Audio Research LS-3 power transformer

[Thomas Giz]

Does anybody know what are the spec for power transformer in Audio Research LS-3 ?

 

[ticknpop]

Several of the r- core transformer sellers have something suitable - you can get one with both the HV and heater voltages if you select the right model

100W R-Core Transformer 115V//230V ,Pre amplifier / DAC

 

[Thomas Giz]

Thanks for the link.
This power supply looks like for the tubes but in fact, it powers solid state circuit of LS-3 line stage and here comes my problem. I need to replace broken transformer but don't know the exact spec.
Guessing, one of the shown in the link transformers with secondaries of the 300V and 18V should do. Unless going with more current, maybe 280V or less would be also ok ? What's your recommendation ? Maybe R80-63 ?
Not sure about that 250mA though.

 

[ticknpop]

Yes your choice of transformer is in the right area.

360 VDC = 254 VAC rectified

You could add series resistance to HV secondary to lower the DC voltage if it's too high when loaded. 250 ma shold be more than enough.

The 2 of 9 volt supplies wired in series should be good for 24 VDC when rectified - you may need a bigger heatsink on the 12 volt regulator becuase of the higher raw VDC ( 24 instead of 20 VDC) and you want a cap with at least a 35 VDC rating for R19

 

[Thomas Giz]

Yes, around 260Vac should be enough. However there is something I don't quite understand. Measured secondary of the existing transformer under the load shows 290Vac ! Does it mean transformer is underrated and they pushed secondary Vac to offset for sagging ?

Thanks

 

[ticknpop]

The existing transformer likely is a little high in voltage to allow for low line in countries with low line voltage. The 280 VAC of the replacement transformer sounds fine if your measuring 290 VAC on the original one. You keep worrrying about it or the original being overloaded ? 250 ma will be way more than the circuit draws

 

[Thomas Giz]

Thanks thicknpop.
I went ahead and ordered transformer.
Meantime need to resolve unusually high floor noise on the output. Could be one of the MOSFETs in gain stage. Replaced rectifiers, all electrolytics and regulator MOSFET already. It helped to some extent but hiss is still audible...😡

 

[ticknpop]

Is it the same hiss on both channels,? How efficient are speakers? How much gain on your power amp? Tube or solid state? I would guess the resistors in both amp and preamp are metal film of some sort? Does preamp have hi/lo gain switch? (Try low). With or with out the volume control turned up? If hisses even with volume down it's the circuit design if it's not the parts ( resistors, jfets, ?)

 

[Thomas Giz]

Both channel same level hiss. Volume control brings no effect unless crank up all the way up. It has definitely too much gain. This is my second LS3 unit and as I remember, former one did not output so much noise. Both used in exactly same system. This is the solid state design only and tonight I will try to lower gain at least half. There is a clearly voltage divider at the feedback loop.

LS-3B is pretty much considered quite good. Old design but still musically very reliable. My unit has additional board for balanced output.