Quad 'Current Dumping' amps

[Paul R]

I did look for a Linn patent application regarding 'Chakra' with no success. There are loads of 'Linn Products' patents, rather a lot to do with fishing reels, but I didn't see the feedforward power amp stuff.

There is an interesting feed-forward/SMPS one though. Not that I understand it,

(This is on the free index linked from the UK Patent Office)

Paul

 

[LesW]

Thanks to Andrew for the 'denoument' of feedforward although I've yet to encounter an audio circuit with a unidirectional characteristic but am probably too thick to see the inner workings.

The reactance bridge is indeed frequency dependent and as such, presents a limiting factor in expanding the performance of the so called current dumping circuit.

At some time in the past, I added a quite separate regulated power supply to the class A amp and left the dumpers to their own devices attached to a simple, unregulated high current power supply. The results were entertaining to say the least and this beast drove my Gale 401s with not a little verve.

Paralleling devices is possible as demonstrated by the 606 and the nature of the feedback loop may mean that there is little interaction from the non linear characteristics of the devices as with conventional designs - an interesting thought..........

It's probably time to revisit the project as it's still tucked away somewhere in the storeroom, untouched for around twelve years.

 

[Andrew L Weekes]

I've just realised that one bit I wrote above is misleading: -

(e.g. the changing load presented by the current dumpers as they turn on and potentially present a widely varying load to the class A stage).

This is a bit misleading, as one may interpret that the load presented by the base current into the dumpers could cause non-linearity, but this actually isn't the case, as it's also cancelled by the bridge, but the large changes in gain, with varying current in the dumpers, can give rise to non-linearity due to large gain variations in the o/p devices and the non-zero o/p Z of the class A stage. Even this though is theoretically at a very low level in the Quad circuit.

We can address this in several ways though, better choices of output devices now available will help, but the clever way is to see if there is a further modification to the bridge circuit that can remove this problem.

I have some papers somewhere that present just such an idea, hence my keen interest in the circuit, I wonder what Linn have done.

I even have some info on a Class D amp with feedforward correction too!

Andy.

P.S. Paul, I've just found that Linn patent on the PSU, written by none other than Bill Miller - looks interesting!

P.P.S. the Troika body is patented too! (GB2188718)

 

[Zombie]

At some time in the past, I added a quite separate regulated power supply to the class A amp and left the dumpers to their own devices attached to a simple, unregulated high current power supply. The results were entertaining to say the least and this beast drove my Gale 401s with not a little verve.

It's probably time to revisit the project as it's still tucked away somewhere in the storeroom, untouched for around twelve years.

Would be very interested the day you dig it out...

 

[clive7164]

Andrew,
have a look at net_audio.co.uk they do mods for quad amps,it may give you a few ideas of how they improve them
Clive Smith

 

[Zombie]

not much exciting there, except the PSU mod for 405

 

[Disbeliever]

I am experiencing protection relay problems with the new Quad QSP amp. After pressing standby to ON, I get a loud pop through speakers, when switching off directly from wall socket, sound continues 4/5 seconds unless first going back to standby, a pain,even then slight residual shudder. This problem does not occur when switching on/off my Sony DA5400ES receiver all I hear is the relay click, No Pop from Speaker Obviously Quad protection system inadequate needs update. However QSP amp sounds excellent.

 

[Disbeliever]

What about the protection circuits for Dumpers inadequate on new Quad QSP amplifier

 

[Robert]

I am experiencing protection relay problems with the new Quad QSP amp. After pressing standby to ON, I get a loud pop through speakers, when switching off directly from wall socket, sound continues 4/5 seconds unless first going back to standby, a pain,even then slight residual shudder. This problem does not occur when switching on/off my Sony DA5400ES receiver all I hear is the relay click, No Pop from Speaker Obviously Quad protection system inadequate needs update. However QSP amp sounds excellent.

That is normal for nearly all power amplifiers.
If the unit has a standby function (or soft off) and you are bypassing this by using the mains as on/off then noise is not unexpected. It isn't how the amp is designed to be used.

The slow fade out is also normal.
Some amplifiers such as the Sony receiver will mute immediately on power up and power down for several seconds.

Neither is a 'problem' as such, just how the designs operate.

 

[Audio Images]

devialet d premier......... like a 405 with 37 years of evolution??

 

[martin clark]

Quite possibly. The possible application of the feedforward design using a class-A stage with an analogue switching (class D) amp for the dumper stage was suggested in the Vanderkooy /Lipschitz JAES analysis of the 405 (Jan/Feb 1980)

 

[Cable Monkey]

Am I right in saying that my understanding of most conventional class D applications is essentially that they are the class A/D topology as described above? I made that connection when reading up on dumping after I had initially read up on class D amplification but it may have been just the way a particular amp was configured to work.

 

[martin clark]

Not at all, in Class D (= analogue switching amplifiers) the 'output stage' switches between rails at high frequency. There are of course several different approaches to how that switching and/or frequency is controlled to produce the output signal, but it's not remotely the same thing.