2. amps-preamps
  3. 157647-13968

Preamp and impedance question

I am asking a custom builder to build an amp using Luxman mq-300 schematic.  I have two questions:

1) the Mq-300 is a power amp.  I don’t have a preamp.  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?

2) 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?

thanks.
gte357s

Showing 2 responses by erik_squires

erik_squires
16,806 posts
 
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
erik_squires
16,806 posts
 
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.

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