View Full Version : Quad 405 Hum
I thought I would get my 405-2 out for a spin. I hadn't really used it since it was serviced by Quad 2 years ago.
I connected it to my 34, using the 4 pin din and switched electrical connector from the 34 and switched it on and I get quite a loud hum through the speakers. When I disconnect the 34 so only have the 405-2 running no hum.
I suspected an earth loop so, making sure that the 405-2 was earthed via the 34, connected the 405-2 to the electric with the earth dosconnected. No hum, not that I want to run it like this as I may want to use a passive preamp for instance.
So I do have an earth loop. Any suggestions on how to eliminate it safely!
It is unlikely IMO that an earth loop will be designed into the pairing of the 34 and 405, and so it is probably an externally applied problem.
Very difficult to diagnose from this remote a position to more information.
If it helps it happens even if nothing else is connected to the 34.
It does seem weird, and I have tried 2 different 4 pin din connectors between the 34/405 and it happens on both.
If it helps I also have 4pin to rca, and I just connected pin to ground and connected it to the 405 and dont get hum, so it looks like it is when I connect the 405 externally.
I have just connected the 405 to a passive pre - no hum.
Connected the cdp to the passive no hum, but I dont think tells us anything as cdp's are double insulated and don't connect to earth.
I have a Quad 306 which I have just connected to the 34 and I don't get any hum.
So, assuming that you have the pre connected to the power and get hum from both channels, does changing the configuration of the pre change anything? (Different I/P sources, tone controls in/out etc).
Light Dependant Resistor
18-04-2020, 09:19
Only three pins of the 4 pin cable on a 34 are used. It is possible to insert the 4 pin din incorrectly.
When inserting observe the indent in the connector aligns with the socket indent.
Also always use the standard 4 pin din to 4 pin din cable, it is quite likely if RCA's are being used via the 4 pin din that,
one of the pins is incorrectly allocated relative to the 34's wiring,
A passive pre including LDR types though will be better.
No the hum doesn't change. I think that there is something wrong with the 4din input on the 405. Over the weekend I'll put in place some RCA inputs and try that.
From memory, and I had a 33 from '72 to'91, the four pin din is one of two standards which are different in pin positions.
The pin configuration was on both, a quadrilateral, but of differing dimensions.
If there is no hum from the 405 with nothing feeding into it, and when you carefully using a needle between your fingers touch each +ve I/P individually, that will produce an O/P hum on your speakers from each respective channel.
That will show, (or not), that each channel is amplifying and functioning correctly, and when nothing is going into each I/P, there should be no audible noise with your ear next to each speaker.
That eliminates malfunction from the power amp.
I don't believe either of my 4pin din -> 4pin din cables are faulty. I have checked them for continuity and both are fine and match the markings onboth the output of the 34 and the input of the 405.
I have just directly wired the right channel from the 34 to the power board with no hum and works fine. If I'm brave I may unsolder the 4 pin din input on the 405 and wire some RCA in's and then try the cable that is fine with the 306.
It was suggested a faulty DIN and that is what it was. I removed it from the 405 and spotted that there were shared of solder on the live which touched the earth. I removed those and it appears to be OK.
Many thanks for those who took part in this problem solving
Be Safe
Lee
Nah, spoke too soon. With the din plug not screwed in - no hum. The moment I screw the din plug back in place I get hum.
I've been reading around the web. I have boards M12565.7. These versions were when QUAD started to use RCA on the 405 and on Keith Snook great site it states "M12565 iss. 7 PCBs (QUAD 405-2) used for serial numbers from 85000 onward - Note input earth change with phono input connectors", where on the schematic it states "Ground Isolated at PHONO"
So I get no hum when isolated but hum when I don't.
Now I don't know what to do. I might just dump the din and put isolated phono sockets on and be done with.
Given the current situation isolated is the way to go:D
I find this frustrating, I'm sure that if it was in front of me I could solve it quickly, but the extra link of verbal coms makes it much harder.
Yeah a trip to the coast would be nice in normal circumstances :(
But for this current malaise, (no pun intended), you would be welcome.
On all 405s using a 4-pin DIN input socket, pin 2 of the socket is both the chassis earth and the signal earth.
On 405-2s fitted with RCA sockets, the sockets are isolated from the chassis earth and the outers are used for the signal return only. The chassis earth is directly connected to the 0V rail and via 100nF capacitors to the +/- ve rails.
According to Keith Snook, there is also no distinction between the chassis earth and signal earth for the Quad 34 preamp. So there is a possibility of an earth loop being created if the earth continuity conductor is maintained in the mains lead connecting between the pre and power amp. I assume that with the Quad interconnecting mains lead, this earth connection is omitted - is this correct for the lead you are using?
That sounds an entirely reasonable hypothesis, have you had the combination previously working without exhibiting the hum problem Lee?
The interconnecting mains lead does NOT omit the earth connection. Barry I understand your reasoning and that appears a solution. However I don't get a hum when I use the same cable with a 306. Also this is the first time I have used the 34/405 combination. When I sent the 405 for service I didn't have the 34, so when it came back I tested it with a passive, so assumed it was ok. I do have another amp with pre out, but that is double isolated so no problem. SO it was only this week that I tried the 34/405 for the first time.
At Keith Snooks site he has a download of the evolution of the 405 http://keith-snook.info/amplifier-hifi-schematics/QUAD%20405%20schematic%20evolution.pdf
My boards are 12565.7 with DIN which are on page 8. On page 9 it is the same board but with phono. The only difference I can see that the din has signal and chassis joined but rca is isolated from chassis.
So based on that I moved signal ground from the chassis. But rather than keep it isolated I joined signal ground to chassis with a 100 ohm resistor and that works fine. I will try to source a 10 ohm resistor to mimic the same value as R2 which is used to join them else where in the circuit. I've been measuring the DC offset whilst doing this and the changes are negligable and all down at a couple of millivolts.
A 10 Ohm resistor to join the signal earth to chassis is a good way to go.
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