Sunday, August 08, 2010

Improving a new Quad 909

One of our French customers, who is working in professional audio, bought a new Quad 909 to drive his Tannoy studio-monitors.

He has always used Quad 303 monoblocks with Dada High-end boards and was very pleased with the result so, as he needed more power for another project he bought a 909.

The Tannoys are very sensitive and transparent (as studio monitors should be). When he connected the 909 he heard a hiss and distortion in the mid-tones.He brought back the amplifier to the reseller and it was tested by Quad. They said everything was working within specifications so there was nothing they could do.

So he sent the amplifier to Dada Antwerp and asked us to find the reason of the problem and to see what we can do about it.

Joost and me took an afternoon to do some tests with Virtins Multi-instrument Audio analyser and the other lab-equipment.

First we did a listening-test. The sound was rather "harsh" and "metalic" compared to other (revised) Quads.

We measured a Total Harmonic Distortion of 0,8854% (!) and a Signal-to-Noise Ratio of 81,4dB at full power with 8 Ohm load. The total power before clipping was 120W RMS. Very bad figures for a high-end amplifier... And much worse than the "classical" 606 the 909 is based on.

We decided to take the amplifier apart and to concentrate on 3 possible sources of distortion:

  • The power-supply

  • The amplifier-boards

  • The input-board for the Quadlink

The power-supply board is the "classical" board known from the 606-II and the 707 with a Toroïdal transformer and 4x 15000µF capacitors in a dual-mono design. Only, the original BHC Aerovox capacitors have been replaced by Chinese CapXon capacitors.

We took them out to test them with the Hameg LCR meter.

The BHC Aerovox was the ALP22A 63Volt, the CapXon has no type-indication, it is a plug-in two pin 80Volt 105° type.

We measured the capacitance at 100Hz, 1KHz, 10KHz and 25KHz with a 1V DC bias voltage. Both were within specifications (15mF or more) up to 1KHz. At higher frequencies the BHC scores better with 3x higher capacitance, for example 1100µF at 10KHz instead of 300µF for the CapXon.

The measured internal resistance is about the same and around 10mR at 1KHz.

This leads us to the conclusion that both capacitors are very comparable with a slight advantage for the BHC specifically in the higher frequencies. Of course this test doesn't say anything about the ageing of the capacitors after several hours of usage. The BHC has an excellent reputation for ageing. We decide to replace the 4 CapXon in the power-supply with BHC Aerovox.

Then we take a look at the amplifier boards. We replace the low-cost capacitors as usual, the electrolytes with Nichicon, the range between 1nF and 1µF with Wima MKS and the range below 1nF with Silvered Mica.

We also recalculated the bridge and used the exact values for the bridge components. Joost will comment on this later.

We connect the driver-boards to the power-supply. Now we measure a THD of 0,025% and a S/N Ratio of 87dB which is a lot better yet. A listening test gives the rich and transparent sound we are used to with Quad amplifiers.

Finally we take a look at the input-board that also contains the switch-on delay and the symmetric -> asymmetric conversion for the Quadlink.

The asymmetric signal from the RCA-inputs also passes through the output-stages of the preamplifier and again we find some low-cost capacitors on board. At a low input voltage the THD of the board is 0,003% but increasing the input-voltage increases the THD of the board. Around 10V we have 0,4% THD.

As the customer won't use the Quadlink we decide to remove the board from the signal-line and to connect the RCA-inputs directly to the amplifier-boards like in the 606.

The conclusion is that every detail has it's importance for the final sound-quality, specially the quality of the components and that it is best to avoid the Quadlink if you don't need it.


Practical: Component list

The 909 PCB is a thhrough-metalised PCB. You should use a good desoldering-pump or a desoldering-station to remove the old components.

For soldering on the PCB's only use lead-free solder. Tin/lead solder won't mix with the solder on the board.

You can click on the components below to go directly to the Webshop for ordering or for seeing more technical information or price and availability.

This way we have replaced all electrolyte capacitors with the best quality and we have adapted the bridge to the optimum value.

The specifications of the 909 transformer are: 2x 80,5 Volt 500VA

Measurement of a 606 MKII (my own) as a reference:

Vout is 32.2 V, no load, left and right channel:

THD 0.0009 and 0.0010

IM Din 0.0045 and 0.0041

Vout is 32.2 V, load is 8 Ohm:

THD 0.0129 and 0.0059

IM Din 0.0342 and 0.04

So the left channel is out of spec, THD should be lower than 0.01 some work ahead!

Joost Plugge


Anonymous Peter said...

Shame on Quad!No wonder people find their way back to DIY!If you can't buy good stuff for good money,the only way is to do it yourself.
But anyway,could you publish a little more on the test setup?I just can't imagine one can do such measurements with a standard PC audio input/output and a piece of software!

3:47 PM  
Blogger Stefaan said...

