QUAD II Classic monos input sensitivity

[Nagraboy]

I’m thinking about getting a pair of these to use between my Nagra PL-P preamp and Falcon LS3/5a speakers.

I’ve noticed that input sensitivity seems to affect background noise in the system, eg. the Quad 303 with 500mV leads to a bit of noise through my speakers unless used with a Quad 33/34 preamp, whereas my Sugden A21ap is much quieter via the Nagra pre than the 303 is.

Unfortunately, when looking for the input sensitivity spec for the Quad II Classics it seems there are multiple different figures quoted. Some dealers websites have quoted: 570mV, 775mV and 1.4V. Quad’s own website says it’s 775mV, the 2005 Stereophile review says 570mV. Quad’s service department quotes 1.4V for 13 Watts, even though everywhere else says the figure is for 15 Watts.

Thinking about it, I can’t see that the new amp would have a higher sensitivity than the original (ie 1.4V) unless they’d changed the circuit quite a bit, although I’m no expert. I know they now use a GZ34 rectifier instead of the original GZ32 now but I don’t know what else has changed.

If it really is 1.4 Volts for full output, will that work properly with my Nagra valve pre?

 

[Tony L]

If it really is 1.4 Volts for full output, will that work properly with my Nagra valve pre?

1.5V is pretty much the standard these days so I’d expect it to work fine. I’d go with Quad’s figure as to the actual figure, one would assume they knew.

 

[Nagraboy]

1.5V is pretty much the standard these days so I’d expect it to work fine. I’d go with Quad’s figure as to the actual figure, one would assume they knew.

I’m not so sure Tony: the current Artera power amp is 980mV, the valve Quad II-forty and eighty are 1 Volt and the older 909 is 775mV. So they’re all over the place and significantly below 1.5V.

Quad’s own website (and the PDF of the owners’ manual) states 775mV for the II Classics yet their service dept told me 1.4 Volts and didn’t explain the discrepancies I’d found.

I’m hoping they’ll lend me a pair to try for myself.

 

[Tony L]

I know proper Quad IIs from the 1950s are somewhere around 1.5 to 2V (can’t remember which). They’d definitely be fine with your Nagra, and to be honest a nicely restored pair would be my choice. I think the modern ones vary model to model, e.g. a pair of ‘II Classic’ are different to a ‘Classic 40’ or whatever. To be honest I just don’t know as my own interest really only encompasses the Walker-era kit.

 

[GT]

Nearly all power amps today, with the exception of a one or two, have insensitivities of less than 1 volt. Most are around 750mV. Original Quad IIs are around 1.5v.

 

[Nagraboy]

The II Classics are the remake of the originals, just with a GZ34 rectifier (for some reason). The 40 and 80 Classics aren’t what I’m after, as I like the idea of the old amp but nice and new. From looking around I’ve read that the original Quad II was 1.4V - which is the same figure Quad gave me today for the modern reincarnation. It would make sense that they’re the same if they are almost identical circuits.

The Nagra pre has its dB markings on its dials set 0dB for a 1V input and output (separate input and output controls). I could output 1.4V from the Nagra but it would be very near the end of the volume control range and I’d be concerned about potential noise, but it’s a big unknown really. This makes it hard to risk spending the almost £2000 on the Quad monos.

 

[GT]

The II Classics are the remake of the originals, just with a GZ34 rectifier (for some reason). The 40 and 80 Classics aren’t what I’m after, as I like the idea of the old amp but nice and new. From looking around I’ve read that the original Quad II was 1.4V - which is the same figure Quad gave me today for the modern reincarnation. It would make sense that they’re the same if they are almost identical circuits.

Stick to the original Quads IIs if you can and get them rebuild by someone who knows them well to extract the best from them. You can't beat the originals if done correctly.

 

[Tony L]

I could output 1.4V from the Nagra but it would be very near the end of the volume control range and I’d be concerned about potential noise, but it’s a big unknown really. This makes it hard to risk spending the almost £2000 on the Quad monos.

You are far more likely to get noise using to much gain into a sensitive power amp than the other way around, e.g. trying to use a typical valve preamp into a 0.125V Leak just finds all the hiss and noise in the preamp.

 

[Nagraboy]

You are far more likely to get noise using to much gain into a sensitive power amp than the other way around.

Yep, good point.

 

[GT]

I could output 1.4V from the Nagra but it would be very near the end of the volume control range and I’d be concerned about potential noise, but it’s a big unknown really.

I am sure the Nagra preamp should be capable of outputting more than 1.4 volts.

 

[Nagraboy]

Stick to the original Quads IIs if you can and get them rebuild by someone who knows them well to extract the best from them. You can't beat the originals if done correctly.

I’ve thought about doing this Graham and I’d be quite keen to do it, but it’s hard to know if you’re buying a pair with good transformers after so long isn’t it - although I’ve seen you can buy brand new mains transformers from somewhere? Then I’d likely want to have modern sockets fitted (Switched IEC, RCA socket) and a hammertone respray etc. Quite a bit of work.

