Preamp for QUAD 405

[Merijn]

Hi,

Since half a year or so, I own a vintage QUAD 405-1. I like the amp a lot, but my current integrated amp's line out is not satisfying at all (NAD C320). By searching the archives here at diyaudio I came across a posting by AndrewJ, who said that this Quad could easely be drivven by a passive lineamp. Since I just like to build an active amp, what would be a candidate? I personally like the Aleph L by Nelson Pass, but I am curious for other designs. Maybe SoZ or something like that...

thanks,

Merijn

 

[Brett]

The Quad 405 has an input sensitivity of 0.5V for full output, so if CD is your only source, then a passive pre will do. Phono will need a gain/RIAA stage. There are a couple of recent threads on this board, and several over at audioasylum on DIY passive pre's that you can make for little cost. I think all of the Pass designs have around 20dB or so of gain which you don't need, and will only have to attenuate away, but would be very good proven and effective designs for yo to try cheaply, with more performance than you could get for similar money almost anywhere else. A standard 2V CD o/p will have 12dB too much level for the 405 to start with. The 405 is not a very quiet amp and adding an active stage will reduce the SNR further. I own a 405 myself, though it's not in use.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=2608

Theres also a great article by Bernd Ludwig about improving the Quad 405 series, including reducing it's gain to some benefit, at
http://www.audiocircuit.com/9121-so...ommercial/Quad-QUA/9121CMQUA-MOD-405-BL.htm#A

 

[Nelson Pass]

I personally like the BSOZ best, and it does not have all
that much gain. Be sure to use both input and output
pots, the inputs to set the gain range, and the outputs
as volume controls.

 

[Asen]

Quad 405

I've tried the 405 with both passive and active preamps. The best results were obtained with a SRPP tube pre. After that I built the JLH SE 10W amp and I'd never used the 405 since. The 405 is a very good amp, indeed, and I have friends that still enjoy its stability and control. And definitely it performes well in some systems. My own opinion is that it sounds a bit compressed compared to more simple circuits.

Regards

Asen

PS - Bear in mind that not all the mods described by Bernd Ludwig work well. Especially the mod with the 36V zeners. In spite of all that the article is great.

 

[Merijn]

Thanks a lot for all the info! I think I am now allmost in the wrong thread... Asen, can you post me a link to this SRPP preamp. Is it by coincidence made with the 6072 tube?

kind regards,

Merijn

 

[Asen]

SRPP

The tube I use is E88CC

Here is a link:
http://www.diyaudio.com/forums/showthread.php?threadid=250

Asen

 

[Asen]

Quad 405

One more thing! The 405's output is out of phase, so it goes well with the SRPP, because both working together will give you an in-phase output.

Asen

 

[LaZarus]

Merijn,

Did you built that tube amp already? I own a quad 405-1 too and I am looking for a preamp design to built. I am planning to make the rod elliot project 88 preamp.
I already lowered the input sensitivity and replaced the opamp with an opa134, with good results.