Hello all, this may be a shot in the dark but i have read some excellent threads here so i know there are some well informed and experienced folk around !
I have a Quad 405 (mk1) which was modded some moons ago by a gentlemen named Ivan who was residing in London at the time.
Unfortunately i cannot remember what mods i had done exactly, but i do know it has dual power supplies* and rca inputs added.
(I've added pictures of the internals of the amp to this post)
Unfortunately i lost contact with him and have been unable to locate him.
I am going to try and use a potentiometer (alternatively named a passive preamp) inbetween my audio interface's DAC and my Quad 405.
The audio interface is a Fireface800 and the Output Impedance is 75 ohm.
So going by the rule of 20x, i am going to use a potentiometer at 1500 ohm, and then multiply that by 20 for the Quad = 30,000.
I believe the Quad 405's original Input Impedance is 20,000k, and i would be ok with that, however Ivan informed me back then that he added a resistor to the Input Attenuator which brought the Quad's Input Impedance down to 6800k.
(ah, i've just remembered that the Quad's Input Sensitivity was adjusted to 1.5v)
So my question is, can i simply remove this resistor in hopes of returning it to 20k, or does it involve making other adjustments ?
Or maybe even -can i increase this amount to 30k without complex work.
I am not proficient at circuits and electronics, but i am competent and experienced with soldering and getting simpler tasks done.
Any input at all would be most appreciated, thank you !
I have a Quad 405 (mk1) which was modded some moons ago by a gentlemen named Ivan who was residing in London at the time.
Unfortunately i cannot remember what mods i had done exactly, but i do know it has dual power supplies* and rca inputs added.
(I've added pictures of the internals of the amp to this post)
Unfortunately i lost contact with him and have been unable to locate him.
I am going to try and use a potentiometer (alternatively named a passive preamp) inbetween my audio interface's DAC and my Quad 405.
The audio interface is a Fireface800 and the Output Impedance is 75 ohm.
So going by the rule of 20x, i am going to use a potentiometer at 1500 ohm, and then multiply that by 20 for the Quad = 30,000.
I believe the Quad 405's original Input Impedance is 20,000k, and i would be ok with that, however Ivan informed me back then that he added a resistor to the Input Attenuator which brought the Quad's Input Impedance down to 6800k.
(ah, i've just remembered that the Quad's Input Sensitivity was adjusted to 1.5v)
So my question is, can i simply remove this resistor in hopes of returning it to 20k, or does it involve making other adjustments ?
Or maybe even -can i increase this amount to 30k without complex work.
I am not proficient at circuits and electronics, but i am competent and experienced with soldering and getting simpler tasks done.
Any input at all would be most appreciated, thank you !
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First, I would worry about the terrible solder joints on the speaker connectors.
It isn't possible to convert a 405 to monoblocks unless you use two of them.
Source loading impedance must be according to the mfr spec of your source component, which is most often 10k, or more.
Otherwise, huge coupling capacitors would be necessary. This is why volume controls under 10k are seldom seen.
It isn't possible to convert a 405 to monoblocks unless you use two of them.
Source loading impedance must be according to the mfr spec of your source component, which is most often 10k, or more.
Otherwise, huge coupling capacitors would be necessary. This is why volume controls under 10k are seldom seen.
"First, I would worry about the terrible solder joints on the speaker connectors."
😆 Yep, it made me cringe aswell seeing that today, but because it has always just worked without hiccup i never troubled it !
I will definitely be doing a resolder.
"Source loading impedance must be according to the mfr spec of your source component, which is most often 10k, or more.
Otherwise, huge coupling capacitors would be necessary. This is why volume controls under 10k are seldom seen."
This is great info thank you. I was wondering why 10k was the lowest i had come across.
I've just seen a mod where a 3k series resistor is added to a 10k pot, would that be the way to go about it ?
😆 Yep, it made me cringe aswell seeing that today, but because it has always just worked without hiccup i never troubled it !
I will definitely be doing a resolder.
"Source loading impedance must be according to the mfr spec of your source component, which is most often 10k, or more.
Otherwise, huge coupling capacitors would be necessary. This is why volume controls under 10k are seldom seen."
This is great info thank you. I was wondering why 10k was the lowest i had come across.
I've just seen a mod where a 3k series resistor is added to a 10k pot, would that be the way to go about it ?
If you remove the resistor you will lose the gain change and you will also possibly move the LF point of the 405.
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I would revert to the original Quad input circuit. There is no input attenuator, rather it is a "virtual earth" input circuit.
This should not be changed.
