Wayne's BA 2018 linestage

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#741
The WBA18 works at +/-30v in the Yarra, although some of TO92 pairs are running pretty toasty hot at 70C. Bias current is steady and identical in both channels at about 14mA. DC offset drifts about +/10mV depending on ambient temp and air flow across the module. Nonetheless, the preamp is now verified to play music. I am using it to drive a 0dB power amp (MOAMOFO) so changed the feedback shunt resistor from 10k to 1k for 29dB gain. It sounds very nice. Boards are ready to go to production for the GB.

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#745
Those measurements look great X.

And overall gain can be a good compromise as well with the M2X. Closed loop gain of the M2X is 14-15 dB. Closed loop gain of Wayne’s pre is about 12dB (default build). So total of 26-27dB is perfect for many systems especially with passive pre, digital setups.

Whoever can test it with their M2X, it will be much appreciated.

Anyone know where the BOM for the WBA18 is for a default 12dB gain? Whoops! Found it in the WBA18 thread!

Thanks,
Anand.
 
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#746
Howdy, here's a quick reminder from the First Watt table of their products, that M2 and M2x have no feedback. So "closed loop" gain isn't quite right, there's no loop to close.

M2's input to output gain is (IPSgain) x (autotransformer gain) x (OPSgain)

which, numerically, equals (0.99) x (6.0) x (0.95) = 5.64X = +15.0 dB

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#747
The WBA18 looks like a good preamp stage for the Yara. I have ordered a pair and plan to try them in addition to the original Melbourne. However, since there is no provision for an output coupling cap on the WBA18, I am unsure if it is a good idea to use this with the M2x. Perhaps I am being overly cautious about magnetizing the Edcor transformer, but if it takes a several minutes for the circuit to reach thermal equilibrium and low offset voltage, then that might be a problem.
 
#748
You could cut the trace on the M2X between the output bolt and the Edcor, scrape off the solder mask, and just solder a 220uF (audio grade bipolar) cap there. You can always remove and jumper it for use with other input boards. Or bypass the Edcor with a jumper and boost the overall gain of WBA18 to 22dB to 29dB depending on how you like your amp gains to be. It will be a totally different sound and lost the DNA of the M2 (which is basically H2 and H3 about same level and THD in 0.05% range). It will still be a zero feedback power stage though and that sounds good in general.
 
#751
Post 744 is just the WBA2018 preamp. It is 2nd order dominant.

Regarding M2 distortion: the actual measurements that I have made plus several others that I have seen now show a significant amount of 2nd order and 3rd order. Not purely 3rd order - despite the the differential O-scope image from the manual.

For example, member Elwood625’s recent measurement with Ishikawa input stage with M2X driving 1w into 4ohm load:
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#752
You could try different input stage boards to find out how large or small their effect is, upon the graphs which interest you. One possibly revealing input stage board to try is: none at all, i.e., completely remove the board and directly connect a bare copper wire from the input bolt to the output bolt. Four wraps around the input bolt, three or four figure eights between the two bolts, and four more wraps around the output bolt, ought to do it.
 
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#755
The Edcor’s impedance is 600ohms and the offset on the WBA18 can be as high as 200mV to 300mV when first turned on an warming up. 300mV/600ohm = 0.5mA. Is a 0.5mA current enough to magnetize the Edcor’s core? Safest thing to do is figure out how you can sneak a coupling cap in there. Either cut the trace on the main M2X PCB and add a cap. Or cut the trace on WBA18 beyween ouriur and boot hole and add a cap. I think you have more room on the M2X mainboard.
 
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