[ecwl]

Sadly, Gaggan with the 25-course meals has been closed since 2019. I tried to get in when I visited in 2019 but wasn't able to get a reservation due to my poor planning. On the bright side, there is a more casual restaurant he opened called Gaggan Anand which I look forward to trying in my lifetime.

 

[548184]

tidying up... ignore...

 

[Jawed]

@Rob Watts I'm interested in your opinions regarding the "stuff" that is commonly seen in the output of power amplifiers.

As far as I can tell it's quite common for there to be a "Zobel network" or "Boucherot Cell" (or both?) in power amplifiers. One of the considerations here relates to the control of high-frequency oscillations which can "run away" or destroy the amplifier (over-current, presumably).

Usually this is stuff seen with power amplifiers that are used to drive loudspeakers. The cable used to connect the speakers is an "unknown" and the speakers themselves are also an unknown. The combination of these unknowns presents the designer of the amplifier tricky questions about maintaining the safety margins of the amplifier.

Sometimes, it seems, there's discussion of RF induced into the speaker cables having an interaction with the feedback circuitry of the power amplifier.

I'm guessing that in the design of your DACs with headphone-capable outputs, you do not use any of these techniques to control the behaviour of the output circuit. I suppose if nothing else these extra elements will cause reduced transparency. On the other hand, perhaps RF induction is a problem that needs to be solved?

Presumably, "well-behaved" cable constructions and headphones minimise problems for your headphone drive amplifiers. But still you have to solve the problem of all possible, potentially "unsafe" connections and with TT2 in particular, given the fact it happily drives speakers, there's greater scope for problematic connections which might be dangerous for TT2.

So, how should amplifiers tackle the variables of cables and speakers that they are connected to? How do you ensure that amplifier safety is always exceptional?

 

[Reactcore]

Do owners using qutest to directly drive low impedence headphones cause issues?

I'm driving my HP directly out the RCA's for quite some time regulating volume on F2K and it works flawlessly.

I even added buffer caps to Qutest internal OP 8V power section to deliver for current peaks.

My used HP is 300ohm.
I tried with 16ohm earbuds too but they get too loud fast.

 

[Rob Watts]

@Rob Watts I'm interested in your opinions regarding the "stuff" that is commonly seen in the output of power amplifiers.

As far as I can tell it's quite common for there to be a "Zobel network" or "Boucherot Cell" (or both?) in power amplifiers. One of the considerations here relates to the control of high-frequency oscillations which can "run away" or destroy the amplifier (over-current, presumably).

Usually this is stuff seen with power amplifiers that are used to drive loudspeakers. The cable used to connect the speakers is an "unknown" and the speakers themselves are also an unknown. The combination of these unknowns presents the designer of the amplifier tricky questions about maintaining the safety margins of the amplifier.

Sometimes, it seems, there's discussion of RF induced into the speaker cables having an interaction with the feedback circuitry of the power amplifier.

I'm guessing that in the design of your DACs with headphone-capable outputs, you do not use any of these techniques to control the behaviour of the output circuit. I suppose if nothing else these extra elements will cause reduced transparency. On the other hand, perhaps RF induction is a problem that needs to be solved?

Presumably, "well-behaved" cable constructions and headphones minimise problems for your headphone drive amplifiers. But still you have to solve the problem of all possible, potentially "unsafe" connections and with TT2 in particular, given the fact it happily drives speakers, there's greater scope for problematic connections which might be dangerous for TT2.

So, how should amplifiers tackle the variables of cables and speakers that they are connected to? How do you ensure that amplifier safety is always exceptional?

Yes the half Zobel (series R and C) or full Zobel (series R and C followed by an OP inductor) is very common. If you use an inductor that is air cored, then it doesn't degrade the SQ too much; but a ferrite core definitely degrades performance as it upsets transient timing due to the inductance changing with signal current. I design power amps to avoid half and full Zobel, but it means that the open loop OP impedance needs to be ultra low at HF (normal for my designs anyway) and you need more phase margins. Plus you need to test with lots of inductive and capacitive loads to ensure stability.

