dedicated regulators.

Now it's studio proof and modern without noise.

[edit]

Technically that's not correct. It's without external noise. SID is pretty clumsy so its internal world can still be heard. Its envelopes are not even close to 100% kill and internal control data still is heard. But this is a feature. All that clumsiness and "spice" is why one would ever use SID in 2018.

By the way I removed the 2SC1815 output stage mentioned in the first post. Its only purpose was to act as a buffer for a 15khz 1st order low pass filter. I no longer need it since the dedicated ground and regulators of the SID seem to guarantee there's no RF crap leaking at the outputs.

Ok so the takeaway is cutting the chip off from all power and giving it dedicated 20 and 5v with a proper ground.  What were you using to control it?  Id like to see if this could be done via raspberry pi or something more compact than that dinosaur carcass its attached to.

Oh god, how much time have I spent with these, Bubble Bobble anyone? /memory flash off
Kingston I´d really like to listen to some of the things you are going to do with this dinosaur carcass!

sr1200 said:

Ok so the takeaway is cutting the chip off from all power and giving it dedicated 20 and 5v with a proper ground.  What were you using to control it?  Id like to see if this could be done via raspberry pi or something more compact than that dinosaur carcass its attached to.

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You could, but why redesign the wheel?  There have already been hardware synths using SID chips such as the SIDstation and SammichSID.  I bet if you do some searching there may even be an open source project.

wasnt aware of those!  Thanks gonna look into that.

sr1200 said:

What were you using to control it?  Id like to see if this could be done via raspberry pi or something more compact than that dinosaur carcass its attached to.

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I'm controlling it with the C64 itself of course. Hooked to the rest of the studio through MIDI with http://www.mssiah.com/features.php I always use that mono synth. mssiah can do a whole lot more of course.

Trust me, you want the dinosaur carcass with all its keys and extra buttons - instead of some tiny gadget like sidstation. This chip is basically a minimoog. Imagine playing a minimoog through a goddamn phonepad. Yeah, didn't think so.

It's a live instrument now.

I still stick by my earlier statement... bad ass.

Kingston said:

.....
This chip is basically a minimoog.  Imagine playing a minimoog through a goddamn phonepad.
.....

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Someone obviously doesn't know what a minimoog is.  Can you please advice me where I can find a 24db-VCF in the SID-chip ?

analogguru said:

Someone obviously doesn't know what a minimoog is.  Can you please advice me where I can find a 24db-VCF in the SID-chip ?

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I guess someone woke up on the wrong foot. They are both simple and limited childrens 3-osc + noise monosynths although SID can also be polyphonic and has additional filter modes. One of them has historic weight and myth attached to it that doesn't withstand technical scrutiny.