 

[GT]

I’ve thought about doing this Graham and I’d be quite keen to do it, but it’s hard to know if you’re buying a pair with good transformers after so long isn’t it - although I’ve seen you can buy brand new mains transformers from somewhere? Then I’d likely want to have modern sockets fitted (Switched IEC, RCA socket) and a hammertone respray etc. Quite a bit of work.

I used to do all that years ago... QUAD Parts

 

[Nagraboy]

I am sure the Nagra preamp should be capable of outputting more than 1.4 volts.

Just took this photo of the front panel. You use the two dials on the left to set the input attenuation of the input source so that it doesn’t go over 1V at the highest peaks to reduce distortion, then the output level (volume control) will output a max of about 1.5V from what I can gather.

 

[GT]

On the Nagra Classic power amplifier they have a sensitivity switch for 1v and 2v settings. I can only assume there is a huge amount of feedback used in the PL-L design, plus a lot of signal attenuation if it only outputs 1.4v and has an ECC83 and an ECC81 in the line stage circuit.

 

[irb]

I’m thinking about getting a pair of these to use between my Nagra PL-P preamp and Falcon LS3/5a speakers.

I’ve noticed that input sensitivity seems to affect background noise in the system, eg. the Quad 303 with 500mV leads to a bit of noise through my speakers unless used with a Quad 33/34 preamp, whereas my Sugden A21ap is much quieter via the Nagra pre than the 303 is.

Unfortunately, when looking for the input sensitivity spec for the Quad II Classics it seems there are multiple different figures quoted. Some dealers websites have quoted: 570mV, 775mV and 1.4V. Quad’s own website says it’s 775mV, the 2005 Stereophile review says 570mV. Quad’s service department quotes 1.4V for 13 Watts, even though everywhere else says the figure is for 15 Watts.

Thinking about it, I can’t see that the new amp would have a higher sensitivity than the original (ie 1.4V) unless they’d changed the circuit quite a bit, although I’m no expert. I know they now use a GZ34 rectifier instead of the original GZ32 now but I don’t know what else has changed.

If it really is 1.4 Volts for full output, will that work properly with my Nagra valve pre?

The Quad 303 (45w into 8 ohms, from 0.5v input) has a gain of about 31.6 dB, by my calculation. Which is highish, by modern standards, certainly. If the specs I've read online for the Sugden are correct (25w into 8 ohms from 0.65 v input) it has about 26.7 dB gain. As for the Quad II Classic, the official specs (15W into 8 ohms from 0.775v input) imply a gain of 23 dB. If Stereophile's specs are correct, the gain is about 25.7 dB. JA measured the gain in the Stereophile review at 26.55 dB. WHichever is correct, the Quad II Classic should hopefully be OK.

 

[Nagraboy]

On the Nagra Classic power amplifier they have a sensitivity switch for 1v and 2v settings. I can only assume there is a huge amount of feedback used in the PL-L design, plus a lot of signal attenuation if it only outputs 1.4v and has an ECC83 and an ECC81 in the line stage circuit.

Mine’s a PL-P which has a three valve input stage, plus a three valve output stage. I don’t really know how much they output, it would seem unusual if it were only 1.4V.

 

[Nagraboy]

The Quad 303 (45w into 8 ohms, from 0.5v input) has a gain of about 31.6 dB, by my calculation. Which is highish, by modern standards, certainly. If the specs I've read online for the Sugden are correct (25w into 8 ohms from 0.65 v input) it has about 26.7 dB gain. As for the Quad II Classic, the official specs (15W into 8 ohms from 0.775v input) imply a gain of 23 dB. If Stereophile's specs are correct, the gain is about 25.7 dB. JA measured the gain in the Stereophile review at 26.55 dB. WHichever is correct, the Quad II Classic should hopefully be OK.

Well, if these gain figures are correct then yes the Quad II Classic should be fine, as the Sugden is acceptably quiet through the Nagra + Falcon LS3/5a combo, whereas the 303 is not (unless I use a 34 pre). However, I’ve been quoted 1.4 Volts today from the head of Quad service dept, so I’m none the wiser really...although it is perhaps encouraging that JA has actually measured the amp and come up with a figure that would be about right.

 

[Arkless Electronics]

Most valve pres will output tens of Volts without clipping into a reasonably high impedance load. It should be just fine and the "pre attenuators" before the vol control (if I'm correctly understanding this feature) could be turned up to get useful vol control range.

 

[Tony L]

I’m reading it as like a channel-strip on a mixing desk in that the stereo ‘input levels’ are the gain knob at the top and the ‘output’ is the fader. I’d expect to be able to set it for pretty much any input and power amp scenario, but where the noise would be on a very sensitive power amp would need assessment, especially if using highly efficient speakers. After living with 105db Klipsch for a while I learned a lot about noise upstream!

 

[Nagraboy]

Most valve pres will output tens of Volts without clipping into a reasonably high impedance load. It should be just fine and the "pre attenuators" before the vol control (if I'm correctly understanding this feature) could be turned up to get useful vol control range.

There’s this block diagram on the lid. There are two amps, one for input and one for output: