New Quad QSP / QMP Pwr amps

[spidertooth]

Common to overlook the 77 series due to its strange 2-way remote control system, but it is extremely good-sounding kit and no reliability problems with the final set of ROMs. (Ironic that like many esoteric designs, they finally made it work after they'd already decided to ditch it).

The amplifiers (the non current-dumpers) in particular are exceptional value for money even at the original price. In my systems they have seen off a number of potential "upgrades", some of which on paper should be in a different class.

I believe they designed by Jan Ertner, as something of a break with the current dumping tradition (which was continued in the 707). So how was it, then, that they ended up being so very Quad?

 

[Disbeliever]

Hi John W
QSP there is no AC detect.

I am confused when you say as with the original Quad designs, there's no output protection relay, however you end your very interesting post by saying : Its ironic then that a simple mod. of the existing output stage protection circuit based around T4 would have made a better"shut down" mode and was already available....

 

[JohnW]

Hi John W
QSP there is no AC detect.

I am confused when you say as with the original Quad designs, there's no output protection relay, however you end your very interesting post by saying : Its ironic then that a simple mod. of the existing output stage protection circuit based around T4 would have made a better"shut down" mode and was already available....

There are Pros and Cons to using Relays on the speaker output -

Cons: some believe can effect Audio Quality and relays are known to be unreliable + added cost!

Pro: Added layer of protection* and can help insure "silent" power cycling.

*On the 606 based designs (QSP is a 606 based design) - the speaker is effectively AC coupled - so inherently DC protected.

I was commenting on the circuit topology - that there was already an accessible node where the amplifier could be shutdown - without need to added the Lower side "clamping" circuit... Nothing important - maybe just thinking out aloud...

John

 

[IanG-UK]

Slightly off topic but when I went into the IAG room at the Munich show (marked "VIP invited guests only" but that did not deter me) the three IAG representatives were entirely uninterested in promoting their goods whether that be Audiolab or Quad or Wharfedale or ...

In fact they were releasing a new Quad electrostatic (the 2912 - looks like a 2905 and probably is much the same) as well as the Platinum series of Quad electronics. Normally that would be big news for the audio community. I haven't heard a thing about it since Munich, whether in the hifi press or on the internet.

If this thread and my experience is representative, I shall not be buying any more Quad gear - having owned it for 30 years.

What a pity such a brand is being tarnished ...

 

[JohnW]

Walker and Albinson didn't of course see them as practical limitations in the real world.
So yes you could fix come things but those only matter if the results are audible, and they both argued that the design was 'good enough' - it's the old transparent is transparent and you can't make it more transparent argument.
For example you could have applied far better component tolerance selection to the bridge components but back in 1975 that would have pushed up costs for what they regarded as inaudible benefit.

The 'C' mod certainly lowers distortion and it costs nothing to implement, but the reduction achieved is again very modest.

At the end of the day, you have something that even in it's flawed and limited technical state (and nothing is perfect) returns THD and IMD well below audibility and will drive most loudspeakers without clipping. Certainly a serviced 405 will deliver <0.002% (excluding input stage noise) and is flat across the audible range. Better is moot IME.

Walker's engineering ethos centred on reliability, consistency and value in a design that was good enough for the job required. In essence, they wouldn't significantly over-engineer where they believed there would be no audible benefit to most users, and no improvement to reliability. Such action was considered to simply raise the cost to the customer for no useful benefit.

Nevertheless, that was 30 years ago and there can be no doubt that if both men were still refining their amps and the CD circuit today it would likely look a little different. Things do progress, though where things like the 405 are concerned it would be refinements to an already good performance IMO.

With the greatest respect to Walker and Albinson!!! the last thing the world needs is another "Arm Chair" designer (referring to my comments)!!!

With today's powerful simulation tools which where not available when the original 405 was invented - its easy to simulate and pull apart the design... But also brings a smile to my face when I unravel clever "hidden" circuit tricks used by the original designers...

As I say, I'm nothing but an "Arm Chair designer" until I actually have a reliable amplifier product for sale - and this still world not necessarily put me anywhere near the same league as Walker and Albinson!!!

I'd not agree with your comments upon sound quality - as I find little correlation with THD and subjective sound quality - but we can happily agree differ on this subject as the last thing anyone one of us needs like a hole in the head is to waste our precious time on an subjective verse " measurement" debate!!!

John

 

[Robert]

Indeed John.

I'm certainly not against revising the circuit - for that is exactly what Walker and Albinson did over the years and would still be doing today. The 6 series are evolutions of the 405 though it isn't clear who revised them.

So long as the results are related directly to how the thing actually sounds against peers or works in everyday use, there is no objection from me.

 

[Disbeliever]

There are Pros and Cons to using Relays on the speaker output -

Cons: some believe can effect Audio Quality and relays are known to be unreliable + added cost!

Pro: Added layer of protection* and can help insure "silent" power cycling.

*On the 606 based designs (QSP is a 606 based design) - the speaker is effectively AC coupled - so inherently DC protected.

I was commenting on the circuit topology - that there was already an accessible node where the amplifier could be shutdown - without need to added the Lower side "clamping" circuit... Nothing important - maybe just thinking out aloud...

John

Cost the amp is expensive enough to be able to include the added cost. My existing 120 watt amplier is 14 years old and never had a problem with relay reliability. IAG's Rob tells me they have a solution to my complaint ,will be implemented on the two amps I rejected, will be interesting to learn exactly what it is ?

 

[JohnW]

Cost the amp is expensive enough to be able to include the added cost. My existing 120 watt amplier is 14 years old and never had a problem with relay reliability. IAG's Rob tells me they have a solution to my complaint ,will be implemented on the two amps I rejected, will be interesting to learn exactly what it is ?

Rob at IAG is a very sound guy - so I'd trust in the solution,

John

 

[JohnW]

I must hasten to add, that EVERYONE at IAG UK are a great team...

 

[Disbeliever]

John W I am advised by a technical friend that the 606 is NOT AC coupled, he has the CCT , he says you are thinking of the 303

 

[JohnW]

Your friend is mistaken (basically made a simple assumption based upon "typical design" topology) - ask him to trace where the Ground connection of the speaker is returned - notice there is no direct connection to the transformers secondary side "mid point" typically used as a PSU Ground reference point... the Speaker is "center tapped" between the center points of the Bulk reservoir Caps - but this "Center connection" is not returned to the transformer center point (atleast not directly - its actively "centered").

The dual transistor "active" potential divider sets the "mid point" reference point - well its "offset" slightly to facilitate "balanced" output voltage swing - this configuration allows inherent DC offset protection.
John

 

[sergeauckland]

Your friend is mistaken (basically made a simple assumption based upon "typical designs") - ask him to trace where the Ground connection of the speaker is returned - notice there is no direct connection to signal Ground... the Speaker is "center tapped" between the center points of the Bulk reservoir Caps - but this "Center connection" is not returned to signal Ground (atleast not directly).

The dual transistor "active" potential divider sets the "mid point" reference point - well its "offset" slightly to facilitate "balanced" output voltage swing.

John

My Behringer A500 amplifiers use a similar topography. The junction of the two output transistors goes to one side of the loudspeaker load whilst the other side goes to the junction of the main reservoir capacitors which is NOT returned to ground. This makes the output AC coupled, but with the AC coupling within the feedback loop that results in a very low output impedance even at low frequencies.

The Quad 303 had a conventional capacitor-coupled output in which the coupling capacitor was outside the feedback loop and therefore the output impedance rose at LF.

S.

Edit: Some amplifiers did have the output coupling capacitor within the feedback loop by the expedient of taking the feedback tapping point from the loudspeaker terminal rather than the junction of the output transistors. However, this would require a separate DC feedback path for stabilisation, and would also mean a small DC offset on the output that might mark the amplifier down in magazine reviews.

 

[Robert]

306 also shares the same psu topology as the 606.
Virtually identical circuit in fact using just one pair of dumpers instead of three.
Nice little power amp as you can pick them up for £150.

Not looked at QSP schematic to see if they retain this method.

 

[Arkless Electronics]

QSC have long used this topology (The Behringer amps are an exact copy to the extent that you can use the QSC service manual for the Behringer! Don't know if they ripped off QSC or some deal!?). The ATC power amps also use this topology and I believe Hafler used it quite early on.
Seem to recall an article by Prof. Cherry going back further though....

 

[Disbeliever]

306 also shares the same psu topology as the 606.
Virtually identical circuit in fact using just one pair of dumpers instead of three.
Nice little power amp as you can pick them up for £150.

Not looked at QSP schematic to see if they retain this method.

