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Help Required - Looking for the real Greg Ball - designer of the Eidetic GB-1a and GB-1b Power & Pre-amps!


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Hi everyone, the title says it really, I have picked up an Eidetic GB-1b power amp that is undergoing some much needed work to bring it back to top working order again and trying to find a circuit diagram or find the designer has not been successful so far. The LC of the amp has some 500mV of DC on the output and will hum after some 10-15 minutes of running, RC also has around 350mV of DC but doesn't hum .. well not after 15 minutes anyway. It came with no user manual and had been 'fiddled' with. LED hanging out the front of it, another power switch put in that is too small for the cutout and is just push to make rather than the left-right on/off switch that ran horizontally. A cable from the RC board that was supposed to connect to the ground bar was just flopping uselessly around the output terminals. 

 I have no idea how the 1-2 ohm and 6-8 ohm plug at the back works other than it pulls out, maybe the 1-2 ohm setting is with the plug pulled, best guess I can make at the moment. 

After all these years and by the looks of it some abuse, I am replacing all the caps with Elna Silmic II's on the channel boards. I don't know about the bank of 16 filter caps yet, they are in unmarked silver cans and I have no idea what value they are until I pull a couple  and check them. 

 This version seems to be an early model with the LF357 op-amps installed, though I know the version 2 model uses the OPA627's instead, if I did that I have no idea without circuit information on any other minor changes around that op-amp may be needed. For the moment I'll stick with the LF357s. 

 ALSO, the outputs for DC and bias adjustments are unidentified for the time being, bias level is a mystery so far. 

As of the moment, all caps on the LC measure fine for capacitance but I am unable to measure leakage for now. A leakage meter would be handy! I haven't got to the RC yet. 

Any help here would be much appreciated!

PS I would really love the Eidetic GB-1a pre-amp to match up! Even one that needs some TLC is ok. I'm in enforced retirement, so big $$ don't apply at this time :) 

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From the depths of my memory, I think he went under the name SKA, and sold DIY kits and stuff. That was a long time ago, though, and he's probably long since moved on. Best I've got, sorry.

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3 hours ago, The Rock Puppy said:

From the depths of my memory, I think he went under the name SKA, and sold DIY kits and stuff. That was a long time ago, though, and he's probably long since moved on. Best I've got, sorry.

It appears he's gone to ground somewhere, but may yet pop up! I did hear that he wasn't well. 

Still, someone here must surely have had some dealings with this amp and pre-amp, they were quite famous in their day, there MUST be a proper circuit diagram somewhere, surely. 

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These may give you some info to help your detective work:

 

https://www.stereo.net.au/forums/topic/5411-introducing-greg-ball-aka-ska/   

+ link to research article " Distorting Power Supplies" published in Wireless World (Dec 1990) he referenced in the above post as being the basis of the

Eidetic GB1 and 2 pre and power amp

https://worldradiohistory.com/UK/Wireless-World/90s/Wireless-World-1990-12.pdf  (page 1084+)

 

 

https://www.diyaudio.com/forums/solid-state/67151-eidetic-amplifierguru.html

 

https://www.diyaudio.com/forums/solid-state/65286-simple-killer-amp-14.html   (link to one of his articles at the end)

 

GB1a preamp currently advertised on Gumtree, images has brochures and review  https://www.gumtree.com.au/s-ad/ashmore/guitars-amps/amplifier-eidetic-gb1a/1217455088

 

ETI advertorial:

https://worldradiohistory.com/hd2/IDX-AUSTRALIA/IDX/Archive-ETI-AU-IDX/IDX/ETI-1987-12-OCR-Page-0046.pdf

 

GB1b brochure: https://www.stereo.net.au/forums/topic/307761-eidetic-gb1b-power-amplifier-vertical-mosfet-made-in-brisbane/

 

 

 

 

 

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9 hours ago, audiofeline said:

These may give you some info to help your detective work:

 

https://www.stereo.net.au/forums/topic/5411-introducing-greg-ball-aka-ska/   

+ link to research article " Distorting Power Supplies" published in Wireless World (Dec 1990) he referenced in the above post as being the basis of the

Eidetic GB1 and 2 pre and power amp

https://worldradiohistory.com/UK/Wireless-World/90s/Wireless-World-1990-12.pdf  (page 1084+)

 

 

https://www.diyaudio.com/forums/solid-state/67151-eidetic-amplifierguru.html

 

https://www.diyaudio.com/forums/solid-state/65286-simple-killer-amp-14.html   (link to one of his articles at the end)

 

GB1a preamp currently advertised on Gumtree, images has brochures and review  https://www.gumtree.com.au/s-ad/ashmore/guitars-amps/amplifier-eidetic-gb1a/1217455088

 

ETI advertorial:

https://worldradiohistory.com/hd2/IDX-AUSTRALIA/IDX/Archive-ETI-AU-IDX/IDX/ETI-1987-12-OCR-Page-0046.pdf

 

GB1b brochure: https://www.stereo.net.au/forums/topic/307761-eidetic-gb1b-power-amplifier-vertical-mosfet-made-in-brisbane/

 

Thank you for all of that, some of it I can understand and other sections nada, not that techy left-brained :)

The Gumtree ad is still up and running, but I cannot afford both amp and pre-amp, even though the amp is version 2 with the upgraded op-amps etc. 

Still no-one seems to have an actual full circuit diagram, so I will just have to persevere on my own for now. 

On removing the heatsinks clips, I can see that all the mosfets are gain matched to virtually identical tolerances, they are still marked on the back with their values. Great stuff. The small value bjt's used for bias stability etc are all common transistors, so easy to replace if necessary. 

One thing's for sure, this is NO ordinary circuit design! 

 

 

 

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Expand the above post for Lightstream's comments and updates on his amp exploration, they are at the bottom of the quote.

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