advertisement


Effed up Quad 909 (UK made)

John,

You posts about the Quad power amps are very valuable. Would you be willing to write some notes on them, maybe on the quality differences between UK and China built models, how to determine where an amp came from and perhaps options for improving performance. It would be an excellent addition to our Reference section.

Thanks

Jo
 
Jo, AFAIK the English built amp has a card with the people involved building it. Haven't heard the Chinese built amps have that.
The biggest problem with the 909 is the power on thump...
 
Zombie said:
Jo, AFAIK the English built amp has a card with the people involved building it. Haven't heard the Chinese built amps have that.
The biggest problem with the 909 is the power on thump...
Click to expand...

I got the thump problem with the QSP, however I always turn my amps on & off directly from the wall socket, if an amp can not accept this its no good to me.

JOHN W has previously said On the Quad QSP there is no AC detect, so the design can not shut itself down in a controlled manner- without this circuit everything else is going to be "Micky Mouse" IMO ... Reviewers never mention these problems ? As for the Quad Platinum great reviews but I can not use it due to the hum problem I encountered.
 
JohnW said:
Thomas,

So channels working, typical Quad perfromance:-

https://dl.dropboxusercontent.com/u/86116171/QUAD909 1W 0.0068% -76.79dB Awtd.jpg

In the above FFT you can see the spread of the Mains hum components (typical of Quad amps).

1W 1KHz, 8 Ohms, THD 0.0068% (With spread of higher order crossover distortion).

SNR Awtd -76.79dB Ref 1W 8 Ohms (Input signal level 40mV).

I'd like to improve the performance if its OK with you? - what system do you use with the AMP as its input Gain is very high?
Click to expand...

Sorry for the delay, I've sent some time on this Amp - partially because I'll be working on an Amplifier design in the very near future, and its always good to get the "feel" of ones peers...

In all honesty, I'm still not very impressed with the end results - basically there is little that can be "easily" modified to resolve the designs deficiency - I know how to resolve them, but not how to add them "cleanly" to the original PCB layout....

Anyway, on the Left channel I've been working on:-

THD 0.0009% 1W 8Ohms (Unmodified was 0.0068%)

SNR Awtd 92dB Ref 1W 8 Ohms, a 15dB improvement over the standard unit.

FFT of the modified PCB, 1W 8 ohms:-

https://dl.dropboxusercontent.com/u/86116171/QUAD909 Lch Mod 0.0009% THD -92db Awtd Ref 1W 8R.jpg

Its a shame that the Noise floor (Hum) is still so high... it seems quite the norm to have such poor results from Power amplifiers... which as a designer is quite surprising (and poor game IMHO)...

Still you now have the best measuring (and hopefully also the best sounding) Quad 909.

BTW, I checked the main PSU Bulk Caps. and they are still good, measuring about 12,900uF each (and good low ESR), still within the +20% / -20% rated tolerance.
 
JohnW said:
Its a shame that the Noise floor (Hum) is still so high... it seems quite the norm to have such poor results from Power amplifiers... which as a designer is quite surprising (and poor game IMHO)...
Click to expand...

How high in real terms? I'm curious as I've got a Quad 306, which as I understand it is a related design (just a lot smaller) and it's the quietest amp I've ever used. As an example I can hook the thing up to my Klipsch La Scalas (one of the most efficient speakers out there at 105db) and not hear any hiss or hum. By comparison my 303 does a bit of both in that context, I can hear T-Amp switch-mode PSUs whining and my Leak Stereo 20 sounds like a slightly malfunctioning white noise generator with added random crackles etc. I'd have thought a factory spec 909 should be at the 306 end of that list or better?
 
JohnW said:
Anyway, on the Left channel I've been working on:-

THD 0.0009% 1W 8Ohms (Unmodified was 0.0068%)

SNR Awtd 92dB Ref 1W 8 Ohms, a 15dB improvement over the standard unit.

