Wouaw !! This is a perfect case for a DIY project with switching power supply !
My amp have IC sockets on boards (the 405-2 havn't, I think). So, well, if I have nothing else to change on the bords to put an OPA604 instead the LM301, why not ? I can try. But first, I have to wonder about the caps, to get an original 405 working like new! This is your advice (a good one !!!) 🙂
I understand your point of view, really. But I have a genuine 405, so I don't want to make to many changes on it. I can think that, made today, the same amp would use better OP amp, I know the LM301 is obsolete, so I have no problem to replace it. I keep the "philosophy" of this amp.
If I want, later, to experiment around the 405, so I will buy a chinese case copy, 2 chinese boards copies, 2 SMPS, and then I'll play. And I agree with you, all I could read about these modifications tell about a great improvment about dynamic, bass response, etc. But, as you wrote, it is no more a 405. I will like to do it. But I don't want to dismantle a genuine and respectable 405 for that. If my amp was already modified, why not ? But it is not the case. I choose to buy THIS ONE because nothing was touched in before. I will service it, but not really upgrade it.
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I agree. And the 'philosophy' of this amp (and any other) has nothing to do with what opamp is used. The 405 is based on a unique concept of a class B output stage and a variable feedback factor driven by a bridge. This unique concept does not change with a different opamp or different output devices. It was one of the very few, maybe the only, real new concept in audio power amplification in the last half century.
Jan
Wouaw !! This is a perfect case for a DIY project with switching power supply !
My amp have IC sockets on boards (the 405-2 havn't, I think). So, well, if I have nothing else to change on the bords to put an OPA604 instead the LM301, why not ? I can try. But first, I have to wonder about the caps, to get an original 405 working like new! This is your advice (a good one !!!) 🙂
😎 isn't it.
And the matching preamp:
New Finished Classic QUAD34 Clone Preamplifier Stereo Preamp HIFI AMP | eBay
Worth it just for the case by look of it 😀 although a bit naughty that is says 'Made in Huntingdon by Quad' on the back.
Yes, get your 405 all working correctly first with new caps, then think about trying other stuff.
Ah ah !! These chinese copies look nice !
But when the same seller propose "Original new ELNA capacitors"... can we trust him ? Originals or copies ?
Actually I haven't listened much to QUAD 405s in domestic circumstances. I have in a radio studio, there it worked to everyone's satisfaction.
In 1997 I became a volunteer at a local radio station; in late 2001 I finally noticed that an opened-up piece of equipment that had stood on a shelf for as long as I had been a volunteer there had "The Acoustical Manufacturing Co., Huntingdon, England" printed on its back side. I immediately took it of the shelf, turned it around and found out that it was a broken-down QUAD 405.
I took it home to see if I could repair it. The protection triac of one channel had become a short circuit, as had the 100 uF tantalum capacitor of the DC feedback, and someone had replaced the supply electrolytic capacitors with models having four pins, but had by mistake connected the pins with no internal connection. The LM301s had been replaced by TL071s.
I fixed what needed to be fixed and made most of the modifications that the Acoustical Manufacturing Co. themselves made in the later QUAD 405-2. I made one small additional modification to the DC bias loop: instead of 4.7 kohm - 100 uF - 22 kohm, I used 68 kohm - 2.2 uF - 68 kohm, so I could use an MKT capacitor rather than a tantalum capacitor. The bass response with 68 kohm - 2.2 uF - 68 kohm is nearly the same as with the original values and with a FET op-amp, the higher resistors also don't cause any extra offset.
This amplifier was used in the studio for driving the monitor loudspeakers from 2002 to 2010. After that it wasn't needed anymore because the radio station bought active monitors. Another volunteer took it home and has used it ever since. He is very happy with it, and he is used to listening very critically as restoring recordings is what he does for a living.
I've just redone the calculation that I did in early 2002.
If you want to reduce the sensitivity by a factor of x, add a series resistor of x-1 times 20 kohm. The time constant of the series resistor and the series capacitor must be equal to the time constant of the 330 kohm-47 nF feedback across the op-amp inside the QUAD 405. That is, C = 15.51 ms/R.
For example, if you want to change the input signal required to fully drive the amplifier from 0.5 V to 1.5 V, then x = 3, so you need a 40 kohm series resistor.
