Hi, just needing a little help. I'm a tube guy so am lost around transistors!
I have a Quad 77 integrated amp that was going well until I moved it on the shelf while it was going. I heard a few arcs in the power cord socket and now the unit is dead. 😡
Fortunately there are a lot of internal fuses so I've been able to isolate the problem to the main power amp. All the preamp and logic control stuff seems ok.
When ever I put the power amp fuses in, the main fuse blows. I've removed the output transistors (MJL21193/4) and the fuses have stopped blowing. The power amp voltages are +/- 45v. I don't measure any shorts in the output transistors.
Do you think the output transistors are at fault of is it something else ie a driver fet/transistor?
thanks!
I have a Quad 77 integrated amp that was going well until I moved it on the shelf while it was going. I heard a few arcs in the power cord socket and now the unit is dead. 😡
Fortunately there are a lot of internal fuses so I've been able to isolate the problem to the main power amp. All the preamp and logic control stuff seems ok.
When ever I put the power amp fuses in, the main fuse blows. I've removed the output transistors (MJL21193/4) and the fuses have stopped blowing. The power amp voltages are +/- 45v. I don't measure any shorts in the output transistors.
Do you think the output transistors are at fault of is it something else ie a driver fet/transistor?
thanks!
My mistake, it still blows the main fuse when the main amp is connected but with no output transistors in circuit. Bugger.... At least the output transistors measure OK.
Very embarrasing for me...
It had originally blown the 3.15A mains fuse. I only had 2.5A replacement fuses on hand.
It took me a while to realise that the unit is always on ie the standby mode is really just a mute function. That means that the inrush current at the point the power was turned on exceeded the 2.5A fuse rating even though the unit was in standby mode at turn on. It seems that the 3.15A fuse that Quad specified has very little headroom based on New Zealand mains voltage. Bottom line....get the right fuse...
That's all folks.
PS does anyone have the integrated power amp schematic and output stage biasing instructions?
It had originally blown the 3.15A mains fuse. I only had 2.5A replacement fuses on hand.
It took me a while to realise that the unit is always on ie the standby mode is really just a mute function. That means that the inrush current at the point the power was turned on exceeded the 2.5A fuse rating even though the unit was in standby mode at turn on. It seems that the 3.15A fuse that Quad specified has very little headroom based on New Zealand mains voltage. Bottom line....get the right fuse...
That's all folks.
PS does anyone have the integrated power amp schematic and output stage biasing instructions?
A motor or transformer started direct on line needs a fuse rated at roughly three times the equipment's normal maximum current.
i.e. a 225VA 230Vac transformer has a maximum rated current of pretty near 1Aac.
The direct online fuse rating would usually need to be around 2.5A or 3.15A
If it were a 300VA transformer then a 3.15A or 4A fuse would be used.
Instead one can use a soft start circuit that allows a T rated fuse with a rating roughly equal to the normal maximum current to be used and remain reliable for cold starting for years of operation.
I suspect the Quad has not implemented a soft start circuit and uses a transformer of roughly 300VA
i.e. a 225VA 230Vac transformer has a maximum rated current of pretty near 1Aac.
The direct online fuse rating would usually need to be around 2.5A or 3.15A
If it were a 300VA transformer then a 3.15A or 4A fuse would be used.
Instead one can use a soft start circuit that allows a T rated fuse with a rating roughly equal to the normal maximum current to be used and remain reliable for cold starting for years of operation.
I suspect the Quad has not implemented a soft start circuit and uses a transformer of roughly 300VA
AndrewT you are correct, Quad did not use a soft start and I estimate the toroidal transformer at 500VA.
I'm surprised that a 500VA can start direct online with a T3.15A mains fuse.
That's a factor of only 1.5 times the normal maximum operating current of ~ 2.2Aac
I don't know enough about transformers, but the transformer Designer can adopt some techniques to reduce the starting current at power ON. Maybe Quad specified such a design for their amplifier.
That's a factor of only 1.5 times the normal maximum operating current of ~ 2.2Aac
I don't know enough about transformers, but the transformer Designer can adopt some techniques to reduce the starting current at power ON. Maybe Quad specified such a design for their amplifier.
I know a Quad 306 arranges a circuit breaker on the active 240V side of the transformer that protects against speaker shorts and overload. It is very effective. It appears to act well before the fuse,
and to cover almost every circumstance.
But looks like Quad took this a stage further in the 77 integrated amp,
https://www.hifiengine.com/manual_library/quad/77.shtml
"The protection is controlled by a microprocessor which constantly monitors the state of the amplifier"
and to cover almost every circumstance.
But looks like Quad took this a stage further in the 77 integrated amp,
https://www.hifiengine.com/manual_library/quad/77.shtml
"The protection is controlled by a microprocessor which constantly monitors the state of the amplifier"
A 500VA toroid weighs in at around 5kg yet the total weight of a QUAD 77 is only 6.3 kg. Given that an amplifier designed for a spec. of 2 x 84W/8R only requires about 250VA and it's not spec'd like the 606-909 series to power ESL speakers, I'd suggest the transformer isn't really much more than 300VA. That might make the 3A fuse rating a little more credible.
Last edited:
Doesn't the protection referred to relate to the amp output? Certainly when I had my amp apart there was no evidence of mains monitoring via microprocessor. The psu looked very simple and traditional. Ie big transformer, bridge rectifier, 20,000uf filtering. The lower voltage logic control psu and preamp psu had Lm317 type regulation.
Yes, as described, it's a circuit breaker which takes the place of a mains fuse and doesn't specifically protect the speakers or anything else until its steady mains current rating is exceeded. Being a thermal/magnetic device, it actually responds relatively slowly, ignoring fast pulses and behaving a little like a slow-blow fuse. It is rated only 0.5A which is OK for a nominal 50W/channel amplifier operating from a nominal 230V AC supply and you may notice this is a lot less than the 3A glass fuse rating needed to protect your 84W/channel model 77. This rating makes a joke of its ability to protect against anything other than disasters but is needed to avoid nuisance blowing with the high starting surge current.
A relatively slow device like the circuit breaker will only protect a very robust output stage such as in the older 306 or 606 models and in my view, might not be a wise modification to the more conventional Quad 77 design. Now that the problem is corrected and the original fuse replaced, you don't have much reason to make further changes anyway.
A relatively slow device like the circuit breaker will only protect a very robust output stage such as in the older 306 or 606 models and in my view, might not be a wise modification to the more conventional Quad 77 design. Now that the problem is corrected and the original fuse replaced, you don't have much reason to make further changes anyway.
- Status
- Not open for further replies.