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Old 23rd January 2011, 10:34 AM   #11
diyAudio Member
Join Date: May 2004
Location: Hertfordshire
The topic of the 405 comes around from time to time. Are you aware of Bernd Ludwigs mods here?
The later Quad current dumpers seem to have quite different circuitry to the 405 but whether the topology is the same or not as the 405 I am not sure.
As to why they have not taken over the world I think it would be patent issues initially. And now people have forgotten the technique plus a marginal cost increase from a bog standard design.
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Old 23rd January 2011, 11:12 AM   #12
diyAudio Member
Join Date: Feb 2009
Location: UK
Hi Firestorm

To be honest, I haven't tried my software with anything other than an op amp-based headphone amplifier which gave very low THD. However, I did try the sound card with a direct loop back and found that its own distortion and noise are not quite as negligible as I thought.

My next steps are to make the input attenuator variable (it's silly that I'm not driving the input to full scale all the time) but with sufficient impedance to prevent blowing the input up accidentally, and to characterise the system's inherent noise and distortion to compensate for them. I figure that I will have to normalise the harmonics in the FFT to the phase of the fundamental so that I can then subtract the sound card's THD in the complex domain. (Conceivably, distortion in the sound card could actually be reducing the measured THD of the amp, by compensating for a 'wiggle' caused by the amp in the opposite direction. Unlikely, but possible.)

I will assume that noise in each frequency bin sums with the power of the magnitudes, and that if I average enough FFTs together over time I can simply subtract the power of the sound card's noise. Or something like that! I also think I may be contributing noise/distortion to my generated sine wave with rounding errors, so I will look at tweaking that.

One thing I have realised about such a sensitive system is its usefulness in neutralising imaginary worries such as whether the two amplifier channels are matched or one is damaged, or whether a component change or a change in grounding has been beneficial or detrimental. For example, I am planning to fit a triac-based crowbar circuit across each amplifier output, and without this software I might have imagined that it was, in some way, contributing distortion or shaving a bit off the top end. I will be able to check in real time whether there is any effect whatsoever if I touch the wire on and off.

As regards the sound of the unmodified 405 clone to my ears it sounds excellent. There's only a slight 'thump' at start up, and it seems well-behaved at power down. (I found that changing the op amp to a TL071 or OPA134 without any other changes led to a slight squeal when powering down with the source still playing when I tried it, but that was before I corrected the Zobel error on the PCB.)

My one concern is the notorious SOA limiting of the Mk I 405 and whether it is cutting in at high listening levels on my 'good' speakers, which are nominally 6 ohm impedance but which may drop under certain circumstances (who knows?). I would like to either remove those components (dangerous?) or replace them with the Mk II limiters (a bodge). Obviously I'm not too worried about blowing up the amps themselves (25 a pair inc. P&P) but I am worried about my speakers. Hence the desire to fit crowbar circuits like Quad fitted themselves. I can't think how to guard against HF oscillation blowing up the tweeters except with an active circuit which shorts across the output with a relay when it detects it. I would like to avoid relay contacts in series with the signal path if possible.

Or do I go the whole hog and design fresh PCBs to the Mk II circuit, including the crowbars? I am convinced that the Quad current dumping design is the way of the future and I can't imagine bothering with those other primitive topologies from now on!
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Old 23rd January 2011, 11:20 AM   #13
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Join Date: Feb 2009
Location: UK

Thanks, yes, I've been looking at some of the many 405 mods around the interweb. Some of them seem quite significant like changing the op amp stage from inverting to non-inverting which reputedly improves noise and distortion quite a lot. Whether or not I like the idea of changing Quad's basic design I'm not sure - even if Quad did drop quite a howler (literally) with their Zobel grounding error in the original design. Quite how they missed that one is beyond me.
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Old 23rd January 2011, 11:26 AM   #14
frank1 is offline frank1  United Kingdom
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Join Date: Apr 2006
Location: Devon UK
Default Lazy?

