Question about SID caps

[pay_c]

Hi there!

I´m still over my SID. I´m going to change kinda little stuff until its finished at last (using TK´s PSU recycle & changing to 8580 & better ground shielding). Now I need new caps:

When I built the SID with the 6581 I used normal ceramic caps ("Kerkos" in german). They should have a tolerance either like +/- 10% or 20% (lets assume the worst case 20%).

The SID works with 12dB filtering, right? One cap for HP, one for LP, right? (I think so at least).

Mathematics: About some filter formulas and so on, check your electronic skills!  ;D ;)

SO: If you want a center frequency of 1000 Hz (in every SID & filter) the values with +/-20% can go from 833 Hz up to 1250 Hz (!!!!)!

Is that the reason, why my 6581 filters sounded veeeery different from one to each other? Should be it!

SO: Use low tolerance caps (like Styroflex caps - relative cheap & reliable, +/-2,5% (!)). Did anyone try? I will anyhow, but I just wanted to share this thought (and perhaps bring it in to the partlist of the SID).

Greets & thnx for answers! 8)

[TK.]

Hi Pay_c,

yes, this could be the reason,

From the SID datasheet: "CAP1A/CAP1B/CAP2A/CAP3A - these pins are used to connect the two integrating capacitors required by the programmable Filter. C1 connects between pins 1 and 2, C2 between pins 3 and 4. Both capacitors should be the same value. Normal operation of the filter over the audio range is accomplished with a value of 1000pF for C1 and C2. Polystrane capacitors are preferred. In complex polyphinic systems, where many SID chips must track each other, matched capacitors are recommended. The frequency range of the filter can be tailored to specific applications by the choice of capacitor values. For example, a low-cost game may not require full high-frequency response. In this case, larger values for C1 and C2 could be chosen to provide more control over the bass frequencies of the filter. NOTE: Cut-off frequency variation may occur from chip to chip due to process variations, and power supply voltage. Capacitor values and voltage regulation can compensate for these variations."

Sidenote: the suggested C1/C2 values are the same which are used in the original C64 designs. As the datasheet says, the best value differs with the chip version.

Best Regards, Thorsten.

[pay_c]

OK, I´ll try those 2,5% styroflex ones. I´ll tell you then, how they work.

Besides: Integrated caps are veeeery similar (the same with transistors - very low tolerance). I really do think low tolerance caps should be used there (within the partslist).

Greets

[TK.]

Do you have an explicit suggestion? (Reichelt part number)

Best Regards, Thorsten.

[pay_c]

"Styroflex 6,8n" 2,5% and 30 cent a piece (which is okay, I think). Theyre just a little big (16,5 mm long and 6,9 mm diameter).

[pay_c]

Yepp, they work great! Every single filter line now correct on all SIDs (I checked it even in Wavelab). Really nice now!

btw: Those 8580 can vary from chip to chip veeery far! I got 5 and now three of them are sounding great and one is like... ... just OK (the filtering sounds broken...) and the other one is just broken (not one beep). They were all 8580R5 !!

Suggestion: Putting those Styroflex in the parts list?

So, my SID is finished! Pics will follow in January!!

Only Prb: I'm getting some random sounds (see testing/TS forum). But thats something else... :-/

Happy Xmas!!

[TK.]

thanks - I've changed the orderlist.

Best Regards, Thorsten.