DC blocking capacitors audibility.

[solderdude]

perhaps they do know, but are not inclined to say so.

All famous designers and high-end designers know it too but keep it to themselves in order to sell products that sound better than those of any competitors.
These competitors, of course, also posses knowledge but use slightly different ones. You know, just to be different, to use different components or to not shamelessly copy 'ideas' of other designers. They all know the secrets to ultimate sound quality.
Some prefer NOT to use coupling caps, some purposely use them to 'create a house sound', some use specific brands and types, other find DC-servo is the way to go.

Oh I know they know and know what they know too. But... I am just not inclined to say so and let others figure it out for themselves too why spill the beans... I had to find it out for myself too... why share that knowledge.

 

[mhardy6647]

It is often like that. The best people are busy with complicated things. So, that one doofus guy gets the job...

I always think about that passage, I must admit, whenever I try to read a page or two of a manual and then give up in frustration.

... and, now, with this corollary thought, thanks to a post (which I cannot find at the moment) from the "Humor" thread here at ASR. \

 

[Angsty]

I’m no electrical engineer, but I do notice what some of them do.

My favorite phono preamp designer is Ron Sutherland, a thoughtful engineer who extensively uses op amps in his designs. In his Insight preamp at $1400, he uses a blocking capacitor. In his $2200 20/20 preamp, he used a DC servo. Same for his higher-end Duo preamp.

Seems like a blocking capacitor is a legit design choice that can be bettered if you choose to spend more money in the circuit design.

 

[solderdude]

Seems like a blocking capacitor is a legit design choice that can be bettered if you choose to spend more money in the circuit design.

I can assure you that a DC servo is muuuuch cheaper to build than the usage of 1 or 2 'audiophool' coupling capacitors and takes up less board space as well.
In cases where 2 stages have a substantial DC difference (think tube pre-amps etc.) DC servo is not an option and coupling caps are the only way to go. Damn the terrible and audible loss in sound quality.

All you need is a low noise opamp with low offset and high gain, some small capacitance values and a few resistors.
Easy peasy and definitely does not warrant a $ 800.- price hike. But hey.... you can boast about having 'DC-servo' and charge a lot more for the device while reducing production costs.

 

[fpitas]

definitely does not warrant a $ 800.- price hike

!

 

[Angsty]

I can assure you that a DC servo is muuuuch cheaper to build than the usage of 1 or 2 'audiophool' coupling capacitors and takes up less board space as well.
But... if you do and are serious about protecting downstream gear (transducers) you would have to add at least DC protection.
All you need is a low noise opamp, some small capacitance values and a few resistors.
Easy peasy and definitely does not warrant a $ 800.- price hike.

Thanks for the information on manufacturing costs; I don’t get to see that side of things directly.

Regarding the cost difference, there’s a lot more difference than that between the designs. What’s remarkable is that Ron has managed to keep the price on both components the same since introduction in 2014 and 2010, respectively.

 

[solderdude]

Of course there is. Just saying that DC-servo is not expensive and there is no reason to only use it in the higher price segment as a 'sound quality enhancing feature'.
If it were possible then there would be no technical nor financial reason not to use DC servo as that costs peanuts.
There are always filter capacitors in the phono-preamp path anyway which are even more important than coupling caps so its not that he can avoid capacitors in the audio path.

 

[mhardy6647]

!

Year 4 Writing – Letter Writing - New Century School

www.newcenturyschool.com.au

• In 1862, the novelist Victor Hugo famously wrote the world’s shortest letter. It was written to his publisher to ask how his new book was selling. All he wrote in the letter was this: “?”. His publisher simply replied: “!”.
• Funnily enough, the book he was asking about was over 1200 pages long!

EDIT: It is worth mentioning that I'm pretty darned sure that the "new book" in question was Les Miserables.

 

[fpitas]

Les Miserables

Those who discovered they paid $800 for $5 of parts.

 

[mhardy6647]

Those who discovered they paid $800 for $5 of parts.

"... where there is weeping and gnashing of teeth."

 

[fpitas]

"... where there is weeping and gnashing of teeth."

View attachment 322321

Now THAT'S a party!

 

[Roland68]

Thanks for the information on manufacturing costs; I don’t get to see that side of things directly.

Regarding the cost difference, there’s a lot more difference than that between the designs. What’s remarkable is that Ron has managed to keep the price on both components the same since introduction in 2014 and 2010, respectively.

Coupling capacitors have been around since the beginning of audio amplifiers and DC servo circuits are listed in OPamp documentation as early as '89, but I suspect they were around earlier.

With the phono preamplifiers you list, you pay a large part for the development costs, which is completely legitimate, especially for small quantities. Otherwise these small companies/developers would not be able to survive at all. The proportion of electronic components is around 10%.
And as @solderdude said, the difference is marginal between DC servo and coupling capacitors. Even with the very expensive AD797 for DC servo, the price difference is in the low 2-digit range.

 

[SIY]

the very expensive AD797 for DC servo

…which would be a terrible choice for a servo from a performance standpoint. Cheaper low offset FET input have higher performance in that application.

 

[Roland68]

…which would be a terrible choice for a servo from a performance standpoint. Cheaper low offset FET input have higher performance in that application.

That could be the case, the circuit was over 20 years ago and I was just helping. The whole thing also comes from old data sheets. It may be that an NE5532 / NE5534 was used for the DC servo in the later circuit.
But it was the most expensive OPAmp I have seen in a DC servo circuit and the cost difference compared to coupling capacitors cannot be greater.

btw, based on your experience, which OPAmp would you currently recommend for a DC servo circuit

 

[LTig]

That could be the case, the circuit was over 20 years ago and I was just helping. The whole thing also comes from old data sheets. It may be that an NE5532 / NE5534 was used for the DC servo in the later circuit.
But it was the most expensive OPAmp I have seen in a DC servo circuit and the cost difference compared to coupling capacitors cannot be greater.

btw, based on your experience, which OPAmp would you currently recommend for a DC servo circuit

I used an OP177 for the DC forward compensation (not a real servo) in my DIY phono preamp. That was in 1997, don't know about its availability/price today.

 

[SIY]

That could be the case, the circuit was over 20 years ago and I was just helping. The whole thing also comes from old data sheets. It may be that an NE5532 / NE5534 was used for the DC servo in the later circuit.
But it was the most expensive OPAmp I have seen in a DC servo circuit and the cost difference compared to coupling capacitors cannot be greater.

btw, based on your experience, which OPAmp would you currently recommend for a DC servo circuit

Current, I don't know (I haven't kept up), but back when I was using servos for preamps, things like LF412 or AD712 worked very well: low DC offset, ultra-low bias current.

 

[DonR]

Taken from a PDF of Zen and the Art of Motorcycle Maintenance that I - ahem - found in some obscure corner of the interwebz.

I wrote my fair share of user documentation when I was wet behind the ears. It is so true.

 

[egellings]

I always think about that passage, I must admit, whenever I try to read a page or two of a manual and then give up in frustration.

... and, now, with this corollary thought, thanks to a post (which I cannot find at the moment) from the "Humor" thread here at ASR. \

View attachment 322310

...or he was too ignorant to be even able to read it, try as he might.