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Old 28th Nov 2019, 1:12 am   #1
bikerhifinut
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Default Quad 405 audible distortion, where to start looking?

Hi I have a pair of quad 405 that have been converted to monoblocs with what look like genuine quad conversion kits so could even have been factory conversions. I am fairly certain they have been ex pro use as PA amps. All the electrolytics look to have been replaced with good quality modern "Nichicon" brand units and original values used.
One of the monos had a noisy transformer which I replaced with one from a 405-2 stereo and all seems well there.
However one of the pair is showing a fault which is an audible distortion on transients, I suspect, but can't yet prove, at about the point where the ClassA drivers pass the signal on to the larger output pair, if I understand the current dumping circuit at all.
The other significant information is that I am using these amps to drive a pair of recently serviced Quad ESL989 speakers. They are very insensitive beasties even by ESL standards which is why I bought the 405 monos as being more than suitable to cope with the difficult load of the speakers.
We can eliminate the speakers as they work just fine with my Valve amps, even the Leak stereo20 although that is really working hard at it's modest, but genuine, 10W. I tried my lovely old 80s Mitsubishi silicon power amp which I don't try to torture with the ESLs as its relatively easy to get it's protection relays to energise with the ESL, and it also drove the speakers just fine at the levels I could hear the sound breaking up with the 405. The other valve amps which are meaty KT88 and EL34 jobs, both delivering a genuine and measured by me 35W plus also handle the quads without this fault showing up.
So is it a crossover distortion issue when the Class B circuit chips in or is it the clamping circuit that the amps are fitted with? I note from the quad service notes that there's a procedure to adjust the amps with the preset pot across the inputs when mono bridged and could it be simply an imbalance here?
A lot of questions I know, and I will readily admit t hat I am prepared to let a professional engineer examine the amps if there isn't a simple fix as they are worth a lot to me and I did get them for a very reasonable price.
Thanks for your patience.

Andy
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Old 28th Nov 2019, 8:05 am   #2
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Default Re: Quad 405 audible distortion, where to start looking?

Hi Andy.

Just a first, maybe silly thought. Do your modified 405’s still have the Triac crowbar fitted across the ‘speaker outputs? Maybe some back EMF from driving the ESL’s is causing some strange effect here?

Probably miles off, but it’s a thought.

Have you injected a sine and/ or square wave and examined the output on a scope yet?


Cheers. SimonT.
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Old 28th Nov 2019, 6:32 pm   #3
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Default Re: Quad 405 audible distortion, where to start looking?

Thanks for the ideas, Yep I will dig out the sig gen and dummy loads and get a look at the scopetrace. I imagine I am looking for the sine wave clipping, perhaps asymmetrically?
I hope the fault isn't a serious one, I'm certain it was fine a couple of days ago.
They certainly sound really good to my old lug oles and I would like them working as they should be.
I will have a look inside again as I now can't remember which amp I swapped the mains transformer on and I wonder if I disturbed something doing it. If it's the same amp. The clamp circuit was one of my suspicions too but I am loath to disconnect a safety feature as the ESLs clamp circuits can show as a short if they activate, even momentarily and I worry that would be curtains for an unprotected output circuit. I assume that's the same as the crowbar circuit?


A.
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Old 28th Nov 2019, 7:57 pm   #4
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Default Re: Quad 405 audible distortion, where to start looking?

Hi Andy.

For testing, I always connect my test speakers through a 4700uf electrolytic, so if the output stage goes caput, then at least the speakers are protected from a potentially heavy DC current. This will allow you to temporarily disconnect the triac crowbar to eliminate.

I’m not familiar with the circuit of the ESL 989’s so not sure how they are internally configured.

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Old 29th Nov 2019, 11:30 am   #5
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Default Re: Quad 405 audible distortion, where to start looking?

A couple of big capacitors in back-to back series might be an improvement... you never know which way the output will ram itself if power transistors go in a DC coupled amp.

David
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Old 29th Nov 2019, 11:48 am   #6
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Default Re: Quad 405 audible distortion, where to start looking?

Time for some hard and fast diagnostics. If the distortion is audible, it should be visible on a scope trace without need of a distortion analyser.

Have you downloaded the 405 service manual?

Craig
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Old 29th Nov 2019, 1:15 pm   #7
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Default Re: Quad 405 audible distortion, where to start looking?

I have looked online and I can get the manual off Keith Snooks site so shall print it off.
I have a couple of Oscilloscopes and a decent sig gen, but my dummy loads are only good to 40W so I shall have to get a pair of heavy duty 8 ohm resistors that can handle 200W as the monoblocs are rated at 180W according to Mr Quad. I know it seems an awful lot of muscle for a domestic setup, but the pair were less than most 405 stereos I have seen and they were both working fine when I got em so I hope the issue isn't too unfixable but I get the impression the 405 is a very serviceable amp.
I guess I look for clipping or other effects on the sinewave? I did change the noisy (buzzy) transformer on the affected amp and I fear I may have inadvertently disturbed a setting? for instance if the preset pot on the input was disturbed whilst dismantling the chassis would that unbalance the bridge and cause distortions? I was careful not to disturb the boards but it was tight when unscrewing the heatsinks to get at the front transformer screws.
I'm OK with basic stuff but I am wary of any major work and would happily pay a reasonable fee for a pro to set them up/repair as I really like these amps and for me they are as good as it gets notwithstanding their 80s heritage.

