Preamp and impedance question

Can I just add a volume pot to a power amp schematic? I did some research about passive preamp, but it seems even a passive preamp has a circuit and a transformer. Is it better to buy a separate passive preamp?

Depends on what’s driving it. Passive preamps are not all transformer based, some are resistive.

If you end up with a suitably high resistance for the source, sure.

I warn you that the magic in those old amps is in the quality of the transformer. Rather than attempt a clone, you might be better off with some other kit with suitably matched output transformers.

gte357s
Can I just add a volume pot to a power amp schematic?

Of course you can, the Lux is 240kohm!! input impedance with 0.49v input sensitivity!!! so absolutely no need for any active preamp with even more gain here.
Just get your tech to use a 20kohm stereo Alps (Blue Velvet) volume pot on the input of the Lux.

The transformer is designed for 8 ohm.  If I want it to support 4 ohm, I need to upgrade the transformer.  I know that the speaker impedance curve can dip to 5 ohm at 30Hz.  Is it important to have 4 ohm support?

A 4ohm tap would be better, you'll halve your wattage though.
Depends on what -phase angle is associated with that 5ohms, what speakers are they? Hopefully there's a phase graph done on them.


Cheers George

@erik_squires Currently, he will use Tango XE-20s transformer.  I hope it is a good quality transformer.  But if I want to have a 4 ohm speaker output, I need to add $500 to upgrade to Tango FC-30-3.5s.  So, for the additional $509, I get a 4ohm output and likely better sound, but don’t know how significant it is.  The ultimate question is, should I do this upgrade .... I think I don’t need 4 ohm.  My current speaker has a power subwoofer.  So, it seems the dip in impedance at low frequency is not a concern for now.  but I may change my speaker, or I may sell the amp.  To ask the question a different way, when you buy an amp, is it important to you for the amp to output 4 ohm?

@georgehifi 
thats good news that I can just add the volume pots. =)

my current speaker is a Spatial Audio X5.  It has a power subwoofer.  When I say my speaker dips to 5 ohm at 30Hz, it is for my old Zu Omen Def.  And that is based on my memory, so, number may not be exact.  I just use that as an example, easier to give some numbers.

and I remember when I use my Zu, I feel the 4 ohm tap gives more bass.  But it can also be a psychological impression, nothing scientifically measured.

Spatial Audio X5

Na, nothing measured on these, but just by the looks of of them, I'd say a 4ohm tranie tap will be fine for these, btw the real Lux has 16-8-4ohm taps on it's transformer.
Cheers George