FFT of the modified PCB, 1W 8 ohms:-

https://dl.dropboxusercontent.com/u/86116171/QUAD909 Lch Mod 0.0009% THD -92db Awtd Ref 1W 8R.jpg

Its a shame that the Noise floor (Hum) is still so high... it seems quite the norm to have such poor results from Power amplifiers... which as a designer is quite surprising (and poor game IMHO)...

Still you now have the best measuring (and hopefully also the best sounding) Quad 909.
Click to expand...

How much of the 15dB improvemment in noise and the distortion reduction was due to the gain reduction?
 
S-Man said:
How much of the 15dB improvemment in noise and the distortion reduction was due to the gain reduction?
Click to expand...

Reducing the gain on the PCB does not result in a linear reduction in noise due to poor PCB layout and noise injection on the PCB.

The THD reduction was due to the relocation of the feedback takeoff point and fitting a more linear inductor on the Emitter of the FB transistor (Improvements to grounding also helped).
 
Thanks John!

Did you leave the floating power-ground scheme (AC coupling)? I believe Les W used to do this mod to Quads and claimed improved sound.
 
Tony L said:
How high in real terms? I'm curious as I've got a Quad 306, which as I understand it is a related design (just a lot smaller) and it's the quietest amp I've ever used. As an example I can hook the thing up to my Klipsch La Scalas (one of the most efficient speakers out there at 105db) and not hear any hiss or hum. By comparison my 303 does a bit of both in that context, I can hear T-Amp switch-mode PSUs whining and my Leak Stereo 20 sounds like a slightly malfunctioning white noise generator with added random crackles etc. I'd have thought a factory spec 909 should be at the 306 end of that list or better?
Click to expand...

I have Quad 240's which are much quieter once modified (basically a 306 design) - the 606/707/909 PCB layout is particularly poor.

I find the low level spurie components you would believe are "low enough" still leave a sonic fingerprint to the sound - so those multitude of 50Hz hum related spurie impact the Bass resulting in a defused but "fuller" sound - lacking tightness and "speed" / impact. However making smaller Box speakers seem more Bassy (but also a little "one note").

The spread of higher order crossover distortion indicates potential phase modulation problems around the zero crossing point which robs the design of being able to recreate a "holographic" sound stage....
 
S-Man said:
Did you leave the floating power-ground scheme (AC coupling)? I believe Les W used to do this mod to Quads and claimed improved sound.
Click to expand...

I'm no fan of the floating power-ground scheme - in fact in my book it would be a big no no!

Sadly it cannot be modified on the 909 design without a new transformer with centre taps - and then what to do for DC protection without an output relay?

I guess Quad chose it for a couple of reasons:-

1. Inherent DC offset protection on the output

2. A "centre point" offset voltage could be added to the "virtual earth" circuit to compensate for the non-symmetrical clipping behaviour of the Quad circuit - thus insuring apparent symmetrical clip.

In an effort to improve PSRR (improve its hum rejection) of Thomas unit I modified the circuit to improve its open-loop gain at LF - the extra feedback gain will help to improve Bass performance of the AC Coupled output, but its not a very effective solution sonically it does help to improve the SQ, but it would be better if the design was not AC Coupled in the first instance...
 
Great John, thank you. I assume the input sensivity is still the same to suit the pre output of 0,775 V?
What do you suggest now? Send it back or modify it further?
 
Zombie said:
Great John, thank you. I assume the input sensivity is still the same to suit the pre output of 0,775 V?
What do you suggest now? Send it back or modify it further?
Click to expand...

Thomas,

I'd love to work on it more, but I need get back to working on the MDAC2 project + a Tube Amp for a good friend that I promised months ago!

The Gain will be slightly different then via the AMP Bus - but its set to the standard power amplifier gain of 27dB.

I'd like to ship it tomorrow if I can complete the second channel tonight.
 
You must log in or register to reply here.


advertisement


Back
Top