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Old 12th January 2011, 12:51 AM   #1
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Default Cheap eBay Quad 405 Clone measurements

I wondered if anyone would be interested in the results I just got from measuring the distortion of one of those 25-a-pair Quad 405 clone boards 'out of the box'.

My test setup comprises a PC with M Audio Audiophile sound card feeding the amp and monitoring the output. It's my own software that runs at 16 bit resolution, 44.1 kHz - I must get it going at 2496 if possible! The amp is powered off a +/-50V supply with 10000uF across each rail.

I tested the amp with a 1kHz sine wave and an 8.2R load resistor, plus a real speaker for comparison (a big rubbishy Sony thing from the '80s). I tested 1W, 10W and 35W output powers.

As far as I can tell, the amp is made to the very original 'pre-Mk1' design as shown in Peter Walker's original paper to the AES in 1975 so many, many improvements are apparently possible. Like the early reviewers, I found the noise performance to be too bad to make distortion measurements at very low powers. Apparently changing the op amp topology from inverting to non-inverting works wonders with this. Several other mods are supposed to further improve the distortion.

Nevertheless, the distortion seems very low to me...

Should this amplifier sound any good, or do those spectral lines scream that it will sound terrible?

Something that I thought might be of interest is the difference in results between the test resistor and a real speaker which shows a factor of 2 difference in distortion at high power.

Another thing I'm interested in is whether it is possible to isolate the 'residual' distortion by removing the fundamental from the complex FFT and calculating the inverse FFT. Has anyone here done this already?

(A point concerning the screenshots: there is an apparent discrepancy between the heights of the green bargraph and the indicated harmonic dBs. Although the green bargraph display shows the raw windowed FFT response, the 'H2 -98.4'' etc. indicate the relative total dB level of each harmonic, summing the power over several adjacent bins to correct for the effects of windowing.)
Attached Images
File Type: jpg screengrab_1kHz_1W_8R_res.jpg (138.5 KB, 1365 views)
File Type: jpg screengrab_1kHz_1W_8R_spk.jpg (138.0 KB, 1294 views)
File Type: jpg screengrab_1kHz_10W_8R_res.jpg (150.7 KB, 1250 views)
File Type: jpg screengrab_1kHz_10W_8R_spk.jpg (150.5 KB, 1225 views)
File Type: jpg screengrab_1kHz_34W_8R_res.jpg (162.1 KB, 1211 views)
File Type: jpg screengrab_1kHz_34W_8R_spk.jpg (168.7 KB, 247 views)
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Old 12th January 2011, 01:02 AM   #2
tomchr is offline tomchr  United States
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Looks like a run of the mill semiconductor amp distortion signature to me. All the "grass" is either the noise floor of your measurement setup or caused by supply ripple in the amp. Noise would be uniformly distributed. Ripple would show up at multiples of 50/60 Hz and IM products caused by the ripple would show up at multiples of 50/60 Hz of the input signal frequency and its harmonics.

~Tom
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Old 12th January 2011, 07:43 AM   #3
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Tom, thanks. Yes there's definitely some mains stuff at the bottom end.

This current dumping amplifier is noted for its noisiness in general in its un-modified form, but is supposedly good for its low distortion, which can be nulled out further if care is taken with component selection and other potential mods.

If the distortion figures I've shown are just run-of-the-mill I think I may just give up and buy a cheap Japanese amp on eBay!

(of course I won't)
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Old 12th January 2011, 07:13 PM   #4
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How do you calibrate the THD of the sound card? Did you subtract the soundcard distortion from the amp output (multiplied by the gain of the amp)? Also how did you calculate SNR I don't see any noise above -93 dB or is it SINAD

Also as Tom mentioned the noise looks like power related, you cannot blame the amp for a noisy power supply, I would nullify the power hum related components as well.

