Has anybody built one of these? I have two PCB's with solder mask and everything for a Quad 707, and I'm planning to build it for a long time.
So, what's the problem, you ask? First of all: was the 707 actually sold by Quad or is it a mod of the 606, or the 606 MKII? I've checked the original 606 schematic against the one found here http://iquebec.ifrance.com/quadfranco/schemas/amp_707.gif and there are some differences.
Second of all, since I'm not interrested in using the full power rating, I'm thinking about using only one transistor instead of three in parallel in the ouput stage, reducing also the power of the mains transformer and consumption (2x240w output power in class A? I don't think so, this is like keeping the microwave working all the time and one should mind about electricity bill). However, someone told me that thins might affect the "current dumping" (whatever that is). Do you know something about this?
So, what's the problem, you ask? First of all: was the 707 actually sold by Quad or is it a mod of the 606, or the 606 MKII? I've checked the original 606 schematic against the one found here http://iquebec.ifrance.com/quadfranco/schemas/amp_707.gif and there are some differences.
Second of all, since I'm not interrested in using the full power rating, I'm thinking about using only one transistor instead of three in parallel in the ouput stage, reducing also the power of the mains transformer and consumption (2x240w output power in class A? I don't think so, this is like keeping the microwave working all the time and one should mind about electricity bill). However, someone told me that thins might affect the "current dumping" (whatever that is). Do you know something about this?
Hi Mr Push-pull,
If you don't use all the transistors, then the one you leave running alone will go *pop* because it will exceed SOA.
This is not a class-A design but a current dumper, and unless you get everything as original it might not work as well as the Quad ones do.
Cheers ............. Graham.
If you don't use all the transistors, then the one you leave running alone will go *pop* because it will exceed SOA.
This is not a class-A design but a current dumper, and unless you get everything as original it might not work as well as the Quad ones do.
Cheers ............. Graham.
606 rebuild
Hello Mr push_pull;
you should look for a QUAD 306 schematic. The 306 is the smaller version of the 606, also employs current dumping and it is often told that this amp "sounds better". This amp employs only 2 O/P devices instead of 6 and uses a smaller transformer. In my opinion, it is better than the QUAD 405 (which was QUAD`s first current dumping amp).
Current dumping, in simple words, contains a tiny class A precision amp which drives a large "unprecise" amp, these are called "current dumpers". You may find a lot of articles on the web, and you may also find a paper where P.J. Walker describes the principle when announcing it at a AES convention somewhere in the 70s.
606, 606-II and 707 are almost the same as far as the circuity of the power stage is concerned (as far as I know). However, these models are different in a few other details.
cheers, audiomatic
Hello Mr push_pull;
you should look for a QUAD 306 schematic. The 306 is the smaller version of the 606, also employs current dumping and it is often told that this amp "sounds better". This amp employs only 2 O/P devices instead of 6 and uses a smaller transformer. In my opinion, it is better than the QUAD 405 (which was QUAD`s first current dumping amp).
Current dumping, in simple words, contains a tiny class A precision amp which drives a large "unprecise" amp, these are called "current dumpers". You may find a lot of articles on the web, and you may also find a paper where P.J. Walker describes the principle when announcing it at a AES convention somewhere in the 70s.
606, 606-II and 707 are almost the same as far as the circuity of the power stage is concerned (as far as I know). However, these models are different in a few other details.
cheers, audiomatic
current dumping refers to using feedforward rather than feedback correction. A class A amplifier provides ALL of the voltage gain but only a small amount of current. When more current is needed a class B output stage provides the muscle and the error of the class B is fed back to the input of the class A to cancel the distortion. A clever bridge arrangement does this at all the audio frequencies. As long as the Class B is fast enough to keep up with the Class A the system works well. It has the great advantage of not requiring a bias set up, so every one built with the right tolerance components performs like every other.
The original 405 was let down by a too rigid current limit designed for 8 ohm speakers at that time, this was corrected to give more into 4 ohm in the 405-2. Also the input op-amp was 30 year old design and the 405 performs much better with a modern OPA134 or similar. Other small circuit tweaks were done over the years ending in the later designs with parallel output devices and discrete input circuitry instead of an op-amp etc.
The original 405 was let down by a too rigid current limit designed for 8 ohm speakers at that time, this was corrected to give more into 4 ohm in the 405-2. Also the input op-amp was 30 year old design and the 405 performs much better with a modern OPA134 or similar. Other small circuit tweaks were done over the years ending in the later designs with parallel output devices and discrete input circuitry instead of an op-amp etc.
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