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Quad 2 - non-standard capacitor values

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marantz123:
Fellows

Need some advice regarding capacitors used in the Quad 2.  C1, C2 and C3 are specified as 100nF.  I have come across a configuration where C1 has been chosen as 220nF and both C2 and C3 as 680nF. I gather that a characteristic of the Quad 2 input stage is that it requires a preamp output/input signal capable of up to 1.4 V of swing for satisfactory performance, which I understand is a characteristic of the QC2 preamp designed to be used with the Quad 2.

Understandably a larger capacitor (C2, C3) would allow a higher amplitude of signal to pass across it. Is it possible that the mentioned higher capacitor values were "chosen" such that the Quad2 would perform well enough with a modern preamp output, assuming 1V of output swing as an example ?

Would the larger C1, C2 and C3 affect the lifespan of the tubes or other aspects of the amplifier ?
What would be a suitable alternative preamp for the Quad 2, would the preout from a Marantz 1060 perform well enough with the original configuration ?

Is there any merit in the non-standard capacitor configuration mentioned ?

Thanks
Marantz123.

Moonraker:

--- Quote from: marantz123 on September 27, 2021, 10:36:10 PM ---Understandably a larger capacitor (C2, C3) would allow a higher amplitude of signal to pass across it.

--- End quote ---

Huh?  Very definitely, NOT.

What is 'it; here? 

An 'amplitude of signal' can not 'pass' 'across' anything.  The 100nF capacitor's reactance is, and is supposed to be, significantly lower than the input impedance of the output stage in the frequency passband.

Hence larger capacitances will have 'no' effect, except extend the LF response of that part of the circuit.  This could create problems such as 'motorboating' or saturation of the output transformer.

The main thing to ensure with a preamplifier for the QUAD2, is the absence of DC.  The QUAD2 input is DC coupled.

marantz123:

--- Quote from: Moonraker on September 28, 2021, 09:58:40 AM ---Huh?  Very definitely, NOT.

What is 'it; here? 

An 'amplitude of signal' can not 'pass' 'across' anything.  The 100nF capacitor's reactance is, and is supposed to be, significantly lower than the input impedance of the output stage in the frequency passband.

Hence larger capacitances will have 'no' effect, except extend the LF response of that part of the circuit.  This could create problems such as 'motorboating' or saturation of the output transformer.

The main thing to ensure with a preamplifier for the QUAD2, is the absence of DC.  The QUAD2 input is DC coupled.

--- End quote ---

@Moonraker : Welcome to the forum !  Lets look at an example. At 1kHz, a 220nF capacitor will have a reactance of 723 Ohms. At this same Hz, a 100nF capacitor has a reactance of 1591 Ohms. Assume an applied voltage of 1V, the resultant current would be 1.38mA for the 220nF capacitor and 0.62mA for a 100nF capacitor, however to simplify this, the Power across the 100nF capacitor at 1kHz is 0.62mW compared to the 220nF which calculates at 1.38mW of power across this capacitor.

Is it then fair to also say that the 680nF capacitor ( instead of the specified 100nF capacitor) at 1V , 1kHz, will allow "more signal" to be presented to the power stage of the amplifier ?

Please feel free to support your understanding. Substantiate mathematically if possible please.

Thanks
Marantz123.

handsome:
Viewed at single frequency yes, the capacitor size will attenuate the signal. But amplifiers are designed to pass the many frequencies in the audio bandwidth at once and so, in the real world your attenuation is manifested as a rolling of of low frequencies. Changing capacitor size therefore as Moonraker correctly asserted, affects low frequency performance. In a nutshell the coupling cap in tandem with the input impedance of the stage (and technically the output impedance of the previous stage) creates a high-pass filter.

Substituting larger values will allow for more bass - but you are now second-guessing both the designer’s intentions and his knowledge of the performance of the rest of the circuit’s components. However it can be successfully argued that when the Quad II was designed and sold, “doof-doof” music had none of the adherents it has today and very few if any loudspeakers of the time could reproduce those bowel-churning frequencies. So increasing the value of the input capacitor would be acceptable because a) almost modern music has a “doof-doof” component b) some loudspeakers and all subwoofers will gaily have a go at reproducing those bits and c) the input capacitor is not in the feedback loop ergo Mr Walker may relax, for now, within his grave. This means your Quad II would have a better chance of powering a rave which is probably what it has been fantasising about when you listen to your Marantz.

marantz123:

--- Quote from: handsome on September 28, 2021, 02:18:41 PM ---Viewed at single frequency yes, the capacitor size will attenuate the signal. But amplifiers are designed to pass the many frequencies in the audio bandwidth at once and so, in the real world your attenuation is manifested as a rolling of of low frequencies. Changing capacitor size therefore as Moonraker correctly asserted, affects low frequency performance. In a nutshell the coupling cap in tandem with the input impedance of the stage (and technically the output impedance of the previous stage) creates a high-pass filter.

Substituting larger values will allow for more bass - but you are now second-guessing both the designer’s intentions and his knowledge of the performance of the rest of the circuit’s components. However it can be successfully argued that when the Quad II was designed and sold, “doof-doof” music had none of the adherents it has today and very few if any loudspeakers of the time could reproduce those bowel-churning frequencies. So increasing the value of the input capacitor would be acceptable because a) almost modern music has a “doof-doof” component b) some loudspeakers and all subwoofers will gaily have a go at reproducing those bits and c) the input capacitor is not in the feedback loop ergo Mr Walker may relax, for now, within his grave. This means your Quad II would have a better chance of powering a rave which is probably what it has been fantasising about when you listen to your Marantz.

--- End quote ---

Viewed at any frequency (not just a single frequency) in the audio spectrum, the capacitor value would have a resultant change in signal attenuation. (Based on the capacitive reactance curve, the manifestation you speak of at bass frequencies is understandable) Considering  "bass" frequency of 100Hz, a 680nF capacitor as found in circuit would have a reactance of 2340 Ohms compared to a 100nF capacitor which would have a reactance of almost 16k Ohms. I can agree with the noticeable roll-off or increase in bass as you mention, considering the change in reactance from 2k to 16k Ohms with the 680nF vs 100nF.  The human ear notices an increase in bass as it is immediately noticeable and dominates the content modern music to an extent, however collectively all frequencies in the audible content are affected by the change to some extent.

Yes, somebody has second guessed the designers choice, but how does the "degree" of change affect the health of the amplifier and the lifespan of the tubes? @Ampdog has contributed Quad 2 content on other forums over the years, as well as others here along with other members. I have have read up on everything I could find thus far, including Keiths webpages. Theres no substitute for experience.. thanks guys.  :pray:

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