Hi, Jamie,

You caused me to go look at the CAT22 schematic again and I noticed some 
interesting things.

(1) You're right that there are odd diode-ninja things in he drive to 
the FETs, but there are also two FETs driven from the same line, 
producing, what would appear at first glance, a steeper attenuation slope.

(2) There is another nasty thing--fairly hard diode clipping (which 
isn't as hard as digital clipping as you know) on the base of the 
compressor's audio output emitter follower. Joy!

I'm glad I've got a bunch of CAT22 cards to carry me into the future.

Fortunately, failure modes appear to be MOSTLY coupling caps.



On 2012-01-11 3:12 PM, Jamie Howarth wrote:
> They also do some weird attack and release ninja with diodes that is really hard to mock up. The while enchilada has strange xfer curves beyond just the compression ratio, bandpasses, and thresholds-  which is why they invert-added through the same screwball circuit - building two of these won't match.
> Knowing what I know now about this self-nulling circuit and tape compression there's no way it can't be heard mistracking. Comparing a dolby a encode-immediate-decode through wire is audible if the dolby dot levels are even slightly mismatched. Off tape it's hilarious. Of course who likes hiss...
> But that's why hot 15IPS or hot 30IPS was the preferred "evil" ... Dolby a changes stuff a lot.

Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
Quality tape transfers -- even from hard-to-play tapes.