As to RF the Zobel network is not effective - they are there for frequencies to a few MHz only. You need other techniques to minimise RF going back to the feedback loop.

 

[Kentajalli]

@Rob Watts I'm interested in your opinions regarding the "stuff" that is commonly seen in the output of power amplifiers.

As far as I can tell it's quite common for there to be a "Zobel network" or "Boucherot Cell" (or both?) in power amplifiers. One of the considerations here relates to the control of high-frequency oscillations which can "run away" or destroy the amplifier (over-current, presumably).

Usually this is stuff seen with power amplifiers that are used to drive loudspeakers. The cable used to connect the speakers is an "unknown" and the speakers themselves are also an unknown. The combination of these unknowns presents the designer of the amplifier tricky questions about maintaining the safety margins of the amplifier.

I do remember some of the early Naim amps were sensitive to capacitive cables, they even used to warn users that "your cable can blow up your amp"!
One of the reasons, why I favour mono-blocks with very short speaker cables.

 

[Kentajalli]

A question for Mr. Watts @Rob Watts .
I Use my Mojo with my mobile phone exclusively.
I was fortunate that I never noticed the effects of RFI on my sound, but a change of headphones means that now I can hear it, ain't ignorance bliss!
I make my own USB interconnects and they are long (1 meter), so the phone never comes close to Mojo.
Here is the question:
I am about to use some RFI blockers on the cable, my choices are small radial ferrite chokes, common-mode ferrite chokes and small capacitor built and heat-shrunk sleeved into the cable, near Mojo end.
How is the RFI affecting Mojo? is it on the ground and 5V lines? id it affecting the data lines? both?
I was thinking of a CM choke on ground and 5V line and a small capacitor afterwards, would that be enough? or do I need chokes on both ground and 5V lines and a CM choke on data lines?
I appreciate your reply. If anyone knows, it is you!

 

[Rob Watts]

I wouldn't try making your own USB cable, as it has to be impedance matched - I would stick too certified USB cables. Many audiophile USB cables are engineered to increase RF noise, making them brighter sounding. Quality audiophile USB cables sound the same as generic certified cables...

The situation is RF current flow from the source through the DAC into the DAC's ground plane; this can only happen if the source is grounded, and Mojo's output is grounded too. So a battery operated source will not create these currents. Mojo also has ferrites built into USB ground, to attenuate these currents too.

I suspect that your real problem is your new headphone, and nothing to do with RF. My DACs really do make the differences between transducers much more apparent, as the DACs fundamentally have little of their own sound signature, making it a lot easier to hear problems with your headphones. There are a lot of devices on the market that have high audiophile reputations, but actually have serious sound quality problems.

If you want to rule out RF as an issue then try an optical source. If it sounds the same as USB with generic certified cables, then it's definitely your new headphone.

 

[Kentajalli]

I wouldn't try making your own USB cable, as it has to be impedance matched - I would stick too certified USB cables. Many audiophile USB cables are engineered to increase RF noise, making them brighter sounding. Quality audiophile USB cables sound the same as generic certified cables...

Unfortunately what I require I can not buy pre-fabricated. But I did try standard cables, they did not fare any better. they are bulky, and I also would need a USB-C OTG dongle, which makes the whole contraption way too big.

The situation is RF current flow from the source through the DAC into the DAC's ground plane; this can only happen if the source is grounded, and Mojo's output is grounded too. So a battery operated source will not create these currents. Mojo also has ferrites built into USB ground, to attenuate these currents too.

The situation is when I put the phone in full plane mode (all transmitters off) the effect goes away, am I imagining it? don't think so. when the phone is transmitting, different cables make a difference. I have made a few, two are pictured below.
The one with just a simple ferrite cylinder makes a tiny improvement, the other with two multi-turn ferrites (one for ground, another for 5V) has made the largest improvement. phone on plane mode has the highest effect.

I suspect that your real problem is your new headphone, and nothing to do with RF. My DACs really do make the differences between transducers much more apparent, as the DACs fundamentally have little of their own sound signature, making it a lot easier to hear problems with your headphones. There are a lot of devices on the market that have high audiophile reputations, but actually have serious sound quality problems.

Headphone is a Hifiman Sundara, which sounds pretty good with phone on plane mode and the better cable.

If you want to rule out RF as an issue then try an optical source. If it sounds the same as USB with generic certified cables, then it's definitely your new headphone.

I do not have that option with a phone, but a PC optical sounds same as phone on plane mode with the better USB cable - but only to my ears, and not in a quick A/B test.

Lastly may I thank my lucky stars, that I can get answers from designer of my equipment, this is really rare! and appreciated.
Thank you @Rob Watts for taking the time at 7am to reply.

 

[antastik]

I wouldn't try making your own USB cable, as it has to be impedance matched - I would stick too certified USB cables. Many audiophile USB cables are engineered to increase RF noise, making them brighter sounding. Quality audiophile USB cables sound the same as generic certified cables...

The situation is RF current flow from the source through the DAC into the DAC's ground plane; this can only happen if the source is grounded, and Mojo's output is grounded too. So a battery operated source will not create these currents. Mojo also has ferrites built into USB ground, to attenuate these currents too.

I suspect that your real problem is your new headphone, and nothing to do with RF. My DACs really do make the differences between transducers much more apparent, as the DACs fundamentally have little of their own sound signature, making it a lot easier to hear problems with your headphones. There are a lot of devices on the market that have high audiophile reputations, but actually have serious sound quality problems.

If you want to rule out RF as an issue then try an optical source. If it sounds the same as USB with generic certified cables, then it's definitely your new headphone.

When connecting a source to a Qutest via spdif, would you recommended the source be grounded?
Thank you for your very informative posts

 

[Ards]

Many audiophile USB cables are engineered to increase RF noise...

That's quite a statement.

 

[flyte3333]

The situation is RF current flow from the source through the DAC into the DAC's ground plane; this can only happen if the source is grounded

@Rob Watts What about leakage currents with un-grounded USB source that is SMPS powered ?

For Mojo and Hugo2, which don't feature USB galvanic iso.

 

[ecwl]

The situation is when I put the phone in full plane mode (all transmitters off) the effect goes away, am I imagining it? don't think so. when the phone is transmitting, different cables make a difference. I have made a few, two are pictured below.

If I place my old iPhone too close to the Mojo, and cellular is on, I get a faint Morse code like noise intermittently, especially if the phone shows 1-2 bars and less often if it is showing 5 bars. If I move the phone further away (because my cable is long enough) the noise is gone. I wonder if you can try the same. My assumption was that with older phones and weak signal, the phone ramps up the power so much that it generates too much EMI/RFI in the surrounding air that the casing of the Mojo cannot block. In my case, the noise didn’t have anything to do with the USB cable. But it’s always possible your situation is different than mine.

 

[Kentajalli]

If I place my old iPhone too close to the Mojo, and cellular is on, I get a faint Morse code like noise intermittently, especially if the phone shows 1-2 bars and less often if it is showing 5 bars. If I move the phone further away (because my cable is long enough) the noise is gone. I wonder if you can try the same. My assumption was that with older phones and weak signal, the phone ramps up the power so much that it generates too much EMI/RFI in the surrounding air that the casing of the Mojo cannot block. In my case, the noise didn’t have anything to do with the USB cable. But it’s always possible your situation is different than mine.

Thanx. I am aware of this.
my phone never comes close to mojo.
the effect I am complaining about is a hardness to the sound and muddying and almost sibiliant effect. it is very mild, but once you know you can have better ....
I do not hear any actual noise, clicks or otherwise.

 

[ShenaRingo326]

I wouldn't try making your own USB cable, as it has to be impedance matched - I would stick too certified USB cables. Many audiophile USB cables are engineered to increase RF noise, making them brighter sounding. Quality audiophile USB cables sound the same as generic certified cables...

Any recommendations (brands/stores) for good quality certified USB cables?

thanks!