Sunday, June 11, 2017

Norm's GAS Son of Ampzilla (SOA)

GAS Son of Ampzilla Amplifier

Today were showcasing the restoration of a customer GAS (Great American Sound) SOA (Son Of Ampzilla) amplifier! The GAS SOA is considered by many to be a top favorite amongst GAS owners. Produced from 1977 to 1982 with or with a PS the SOA is reported at 80Wrms @ .08% (8OHM)!

One of my favorite parts of the GAS SOA is the extremely simplified power supply stage. The 10,000MFD is a -/+45V rectified rail source and a two separate ac/dc bridge stages for the -/+52V driver rails.

The original 10,000MFD capacitors were replaced with a high temp computer grade Chem-Con CDE capacitor (I do NOT recommend increasing the filtering capacity here due to in-rush current limits and stressing the PS/Pformer in this particular design.)  

The original 52V AC/DC diodes were replaced with a Ultra-Fast (UF) type axial Fairchild diodes for cleaner rectification and transients. The 2200MFD capacitors were both replaced with a high grade Fine-Gold Nichicon capacitors with an increase in operating voltages. 


Their are several versions of the GAS SOA amplifier circuit out there. Many of the series feature the molex pin connectors to the output circuit PCB which makes access convenient for once.
The output stage is driven from the -/+52v driver supply and -/+45v filtered rail source via separate AC/DC source. The driver stages features a nice dual differential stage and clipping adjustments. The bias control like most GAS amplifiers are driver via the MPQ6001IC (GAS100A)

One of the major issues I have with GAS is the use of component header mounts used for the differential stages and other places in the GAS Grandson and of course the use of proprietary numbering. GAS would rename BJT devices, a late model cross reference is available but in my opinion this is a poor practice especially with an unverified cross reference which adds significant time when upgrading these and a bit of a PITA. 

Differential Matching

The differential mounts was removed and  replaced with modern low noise TO-92 Fairchild 1% matched differentials (see above) which were soldered directly to the PCB. The driver stages which are known to fail on these units were updated with modern robust On-Semi TO-220 MJE series devices also replacing the original TO-6 device along with new MICA insulators (TO-220) and thermal compound.  The bias and offset potentiometers were replaced with precisions Bourns devices.

The electrolytic were replaced with a audio grade Fine-Gold and low impedance PW Nichicons with an increase in operating voltages. The input/output and bypass films were all replaced with high grade WIMA polypropylene film capacitors.



On the output PCB is housed with GAS100A IC and replaced new Bourns potentiometer, I do like to replace the original ceramics with high grade WIMA polypropylene films as shown. 




The original input wiring was of very low quality and again a push connector, as well as loose and corroded input hardware.  New gold plated Tiffany-Style RCA inputs were added along with heavy shielded co-axial constructed input wiring installed to reduce intrinsic potentials.



(Note the impressive frequency response test out to 70Khz! extremely linear and accurate amplifier) 

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