Thanks to all so far, I know I'll get sensible advice here.
Andy.
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Old 2nd Sep 2020, 1:41 pm   #8
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Default Quad 405 monobloc fault

I originally posted about this late last year but that thread is closed now.

Finally its off the roundtuit pile.
The amp has a very audible distortion on certain transients.
It's got the Quad 410 input and out put boards for bridging so is as per the service manual.
The boards have been refurbished in the past with good quality branded electrolytics and looks like some resistors were replaced, along with IC sockets and OPA134 chips now in. That's how they came to me, personally I ain't convinced they make much difference.
I got the amp on the dummy load and scoped it at 1kHz, and at around 18V peak to peak across the 7.5 ohm resistive load the waveform went from a nice sinewave to a complete hash, unfortunately I couldn't capture the trace but it went complex and the signal level collapsed.
Square wave showed the trailing edge of the wave was rounding off with a slight "peak" at the very end at most levels so that indicates an issue.
I took the frequency down to 500Hz and the distortion occurred at a much lower level.
By 300 Hz I didn't get the sig gen above about 100mV if that before there was a bang and flash as the 4A Fuses on the LH amp blew.
So I think the issue must lie with that board, it did seem to be clipping asymmetrically before the failure.
If its more involved than a reset of the balance on the input adjuster then I readily admit its beyond my skill set to start hunting around the amp board as the amp is worth a lot to me personally.
I can remove the boards and so I wondered if it would be a solution to get them both checked over and serviced by a competent person.
I really like these amps, they do what it says on the tin so to speak.
I got them for a very reasonable price last year to partner my very insensitive Quad 989 loudspeakers and I know the stereo version will do the job handsomely but I would like to keep these as monos as they were originally converted. No doubt they were originally pro used, I can't think of many domestic setups needing 180 genuine watts!
Andy.
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Old 2nd Sep 2020, 4:13 pm   #9
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Default Re: Quad 405 audible distortion, where to start looking?

Have a look at the DADA site for diagram and don't forget R7/8 a 3k3 resistor that is under rated ( fit a 1w version off set from the PCB ) https://www.dadaelectronics.eu/downloads
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Old 4th Sep 2020, 6:19 am   #10
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Default Re: Quad 405 audible distortion, where to start looking?

As far as fault finding the amp boards goes, it may be easiest if you lash up a board one at a time with the dummy load on the bench, and ignore the Q410 pcbs until you have the 2 separate amps sorted.
Mine ran quite happily for a few minutes without an additional heatsink, but I used a g-clamp to fasten it to a piece of extrusion for a longer test.
NB You need a 0V to the alloy bracket if testing outside the chassis.
Regards, BC
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Old 4th Sep 2020, 11:56 am   #11
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Default Re: Quad 405 audible distortion, where to start looking?

Very good sense!

It's likely that one of the bridged amps has gone sour. The chances of similar faults happening in both at once are somewhat lower unless common-denominator damage.

Find which one... fix it.... restore to bridge config.

David
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Old 6th Sep 2020, 8:52 pm   #12
bikerhifinut
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Default Re: Quad 405 audible distortion, where to start looking?

Indeed it does look like the RH amplifier board is the culprit.
I had it on the scope last week and it went very pear shaped by the time there was 18V on the dummy load at 1kHz.
It got progressively worse as I swept the frequency down and by 300Hz there was a flash and bang as the fuses blew on the RH board.
I wouldn't discount a problem on the LH board either but it does point at that board.
Being a bit of a coward and not wanting to make a bad situation worse, I am minded to get a professional to check the amp over as I think everyone would agree that the Quad 405 is an amplifier well worth getting sorted out.
Just looking at the output board makes me sweat as it wouldn't be an easy job to remove it for any repairs in case the clamping circuit is faulty.
I got some more 4A fuses but I'm very cautious as to whether I should investigate further.
Can I assume here that if the other board is ok, by disconnecting its output from the output board and clamp and connecting it across the 7.5 ohm dummy load, I should be able to run it up on the scope from the signal generator and check the waveform out?
If its just one board gone awol maybe that's all the problem is but I am a bit suspicious of the clamp boards.
Thanks gents, I feel I am somewhat out of my depth here though.

Andy.
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Old 6th Sep 2020, 10:53 pm   #13
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Default Re: Quad 405 audible distortion, where to start looking?

Bridging an amplifier doubles the voltage that can be provided to the load, but does nothing to boost the current that can be taken before boom.

So in going to a bridged config, you probably want higher-Z speakers unless you want to live close to the edge (down by the corner, seasons will pass you by..... nonsense words following the phrase 'close to the edge', by Jon Anderson)

David (an old Yes rocker)
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Old 6th Sep 2020, 11:13 pm   #14
Craig Sawyers
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Default Re: Quad 405 audible distortion, where to start looking?

