Modifying Audio In / Out of MidiBox Sid

[zenguru]

Hello everyone! Here is some questions from a newbie MidiBoxer...

I have understood that the Audio In/Out circuitry is copied from the original design of the C64. Am I right?

The Audio out uses a "general purpose" transistor BC547, which is used in a common collector (emitter follower) circuit to prevent damages caused by shorted pins. Am I right?

If the transistor is replaced with a low noise audio transistor or a low noise opamp buffer, the signal will be clearer. Am I still right?  ;)

Also the Audio In could have an opamp buffer...

Does this make any sense? Has anyone tried different component setups? Or is the amount of noise coming from the transistor so small compared to the overall noise that it can be neglected?

Thanks!

-zenguru-

[moebius]

Don't know... Can't say for sure... Probably ;)

Although, now looking at schems. (not the sid specs.) it looks like SID output probably is "current out", so if you want to replace the output electronics... make sure that (hi-fi grade) opamp sees some voltage at the input.

It makes sense, alright. Just can't say how much a single transistor in the beginning of the signal path does for the overall signal quality  ::)

[Artesia]

The response characteristics of one simple circuit can seginficantly effect the sound quality. It doesnt have to be a complex one to comprimise the signal. The performance of the overall system is only as strong as its weakest link. To make an anaology to (british) public transport. It only takes one train thats consistantly 'unreliable' - to hold up an entire trainroute.

Though the one thing to consider, is that due to the age of the sid - that its capable sound quality may not exceed that of the output transistor section. Also this may well have been done to ensure a 'authentic' reproduction of the sid sound.

Old synthsisers like the moogs are defined in their character from the next synth by their conveiniently nice sounding 'imperfections' in signal reproduction. Which could be compared to the fact that with guitars its not just the string you hear - but also all the incidental resonances of the guitar body.

Having said that, in order to obtain a different; maybe cleaner sound - the best possible solution would probably be to use a half dencent op-amp circuit instead. Maybe a TL074 ? :)

[zenguru]

About those "imperfections"... I believe it's the harmonics that make the moog sound so good. Although the harmonics are sometimes considered "imperfect", especially when a pure sine wave is wanted,  they still give the guitar, piano, synth etc it's characteristic voice.

When using transistors or  vacuum tubes (or any type of components, really :) , there's always some  harmonics, caused by non-linearity, and noise present. Depending on the component type, the harmonics can be "musical" or "non-musical". That's ok.

What isn't so ok, is the noise. Noise is always considered a "bad thing" and an imperfection. (Well almost, some analog synths use noise sources to make different types of sounds, for example) Sources for noise are resistors and semiconductors. Noise affects the signal-to-noise-ratio, SNR, which is often expressed as desibels, dB. SNR is used in pro and often consumer audio electronics to describe the amount of the wanted audio signal compared to the not-wanted noise.

I think that the BC547 in the output circuit could be replaced with BC549 or BC109. They both are "low noise general purpose" transistors. I will check it later.

I don't fully understand the functionality of the original output circuit, but I will work on that... Is it just for preventing overloads (=short circuit), in normal operation the transistor doesn't conduct...? Hope someone knows better!

Oh, and the input circuit looks strange too... there's basically two capacitors in series...!?! Why? The other is a coupling cap, that's for sure, for blocking DC-current. But the other one...  ???

Well, as I said, I will work on that. If there's any interest, I'll be happy to share my information to others.

[d2k]

welcome zenguru :)

sounds good to me - i thought about a simple JFET gain stage a while back but havn't done anything since...

I don't fully understand the functionality of the original output circuit, but I will work on that... Is it just for preventing overloads (=short circuit), in normal operation the transistor doesn't conduct...? Hope someone knows better!

I think your right there ye - the transistor there just to prevent dc from leaking in...

For the input side i dunno mebbe just a simple opamp buffer?

Cheers

[moxi]

Noise affects the signal-to-noise-ratio, SNR, which is often expressed as desibels, dB. SNR is used in pro and often consumer audio electronics to describe the amount of the wanted audio signal compared to the not-wanted noise.

Hi ,i think ther is no realy improvement in using a better transistor on the output ,and you won't get a better SNR than the one of the SID pic himself ,so the noise added by this transistor is really unimportant.

maybe it's better to spend time on a well filtered power supply ,the improvment will be more significant! -or add a tube stage on the output ,to add nice noise!

;)

[zenguru]

Hi ,i think ther is no realy improvement in using a better transistor on the output ,and you won't get a better SNR than the one of the SID pic himself ,so the noise added by this transistor is really unimportant.

I think you're right, moxi...  in the case that the transistor really isn't conducting. And that really seems to be so.

Maybe I'll just put that bc547 there and consentrate on the psu, like moxi suggested.  ;) Of course, a controllable input/output would be nice...  ::)

By the way, what's the max amplitude of the output signal?

Ok, thank's everyone!