Preamp and impedance question

@georgehifi yes, can’t find the curve for the X5.  Actually, the current price without upgrade only have 8ohm tap.  In your opinion, is 8 ohm ok, or is it worthwhile to add $500 to upgrade the transformer, which will then have a 4 ohm tap and likely will improve the sound?  

While reviewers tend to prefer 4 Ohm taps, the reason for these taps is to ensure equal power output. My mind is hazy, but I think the main reason for this is that without the tap your amp’s power output drops at lower impedances.  @atmasphere is the real tube/transformer expert here.

Best,
E

You can put a volume pot on the amplifier, the only reason most amps do not have one is it limits their market to customers with only one source. What you have to take into consideration is what input impedance your source wants to see. Anything above 10k ohms should work fine. I would go with a shunt volume control as it only puts one (series) resistor in the signal path and I would use a series resistor of 15K ohms, high enough for a respectable input impedance through the volume steps and low enough to keep out noise. You could also include a balance control if you want, just place it upstream of the volume pot.

If your amp has a 4-ohm tap, you can use that with no problem. The power output does not change, but the current draw will be higher as the output xfmr tries to maintain voltage at the lower reflected impedance (power is consumed at the speaker, NOT the amplifier). Any limitations on the amplifier will show up here in the form of voltage drop and distortion as the tubes are asked to work harder. However, it should not affect anything if you listen at reasonable volumes.

@gs5556 
So, my understanding is that, as long as I listen in low to medium volume, paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

Yes it will the bass will be tighter/controlled with the 4ohm than the 8ohm tap, if the speakers are 4-5ohms
Go the $500 extra you can't go backwards once it’s done, and the amp will have far better resale value down the road.
Cheers George