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Replacement Parts for Quad 909
Replacement Parts for Quad 909
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Old 1st February 2017, 01:35 AM   #1
Sonic77 is offline Sonic77
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Default Replacement Parts for Quad 909

So i had a 909 serviced a while back and they changed the main caps and the transformer out, and now it doesn't sound as good lol (when i was expecting it sound better).

The sound is even more laid back and flat (not good lol).

The transformer is presumably an off the shelf of some variety and the caps are Kemet/BHC.

I figure both of these things are effecting the sound negatively, rather than it just being a single bad item.

I know Naim Audio use Talema transformers, if i asked them to make me up a transformer would it likely improve the sound?

Is it possible the Naim transformers colour the sound in a pleasant way, are are they likely to be dead quiet which may possibly be a bit boring?
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Old 1st February 2017, 06:01 AM   #2
Ian Finch is offline Ian Finch  Australia
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Barring shoddy transformer construction or your existing system is in a large area and you need a fair portion of the available power, it's unlikely there will be significant difference in sound quality due to the brand of toroidal transformer.

Talema do make decent quality toroidal transdormers but then, so do many other European manufacturers using probably the same or similar cores and magnet wire. There can be lower noise and with extra features like a belly-band or inter-layer shield but such custom 1-off transformer builds are not cheap. More to the point, what was the reason for changing a major part of the amplifier like the power transformer?

Are you sure that the only change to your system is the service work? How long has it been between listening before and after servicing? Did you subsequently receive the replaced parts or at least a list of them?

The more likely and usual candidate for affecting sound quality is simply renewing capacitors (including upgrading or increasing the amount of smoothing capacitance) though there aren't a lot of them in Quad power amplifiers. The effect you describe is only what I'd expect with a re-cap; tighter, clearer sound and usually the bass is noticeably stronger too - depending on the age and relative quality of the parts replaced.
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Old 1st February 2017, 06:17 AM   #3
Sonic77 is offline Sonic77
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Hi Ian thanks for the reply.

Yes i'm quite sure those were the only changes and i do have the invoice sheet listing those parts. I originally purchased 2 new 909's back in 2007 and only had one of them serviced, so i do a direct comparison and the older/untouched unit definitely sounds better.

Kemet/BHC have a very good reputation but some say they "just don't sound good". Maybe it's the BHC caps. They do have excellent specs, but as we all know you can't really judge something from specs, you need to listen with your ears. Honestly i think it's the BHC caps.

Ian may i ask you are PSU caps considered in the "signal path"?, and what is the definition of "signal path"?
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Old 1st February 2017, 08:52 AM   #4
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by Sonic77 View Post
....may i ask you are PSU caps considered in the "signal path"?, and what is the definition of "signal path"?
This is a real old chestnut - a long and hotly debated issue that is bound to wake up others here and start another fiery debate - like measurements v sound quality and other pointless topics. The question is a semantic one; where it all depends, as your question suggests, on what is meant by "in the signal path".

A naive view is that the signal path is just the AC path(s) drawn as a highlighted or bold line through the amplifier schematic from input to output, as you may see in some service manuals. This only identifies series components and active devices, switches etc. in those lines. In fact, it usually ignores shunt circuit components and the power supply and ground return paths for the signal. These will be just as much in the signal path, whether by series or shunt connections.

From the whole amplifier perspective, everything is in the signal path to some degree because it will have some effect on the output. It's not so simple, understanding the amplifier design because most circuits have multiple functions and the signal path carries various DC components too. What we find though, is the term "signal path" is really just a convenient category into which we place the primary forward signal path, once we've identified it.

Of course, there are also subcircuits like protection, indicator lights, remote control, microprocessors and soft start circuits that may be considered as not being in the signal path but there will always be a case for saying that if we believe it has any perceptible affect on sound, then it may be so

To put the main smoothing electrolytics in a (chest)nutshell, consider they are necessary to shape the the DC supply from a rough sawtooth waveform to virtually smooth DC. Pull the caps out of circuit and just try to listen to the resulting buzz-saw sound effect. It's not audio at all. So, is the PSU in the signal path? Of course it is, since it supplies the very current for amplification that the speakers draw upon. You can't actually have a power supply (and that also means the capacitors in parallel with it) that isn't in the signal path.
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Last edited by Ian Finch; 1st February 2017 at 09:21 AM.
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Old 1st February 2017, 11:56 AM   #5
Sonic77 is offline Sonic77
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Default Opamp change for QUAD 909

Is changing the opamp in a Quad 909 worth doing?

There are some who say the standard opamp (TLC271) acts merely as a DC servo and is not in the signal path, and others who say it is in the signal path and changing it can make a difference to the sound.

Iím tempted to try OPA627 which i'm told should be safe to try, or possibly LM4562.

Any thoughts?

Last edited by jazbo8; 1st February 2017 at 05:17 PM. Reason: Caps
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Old 1st February 2017, 04:43 PM   #6
ivanlukic is offline ivanlukic  Serbia
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Opamp is DC servo and it should not influence subjective performance to a degree that you can call it "improvement". I am sure that QUAD chose adequate OPamp for the job and that any replacement is absolutely unnecessary. Even ordinary TL071 is enough for the required function, as one can see from Doug Self's power amp book. Limitations of 909 are in the topology of the power amp stage and if you are nopt satisfied with it changing DC servo opamp will not make it any better.
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Old 1st February 2017, 05:03 PM   #7
Mooly is offline Mooly  United Kingdom
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Replacement Parts for Quad 909
Quote:
Originally Posted by Sonic77 View Post
Iím tempted to try OPA627 which i'm told should be safe to try, or possibly LM4562.

Any thoughts?
No no no !

What is fitted is perfect for the job and is not in the audio path as such. Its function is to act as an integrator which means it removes all the audio and simply produces a DC correction voltage to bias the amp.

The 4562 isn't even pin compatible (its a dual) and being bjt isn't really suited to this duty anyway.

The OPA627 is a splendid opamp, but one that would be wasted here. Whether it would function as intended isn't guaranteed 100% guaranteed either.
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Old 7th February 2017, 06:49 AM   #8
Sonic77 is offline Sonic77
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Default Quad 909 Which resistors in signal path?

Can anyone tell me which resistors are in the signal path of the 909?

I would like to change out the critical resistors with better quality Vishay.

Thankyou in advance
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Old 7th February 2017, 06:55 AM   #9
Mooly is offline Mooly  United Kingdom
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Replacement Parts for Quad 909
Many of the resistors are in the signal path, but very few would benefit from being changed (if at all).

R1, R2 and R24 would be the classic 'ones to replace' and you could argue R5 and R24.

However in my personal opinion there is no point doing this for this amplifier.
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Old 7th February 2017, 07:21 AM   #10
Sonic77 is offline Sonic77
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Thankyou Mooly. Why do feel it's not worth doing?
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