Quad II owners - can anyone confirm the speaker terminal polarity?

[steveharman]

Owners of Quad II amps "of a certain age@ will probably be aware that many of them have all-black speaker outputs. i.e; not one red & one black - both are black. They're fitted vertically to the chassis of each amp.

I've never found a definitive way of telling which is the signal and which is the ground. Can anyone confirm please?

Thanks,

Steve

[Barry]

Usually any Quad II amp fitted with two black 4mm sockets is a version having a special output transformer, allowing the use of 100V loudspeaker distribution system for Public Adress use (for example British Rail; Rediffusion Services). What does the label on the side of the amp state (does it have A604 marked on it)? If it is one of the 'special' Quad IIs, they are not intended to drive low impedance speakers. A typical label on one of these 'special amps' looks like this:

https://www.vintage-radio.net/forum/attachment.php?attachmentid=111600&d=1439224401



However I have heard of a few 'conventional' Quad IIs being fitted with two black-coloured speaker sockets. These could well be any of the 'special' amps that have had their output transformer replaced (quite possibly by Quad themselves). In those case I would assume the upper socket is the one that would be normally coloured red.

[steveharman]

[QUOTE=Barry;1317515]Usually any Quad II amp fitted with two black 4mm sockets is a version having a special output transformer, allowing the use of 100V loudspeaker distribution system for Public Adress use (for example British Rail; Rediffusion Services). What does the label on the side of the amp state (does it have A604 marked on it)? If it is one of the 'special' Quad IIs, they are not intended to drive low impedance speakers. A typical label on one of these 'special amps' looks like this:

https://www.vintage-radio.net/forum/attachment.php?attachmentid=111600&d=1439224401

Hi Barry,

Thanks for the reply & very interesting information.

I've owned this pair of QIIs for about 20 years, having bought them from a specialist dealer - they're only a few serial numbers apart. They have been back to Quad inHuntingdon for service and no mention was made of them having an unusual output transformer (although I didn't ask specifically!).

The labels on the sides are the "regular" silver type "The Quad II Amplifier" (http://www.audio-nirvana.com/img/q11x.jpg). I wonder if there is some inspection I could do inside to determine the polarity

Thanks again,

Steve

[Firebottle]

Steve, the 'negative' of the output terminals is connected to the circuit common (ground), so if you have a multimeter you should get virtually zero ohms between the negative and the input ground.
From the 'positive' terminal you would read a few tenths of ohms I would think. Whichever reading is the lowest to circuit ground will be the 'negative'.

[Barry]

[QUOTE=Barry;1317515]Usually any Quad II amp fitted with two black 4mm sockets is a version having a special output transformer, allowing the use of 100V loudspeaker distribution system for Public Address use (for example British Rail; Rediffusion Services). What does the label on the side of the amp state (does it have A604 marked on it)? If it is one of the 'special' Quad IIs, they are not intended to drive low impedance speakers. A typical label on one of these 'special amps' looks like this:

https://www.vintage-radio.net/forum/attachment.php?attachmentid=111600&d=1439224401

Hi Barry,

Thanks for the reply & very interesting information.

I've owned this pair of QIIs for about 20 years, having bought them from a specialist dealer - they're only a few serial numbers apart. They have been back to Quad in Huntingdon for service and no mention was made of them having an unusual output transformer (although I didn't ask specifically!).

The labels on the sides are the "regular" silver type "The Quad II Amplifier" (http://www.audio-nirvana.com/img/q11x.jpg). I wonder if there is some inspection I could do inside to determine the polarity.


Thanks again,

Steve

Hi Steve,

If the badge labels on the side are plastic with engraved lettering as you show, then it is safe to say the amplifiers are the usual domestic version. As an aside, you might find this website of interest: https://keith-snook.info/quad-ii-valve-power-amplifier.html (I have a lot of respect for Keith Snook: I have modified my Quad 405s up to 'Level 3' following his recommendations.)

To be certain as to which of the two black speaker sockets is the one which would normally be coloured red, you need to remove the base plate of the amplifier and look at the connections to the secondary side of the output transformer (the transformer is the one nearest the end with the external connectors). The secondary connections are marked 'T', 'S', 'R', 'Q' and 'P'. What should be the red-coloured speaker socket is connected to 'T'. (Also check to see if 'T' and 'S', and 'R' and 'Q' are connected together, which they should be for 7.5 Ohm speakers. If you intend using speakers with 15 Ohm impedance, then the links need to be removed and 'S' and 'R' connected together instead.)

https://www.drtube.com/schematics/quad/quad-ii.gif

[steveharman]

Steve, the 'negative' of the output terminals is connected to the circuit common (ground), so if you have a multimeter you should get virtually zero ohms between the negative and the input ground.
From the 'positive' terminal you would read a few tenths of ohms I would think. Whichever reading is the lowest to circuit ground will be the 'negative'.

Brilliant, thanks Alan - screwdriver & meter at the ready.

Steve

[steveharman]

Wow! Thanks Barry, that's really kind of you + very interesting.

[Barry]

Steve, the 'negative' of the output terminals is connected to the circuit common (ground), so if you have a multimeter you should get virtually zero ohms between the negative and the input ground.
From the 'positive' terminal you would read a few tenths of ohms I would think. Whichever reading is the lowest to circuit ground will be the 'negative'.

A perfect reply: succinct and to the point, unlike my verbose waffle!

Looking at the Keith Snook site (https://keith-snook.info/quad-ii-valve-power-amplifier.html), the measured secondary coil winding resistances are quoted, from which one can estimate the resistance between the 'positive' speaker terminal and ground. With the link S-R in place (for 15 Ohm speakers), the resistance will be ~ 1.5 Ohm, and when T-R and S-Q are connected (for 7.5 Ohm speakers), the resistance will be ~ 0.67 Ohm.

Interestingly, there is a small resistor in series between the terminals Q and P of ~ 0.36 Ohm. This is not marked on the published circuit diagram; the resistor being hidden within the transformer case. I read somewhere that Peter Walker did not want its presence known about. It is thought to be a later addition so the Quad II could be used with the predominately capacitive load of his electrostatic speaker.