Adding a pre amp

[mansr]

31 minutes ago, barrows said:

This process is not all analagous to RIAA equalization, as that (for the most part) is entirely standardized such that everyone knows "what was lost" and how to put it back.

RIAA EQ avoids losing things by boosting high frequencies above the surface noise.

[mansr]

17 minutes ago, KingRex said:

My RIAA comment is probably incorrect as it is known what is being eliminated and brought back.  I thought RIAA was used to reduce the amount of wall Groove cutting required to reproduce Bass notes in the recording.

That as well.

[mansr]

16 minutes ago, Ralf11 said:

I see no reason why a box with a DAC chip in it ("a DAC") cannot provide all the functionality of a pre-amp in a separate box

There are many such devices.

[mansr]

6 minutes ago, barrows said:

The problem here is that a lot of audiophiles seem to think that analog methods of reducing volume are without fault, but that is not the case.  Resistors add noise and distortion, transformers/autoformers add noise and distortion.  Digital volume controls do not share these (analog domain) faults.

What about a variable-gain opamp stage?

[mansr]

14 minutes ago, diecaster said:

You certainly are allowed your opinion. But so am I. My 40 years of experience in audio tells me your position, while logically sound, falls apart in the real world.

 

Even amp designers such as BHK agree that what should be true theory is not in practice. Well designed and implemented preamps, especially tube preamps, improve the listening experience.

So if you put a DAC and a preamp in the same box, you still need a separate preamp?

[mansr]

8 minutes ago, esldude said:

How about that any analog preamp with a volume control in fact is a buffered passive preamp. You did realize that did you not?

 

Passive preamps are the volume control in active analog preamps.  You then follow it with circuitry to make it work properly.  

And whatever circuitry is in a preamp can be placed inside the DAC box. Alternatively, a DAC can be placed inside the preamp box.

 

A preamp serves a few different purposes:

1. Source selection
2. Volume control for sources that don't have one
3. Buffering for weak sources unable to drive the power amp directly

As most sources are digital these days, a DAC with multiple inputs is often a better way to achieve #1. Many DACs have both volume control and a strong line driver, taking care of #2 and #3. With Such a DAC, an additional preamp can only be detrimental.

[mansr]

7 minutes ago, diecaster said:

No. A buffered passive preamp, even one with tubes, has no gain. If there is gain, it is not a "buffered passive preamp".

A buffer provides current gain. That's the definition.

[mansr]

8 minutes ago, esldude said:

Every active analog preamp has a volume control.  A volume control acts just as a passive preamp.  You follow it with a circuit providing a friendly impedance and usually some gain.

That's not strictly true. A variable gain circuit can take many shapes. The end result is the same, of course.

[mansr]

2 minutes ago, diecaster said:

Another quote from Nelson Pass:

 

A buffer is still an active circuit using tubes or transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa.

And that's called current gain.

[mansr]

59 minutes ago, Fitzcaraldo215 said:

But, Pass also makes and sells analog preamps.  So, would his opinion not also be highly biased, as is, of course, your own?

Yes, but it's class A bias.

[mansr]

11 hours ago, sandyk said:

As far as FETs go, they aren't so easy to obtain these days

What are you talking about? Farnell stocks over 10,000 different models:

http://uk.farnell.com/c/semiconductors-discretes/transistors/mosfet-transistors/single-mosfet-transistors

[mansr]

"For every expert there is an equal and opposite expert." -- Arthur C. Clarke

[mansr]

1 hour ago, sandyk said:

I normally don't use suppliers such as UK Farnell . 

As a pensioner I try to get most of my stuff locally or from ebay surplus stock where items like this had dried up last time I checked. I try to get surplus items in reasonable quantities for matching purposes. 

It's often too expensive for me to use sources like Farnell these days, where many items need to be sent out from the U.K. anyway.

Farnell was just an example. Surely there are component distributors in Australia too.

[mansr]

14 minutes ago, Summit said:

One more thing. An analogue VC turns down signal and noise while a digital VC only attenuates signal.

That's true but largely irrelevant. A passive attenuator needs to be followed by buffer which will some noise of its own. A good DAC chip has of SNR of 120-130 dB. Analogue buffers are typically about 10 dB worse. The SNR of a complete DAC box rarely exceeds 115 dB, same for a preamp (they use similar output stages, after all). Attenuating the DAC noise only to have it replaced by preamp noise at the same level is hardly a win.

[mansr]

1 hour ago, Summit said:

Some observations from Paul McGowan

Paul McGowan says himself he doesn't know much about circuit design (that's what he employs engineers for). Why should we listen to him?

[mansr]

3 minutes ago, barrows said:

He is being modest, while he may not be John Curl, Paul is a very competent line level circuit designer

That's not the impression I get from his blog. There he comes off as someone who is more or less making stuff up as he goes along, much of what he says having little or no resemblance to reality.

[mansr]

1 minute ago, barrows said:

Hahahaha!  You do realize that his blog is a self defacing marketing tool, right?  Paul is a highly intelligent man, and he is trying to create a "culture" where average audiophile folks can relate to him.  He appears to be very, very successful at it as well, as he has near cult like following of admirers.

Do not be fooled, he actually does know what he is doing, and is a cunning marketing genius, as well as being a competent low level circuit designer.  I worked at PS Audio for four years.

OK, I believe you. Just seems strange for an intelligent person to portray himself as an utter fool. Not something I'd do. Now if his blog is deliberately stuffed full of nonsense for marketing purposes, it should still not be taken as an authoritative reference. Bullshit is bullshit, whether or not the author is sincere.

[mansr]

8 minutes ago, barrows said:

@mansr, well I agree with you there...  but it is hard to argue with the results!  PS Audio appears to be doing pretty well these days with a lot of employees including a complete engineering staff.  It cannot be easy keeping a company like that going in this business.

Business success and honest marketing are at best unrelated.

[mansr]

25 minutes ago, Ralf11 said:

BTW - don't some pre-amps use digital vol. controls?

Digitally controlled analogue attenuators are common. Is that what you meant?

[mansr]

5 minutes ago, Fitzcaraldo215 said:

Yes, but to avoid confusion, a digitally controlled attenuator for use in an analog circuit is still an analog device, albeit with digital controls.  It attenuates signal purely in the analog domain, and no a-d or d-a is necessary to use it.

Right. A switched resistor network setting the gain of an opamp is one possible implementation.

 

5 minutes ago, Fitzcaraldo215 said:

OTOH, a digital volume control operates entirely in the digital domain by shifting the bits in each PCM signal word.

Bit shifting can only give you 6 dB steps. Finer control requires multiplication.

[mansr]

4 minutes ago, Ralf11 said:

dunno - some say they are; some act like digital - I have not pulled them apart to see

What do you mean by "act like digital"?

[mansr]

4 minutes ago, esldude said:

How many ways can someone misunderstand that a DAC analog output stage has the same job as a pre-amp analog output stage?

14 pages worth.

[mansr]

9 minutes ago, fas42 said:

The buffer within the DAC enclosure is not an ideal circuit, and the PS feeding it is also not ideal.

What makes you believe this? Anything that can be put in box called preamp can just as readily be placed in a box called DAC. Moreover, if the DAC is already that bad, the damage has been done, and nothing downstream can fix it.

[mansr]

Any component with a sound is flawed.

[mansr]

What good is a 32-bit DAC when the noise level is equivalent to 22 bits at best anyway?