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noizyboy 27th February 2012 11:15 AM

Krell KSA 50 Clone PSU virtual ground help
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Hi, I am building a Krell KSA 50 clone amplifier, and I am looking for information on how to design or construct the 55V low current, virtual ground circuit to drive the firstwo stages of the amp. So far on trawling the internet for help, I have only come across low voltage models using op-amps. Any help would be welcome. Attached is the circuit diagram.

analog_sa 27th February 2012 11:55 AM

Not sure why you call this a "virtual ground circuit". Presumably, you just need a regulator for +-55v @100-150mA, right?

noizyboy 27th February 2012 01:51 PM


Originally Posted by analog_sa (
Not sure why you call this a "virtual ground circuit". Presumably, you just need a regulator for +-55v @100-150mA, right?

Thank you for replying.

Well that's what I thought at first, and yes that would work the amp just fine, but wrom what I have deciphered, is the isolation of the small signal ground from the current dispersing mains ground which the speaker is connected. Surely it would keep the music signal clean, and once the speaker has dumped the current to common ground, the music signal is not effected.

analog_sa 27th February 2012 02:52 PM

Separating input and output grounds is a good idea but not in the way you are seeing it. Electrically they have to be at equal potential i.e. tied up together through a very small resistor. Even if you use a "virtual ground" for the input stage it needs to be connected to the speaker ground. Experimenting with this may result blown up speakers. Maybe you want a bridged or balanced amp.

noizyboy 27th February 2012 04:02 PM

OK, I reallise I see things in a somewhat different way, and not always the right way, this is why I ask for help, and value yours. I suppose a balanced amp would be preferable as I do understand I can always single end the inputs with a pin cold to ground link.

You make an interesting point with regards to the virtual ground needing to be connected to the speaker ground. This is a statement that makes me think "why have a virtual ground in a Krell amp"? or why does what I read make me think about things in a wrong way?

I want to understand why this and that particularly the reasons for the VGND.

I have recently recieved the circuits in the post, so the project is a new one.

I am not an electronics engineer, so please accept my appologies for my missunderstandings (plural)

regards, Ian

alexcp 27th February 2012 04:30 PM

Both the input and the speaker must share signal ground as the reference point - otherwise, feedback would work incorrectly and indeed, might apply the full rail voltage across your speaker. A 10 ohm or so resistor is often inserted between the small signal ground and the speaker ground to alleviate problems related to grounding, but it is not always needed nor does it solve all grounding issues. Since you are into Class A amplifiers, check - the article has an example grounding scheme (Fig.6 - Overall wiring diagram) that should clarify the issue.

noizyboy 27th February 2012 06:45 PM

Thank you for your your elaboration on the subject. I will certainly have a good read before attempting powering the circuits up, and report my findings to this thread, for confirmation to my understanding, or missunderstanding.

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