Cheap eBay Quad 405 Clone measurements - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 12th January 2011, 12:51 AM   #1
diyAudio Member
Join Date: Feb 2009
Location: UK
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, 1795 views)
File Type: jpg screengrab_1kHz_1W_8R_spk.jpg (138.0 KB, 1685 views)
File Type: jpg screengrab_1kHz_10W_8R_res.jpg (150.7 KB, 1639 views)
File Type: jpg screengrab_1kHz_10W_8R_spk.jpg (150.5 KB, 1605 views)
File Type: jpg screengrab_1kHz_34W_8R_res.jpg (162.1 KB, 1582 views)
File Type: jpg screengrab_1kHz_34W_8R_spk.jpg (168.7 KB, 313 views)
  Reply With Quote
Old 12th January 2011, 01:02 AM   #2
tomchr is offline tomchr  Canada
diyAudio Member
tomchr's Avatar
Join Date: Feb 2009
Location: Calgary
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.

  Reply With Quote
Old 12th January 2011, 07:43 AM   #3
diyAudio Member
Join Date: Feb 2009
Location: UK
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)
  Reply With Quote
Old 12th January 2011, 07:13 PM   #4
diyAudio Member
Nico Ras's Avatar
Join Date: Nov 2005
Location: East Coast of South Africa
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.
  Reply With Quote
Old 13th January 2011, 01:57 AM   #5
diyAudio Member
Join Date: Feb 2009
Location: UK
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?
  Reply With Quote
Old 13th January 2011, 10:06 PM   #6
diyAudio Member
Join Date: Feb 2009
Location: UK
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.
  Reply With Quote
Old 21st January 2011, 12:07 AM   #7
diyAudio Member
Join Date: Feb 2009
Location: UK
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.
  Reply With Quote
Old 22nd January 2011, 11:47 AM   #8
405man is offline 405man  Scotland
diyAudio Member
Join Date: Dec 2003
Location: South Lanarkshire near Glasgow Scotland
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.

  Reply With Quote
Old 23rd January 2011, 10:01 AM   #9
diyAudio Member
Join Date: Feb 2009
Location: UK
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.
  Reply With Quote
Old 23rd January 2011, 10:12 AM   #10
diyAudio Member
Join Date: Mar 2004
Location: Surrey, UK.
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 ?

  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Quad 405 Piersma Swap Meet 8 25th October 2011 03:23 PM
Kit building Quad 405 Clone JonLawes Solid State 133 10th February 2011 07:34 PM
If I was to buy... (Quad 405 Clone) fusion Solid State 3 17th October 2010 09:29 PM
Quad 405-2 clone al2002 Solid State 19 15th May 2006 06:14 PM
quad 405 hazenhoe Solid State 2 27th December 2003 01:32 AM

New To Site? Need Help?

All times are GMT. The time now is 08:08 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2