SKA GB150D now public domain...

[AndrewT]

and which has been released to the public domain?

 

[still4given]

Hi Andrew,

The schematic I show in post #506 is from the thread in Greg's forum where he releasees it.

 

[AndrewT]

That means the public version has the shared Source Resistor in each polarity.

Does that mean the other diagram showing an RC feeding the front end does not exist as any version?

 

[still4given]

Hi Andrew,

Since you have yours apart, can you tell if yours has the RC feeding the front end in yours?
It would be interesting to see if Greg had it that way at one time.

Thanks, Terry

 

[Mike Gergen]

There has never been an RC feeding the front end on the stock GB150. In any version.

 

[borys]

I was symulating some RC filtering before I have made a board but results were bit worse with filtering, the supply rails are reference for driving the mosfets here so with filtering this ''reference'' would be different for part before RC and after RC filter. In spice symulations it was a bit worse anyway.

 

[Bigun]

This topology can not have RC filters in the rails to the front end. Simply, the whole amplifier is the front end. The FETs are the VAS stage. Best thing is to make the power supply better if you're concerned that the amplifier PSRR is too low.

 

[TonyTecson]

Rubbish.
I just went up and dismantled the 150 so that I could access the underside of the PCB.
The V+ trace is directly connected to the nearest Source Pin and via a short trace to the further Source Pin. The V- is similar, just slightly longer traces due to the S Pin being on the other side of the device.

There is clearly no Source Resistor.

As a double check, I measured the resistance from V- to Source Pin and it reads 0.14r just exactly the same as the shorted probes reads.

i suggest that you contact Greg Ball and ask him yourself Greg is the best person to answer your concerns....
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[Bigun]

I'm not an expert on this but I believe the older versions of SKA did not have the source resistors and Greg added them to the design later. One of the reasons I believe he added them was to reduce the turn-on surge at power-up.

 

[TonyTecson]

with gate thresholds of 2 to 4 volts compared to 0.6 volts for a silicon bjt's,
i wonder what a 0.22 ohm source resistor can do....what current is needed?
there is a reason source resistors are not used.....

the experts here can shed more light....

 

[AndrewT]

i suggest that you contact Greg Ball and ask him yourself Greg is the best person to answer your concerns....
SKA sponsored Audio Forum - Index

I don't have a concern.
I see contradictory information being posted.
I see a link that purports to clarify the situation.
The concern should be with those that are trying to clone the "for public" amplifier.

I don't need to clone it.
I have one and as a result I have access to all the IP that the designer wants to release to his customers.

 

[TonyTecson]

so then if you are not building and you have no concern, that's fine, thanks for confirming...

i am building this amp and i have no issues....yet...

 

[Bigun]

with gate thresholds of 2 to 4 volts compared to 0.6 volts for a silicon bjt's,
i wonder what a 0.22 ohm source resistor can do....what current is needed?
there is a reason source resistors are not used.....

the experts here can shed more light....

There is an initial turn-on surge when you power-up. In simulations (using Cordell models) this surge current is quite large. The capacitors on the CCS front end will not control this part of the turn-on because the CCS wakes up too slowly. A few people have posted into the SKA forum that they have used a soft-start module in their power supply to kill this turn-on thump. YMMV

As far as I can see the only reason why there isn't a source resistor on each FET is because the use of source resistors was an after-thought and Greg doesn't have enough space on his pcb to fit 4 power resistors and keep the neat traces. You need neat traces on this amp - it has at least two different frequency ranges it likes to oscillate if you get the pcb 'wrong'.

 

[Ian Finch]

A couple of years ago the GB150 was modified to accommodate new output Mosfets (Greg B. advised the originals were no longer available). Check the original types fitted (as in AndrewT's amplifier) and compare with the currently specified types, if still confused.

 

[TonyTecson]

Ian, did the original GB150 ever used source resistors?

 

[AndrewT]

I checked the supply rails in my 150s.
The Rails do not have any dropping resistors. The transistors are fed direct from the V+ & V-.

 

[TonyTecson]

imho, the 0.15 ohm resistor in series with the rails is too small for normal operation, but big enough to assist in surge protection during startup....

 

[Stuey]

Yes, the 0.15 ohm resistors were added a few years ago and the OE boards were changed. Mine doesn't have them, but I believe Mike Gergen added some to his old-version boards, through some ingenuity with the traces underneath. My understanding is that it helps with the constant bias fluctuations experienced with the old layout, although I can report mine has been completely reliable and has been used daily for some years now.

 

[PauloPT]

imho, the 0.15 ohm resistor in series with the rails is too small for normal operation, but big enough to assist in surge protection during startup....

Is it worthwhile to rise the value of these resistors?

 

[Bigun]

I don't think so. The best way to control the turn-on current surge is a soft-start module at the primary of the power trafo (plenty available on eBay).