Current Dumping. The way forward !

I have had an interest in current dumping amplifiers for many years having built a Quad 450 clone more than 20 years ago. The current dumping principal seems to offer many advantages with no bias adjustment excellent stability etc and there has been much discussion in the past over how and whether it works as Quad claimed. The later Quad amplifiers such as the 606 aka 707 and 909 are often well reviewed and I think would make the basis for a very good amplifier. Many of the Quad designs are compromised by commercial necessity yet still offer excellent performance. All of these amplifiers were designed before modelling was possible. Would anyone be interested in modelling the Quad 606 to try to further understand how it works and to better realise its potential. I have a few spare 707 clone PCBs but I have not yet had the chance to build one.


Disabled Account
2005-05-25 2:25 am
It looks like two amplifiers in one chassis: the upper PNP devices function as a class A amp. When it runs out of juice, the lower PNP-NPN quasi complementory pair (Class B or C as it is underbiased) kicks in.

If the op-amp has sufficient output current capabilities, I am sure you can even eliminate the Class A amp as well.

The key seems to have very fast op-amp so that it can swing extremely fast through the cross-over regions of the Class B amp.

In theory it should work.
Right, it is the article "Current dumping, does it really work" published in Wireless World, June 1978, by Vanderkooy and Lipshitz.

Interesting also is:
It shows several versions of the 405, including PCB layout and how to make the coils.
This site also shows a version with a complementary output (!), from Solntsev (published in 1984, reference 5 in the article).

Hi Pavel,

I'm not sure what you're aiming at. If you mean to ask whether I do like the Quad 405 and derivatives with all their chokes, et cetera? Not really, current dumping is a smart concept, but I doubt it is the best concept for the audiophile.
I like unconventional amplifiers, though, but that is also because I might learn something from these. There are already so many boring mainstream amplifiers.

Hi Steven,
if we take it like smart concept, so yes. Practilal realisation and tuning of this principle is thing, which was " outside my borders ", 'cos it can good measuring equipment, which wasn't in my country available :(. But what is the best concept for audiophile ? Are many looks at this thing and I mean, that most of them are nonobjective and have strong commercial interests. Or do you mean, that can exist something like " Amp's Rose Cross Broterhood " for only knowledgeable ? :cool:
405man said:
Has any one tried any of these mods

The current sink would certainly be suitable for the 606 etc


Hello everyone,

Well, I implemented this modification to a 707 clone.
I built this current source on a separate small PCB.

Now the 707 it sounds really very good. :)
Bass is much deep and highs are more detailed.

Schematic with components used by me is here


I also plan to change 6V8 zenners with LM329.

Any comments/suggestions are much appreciated.

405man said:
Have you thought of changing the value of the 12R resistor in the current sink to increase the area of operation of the class A amplifier..Stuart

Hello Stuart,

I guess you mean to increase the value of resistor in order to reduce current trough class A MJE15033 transistor and move his static operation to a more safe SOA.
Reducing current may have following effects:
- a very low current will make MJE15033 transistor to operate in a nonlinear region
- amplifier will loose power. If lower dumpers will require a low base current (thanks to BD244C), upper (top) dumpers needs at least 50mA to achieve full output power.
- amplifier slew rate will suffer, due C8 47pF is charged and discharged also by class A stage.

Please correct me if my above statements are wrong.

But, in order to achieve full power and increase slew rate a solution is to double current and MJE15033 transistor as per below drawing.


As well this solution is a compromise.
This will require to replace 3k3 (0.4mA) and 330R (2mA) resistors in class A stage with current sources with higher current 0.6-0.8mA respective 3-4mA in order to feed these two MJE15033.