Originally Posted by consort_ee_um View Post
The topic of the 405 comes around from time to time. Are you aware of Bernd Ludwigs mods here?
The later Quad current dumpers seem to have quite different circuitry to the 405 but whether the topology is the same or not as the 405 I am not sure.
As to why they have not taken over the world I think it would be patent issues initially. And now people have forgotten the technique plus a marginal cost increase from a bog standard design.
Its a real shame that they didn't clone a Mark II or even look at all the simple mods that have been applied to these and make the PCB to suit.
Then I "might" have been interested.
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Old 14th February 2011, 10:22 PM   #15
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Join Date: Feb 2009
Location: UK
I got my software and M-Audio Audiophile card working in 24 bit. I enclose the result of a loopback test (phono cable between output and input - ignore the displayed power value). I found that I got a significantly lower distortion value by running the system at 50% full scale amplitude, which tallies with something I read somewhere.

So the intrinsic THD at 1kHz is 0.0005% and at 7KHz 0.0011% (first three harmonics only I'm afraid).

I then re-measured the 405 clone's distortion at 1kHz & 7kHz, at 5W & 50W into a 10R dummy load without attempting to null out any of the intrinsic distortion.

1 kHz:
5W 0.0048%
50W 0.0026%

7 kHz:
5W 0.0163%
50W 0.0101%

Is this in any way similar to a 'real' 405?
Attached Images
File Type: jpg screengrab_loopback_1k.jpg (136.5 KB, 839 views)
File Type: jpg screengrab_loopback_7k.jpg (138.7 KB, 813 views)
File Type: jpg screengrab_405_1k_5W.jpg (129.1 KB, 799 views)
File Type: jpg screengrab_405_1k_50W_.jpg (142.9 KB, 789 views)
File Type: jpg screengrab_405_7k_5W.jpg (116.5 KB, 771 views)
File Type: jpg screengrab_405_7k_50W.jpg (141.7 KB, 66 views)
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Old 14th March 2011, 09:41 AM   #16
diyAudio Member
Join Date: Feb 2011
Hi. Does anyone know how to connect the fifth luger-style connector which is labelled "gnd". Looking at the board i see the two phono input terminals then speaker out then the two 54 volt outputs then there is a gnd connector which my original quad boards do not have. I connected this back to the smoothing capacitors of the power supply via the green wire but just get massive distortion.

Many thanks in advance. John.
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Old 19th August 2011, 11:04 AM   #17
diyAudio Member
Join Date: Dec 2010
Originally Posted by CopperTop View Post
Hi Stuart
Why do you think the current dumping method hasn't taken over the world, particularly in DIY, and people still mess about with preset pots and thermal drift etc.? One factor, I imagine, is that the Quad implementation uses an op amp, and this is considered a 'no no' in audio circles.
Mainly marketing problems. Quad 405 has no sophisticated gimmicks inside. And only a few people realised the beauty of its design.
There are some newer versions without op amp in the signal path. (Imagine how many op amps are in the signal path before the audio conserve (CD, LP) arrives to the listener. And there are some newer designs using the same forward error correction topology (by diyaudio members like boraomega or tvicol).
In eastern Europe the Quad 405 was very populay among diyers as it gives good results with the modest components available there once.
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Old 14th January 2012, 08:27 PM   #18
diyAudio Member
Join Date: Nov 2011
Hello Coppertop,
I am also experimenting with QUAD 405 Cheap Clones from China - which are using the KTD1047 Power Transistor. I have 3 Boards, each giving different distortion numbers using RightMark Audio Analyzer (6.2.3) , specifically under Load (4 Ohm Resistor) the distortion is 'jumping' high. I am impressed by your own measurment SW. Is there a chance that I may use your SW to post my measurements for comparing with your results ?
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Old 15th January 2012, 01:05 AM   #19
Tajzmaj is offline Tajzmaj  Slovenia
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Join Date: Mar 2011
Location: Ljubljana
Hm ...Quad405....
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Old 13th December 2012, 10:35 PM   #20
diyAudio Member
Join Date: May 2010
Location: Benidorm
i'm very interested in results
i gonna make a monster
and 80's technics su-v50 with not longer avaliable burned amp ic svi4004
i will put inside a pair of 25 euro 405 clone pcb's
i was looking the desing, and is exactly the same of circuit 12333 issue 5, the last 405 mk1 mounted in a M12368 9 and 10 bassed pcb
the technics transformer seems to be enough big, the power consumption is 490W
i think it have avaliable 300VA to drive 2 pcb's
output voltaje in 405 power supply is +-50, in technics +-48

i know where pream lines are, i will put a relay speaker protector and a new recicled big sink

i'll let you know the results

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