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Quad 909 Inductors

If one inductor has a core that may explain JW's concern. The magnetic behaviour of the core would then affect performance.
 
Yes, the choice of core is beyond my knowledge as I don`t know the requirements - on pictures of the pcb it appears to be a couple of ferrite beads but I don`t know if they are "good" or "bad" examples.
 
Jim you say you tend to avoid inductors in audio amps, are you saying they are not necessary?

If i removed all the inductors i presume the amp wont work?

What do inductors do in audio amps?
 
The 20uH is low current, so can use thinner wire and more turns.
25mm OD, 12mm ID, 12mm height.
About 100 turns, 4m of wire
 
Jim you say you tend to avoid inductors in audio amps, are you saying they are not necessary?

If i removed all the inductors i presume the amp wont work?

What do inductors do in audio amps?

The inductors in the amp you have are a part of the design. So you certainly should not remove them unless you intend to modify the design. Indeed, the standard 'current dumping' design *requires* one inductor and one capacitor in the 'bridge' circuit to make it work as intended. So your amp certainly wouldn't work if you removed them.

More generally, the can serve various purposes. This is based on their behaviour in terms of two basic properties.

The have an impedance whose value rises with frequency. So they act like a very low resistance at DC but a high impedance at high frequencies. This means they can pass DC whilst tending to block or limit the amount of high frequencies that get though them.

For AC they have the behaviour that the current level is determined by what the voltage cross the coil has been in the past. So for sinewaves, say, the current waveform is a quarter cycle behind the voltage waveform.

These properties can be very useful in filters and in situations where you want to, say, pass dc but impede ac.

However they have drawbacks. These are mainly what have been mentioned. They may produce an external field which can affect things nearby, and also pick up fields from their surroundings. They may also be microphonic. And if they use a core material to get a higher inductance they can be affected by nonlinear behaviour of that material.

For those reasons I, and many other audio engineers tend to avoid them if we can. In practice I've never had one 'inside the loop' of an audio amp. Only used small ones at the output of a power amp as part of an output network. Outside the loop. This can be useful to prevent the amp being directly connected to a capacitance in the high ultrasonic region which presents a needlessy difficult load. Goes with a 'Zobel' to 'signal condition' the operating conditions presented to the amp by users.
 
Ok thanks Jim thats interesting.

May i ask what are the odds of the Chinese inductors being the exact same size and dimensions as the better quality types Quad originally used?
 
Ok thanks Jim thats interesting.

May i ask what are the odds of the Chinese inductors being the exact same size and dimensions as the better quality types Quad originally used?

I can't tell as I don't know what the 'quality' in question actually is. I'd *guess* that - if it were possible - they fit onto the same board. .i.e. same lead spacing and board space. Simply as a matter of convenience for QUAD. But I don't have any specific info on the 909 or what JW had in mind. If you want to know, worth trying a PM to him.
 
I already tried to PM John but my message wouldnt go through because apparently his message box has reached capacity and he needs to delete some of his mails before he can recieve new ones. If you happen to have his personal email would you mind asking him if he would be willing to let you pass it onto me?
 
Try a short message in his MDAC thread in Audio, with a hyperlink to this thread. I'm sure he's more than happy to give you his input, but I'm equally sure he is also too busy to just be browsing PFM :)
 
Chris,
Have you tried calling Quad support? Last year, I was considering a QSP (basically a 909 in a newer box) and had read some negative feedback ref the caps. After not turning up much genuinely comprehensive info, I called Quad direct and chatted with a VERY helpful chap who was also very forthcoming ref the ability for them to service the units for an extremely sensible price. Convinced me to give one a try and I now have an artera in my system which sounds lovely.
 
I already tried to PM John but my message wouldnt go through because apparently his message box has reached capacity and he needs to delete some of his mails before he can recieve new ones. If you happen to have his personal email would you mind asking him if he would be willing to let you pass it onto me?

Afraid I don't have an email contact for him.
 
Whats are the main criteria/specs i need to know about the Quad in order to seek out suitable replacements,

Is it just "inductance" and "ohms" that i need to find out?
 
The basics are (with some tolerance range)

1) The inductance

2) The resistance / current capability

3) Parasitic capacitance

(The above are sometimes proxied by a 'self resonance' frequency and 'Q' values.)

But you may also need a size and shape and to know if the coil is, say, a toroid. Plus orientation wrt the holes in the PCB. Both these may affect pickup. And if it has a core, you'd need some info on the core material, etc.

Similarly, if concerned about microphonics you'd need some idea of what kind of former or mount is used to hold the coil in place.

For small open low value coils replacement tends to be simple and you can base it on inspection. But for higher values which may be encapsulated it may be more awkward. And IIUC you have a situation where you suspect alternative versions you *haven't* seen might be 'better' for some unknown reason. That risks a wild goose chase.

if you can't get info from JW I think you'd either have to experiment or simply accept the coils in situ if the amp works to spec. Or send it to QUAD if it isn't working to spec.

If you wish to go further then some clear photos of the coils *might* help. But we really need some data I think.
 
Chris,

The Chinese manufactured Quads tended to use UK supplied magnetics.

L4 in the Bridge is actually quite decent, L2/L3 are a crude attempt to match the characteristics of the top and bottom dumpers - and is never going to happen. There's so much wrong with the Dumper circuit, L2/L3 would not be on my to-do list.

L1 in the Emitter of the input device is sonically the most critical - its a horrid circuit idea - but without redesign your stuck with it. As Davidsrsb mentioned is subject to magnetic pickup, Quad "Hum Bucked" the inductor, two inductors in series in opposing direction to somewhat cancel the magnetic field.

The biggest single issue with the Quad design is the very crude and "violently abrupt" operation of the Current dumpers...
 
John, I seem to recall you mentioned once that Quad China used locally made and cheaper inductors for the Quad 909, would you be able to recall who was the original uk based manufacture for the inductors?
 
John, I seem to recall you mentioned once that Quad China used locally made and cheaper inductors for the Quad 909, would you be able to recall who was the original uk based manufacture for the inductors?
@Chris88 did you ever get anywhere with this? As a new owner of a Quad 909 I'm keen to know.
 
I've just finished recapping my Chinese built 909.

The four big Capxon caps were all a little down on capacitance value and were high ESR. I replaced these with Kemet ALP22A.

The two 220uF 63v electrolytics are C9 and C11 were replaced with Panasonic FR of the same value.

At C7 I used Panasonic FC 47uF

The polybox MKT at C3 were swapped to Evox SMR

The 330nF at C2 were swapped to Panasonic ECW series.


The recap job has changed the sound of the amp quite dramatically. There's now a lot more detail and the sound is much crisper.

It has lost some of the almost valve like smoothness that I really liked about it though, so I'm not sure it's all positive.

I'll let it settle for a while but I may go back and tinker with the feedback cap at C7 (wet tant?) and the two caps at C9 and C11.
 
Dunno Mike really, but I don't like FC caps. When I tried them on front end PSU of my modded ncc200 I found them nasty. Probably replace them at C7 first.
 
JohnW modified my 909 many years ago and mentioned smth about replacing the inductors and some other bridge stuff. THD is now close to a good DAC.
Btw, my C7 is a 100 uF Cerafine IIRC
 


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