Saturday, December 19, 2015

Great American Sound (GAS) Grandson Amplifier

Great American Sound (GAS) Grandson Amplifier 

Got in something relatively new today to me; a GAS Grandson amplifier! This happens to be the meter-less variant of the series, which I happen to prefer. Upon arrival the customer noted one channel was out which was later determined and outlined below.   The GAS Grandson has some impressive specs, rated at a stabile 2 ohm load even at a full sweep! The only real issue is obtaining information on this unit, it took quite some time to work everything out, even with a schematic simply due to a lack of technical data available. 

Driver Assembly
A major theory of operation I want to highlight is the completely symmetric push-pull DC coupled driver stage, as I mentioned before this is a very clean, simple design which I think goes hand in hand with the idea of true reproduction, or fidelity, pure, simple and clean with no BS attenuations.



Servo Modificiation
This is a modification that can be performed in regards to bias-servo stage. It used a proprietary GAS100A IC which is really a re-branded MPQ6001 IC I discovered after some research. the MPQ6001 is basically two BJTs in a single package, this can be replaced with two darlington MPSA type BJT’s and removing the IC socket.  If possible I always prefer not to use sockets, i prefer a real metallurgical connection vs a physical connection in most cases. When we talk about op-amp rolling and so fourth then a DIP socket it great, but oddly enough GAS used a IC socket for the TO-92 differential pairs, thats  BS cut corner design, especially for differentials! So the differential IC in this case was removed entirely…pft…

All the elektrolytisch were replaced with Nichicon KA audio grade and Nichicon PW low impedance type for higher voltages. The differential pairs were updated to matched Hfe 1845/992 BJT’s vs the original MPS8099/8599 TO-92s. The 2nd stage BJT’s were updated to MJE15XXX series, I want to add here, the initial failure occurred from a damaged MPSU57 PNP BJT, two legs had broken off the body and were resting in place. Fairly typical as some of the MPS type had thin fragile legs.  The 3rd stage BJT’s were updated to more robust TIP41/42 devices and the heatsink trimmed and re-attached. General diodes were renewed with 1N4148 type and the large 30 ohm 2W and 2.7 1W updated with heavy film/MOX type. Bias re-adjusted for about 50mA which is (.44).50 + for 22mV (id write this down, imagine the fun I had determining this) 

Bias Notizen
beim emitterwiderstand R114/R115 fur 25mV

One additional point to make while your updating the IC is to swap out the old bias open-frame type potentiometer, the open-frame especially gunk up and lose tracking easily, we used a bourns precision  1K pot.

Another option to purse is to upgrade the dual Bi-polar FET op-amp IC301, your unit may contain a 1458 or 1558 OP originally I think designed by TI for a general purpose application.  The AD and BurrBrowns variant are a low noise/distortion dual FET application vs a general BP type. In this case we went with a high end (BurrBrown)TI2604 OPAMP along with a new gold 8pin DIP socket for future rolling if desired. 

The filter caps were increased to 12,000 microfarads vs the original 10K, I happen to snag a last few of the Panasonic THA’s for this :)

Man are these amplifiers cool, and luckily, I have two, one for me! Ill be playing around with some OPAMP rolling and trying out some input/output styles so keep an eye out for updates on this one. 

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