Class A Obsession

I knew this would happen. It started as a simple exercise in down-sizing. Moving to an Apartment would clearly mean my Hi Fi had to be rationalized.

In the process, I have bought a Sugden Bijou Headmaster to pair with my Quad 405, which drive my Stax Headphones. However, I have been trying to find precisely what point the 405 dumpers come into play, the hope being I could operate the amp below that threshold, thus keeping it in class A.

Consensus of opinion suggested it probably wouldn’t work. However, early experiments show, maybe I can drive the Stax for comfortable low level listening, without invoking the Current Dumpers. I was “Buoyed” by a comment made by someone on WAM who also has SR - 3 headphones, when he suggested ~10 watts should be enough to drive them.

Having read an article by some Guru on Stax products, his suggestion was to leave the amplifier/SR - 3 & SRD - 5 adaptor on to allow the headphone bias to Stabilize. “This would balance the audio level in each ear - speaker”, which I have found to be absolutely correct.

In doing so, my 405 has been reaching Electric - underblanket temperature when quiescent.

Driving the 405 with at a modest level with classical material, raises this temperature by an, as yet unmeasured amount. However, whilst listening to less intense music, at a soothing level, the 405 heatsink temperature drops below the quiescent value it stabilises at when left on overnight. There is a technical reason for this and I must refresh my memory on the subject: having spent decades working on high Power Broadcast Transmitters, it will take me some time to claw it back from my brain.

Maybe, someone can spare me the task if they themselves can recall what is going on. In essence, I believe, it is because when driving the 405 below the “Class A Threshold”, the RMS power it consumes is less than when it is Quiescent. Thus the reduction in heatsink temperature. I intend to get some temperature probes into the amplifier to take some proper readings.

Which now leads on to what I feared all along, when I first decided to buy a new Pre- Amplifier to replace my 33. I could not believe my luck when I got a pristine Bijou Sugden - the remote control was still unused and on the whole the amplifier looks brand new. I knew this exercise would lead on to yet more expense!!

Initially, I wanted to see if the 405 could deliver, or could be modified to deliver enough class A power for the needs of the Stax. As I say, consensus of opinion and a couple of emails to Quad suggested this wasn’t going to be possible.

It was then I though, if indeed I can currently drive the Stax at low level and keep it operating in class A, at what point then does the Quad 606 transition over to it’s Dumpers. That question remains unanswered as does the one in the direction to which I am being taken - the Quad 909?

Not having used the Stax for years while my Spendors stood idle for some time pending our move, it was a real revelation to find my 405 when married to the Sugden could produce such sublime audio in the Stax. This should also reflect in the fact that, even when driven by the 33, the 405 now holds the Spendors in an iron - grip, extending the bass even lower and bringing superb clarity to the mid - range. So much so, I intend to hook - up the Spendors in the next few days to see exactly what the Sugden does to improve on that.

I have spent the last 2 - 3 weeks trying to find a second hand Class A power amp, to no avail - what is available is too expensive, particularly as there are no guarantees generally given with second hand. I could tolerate that, if prices reflected the chance that amplifiers may need servicing/repairing. But they don’t.

So the price of a second hand 909 will still leave me enough to send it off to Quad for servicing, given they have looked after my 303/33, vintage 1971 and my 405, which I bought second hand in 2004. It seems to me, even with the cost of my Suggie, it and a 909 would still rate as a steal.

Plus, the 909 would be more in keeping with the “Bauhaus” styling of the Sugden.

Why would you leave any item on overnight f it is not being used?
Especially something as inefficient as Class A amps.
Total waste of energy and money, we have planet to think of!

(Please don’t give an audiophile nervosa response!)


It’s cheaper than the gas central heating. Plus, at 76, I’ve done my bit; paid my taxes; and still paying, educated the kids and few others kids. Funded wars that twats like Cameron started - we wouldn’t have boats in the channel if he hadn’t wrecked Libya, allowing all Gadhafi’s arms to go South and Migrants to come North.

I’m no longer on the magic roundabout that Britain has become. So if I can find a Class A amp big enough to burn a hole in the Ozone layer - I will.

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If they are like the SR34 electrets then they need voltage rather than power as the effective impedance does not drop low. I thought the current dumping worked at all levels anyway on Quads?

And if the SRD3 is like the SRD4, swap the thermistors in line for fixed resistors (and stay careful), it makes quite a difference.

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“I thought the current dumping worked at all levels anyway on Quads?” - no. However, I’ll look into the issue of thermistors - next. The SR - 3’s have a bias of 250 volt, the next model on went up to 500 volt, I think? They aren’t the issue though, at the moment, but thank you for the observation.

How very noble of you

Never heard a 405 that didn’t sound soft with any kind of speaker to be honest.


I was being polite. But I wasn’t going to start introducing other issues into the mix. I will look it. However, I have only just got my Stax up and running with my kit and I am both enjoying them and using them as a reference for the changes I am making. So it is not a good idea to change things at this time - not before I can draw any conclusions.

The thermistor issue I wasn’t aware of!

I spent years happily with the Spendors being driven by my 303 and I can see what you mean about “Soft”. Transparent was a term used years ago, but the Bass was never “Punchy”. They are very sensitive to where they are placed in a room with their trolleys, particularly in corners where they could be boomy.

There never seemed much to their design as I have had a good look inside when replacing drivers due to cones collapsing and grating the voice coil. Spencer worked in the BBC sound development department, who Patented the design - LS3/5a? rings a bell. He left the company with the rights to build the BC1 for the BBC, who needed a small studio monitor and to sell to the public.

