No problem 🙂 Just change the transistor to be sure, check the low value resistors around it and then use a bulb tester for initial powering up. Do not attach speakers until it is all proved good.
I will need to order the Tr’s, DADA has a list of equivalents I can use. Worked late so won’t have a chance to chase the fault today. Does anyone know if the output from pins 1 and 3 of an xlr would be a better input than a SE as it would be inverted?
If you believe in absolut phase, if such a thing exsist, change (reverse) the connections of the speakers.
I couldn’t tell the difference when I tried that when I first got the amp.
Ordered the Tr’s from eBay just in case, I hope they are not fakes. 10 of each for less than £7 all from the same seller I think.
Ordered the Tr’s from eBay just in case, I hope they are not fakes. 10 of each for less than £7 all from the same seller I think.
It's not a matter of belief, double-blind listening tests have shown that a polarity change is audible when you listen to very dry, monaural recordings of specially selected music in an acoustically dead room. Swapping the speaker wires is indeed a good fix.
I never noticed the differance, but that is only my experiance. It could be differant for someone else.
Did you ever listen to a very dry monaural recording in an acoustically dead room? Under more normal circumstances, it tends to be obscured by reverberation.
I am still waiting for one set of Tr’s. I have not started testing the board yet.
I have been trying to think of something to do with the QUAD QUAD I bought. It all works apparently so I have 4 amplifier boards. Thinking about buying a cheap 1000W +-40v LLC SMPS and making a 200W per channel stereo amplifier. Would need some resisters, 5k trim pots, a soft start, speaker protection and a cheap 8ohm speaker for testing. It will need to be a cheap Chinese smps and speaker protection circuit as I don’t want to throw loads of money at it. I will start a new thread if I start it and I need to concentrate on fixing my blown board first.
I have been trying to think of something to do with the QUAD QUAD I bought. It all works apparently so I have 4 amplifier boards. Thinking about buying a cheap 1000W +-40v LLC SMPS and making a 200W per channel stereo amplifier. Would need some resisters, 5k trim pots, a soft start, speaker protection and a cheap 8ohm speaker for testing. It will need to be a cheap Chinese smps and speaker protection circuit as I don’t want to throw loads of money at it. I will start a new thread if I start it and I need to concentrate on fixing my blown board first.
If you want to operate two boards in parallel, the extra power will depend on the load, in principle it will deliver twice the current, but only when the load is low enough. At 8 Ohm load there is hardly a current limiting factor, so two boards in parallel will not bring much, especially if you lower the Psu voltage to 40V. If you bridge the amplifier modules, you need some circuitry upfront to do this, or use a special input transformer (not popular these days). In the Quad 405 service manual there is a graph which give information about this monoblocking or bridging and the expected power.
A bit confused about SMPS’s. A 35-0-35 transformer will produce after being rectified 50v; would a smps with v+ GND v- @ 50v be the equivalent? And if I wired that smps to my amps C13 and C14 would that work (it would need a soft start)? I don’t want to do this to my 405 but might do something with the ugly QUAD QUAD when it arrives.
I replaced all the Tr’s. Tr7 and 8 were ok. Tr9 and 10 were bad. It blew the fuses again. No speaker was fitted, could the clamp circuit kick in? Really going to have to do a comparison between the working and none working with my DMM. And also make a Dim bulb tester. And perhaps read up on fault diagnoses….
I replaced all the Tr’s. Tr7 and 8 were ok. Tr9 and 10 were bad. It blew the fuses again. No speaker was fitted, could the clamp circuit kick in? Really going to have to do a comparison between the working and none working with my DMM. And also make a Dim bulb tester. And perhaps read up on fault diagnoses….
Besides, when you connect two channels in parallel, you will get large currents flowing from one channel to the other because of offset differences. Adding a small resistor at the output of each channel can limit the current.
SMPS can seem a bit counter intuitive at first. The ripple from the SMPS is not at 100 or 120Hz but is much higher, many tens of kHz. This means the reservoir caps can (and should) be much much smaller. You may find values as low as 100uF.
The SMPS may object (by failing to start-up) if presented with massive capacitive loading.
If the the SMPS rails are regulated then high value caps can work against the regulation (the feedback loop operation) in a similar way to it not being advised to add large caps to the output of many common linear regulators such as the 78/79 series.
Any feedback loop for regulation needs to respond quickly to changes in load current, a big cap would tend to initially 'help' with supplying a peak in current and then cause problems as the PSU catches up and has to try and replenish the charge in that cap.
Any soft start for an SMPS would normally be 'designed in' and work by altering the initial duty cycle of the switching elements to bring the supply up. Traditional soft start approaches such as a thermistor in the mains input would not normally be advisable and could cause issues. Remember the reservoir cap (across the rectified mains) will be relatively small, probably just a few hundred uF in size.
Marcel, correct, but I was asuming he followed the recipe in the Service Manual, which mention 0.3 Ohm balance resistors, but then I discovered there are differant descriptions with differant kits and solutions, so when he decides what he wants, he can be helped with a working and safe solution.
I have only seen the detailed instructions for running two channels in parallel. Is the virtual or floating ground at 0V always needed with a 405? I have read about large caps being a bad idea for smps. I am listening to a few songs in mono through one speaker before I strip the 405 again.
A 405 does'nt have a virtual ground, only the 520f and the 606 range. A 405 has a normal supply with a centre tap of the TX needed.
I think the 405 used a conventional supply with a centre tapped transformer while the later 606 and 909 used a virtual ground arrangement.
Running two amps in parallel requires the two 0V lines to be joined. The two power supplies for each can then either be one common one or two separate ones... as ever the devil is in the detail though.
Running two amps in parallel requires the two 0V lines to be joined. The two power supplies for each can then either be one common one or two separate ones... as ever the devil is in the detail though.
A standard 405 does have one supply and the two grounds are also connected in standard form, but you are correct about the variations that can be possible when you modify the unit. And the consequenses this may have when you monoblock or bridge the amplifier, a next headache/challence will be the DC protection.
I have had the DMM out. Tr6, Tr7, Tr9, Tr10, CTr10, C16, C15, C5 &C10 are all unserviceable or reading differently to the goof board. My DMM is not very good and is telling me C1 on both board is unserviceable so I have ordered a new DMM and will recheck everything and then check the components off the board.