IAG will not release a schematic of the QSP

 

[Disbeliever]

John W

Vince says I am familiar with the CCT and the power supply arrangement having modified the 606 many times and it is true that the derived earth is artificially created and the 606 was not prevalent to switch-on/off plops, but the QSP is for some reason, I had tried to get the CCT to find out why and assumed that IAG had gone away from this earthing arrangement, because in the original 606 the main caps were inadequate at 6,800uf and the zero point shifted from roughly zero to a non-zero condition dependant the speaker load thus causing compression and also limiting the bass performance, the small PUMP CCT used to maintain this zero condition could only handle a very small current, about 25ma so during the 10m/s intervals the main caps could wander, one taking a charge from the other on non symmetrical audio signals, because of this action on the charge cycle the main caps would receive an equal charge and behave as two caps in parallel however on this discharge, because one cap could steal a charge from thev other,the two caps behaved as two caps in series with double the effective esr etc,this happened because the pump CCT could not supply sufficient current to equalize the charge and maintain zero volts for anything like real speaker loads and the inevitable current drain from the supply, Ross Walker was told about this and my mods. which was to increase the main caps to 10,000uf Elnas and add two reverse diodes across the caps to stop the zero point shifting by more than 0.6 volts, and keeping the pump CCT, the 909 was originally good when Stan Curtis was in charge, but when he left the main caps were downgraded to a cheaper cap which had a very poor high frequency behaviour and distortion was very much in evidence, independent testers vouched for this, and it was widely promulgated on some forum or other. ... Ross Walker stated that since the amplifier met it's specification no modification could make any difference, so the 606 for many became a mediocre amplifier working far below its real potential and sales suffered.. This amplifier has had a chequered history but is fundamentally very good. There were other things that Quad did to the power supply that defies good engineering practice and is almost beyond belief, butb that's history and I don't think people are really interested we are going back into the 80's. Vince Hawtin

 

[JohnW]

John W

Vince says I am familiar with the CCT and the power supply arrangement having modified the 606 many times and it is true that the derived earth is artificially created and the 606 was not prevalent to switch-on/off plops, but the QSP is for some reason, I had tried to get the CCT to find out why and assumed that IAG had gone away from this earthing arrangement, because in the original 606 the main caps were inadequate at 6,800uf and the zero point shifted from roughly zero to a non-zero condition dependant the speaker load thus causing compression and also limiting the bass performance, the small PUMP CCT used to maintain this zero condition could only handle a very small current, about 25ma so during the 10m/s intervals the main caps could wander, one taking a charge from the other on non symmetrical audio signals, because of this action on the charge cycle the main caps would receive an equal charge and behave as two caps in parallel however on this discharge, because one cap could steal a charge from thev other,the two caps behaved as two caps in series with double the effective esr etc,this happened because the pump CCT could not supply sufficient current to equalize the charge and maintain zero volts for anything like real speaker loads and the inevitable current drain from the supply, Ross Walker was told about this and my mods. which was to increase the main caps to 10,000uf Elnas and add two reverse diodes across the caps to stop the zero point shifting by more than 0.6 volts, and keeping the pump CCT, the 909 was originally good when Stan Curtis was in charge, but when he left the main caps were downgraded to a cheaper cap which had a very poor high frequency behaviour and distortion was very much in evidence, independent testers vouched for this, and it was widely promulgated on some forum or other. ... Ross Walker stated that since the amplifier met it's specification no modification could make any difference, so the 606 for many became a mediocre amplifier working far below its real potential and sales suffered.. This amplifier has had a chequered history but is fundamentally very good. There were other things that Quad did to the power supply that defies good engineering practice and is almost beyond belief, butb that's history and I don't think people are really interested we are going back into the 80's. Vince Hawtin

No argument from me concerning the points raised - I'd be in full agreement.

The QSP Grounding arrangements still remains the same - but as I said in my earlier post the 606 was a "carefully balanced design" to try and avoid the issues you are experiencing.

Lets place some trust in the modifications - hopefully they well resolve the issues.

John

 

[Plutox]

QSC have long used this topology

Very impressive kit, QSC. We had set up a big rig, which worked but seemed oddly out of balance between left and right - corrected by adjusting gains. Come the take-down, a junction was found to be wired incorrectly resulting, it was found, in a load of about 1.5Ω for one of the QSC amplifiers, which didn't blink.

 

[Robert]

No argument from me concerning the points raised - I'd be in full agreement.

It would still be interesting to see what effect these things actually have on the output of the 606>, if any.

I do wonder who designed them though, since when the 606 came along PJW was semi-retired.

 

[Disbeliever]

Hi John W

Can you recommend a amplifier that is better than the Quad QSP, without the annoying turn on & off thumps never cured as far as I am aware. I like the Chord SPM 1050 but it is far too expensive.

Regds
Disbeliever