Rounding this to the nearest E12 value, R = 39 kohm. The required value of the capacitor then becomes 397.6923077 nF -> nearest E12 value is 390 nF.
The resistor has to be placed very close to the amplifier to prevent treble loss due to cable capacitance. I therefore put it inside the DIN plug. The capacitor can be placed at some distance if needed, so I put it in the plug on the other side of the cable. In my case that happened to be an XLR plug, so there was plenty of space.
As the coupling caps inside the QUAD 405 are metallized MKT types, there is no point in using anything fancier than MKT for the external capacitor.
By design, the response is a nearly perfect second-order Butterworth high-pass response with 14 Hz cut-off frequency. Peter Walker never saw the point in amplifying signals outside the audible range, in his opinion that could only cause trouble such as intermodulation distortion.
If you want to change the cut-off frequency without changing the concept, scale up the capacitor to ground of the DC feedback, the capacitor in the feedback network of the op-amp, the input coupling capacitor (including the external coupling capacitor if you change the sensitivity in the way I suggested) and the capacitor of the DC protection triac, all by the same factor.
Regarding updating the op-amp: don't use a TLE2071. Due to a bug in its design, it draws too much supply current when its output clips to the negative rail.
Thanks for all the info and insight Marcel. Those that have listened critically such as yourself and Jan all seem to think very highly of the design. A neighbour used to have a Quad system (405 + 34 preamp together with the tuner and Gale 401's) and as someone with a new interest in audio and electronics I was always fascinated by the set up and used to spend ages looking at the circuit diagram (which Quad included in the user manual) and trying to understand how it all worked. I think the source back then was an early Michell Girodeck. Happy days 🙂
I've attached the LT file for the Quad for those wanting to play.
(I wasn't aware of the TLE2071 issue at clipping, in fact that was an opamp I had high hopes for in other small signal projects. Useful to know, thanks)
I've attached the LT file for the Quad for those wanting to play.
(I wasn't aware of the TLE2071 issue at clipping, in fact that was an opamp I had high hopes for in other small signal projects. Useful to know, thanks)
I made a mistake in post #46, I used 82 kohm-2.2 uF-68 kohm, not 68 kohm-2.2 uF-68 kohm.
With 68 kohm-2.2 uF-68 kohm you get close to the original cut-off frequency, but the Q gets slightly too low. With 82 kohm-2.2 uF-68 kohm the Q is almost exactly on target and the cut-off frequency slightly drops to 12.883 Hz (theoretical natural frequency).
Sorry for the mistake. I'm trying to figure out what I did fifteen years ago partly from memory and partly by repeating the calculations I did back then.
With 68 kohm-2.2 uF-68 kohm you get close to the original cut-off frequency, but the Q gets slightly too low. With 82 kohm-2.2 uF-68 kohm the Q is almost exactly on target and the cut-off frequency slightly drops to 12.883 Hz (theoretical natural frequency).
Sorry for the mistake. I'm trying to figure out what I did fifteen years ago partly from memory and partly by repeating the calculations I did back then.
By the way, if you use a fast unity-gain-stable op-amp such as an OPA604 and change the DC bias loop like I did, it is probably a good idea to put a small capacitor (1 pF or so) between the output and the negative input of the op-amp.
When nothing is connected to the QUAD 405's input, the driving impedance of the negative input of the op-amp is then of the order of 50 kohm and the closed-loop bandwidth of the op-amp is then around 3 MHz (20 MHz gain-bandwidth product of an OPA604 and about 6.6 times attenuation in the feedback). A few pF of parasitic capacitance can then add quite a lot of phase shift. With a TL071 I didn't have this issue because of its much smaller gain-bandwidth product.
When nothing is connected to the QUAD 405's input, the driving impedance of the negative input of the op-amp is then of the order of 50 kohm and the closed-loop bandwidth of the op-amp is then around 3 MHz (20 MHz gain-bandwidth product of an OPA604 and about 6.6 times attenuation in the feedback). A few pF of parasitic capacitance can then add quite a lot of phase shift. With a TL071 I didn't have this issue because of its much smaller gain-bandwidth product.
Good advice. I would always recommend that any opamp changes (indeed any changes) are backed up by at least a quick visual check with a scope that all is well.
Very interesting ! I will have to remember this post if in the future I change my OPs 🙂
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