@mmmalmberg besides several variants that disturb the low-frequency response, cause extra output noise or don't work at all, I think three good options surfaced, all with their own advantages and disadvantages:
1. Keep the QUAD 405 exactly as is and put series resistors and capacitors in the connecting cable: 39 kohm and 390 nF per channel for a factor of three reduction in sensitivity. To prevent treble loss due to cable capacitance, the resistors have to be mounted very close to the QUAD 405, preferably inside the DIN connector. The capacitors can be mounted wherever you can find the space. To stay in style with what's used inside the amplifier, the capacitors should ideally be MKT capacitors, but in fact any type of film capacitor will work, as will NP0/C0G ceramic capacitors. Don't use class-II ceramic capacitors such as X7R, X5R, Y5V, Z5U because those are inaccurate and distort.
Advantage: originality of the amplifier maintained
Disadvantage: less output noise reduction than with the other options, it may be difficult to find space for the capacitors
2. Change the op-amp to TL071, remove the 3.3 pF compensation capacitor, add a 4.7 nF capacitor across the Zener for the negative supply, change R9 into a short circuit and change R10 into either 560 ohm or (preferably) a short circuit and use these values:
R6: 100 kohm in parallel with 10 pF
C4: 150 nF
R4: 47 kohm
R5: 39 kohm
C2: 2.2 uF
C1 and R3 as is (680 nF and 22 kohm)
Advantage: this should be the option with least noise and least op-amp distortion, C2 is small enough to use a film capacitor
Disadvantage: you would be the first on the planet to try this, although I have tried something similar with less sensitivity reduction, the filtering of subsonic signals below 3.39 Hz becomes somewhat less effective than in the original QUAD 405 design
3. Joost's proposal: R6 is 100K, R4 is 22K C4 is 150nF and C2 is 33uF. To keep close to Quad designs, replace the old fashioned LM301 with a still period TL071 and remove the compensation cap C3. Although the voltage across C2 is very low, connect the plus of a polarised Cap to ground (mass), or use a non polarised capacitor.
@joost456 Do you want the changes in power supply decoupling like in the QUAD 405-2 added to this or is that unnecessary in your experience?
Advantage: proven recipe, used many times by Joost
Disadvantage: somewhat less noise reduction than option 2, the filtering of subsonic signals below 1.25 Hz becomes somewhat less effective than in the original QUAD 405 design
Just take your pick.
1. Keep the QUAD 405 exactly as is and put series resistors and capacitors in the connecting cable: 39 kohm and 390 nF per channel for a factor of three reduction in sensitivity. To prevent treble loss due to cable capacitance, the resistors have to be mounted very close to the QUAD 405, preferably inside the DIN connector. The capacitors can be mounted wherever you can find the space. To stay in style with what's used inside the amplifier, the capacitors should ideally be MKT capacitors, but in fact any type of film capacitor will work, as will NP0/C0G ceramic capacitors. Don't use class-II ceramic capacitors such as X7R, X5R, Y5V, Z5U because those are inaccurate and distort.
Advantage: originality of the amplifier maintained
Disadvantage: less output noise reduction than with the other options, it may be difficult to find space for the capacitors
2. Change the op-amp to TL071, remove the 3.3 pF compensation capacitor, add a 4.7 nF capacitor across the Zener for the negative supply, change R9 into a short circuit and change R10 into either 560 ohm or (preferably) a short circuit and use these values:
R6: 100 kohm in parallel with 10 pF
C4: 150 nF
R4: 47 kohm
R5: 39 kohm
C2: 2.2 uF
C1 and R3 as is (680 nF and 22 kohm)
Advantage: this should be the option with least noise and least op-amp distortion, C2 is small enough to use a film capacitor
Disadvantage: you would be the first on the planet to try this, although I have tried something similar with less sensitivity reduction, the filtering of subsonic signals below 3.39 Hz becomes somewhat less effective than in the original QUAD 405 design
3. Joost's proposal: R6 is 100K, R4 is 22K C4 is 150nF and C2 is 33uF. To keep close to Quad designs, replace the old fashioned LM301 with a still period TL071 and remove the compensation cap C3. Although the voltage across C2 is very low, connect the plus of a polarised Cap to ground (mass), or use a non polarised capacitor.
@joost456 Do you want the changes in power supply decoupling like in the QUAD 405-2 added to this or is that unnecessary in your experience?
Advantage: proven recipe, used many times by Joost
Disadvantage: somewhat less noise reduction than option 2, the filtering of subsonic signals below 1.25 Hz becomes somewhat less effective than in the original QUAD 405 design
Just take your pick.
Last edited:
Regarding the subsonic filtering, the QUAD 405's DC bias loop is designed such that it behaves as a second-order Butterworth high-pass filter at about 14 Hz, but the slope changes to first-order below a frequency of (1/R4+1/R5)/(2 pi C2). That's 0.411 Hz in the original design, 1.25 Hz in variant 3 and 3.39 Hz in variant 2.
In the 405 service manual Quad stated that the extra capacitor across the negative op-amp suply was added to avoid mild instability when switching off. In most of the Quad designs this capacitor is present even in the ones were the op-amp 'only' forms the DC feedback loop. So yes. Joost Plugge.
There is another advantage of Joost's proposal, which you haven't stated, and BTW which doesn't differ from mine. It is that by increasing NFB by 3x it reduces THD to say 1/3x.
EJP
EJP
Variants 1, 2 and 3 all increase the feedback around the op-amp. 2 has the largest impact, 1 the smallest and 3 is in between.
The feedback around the main amplifier for non-subsonic frequencies is not changed by any of the variants, just the feedback around the op-amp.
The feedback around the main amplifier for non-subsonic frequencies is not changed by any of the variants, just the feedback around the op-amp.
To which cap are we referring, and does this affect the description of "3. Joost's Proposal"?
So D2, yes? I don't see a parallel cap in the schematic nor on my boards. I should add?
Yes add a 4.7nF across D2?
Just making 100% sure - I need to get this amp closed up as we're packing to move up to WA!🙂
How critical is sizing if that, could I get away with .01uF? Closest I have but I’ll Amazon something if I need .047. Hate paying $8. Shipping for mouser but I can amazon prime with free shipping next day delivery. Quality comes into question but…
I was going to buy a Quad 405 clone on eBay yesterday, but saw an auction about to finish with a real one. I got it for £172. I am going to put modern banana plug speaker terminals and rca sockets on it. I am worried about the input sensitivity too. I plan to wire the rca sockets via the back of the din socket and use a 10k dact type 21 step attenuator to adjust the input sensitivity. Is this a good a good way to do this? It does mean I can use the din socket with a 0.5v input or the RCA’s with 1.5v. I might modify it further or put in new 405-2 boards in the future.
The advantage is that you can easily change the sensitivity, the disadvantage that you won't get any of the noise and op-amp distortion improvements discussed in this thread.
Thanks for the reply. I wanted to do a quick and easy mod to get my 405 playing music when it arrives. The minimum I will need to do is the speaker terminals. I will do some improvements and reduce the noise and distortion in the future.
Also, but I have to be carefull here, the potmeter will lower the input impedance as seen from the source, pre-amp, also adds some series resistance, depending on the postion of the pot, which will probably lower the signal to noise ratio. So only add potmeters when you have to, like in a pre-amp.