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Old 23rd June 2008, 06:57 PM   #1
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Default Quad 405 - opinions?

Hi!

My old Marantz amp (PM750) is slowly dying, so i need to change it. I could repair it, but it's quite complicated and it uses very strange audio cables (anyone know what they are? they go from inputs to preamp).

A friend kindly offered me his Quad405-2 layout with some mods. I must say i never heard real 405, except brother's which was built with very bad material and mods are probably not well done.

Can someone please comment the sound of 405-2? Is it high quality amp and can be compared to "better" amps?
What i need is nice neutral amp that can drive my passive KRK ST8 (120Wmax) studio monitors. At some occasions i will use tube preamp with it, to smooth the sound a bit (this monitors sound a bit harsh).
In short words; do you suggest making it for my needs if i get lots of material for free and some help from very experienced friend (he built at least 20 Quads)?
Btw, is it possible to replace input op amp with discrete version? How much gain does it need? I have some very nice sounding DOAs.

Miha
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Old 23rd June 2008, 10:21 PM   #2
alex mm is offline alex mm  Romania
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Default QUAD 405-2 reborn

...Hi, Miha
My first Quad 405 was built in 1981 , I remember , voltage rail only 24 V with 2N3055 transistors . Not long time ago I start to make a new improoved (reborn ) Quad 405 . Here you can find some pictures http://i27.tinypic.com/71h6oh.jpg voltage rail +/-60V and another variant http://i27.tinypic.com/11m40pg.jpg and this one with lateral mosfet http://i26.tinypic.com/1smhrk.jpg finally , the case http://i28.tinypic.com/wr19nt.jpg . . The sound is clear , warm and ...... IC cannot be substitute by discrete componnents , u can use LME 49710 . Regards alex mm .

Sorry for my poor english
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Old 23rd June 2008, 10:44 PM   #3
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Hi Alex!
Nice amp! Congratulations. I think i'm going to build one myself(now waiting for the parts. ).
How warm are you driving it? I mean your heatsinks are not very big how do they handle the heat?
By the way, where did you buy the LM and the power transistors?
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Old 24th June 2008, 02:06 PM   #4
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Hi,
I'm quite sure it's possible to change op amp with non-inverting discrete version. Anyone have experiences with this kind of mod?
I have some experiences with Leach amp, can someone please comment how it sounds compared to Quad? Are they close in sound?
I must say i don't have good experiences with brother's Quad405II. Highs are a bit muddy and "sweet". Lows are not good either, they are somehow exagerated, boomy at around 50 to 80Hz. But amp is built with bad material,so i guess this is the reason for problems i mentioned.
Any more thoughts about sound of this amp when is well built?

Miha
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Old 24th June 2008, 02:47 PM   #5
Mooly is online now Mooly  United Kingdom
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Hi,
I've never been too thrilled with the sound of the Quad 405. Sonically I think (I know !) you can do much better. The whole reason for the current dumping philosophy was to enable a "slow" pair of outputs to be used with no adjustment for quiescent current. From a technical view it was a truly radical new way of thinking. If you want to build one you must stick to the original concept and design. The component values are crucial for it to perform as it does.
Regards Karl
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Old 24th June 2008, 03:23 PM   #6
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by rotation
Hi,
I'm quite sure it's possible to change op amp with non-inverting discrete version. Anyone have experiences with this kind of mod?
I have some experiences with Leach amp, can someone please comment how it sounds compared to Quad? Are they close in sound?
I must say i don't have good experiences with brother's Quad405II. Highs are a bit muddy and "sweet". Lows are not good either, they are somehow exagerated, boomy at around 50 to 80Hz. But amp is built with bad material,so i guess this is the reason for problems i mentioned.
Any more thoughts about sound of this amp when is well built?

Miha


It's a little harsh to say the the Quad 405 were built with 'bad material'. It was pretty much state of the art when it was released.

My first was a low serial number, about 1066 I think, and like a lot of Quad stuff, it did sound warm and 'flat', the Quad sound.
My later 2 both sounded a lot more lively. All benefitted from change of I/O connectors.


There are pages and pages of mods for these amps including upgrading the op-amp.

good luck
Andy
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Old 24th June 2008, 03:33 PM   #7
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Hi,
it was not original Quad, sorry, i should say it. A friend built it for my brother with what he had at hand.
Can you compare it to modern amps?

Miha
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Old 24th June 2008, 05:17 PM   #8
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Based on the schematics I've seen the whole discrete opamp version has been done, Quad called them the 606, 707 and 909.

There is still an op-amp in the circuit, but it's role is that of a normal DC servo.

Stuart
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Old 24th June 2008, 07:33 PM   #9
Mooly is online now Mooly  United Kingdom
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Hello Alex mm,
Just been having a closer look at your handiwork -- very nice job indeed. The "one with a lateral" got my attention (Think you know why ) Did you use the lateral for the Class A output section ? What did you use for the dumpers ? and how true to the original design is it.
Regards Karl
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Old 25th June 2008, 11:14 AM   #10
alex mm is offline alex mm  Romania
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Default Lateral mosfet Quad .....

Hi , Mooly
Sorry, for delay , but I'm bussy at my job . So, let's see first schematic http://i32.tinypic.com/f0opom.jpg and then one of the PCB 's http://i32.tinypic.com/24gnnud.jpg , the rest should be clear .Regards alex mm .
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