Last edited by Nico Ras; 12th January 2011 at 07:19 PM.
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Old 13th January 2011, 01:57 AM   #5
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Hi Nico

Almost certainly there will be some hum loop-related noise in the system. I may temporarily remove the amp's earth to see what it does. However, I can tell you that the amp is noisy when idling. One early reviewer (of a real Quad 405) put it very diplomatically: "Generally it is felt that the noise performance meets realistic requirements", but also rated it as one of the best amps he had tested.

As regards the software, I didn't try to compensate the sound card's own noise and distortion - it is a very good sound card - as I thought that the exact method of doing that might prove controversial. The noise and distortion figures are therefore most likely better than the ones I'm displaying.

Now you may know more about this stuff than me: I fed the sound card's output to the amp via a pot and fed the amp's output back into the sound card via a fixed resistive attenuator (so at the low power tests I wasn't using the sound card's dynamic range very well but I figured the amp was so much noisier than the card it didn't make much difference). I calibrated the displayed Vp-p and VRMS (and therefore power into 8R) readings against an oscilloscope although I might have been able to do that by calculation if I had looked up the sound card's specs.

The spectral display amplitudes and dB readings are referenced to the summed total FFT power where 'power' is defined as FFT bin magnitude squared (and 'magnitude' is defined as sqrt (re*re+im*im)). Each FFT bin amplitude is displayed as 10*log10(bin_power/total_power).

Without referring to the code, from memory THD is 100*sqrt (harmonics_power)/sqrt (fundamental power) and SNR is calculated as the 10*log10((fundamental_power + harmonics_power)/(total_power-harmonics_power-fundamental_power)). No individual noise frequency bin goes above, say, -93 dB, but the sum of all their powers brings the total noise level up to -70 dB or whatever is displayed. Does that sound about right to you?
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Old 13th January 2011, 10:06 PM   #6
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I just found out something quite significant, I think. The 405 clone boards faithfully reproduce the well-known error on the original 405-1 schematics and PCBs: the output Zobel network is grounded to the input signal ground (which is isolated from the power ground by a 10R resistor), not the power ground as it should be, explaining why I experienced catastrophic oscillation which destroyed a tweeter the first time I plugged one of these things in without anything connected to the signal input ground. It happened again tonight when I powered up the second channel without any signal connected - luckily it was driving a test resistor at the time.

Unfortunately it will be a day or two until I can repeat the distortion tests.
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Old 21st January 2011, 12:07 AM   #7
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With the Zobel networks connected to the correct ground, the amps no longer destroy tweeters, and distortion measures just slightly better at high powers.

Tested with a Mordaunt Short MS320 bookshelf speaker as load - working towards plugging in the good speakers! Must be an easier load than the 80s Sonys. THD now 0.004% at 1kHz 40W RMS, and I'm not taking the sound card's distortion into account. Is this a run-of-the-mill figure, or quite good? (I'm pretty sure my software's right).

...

Finally plucked up the courage to power both amps into the good speakers. Sounds bloody good, if you ask me.
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Old 22nd January 2011, 11:47 AM   #8
405man is offline 405man  Scotland
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About 30 years ago a friend and I built some Quad 405 clones. As we did not have suitable test equipment to measure the distortion we modified one amplifier to a non-inverting configuration and used it to provide the gain in a passive LC oscillator, the distortion level measured at 0.0018% which at the time was very good.

Stuart
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Old 23rd January 2011, 10:01 AM   #9
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Hi Stuart

I am going to try and improve my measurement method, including cancelling out the sound card's own contribution to the distortion.

Judging by your moniker, you are a 405 devotee. 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. The cheap Chinese clones I bought are to the original MkI design, so the op amp is an LM301! But switch them on and beautiful music pours forth.

I've changed my mind about the noise performance. It's not an issue in reality. Just a slight hiss from the tweeter if you put your ear right next to it. No audible hum at all.
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Old 23rd January 2011, 10:12 AM   #10
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Hi Coppertop,
interesting post. Good to see your analysis of the 405 clone. How does it compare to other amplifiers, in terms of your measurements with your sound card and your ears ?

What plans do you have for your software ?

Regards
fs
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