Thinking back now, it seems it was the compliance of the main driver (Bexdrene? Cone) and the Crossover Network that sets them apart. The choices of 3khz and 13khz is a good choice, particularly for vocal fundamentals where the phase response is unadulterated by a crossover point. There again, it is also true of the range of the Celestion midrange unit which handles 3 - 13khz. Maybe the combination contributes to the “Softness”.

However, changing from the 303 to the 405 certainly tightened up the Bass and improved the clarity of the midrange. One of the reasons for this is; I believe, the 303 is A/C coupled, with output capacitors, whereas the 405 is Direct Coupled, which appears to create a holding bias on the main drivers. The 303 allows them to do what they wanted under no signal conditions, which clearly must have an effect under dynamic conditions.

Funny really, I never gave much thought to the issue of matching loudspeakers to amplifiers before.

Rogers also were granted the rights to build a small - smaller than the BC1’s - monitor for which the BBC held the patent.

Err the 909 isn’t technically class A

I still use a 34/303 now and then which I really like.

I think a lot of the perceived pipe and slippers of the early quad stuff comes in part from naim in the mid 70s saying if you’re into rock,then you need naim.
As quad was seen as being used by dads/grandads for their classical / Sinatra sessions.


The 405 is a High Quality Class A Voltage amplifier which is supplemented by a current amplifier, which are the current dumpers. The error created by the lack of correct bias of the dumpers. which is fed forward, after being added to the class A signal and cancels out the error (due to the class A and dumper phases, and amplitudes being the same). Thus Zero cross-over distortion). It is a lot more complicated but in general, that is the principle. The error is detected across a “Bridge Circuit”, then added to the Class A signal.

The same principle is used in the 606 and 909.

please elaborate


“I still use a 34/303 now and then which I really like.” - me too!

But after 53 years of service, they deserve a rest. Quad have continually serviced them for me however, which is a testament to them. So, just as with my 405, I am sticking with them, and re - acquainting myself with Sugden.

This is a great article: -!&&p=b0b6fb0cf9e87029JmltdHM9MTcwNjMxMzYwMCZpZ3VpZD0zYjBkMWJjZS0zNzhlLTZlYmMtMzk4YS0wYTRjMzY2ZTZmYTImaW5zaWQ9NTIxNw&ptn=3&ver=2&hsh=3&fclid=3b0d1bce-378e-6ebc-398a-0a4c366e6fa2&psq=Current+Dumping+-+does+it+really+work&u=a1aHR0cHM6Ly93d3cuYW1wc2xhYi5jb20vUkVDT01NTUVOREVEJTIwUkVBRC9DVVJSRU5UJTIwRFVNUElORy9DdXJyZW50JTIwZHVtcGluZyUyMC0lMjBkb2VzJTIwaXQlMjByZWFsbHklMjB3b3JrJTIwRENELnBkZg&ntb=1



For all your Quad answers
Quad (tel:+ 44 (0)1480 452561


Chatgpt please elaborate


Simply put, the key is a Very High Quality Class A Amplifier capable of amplifying the signal to full output voltage swing. It is a low power amplifier which is continuously working and controls the Current Dumpers at all times.

The Current Dumpers are “Off” in the region where cross-over distortion would normally occur with Push - Pull Amplifiers (Classes A.B, B, C, etc).

When the amplitude of the class A signal (audio) increases above a certain threshold, the Dumpers switch on. They supply the bulk of the power as long as the signal is above the threshold.

However, the Dumpers are essentially Emitter Followers, the Voltage Gain of which is ~ 0.98 of the Class A amplifier - due to the characteristic/nature of emitter followers - so about the same amplitude.

There is no “Critical Bias” on the current dumpers so when they switch on, considerable distortion would exist in the region where the signal swings through zero, i.e., +/-.

However, It is the Class A Amplifier that is providing the drive for the dumpers and again because of the nature of Emitter Followers the Output Signal is In - Phase with the Input Signal.

If you applied the Input Signal, which is the distortion free class A amplifiers output, across one end of a 1:1 transformer primary and the Output Signal of the current dumpers, which is 0.98%~ of the amplitude of class A signal, but includes the distortion created by lack of correct bias to the other, the two signals would be identical, with the exception of the distortion due to lack of bias on the dumpers.

As the signals are equal in Phase and Amplitude, they cancel - except for the Distortion Element due to Lack of Correct Bias on the Dumpers.

The only thing - in the perfect case, seen in the secondary of the transformer would be the error, i.e. the Distortion.

Instead of a transformer, a “Bridge Arrangement” performs this function and this Error Signal is then fed into the Class A amplifier, such that it subtracts from the Audio Signal.

When this Composite Audio Signal is “Fed Forward” to the Dumpers, the Error Element of the Composite Audio Signal, which is in antiphase to the distortion created by the dumpers, cancels out the bias error, resulting in zero cross - over distortion.

That the best I can do to explain how it works. Clearly there is more to it than that and debate still rages as to how it actually does work. But they are just different mathematical analyses, which have kept clever people entertained for years. I am only just coming to this, having been perfectly happy with my Quad 303 for decades.

Its successor, the Artera Power Amp. is nominally the same at 140wpc.

Its max power consumption is 750 watts. That equates to an efficiency of 37.33%, but of course this figure includes all the additional supplementary gubbins.

The output Stages of a Class A amplifier is 25% efficient. So all things being equal, there appears to be a reasonable amount of Primary Class A Functionality. Particularly as, Class A.B. efficiency is given at about 60%.

Well that’s my take on it, but unless Quad are willing to come out with absolutes, the definitive amount of power the class A amp supplies remains unknown.