Hi and thanks for reading this post.
I picked up an old quad 405 power amplifier from ebay the other day and noticed a few swollen capacitors inside, especially the power supply capacitors which look really bad the amp is about 40 years old and I am wondering if I should just change the components which are obviously damaged, or change them all. I’m worried because the amp is working for the moment but if it breaks down most places near me charge £40 just to look at an amplifier, the total cost of a repair may well be double that, I could probably replace all the board’s components for that kind of money.
Also there are 2 driver transistors per amplifier board, the RCA 40872 and I am thinking to replace them with the Motorola MJE15031. But the middle leg of the RCA transistor has been removed, and on the board there are only two through-holes for this transistor, so obviously this has been done by the manufacturer. How should I approach this?
Finally there are two Toshiba 2sd424 output transistors per amp board which I am replacing with Motorola MJ15003. I have noticed on a few different amplifiers from different manufacturers, they place a capacitor across the positive and negative power lines of the output transistors. The quad does not have this but could I do this for the quad, I was thinking of 200uf 100volts.
Many thanks in advance. John.
I picked up an old quad 405 power amplifier from ebay the other day and noticed a few swollen capacitors inside, especially the power supply capacitors which look really bad the amp is about 40 years old and I am wondering if I should just change the components which are obviously damaged, or change them all. I’m worried because the amp is working for the moment but if it breaks down most places near me charge £40 just to look at an amplifier, the total cost of a repair may well be double that, I could probably replace all the board’s components for that kind of money.
Also there are 2 driver transistors per amplifier board, the RCA 40872 and I am thinking to replace them with the Motorola MJE15031. But the middle leg of the RCA transistor has been removed, and on the board there are only two through-holes for this transistor, so obviously this has been done by the manufacturer. How should I approach this?
Finally there are two Toshiba 2sd424 output transistors per amp board which I am replacing with Motorola MJ15003. I have noticed on a few different amplifiers from different manufacturers, they place a capacitor across the positive and negative power lines of the output transistors. The quad does not have this but could I do this for the quad, I was thinking of 200uf 100volts.
Many thanks in advance. John.
Agree. No need to replace transistors until they break down. Just replace the caps.
Could the Toshiba indicate a later production? I owned a 405-1 and remember something like BU xx in the power-stage.
Sold the Quad later to buy a Moscode .
You're right about the BDY's
I remember that I switched them out against MJE15023's which sounded better.
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Hum, i have plan to build Quad 405, still looking for last schematics, and best version ...
There is PCBs on eBay, but seems like all of them are first version ...
Also, i got NOS Toshiba 2N3773 and dunno if this is recommended upgrade or Quad was using those transistors @ some point too ...
There is PCBs on eBay, but seems like all of them are first version ...
Also, i got NOS Toshiba 2N3773 and dunno if this is recommended upgrade or Quad was using those transistors @ some point too ...
Look at the second link in this post : http://www.diyaudio.com/forums/solid-state/261554-quad-405-power-amp-advice.html#post4049508
I think that you should look no further if you have NOS 2N3773. It's a good choice.
I also have Chinese QUAD 405 pcbs (by LJM) and it's first version using LM301A input opamp, but you can use any other pin compatible opamp, like TL071 (QUAD's later choice). I intend to use OP27. Output transistor choice is not critical. You can use MLJ2119x if you want.
Yeah ... i have some parts i ordered from UK, like this 0.09 ohm resistor, some small transistors etc, and talking about opamp, i was thinking maybe LME49710 would be a good choice, i even have this neat heatsink for Output transistors ...
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And there is mine 2N3773 collection ...
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An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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Keith Snook explained in his excellent article on QUAD 405 that LME 49710 may not be a good choice because of it's superior bandwidth. With such high speed opamp QUAD 405 may not be stable. You need something with low offset and reasonable speed like good j-fet opamps (OPA134). I shall not use OPA134 because I do not like the sound of j-fet opamps. I think that OP27 will be very good in this application.
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Yes i read this"
" ... The "new kid on the block" the National LME49710 may be a replacement option with only 7nA bias current but it has a high GBWP which could make it unstable and you will not get the published specification in the 405 circuit ~ Many application circuits for these "new" op–amps show current sinks connected from output to ground so the device output does not cross zero and measures better as would almost any op–amp
Beware of bipolar input devices like the NE5534 which is often substituted in the QUAD 405 ~ The NE5534 and the dual NE5532 etc. are very good op–amps and were probably used to produce much of the music you listen too but they may give an output offset of 150mV when used in the 405
C2 removes the a.c. content of the feedback signal as explained next ... "
Now i think i should look for someone who have tested both opamps and to see whats goin on ... Some people says is OK, this is ONLY guy who say is not .... DUNO really
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