There's no reason to add a series resistor before a volume control, unless in normal use the control can be turned up
only slightly, before the volume becomes excessive.
This should not be changed.
There's no reason to add a series resistor before a volume control, unless in normal use the control can be turned up
only slightly, before the volume becomes excessive.
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ejp : "If you remove the resistor you will lose the gain change and you will also move the LF point of the 405."
Ah it's as i suspected, i don't have the experience and know-how to make these adjustments.
I will have to find someone in London or beyond who can do it and i may aswell have a power button added if possible !
rayma : "I would revert to the original Quad input circuit. There is no input attenuator, rather it is a "virtual earth" input circuit.
This should not be changed."
Thank you for the Service Data ! I'll take a look and see if it's something i can do.
I do hope to learn in time (and when i have more free time in future), at the moment i am in the middle of a studio rebuild/rewire and it's driving me crazy lol
Thank you so much for your input guys, you're all fantastic around here
Ah it's as i suspected, i don't have the experience and know-how to make these adjustments.
I will have to find someone in London or beyond who can do it and i may aswell have a power button added if possible !
rayma : "I would revert to the original Quad input circuit. There is no input attenuator, rather it is a "virtual earth" input circuit.
This should not be changed."
Thank you for the Service Data ! I'll take a look and see if it's something i can do.
I do hope to learn in time (and when i have more free time in future), at the moment i am in the middle of a studio rebuild/rewire and it's driving me crazy lol
Thank you so much for your input guys, you're all fantastic around here
The guy you mentioned is a hack and to be honest looking at the amp I would bet those large psu caps are just glued and cable tied to the chassis and that I think would be the tip of the iceberg with it. As mentioned going back to the original circuit would be good as would the dada mods but that's after undoing that mess first.
I’ve seen some of that before. That 2nd rectifier is just held to the ridge on the transformer via one screw and nut and can be pried off easily. It is not secure. And it has no heatsinking.
And that’s just one component.
And that’s just one component.
+
Thank you guys, so i'll leave it be for now and seek to have it re-done in future.
I'm shocked to hear that you know of Ivan ! ..and that after all these years he is still remembered and for being a hack lol.
Do you know if he is still alive ?
Do not ever ship this amp unless it is returned to stock form. It would be destroyed in shipping.
lol, my feelings also, infact i used to be nervous just moving it around and connecting plugs. Fortunately it was kept in a good spot out of sight and was never troubled so i had no issues with it.
To have it re-done i will have to drive it there myself
No, it has dual PSUs.
You might be best off to take or send it to Quad service in Huntington. They will wring their hands just like we are, but they will fix it.
Ah yes sorry, i meant dual power "blocks", please forgive my flakey terminology lol, i'm working on distant memory. I'll edit the post.
Does Quad actually offer modern upgrades to these 405s ? Or maybe they can simply do something for my situation. i'll get in contact.
I have been using this Quad 405 to drive Yamaha NS10s for monitoring in my studio 'forever' as i like the combination, so the idea of the mods was to modernise the amp and keep it as neutral as possible with good imaging. The improvement was obvious and i have always been pleased with it.
Quad will restore it to its original state. I would keep the opamps if new and replace them afterwards. Very little rose really matters.
Quad don't offer 'upgrades' so to speak and im not sure they'll touch that. If you want to to go down the Quad route I would maybe do this as your missing the protect PCB that usually goes on the speaker terminals - contact their service department and buy 2 x Quad 405-2 pcbs which I think they still supply, all parts are now housed on the new boards easy to install, the psu however would be a touch more complicated you will need these caps x2,
https://eu.mouser.com/ProductDetail/KEMET/ALT22A153DE063?qs=BmXAkaUQjrRPbMrM2Qg0ww==&countryCode=GB¤cyCode=GBP
And some 40mm capacitor clamps, some wire - al least the same size and colour as Quad uses and an ability to solder.
https://eu.mouser.com/ProductDetail/KEMET/ALT22A153DE063?qs=BmXAkaUQjrRPbMrM2Qg0ww==&countryCode=GB¤cyCode=GBP
And some 40mm capacitor clamps, some wire - al least the same size and colour as Quad uses and an ability to solder.
This sounds like something i may be able to handle. I'll do some digging. Thank you so much for the guidance and for giving me your time @ everyone.
I need to crack on with the studio for now but once i start on the amp i'll post an update of what i did. Cheers !
Back n the 1980s, I used to have a 405 and did just that. They charged £45 for a pair of 405-2 boards.
kind regards
Marek
I doubt that much work if any us needed on the PCBs. The problems with this amp are in the unsafe butchery perpetrated in the PSU.