Hello Peter,

There will be an article about the test setup. In the end the setup was not so standard, but a high quality (pro) balanced soundcard, a custom breakout box to set the levels, and to conect al the equipment and running it on a 64 bit operating system.

Joost Plugge
DaDa Electronics

12:19 PM  
Blogger Unknown said...

I dont suppose theres a serial number cutoff point where you can identify if you have a 'cheap' 909 or original spec one? sadly.

3:12 PM  
Blogger Stefaan said...

Hello Shaun,

We don't know when Quad started to use other capacitors than the BHC ones.


1:10 PM  
Anonymous heathpw said...

Hi can you publish the full measurments you made for the quad 909, ie at what fequencies you measure the distortion levels and signal to noise ratio vales. I have a quad 909 and am wondering if it actually performs as per the specification, there is little buzz coming out of both speaker at very low lvel when you put your ear to them so the may not be performing per specification, quad manual give the following specs,
Total Harmonic Distortion 0.02% (100W into 8Ohms; 20Hz - 20kHz), Signal to Noise ratio (70W) 108dB (20Hz to 20kHz).
Did the amp perform as the above specification. I would like the know if not how did it perform.

6:40 PM  
Blogger Stefaan said...


We don't have the measurements any more from that particular Amp. But first take care of that buzz. Check if its the 909 or a source connected to it. Remove the input from the 909, check if the humm is still dare. We had reports about 909's with internal hum caused by low qulity coils. Within a few day's I will measure my own 606. I will publish those figures. I currently don't have a 909 on the workbench or on the shelve.


Joost Plugge

6:02 PM  
Blogger heathpw said...

Hi you said that you recalculated the bridge resistors r29=10 and r30=47 to give a total resistance of 47+10 = 57 as opposed to 22 + 22 = 44 in the original circuit design, i assume you measured the value of L4 and found it to be 1.5u Henrys, i mention this because i have a quad 306 which gives the exact same values for the bridge components in the circuit diagram as in the quad 909 circuit diagram, but the vale for L4 = 1.5u henrys is incorrect, i removed the inductor in my 306 and it measured to be 1.15 u henrys which is the correct value for balance. So did you measure all the values in the bridge and what were they.

11:22 PM  
Blogger Stefaan said...


We checked the value, and it was correct. But you are right, when de value is lower than the 1.5uH the original resistor's will give the best result. Because not every body has a acurate inductance meter, we are planning to remove those components from our kits and trust Quad that they did the right thing.

Joost Plugge

DaDa Electronics

5:56 PM  
Anonymous Ryan said...

Hi Stefaan,

so are you saying that the resistor values that u recommended to replace r29=10 and r30=47 are wrong?

Is there any cause for me to worry? As I've already replaced all parts recommended by you in my 909.

4:20 AM  
Blogger Stefaan said...

Hello Ryan,

Ass long we are not certain about the value of the L Quad used, we do not advise to change the resistors in the bridge. Sorry about this.

Regards Joost Plugge

DaDa Electronics

9:26 PM  
Blogger heathpw said...

Hi stefaan i thought you were going to post the measured performance distortion figures of your quad 606.

8:44 PM  
Blogger Stefaan said...

Hello Heathpw,

I will publice the results on short notice, sorry for the delay.

Kind Regards,

Joost Plugge

11:31 AM  
Blogger heathpw said...

Are those distorton figures at 20KHZ, you have said, and the IM distortion seem very high, a poor result. I will test my own Quad 306 and 909 amplifier and post the results for comparison.

1:09 PM  
Blogger Stefaan said...

Hello Heathpw,

I measured at 1Khz. Quad never published IM (Din) distorsion results for the 606 family. So I don't now of this is a "poor" result. We waite for your measurements.

Kind Regards,

Joost Plugge
DaDa Electronics

3:14 PM  
Blogger Unknown said...

Hi Stefaan
I just came across your site; an interesting read although I won't have time to go through your archives. Your results on testing the Quad 909 are quite shocking. When I designed this product I introduced new components throughout with great care and the sound and measured performance were greatly superior to the earlier but almost identical 606. To most listeners it was a different amplifier. Many of the components as specified had to be imported from Europe; inconvenient but necessary.
It seems that the factory no longer bothers and fits generic components. With factories everywhere, not just China, the designer has to keep monitoring the production because the factory will substitute components for convenience; to save money or even to fit a "better" part with the best of intentions.
I adopted the same philosophy as with Rotel; Cambridge Audio and many other companies I designed for where there is no one component or circuit feature that is vital but the overall combination works very well. Change one thing and the balance is lost.
With best wishes to you
Stan Curtis

6:14 PM  
Anonymous Anonymous said...

What about making an AmpBus cable that only has the pin that shuts the 909 on and off connected?

8:25 AM  
Blogger goose said...

So I have removed the Quad link board, connected the led with a 10k resistor is series to the spare DC connection on the pcb.

The 2 amp inputs are now connected direct to the phono sockets.

Put back together fired up and sound great. Thanks for the tips.

4:08 PM  

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