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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
roberttdid
534 posts
 
Why did you make a post, delete it, then create a reply to it like this?  I don't understand.

Post removed Jun 08, 2020georgehifi7,214 posts06-08-2020 4:58pm
Post removed Jun 09, 2020


That's right.
Clean up aisle 4.

georgehifi's avatar
georgehifi
10,312 posts
 
Mistake, get over it, and now the conspiracy theorist!.
gte357s OP
281 posts
 
Guys, I did more research and realized the Zu Omen Def is actually 6 ohm nominal!  No wonder I hear better sound with the 4 ohm tap!  
georgehifi's avatar
georgehifi
10,312 posts
 
Zu Omen Def is actually 6 ohm nominal! No wonder I hear better sound with the 4 ohm tap!
And if that’s nominal, you can bet you b****s, they dip down even further than that.! And even further again when you bring in the -phase angle into the equation as well if it’s high.
So what does that tell you? get the optional 4ohm tap on the MQ-300 output transformer.

Here again is something JA of Stereophile says about combined low impedance with -phase angle (EPDR)
This is on the Emotiva Airmotiv T2 review just put up to read on the web.   Nominal impedance: 4 ohms by the manufacturer. 
https://www.stereophile.com/content/emotiva-audio-airmotiv-t2-loudspeaker-measurements
John Atkinson : Stereophile
  Fig.1 shows that the T2's impedance magnitude does average close to 4 ohms, though the minimum value is 2.85 ohms at 112Hz, and there is an amplifier-taxing combination of 4 ohms and –53° electrical phase angle at 78Hz.


Cheers George
roberttdid
534 posts
 
@georgehifi , Maybe you could explain to gte357s, exactly how this EPDR is "taxing" a tube amplifier knowing that being frequency based, and real music, the time at any of these EPDR minimums is very short.

@gte357s , changing the tap from 8 to 4 ohms will have minor changes on the frequency response, distortion, and damping primarily of the woofer (which also shows as frequency response). There are likely as many who would like the 8 ohm tap as the 4 ohm tap, but of course best to use the one you like.

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