X- amp, centre-tapped vs floating drive.

Circlotron · 22nd January 2005, 01:29 PM

Kinda belongs in the Pass Labs area I think...

Anyway, I was doing a sim of a pair of choke loaded source followers in an X-amp configuration (dreaming up my next project) and it occurred to me should the drive signal be centre-tapped and therefore each end be totally independent, or should it be floating so that the drive signal to either side is referenced to the opposite side and it's nonlinearities.

On the schematic, if I break the link from R13 & R14 to ground the circuit gives basically the same results (unmeasurable distortion

) but the when driving things a bit hard the source waveforms look different depending on the drive.

Seems to me the two drive setups are fundamentally diferent, so which is best?

Circlotron · 22nd January 2005, 01:32 PM

Red is Q2 source with centre-tapped-to-earth drive,
Blue is Q2 source with floating drive.
Green is voltage across load R2 either situation.

Joe Berry · 22nd January 2005, 02:37 PM

I don't see that R13 and R14 do anything useful here. Seems to me that, if they are low enough in value, they will defeat the cross-coupling by acting as shunts around R11-R4-L2-R3 on one side and R10-R9-L1-R1 on the other. OTOH, if they are very high in value, they won't do anything good or bad that I can see.

There would appear to be a small DC voltage to ground on each side of V3, so presumably it too is a floating element - possibly a transformer secondary or the output of a preceding balanced gain stage? I've been a bit absent-minded lately, so if I've missed anything obvious, please don't take it personally!

Nelson Pass · 23rd January 2005, 01:48 AM

Unless I mis-interpret the question, a "virtual" ground will
work just fine with an X amp as long as you don't connect the
output to this ground.

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22nd January 2005, 02:37 PM	#3
Joe Berry diyAudio Member Join Date: Mar 2001 Location: USA	I don't see that R13 and R14 do anything useful here. Seems to me that, if they are low enough in value, they will defeat the cross-coupling by acting as shunts around R11-R4-L2-R3 on one side and R10-R9-L1-R1 on the other. OTOH, if they are very high in value, they won't do anything good or bad that I can see. There would appear to be a small DC voltage to ground on each side of V3, so presumably it too is a floating element - possibly a transformer secondary or the output of a preceding balanced gain stage? I've been a bit absent-minded lately, so if I've missed anything obvious, please don't take it personally!

23rd January 2005, 01:48 AM	#4
Nelson Pass The one and only Join Date: Mar 2001	Unless I mis-interpret the question, a "virtual" ground will work just fine with an X amp as long as you don't connect the output to this ground.

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