Quad 405 sensitivity and 'hum'

[yestertech]

I have a couple of issues with a Quad 33/405 set up :

1 ) the power amp seems far too sensitive for the 33. I'm barely off the end stop on the volume control for reasonable volume and there's quite noticeable bleed through at minimum volume setting. The channel balance this far down the volume control is pretty bad !
I'd always had in my mind that this could be altered via the pluggable 'R11' but it seems I've been wrong all this time ! ( I have no R11 fitted )

2 ) there is a faint 100Hz hum at zero volume from the 405 with the 33 disconnected.
The main PSU caps appear to show fine on the ESR meter. I've seen mentioned that the on-board electrolytics on each amp (C5/C10 ) can cause this. Is this a fairly high probability ?

Any thoughts gratefully received

Andy

[Trevor]

what are you using as your signal source if CD it will be as you describe
Phono an radio should be ok
Caps will need changing by now
If its a 405 mk 1 even the main capacitors could do with a change
Trev

[audiomm]

You'll need to isolate where the faults lie.
First thing to check is the output from the 33 is correct. Which could mean that it actually starts with the inputs. Are you using an input from say a CD player via one of the Radio sockets which could be overloading the 33? If you have a sig. gen then input 100mv into Radio 1 and measure the max output from the 33 and advise. It should be 500mv.
The specifications for input sensitivity for the various 33 inputs are set out in the Operations Manual and if the 33 is working properly then these will be reflected in your measurements.
Secondly, if the 33 has a clean bill of health, then check the performance specs for the 405 in a similar manner to above. An input of 500mv will produce about 28.5 volts at the output into 8 ohm loads ( 100watts = 28.3volts into 8 ohm loads). With regard to the electrolytics C5/C10 - just change these and but use 105 deg C or higher rated jobs. Also, change R7 and R8 (3K3) for 1 watt or higher rated resistors. The originals are half watt and tend to go O/C which can cause a lot of downstream failures.
Let us know how you get on. Regards Mike.

[Ted Kendall]

R11 simply limits the maximum voltage swing at the output for ESLs. Depending on your source, I wonder whether you have the 33 gains set up correctly - raw CD output is too high for the native Aux sensitivity, and usually needs an attenuator. Pickup and tape inputs are configurable.

[Petedox]

First I would check correct input is being used, and that the disc adapter board is fitted in the correct orientation for the input you are using:

Position M1 = Low Output Magnetic, M2 = High Output Mag, C1 = Ceramic.

The S position is 'Special' and can be used as a CD input - best modded according to Quads technical bulletin #36 1991.

[yestertech]

Thanks all
A good point is made about the source level. It is possible that the 33 sensitivities are incorrect. However to clarify, I was using the FM3 via the radio input for the tests.
I will measure the input sensitivity on the 405 and output level from the 33 as suggested by audiomm, to clarify.


Andy

[yestertech]

Originally Posted by audiomm

You'll need to isolate where the faults lie.
First thing to check is the output from the 33 is correct. Which could mean that it actually starts with the inputs. Are you using an input from say a CD player via one of the Radio sockets which could be overloading the 33? If you have a sig. gen then input 100mv into Radio 1 and measure the max output from the 33 and advise. It should be 500mv.
The specifications for input sensitivity for the various 33 inputs are set out in the Operations Manual and if the 33 is working properly then these will be reflected in your measurements.
Secondly, if the 33 has a clean bill of health, then check the performance specs for the 405 in a similar manner to above. An input of 500mv will produce about 28.5 volts at the output into 8 ohm loads ( 100watts = 28.3volts into 8 ohm loads). With regard to the electrolytics C5/C10 - just change these and but use 105 deg C or higher rated jobs. Also, change R7 and R8 (3K3) for 1 watt or higher rated resistors. The originals are half watt and tend to go O/C which can cause a lot of downstream failures.
Let us know how you get on. Regards Mike.

I made the measurements as stated and now I'm frankly puzzled. 500mv fed into the channels of the 405 produces virtually spot on 100W into 8 ohms as measured on my wattmeter.
Equally, 100mv fed into the radio 1 input produces around 520mv at maximum output of the 33.
So both seem as intended.
Using the FM3 connected to the Radio 1 input and the volume on max, the output from the 33 looks about right.
Radio 4, with its lower modulation levels averages around 500mv. The more compressed and processed music stations deliver from 500mv up to 1V.

Yet, when connected as a 33/405 set up, for moderate listening levels, I'm only using the first 90 or so degrees of the volume control and the channel balance this far down the control is all over the place. Its also not possible to reduce the volume to true 'zero'. (There is noticeable breakthrough )
It's as if there needs to be a 6dB ( or more ) pad between 33 and 405 ??
It could just be that the speakers ( a pair of bookshelf Tannoys ) are a might too sensitive.

A.

[Leon Crampin]

My 33 is exactly the same. Much of the problem is due to the law and mechanical backlash between the wipers of the Plessey volume control pot. This makes balance dependent on rotational direction history.

When I bought it, I just couldn't believe that Quad would release a product like this - so I wrote to them. They acknowledged the problem, saying that Morganite pots don't do this - and sent me one.

Words fail.

Leon.

[nutteronthebus]

at least you do not have the transformer hum on the 405 where a new one is needed if you look at the photo the RS part number is on it and I have some angle bracket for £5 if you want it


Dave

[Leon Crampin]

I've had and still have the 405 transformer hum. This is the second one, after buying a replacement from Quad at an exorbitant price for such a poorly constructed component.

My present fix is to surround the 405 completely with books and put it on a rubber mat. I don't care if it gets a bit hot....

Leon.

[yestertech]

Having now replace C5/C10 on both boards the 405 is nice and quiet.(all completely o/c)
Clearly, I can do very little about the range on the volume control as level wise everything seems to measure correctly.
Unless I start altering the input sensitivity around the LM301 ( R6 ) and/or upgrading the volume pot. for something a little more 'high-end'.
I don't think I'll be going down that route !

Andy

[audiomm]

[QUOTE=yestertech;1316304]
Clearly, I can do very little about the range on the volume control as level wise everything seems to measure correctly.
Unless I start altering the input sensitivity around the LM301 ( R6 ) and/or upgrading the volume pot. for something a little more 'high-end'.
I don't think I'll be going down that route !]

Actually, you can do something about the output of the 33. The service manual covers this specifically - see attached extract which shows you have options here. That would solve the issue you have. Mike

[yestertech]

Well I’ve learnt something there - never knew that !
6db should do nicely.

Many thanks Mike !

Andy

6db should do nicely.

I bet you go for 18!

[audiomm]

Originally Posted by yestertech

Well I’ve learnt something there - never knew that !
6db should do nicely.

Many thanks Mike !

Andy

You're welcome
And here's a suggestion from left field.........If you dont want to mess about with the 33 internals - i.e. keeping it stock , then how about making up the attenuator in a 4 pin Din to 4 pin DIN lead. Put the 15k line resistors in one plug and do the other resistors in the other plug. I've done that using .25watt metal film resistors when making input attenuators (DIN) so I can't see why it can't be done at the output end.
Mike