The crowbar was not Quad's finest moment. When it operates it short circuits the output of the amp. So a potentially innocuous fault can trigger the crowbar and blow at least one output transistor, possibly both, and also the driver. Then the crowbar itself destructs. Then the fuses blow.

I fully intend to replace it (by disconnection) on my 405/2 and install either a relay or MOSFET series circuit, so any DC simply disconnects the speaker. Commercial offerings are available from:

http://www.signaltransfer.freeuk.com/protect.htm (a Douglas Self design using relays)
https://neurochrome.com/products/guardian-686 based on a series MOSFET switch
https://www.diyaudio.com/forums/grou...-delay-gb.html another MOSFET design.

Craig
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Old 7th Sep 2020, 10:06 am   #15
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Default Re: Quad 405 audible distortion, where to start looking?

I've not seen one in the flesh but I understood the Quad 410 modification (Q410MOD) was to create a paralleled amp rather than a bridged one ?

Are the amp boards the later type with the integrated crowbar ? - If so have they definitely been disabled correctly as per the mod instructions? (i.e. the solder bridges at the pads provided for this purpose have been removed.)

It's just that blowing fuses only at lower frequecies sounds rather like a bad C17 (10u/non-polarised) in the crowbar.

Cheers
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Old 14th Sep 2020, 1:00 pm   #16
bikerhifinut
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Default Re: Quad 405 audible distortion, where to start looking?

Quote:
Originally Posted by Craig Sawyers View Post
The crowbar was not Quad's finest moment. When it operates it short circuits the output of the amp. So a potentially innocuous fault can trigger the crowbar and blow at least one output transistor, possibly both, and also the driver. Then the crowbar itself destructs. Then the fuses blow.

I fully intend to replace it (by disconnection) on my 405/2 and install either a relay or MOSFET series circuit, so any DC simply disconnects the speaker. Commercial offerings are available from:

http://www.signaltransfer.freeuk.com/protect.htm (a Douglas Self design using relays)
https://neurochrome.com/products/guardian-686 based on a series MOSFET switch
https://www.diyaudio.com/forums/grou...-delay-gb.html another MOSFET design.

Craig
I think you may be very close to the root cause Craig. I was thinking back to when this fault first became apparent and I reckon it was about the time one of my ESL 989 was misbehaving and was traced to a dodgy spark detector aerial which I managed to repair the dry joint on. I think the speaker must have clamped and whereas my other silicon amps which had conventional relay switched protection survived with no ill effects perhaps the clamp circuit on the offending amp is/was faulty and as you say didn't do its job and took out one of the boards.
I have seen a few other alternatives to the amps clamp/crowbar and DADA done that looks awfully like the Velleman kit module. If that would be up to the job then I can easily assemble one and fit it inside the amp. The original 4mm sockets are lousy anyway so a bit of surgery there where the quad clamp is fitted might be worth the hassle.
Nevertheless I still have a "cream crackered" amplifier board and I figure the LH one may need a coat of looking at although I think I am getting a clean sine and square wave off it.
Thanks,
Andy.
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Old 14th Sep 2020, 1:07 pm   #17
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Default Re: Quad 405 audible distortion, where to start looking?

Quote:
Originally Posted by Trigon. View Post
I've not seen one in the flesh but I understood the Quad 410 modification (Q410MOD) was to create a paralleled amp rather than a bridged one ?

Are the amp boards the later type with the integrated crowbar ? - If so have they definitely been disabled correctly as per the mod instructions? (i.e. the solder bridges at the pads provided for this purpose have been removed.)

It's just that blowing fuses only at lower frequecies sounds rather like a bad C17 (10u/non-polarised) in the crowbar.

Cheers
Cheers Trigon, yes that's what it is, its not a conventional bridge but a parallel circuit. You adjust the balance with a pot on the input board. This C17 is it the mucky brown radial capacitor on the boards? I figure if its a 10uF a modern polywotsit should be a better choice than an electrolytic.
It looks like a really difficult job to even just remove that board never mind service it. Doesn't look like a very good bit of production design to me, especially considering the amount of space inside the amp case tomount a board.
A.
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Old 14th Sep 2020, 2:30 pm   #18
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Default Re: Quad 405 audible distortion, where to start looking?

Even if the boards are the later type (M12565) , and the C17s are original, it shouldn't matter because the on-board crowbars should have been disconnected as part of the 410 modification:-

Quote:
To disconnect the clamp circuit on 405s fitted with PCBs M 12565 (serial numbers above 59000) remove both of the side panels. The solder should then be removed from the link pads shown as 'A' in fig 18.
I've never found 'fig 18'... , but the link pad referred to is shown below:-

Click image for larger version

Name:	Crowbar Link.jpg
Views:	106
Size:	41.4 KB
ID:	215673

Cheers
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Old 16th Sep 2020, 9:03 am   #19
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Default Re: Quad 405 audible distortion, where to start looking?

The electrolytic capacitors have been replaced and it is easy to assume they are all OK.

Do you have an ESR meter or capacitor tester to confirm that they are working to spec?
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