English Amiga Board - View Single Post - Line level of the Amiga 600 (and possibly other models)

Stedy · 06 December 2021, 22:18

Hi,

This topic intrigued me and I had a bit of a slow day.
The schematics on Amigawiki.org do not match the original printed versions from Commodore, I should know, I have paper copies of them.

For A600 REV C/June Bug schematics from 1991, R331 and R321 are 360R, the amigawikie_R1.5 and R2 schematics show 1.5K, as does the A1200 schematics.
My printed schematics of the A1200 rev 1D show 1.5K for R331/R321.

C331 and C321 change from 100nF on the earlier Junebug schematics to 3.9nF on the REV 1.5 A600 and A1200. This should keep the high frequency roll-off of the first stage circuit the same.

The other principal difference is the output potential divider. On the A500/A600/A1000 it is R334=1K and R335=390R. This means the output is 0.28x the input.

For the Amigawiki schematics for A600/A1200, the output is R334=1K, R335=2K, so the final stage output is 0.66x the input.

I have created a simulation in MPLAB Mindi/Simetrix but I don't know for sure what the output stage of Paula is to model it. R331 and R321 are used on inverting amplifiers with Avref at 2.5V. This means Paula must output a signal centred around 2.5V and transitions at most +/-1V.
To calculate the gain, it is Rf/Rin or R331/Rpaula and Rpaula is unknown. I'll do some emperical analysis (trial and error) and see what I come up with.

06 December 2021, 22:18	#9
Stedy Registered User Join Date: Jan 2008 Location: United Kingdom Age: 46 Posts: 733	Hi, This topic intrigued me and I had a bit of a slow day. The schematics on Amigawiki.org do not match the original printed versions from Commodore, I should know, I have paper copies of them. For A600 REV C/June Bug schematics from 1991, R331 and R321 are 360R, the amigawikie_R1.5 and R2 schematics show 1.5K, as does the A1200 schematics. My printed schematics of the A1200 rev 1D show 1.5K for R331/R321. C331 and C321 change from 100nF on the earlier Junebug schematics to 3.9nF on the REV 1.5 A600 and A1200. This should keep the high frequency roll-off of the first stage circuit the same. The other principal difference is the output potential divider. On the A500/A600/A1000 it is R334=1K and R335=390R. This means the output is 0.28x the input. For the Amigawiki schematics for A600/A1200, the output is R334=1K, R335=2K, so the final stage output is 0.66x the input. I have created a simulation in MPLAB Mindi/Simetrix but I don't know for sure what the output stage of Paula is to model it. R331 and R321 are used on inverting amplifiers with Avref at 2.5V. This means Paula must output a signal centred around 2.5V and transitions at most +/-1V. To calculate the gain, it is Rf/Rin or R331/Rpaula and Rpaula is unknown. I'll do some emperical analysis (trial and error) and see what I come up with.