Adding a pre amp

[sandyk]

1 hour ago, barrows said:

we already have two very effective RF filters: transformers at both the receiving and transmitting ends.

 

 And they can't add distortion, introduce phase changes, or premature high frequency roll off, especially with high res 24/192 or DSD ?  

My own DIY Class A Preamp for example , is only -3dB at 1.5MHZ.

[sandyk]

49 minutes ago, barrows said:

The output stage of the DAC and the input stage of the amplifier do not ADD distortion as they are necessary.  By inserting an additional component (a preamplifier) between them one is then adding (unnecessary) distortion.

 

 You didn't address my concerns about using 2 transformers, one at the output of the DAC, and another at the input of the amplifier. My main concerns are phase changes and HF roll-off. Transformers are not distortion free either.

A good DAC should not need additional HF filtering, especially by using transformers at both ends.

I.M.E. wider bandwidth sounds better too .

BTW, my Class A Preamp has very close to 4 Zeroes in it's distortion figures. Neglecting the series 100 ohm resistors at the output, which sounds better with most interconnects of 1M or so, it's output impedance at 1KHZ is .002 ohms .

(calculated by an E.E. friend)

Unlike GUTBs recommendation, my preamp only has a gain of around 3.2  x to reduce the need for too much attenuation ,and the loss of more than necessary S/N

[sandyk]

36 minutes ago, Spacehound said:

Any given DAC  (with a volume control) is going have an output stage near enough the same any give preamp.

 

 That is simplistic bullshit !

A decent Preamp doesn't use just a couple of  generic cheap output I.C.s either.

 Many Preamps also use discrete circuitry, as my DIY Preamp does, and a high quality, wide bandwidth attenuator.

Data sheet for DACT audio attenuators.htm

[sandyk]

6 hours ago, jabbr said:

I'm a little surprised that this hasn't been said but one of the best reasons for a pre-amplifier is to to match a high output impedance source with a low input impedance amplifier i.e. to amplify current.

 They are also normally able to properly drive much longer cables, as well as higher capacitance type cables, sometimes even being able to properly drive a 75 ohm cable terminated at the amplifier end.(my DIY Preamp can)  Some recent model solid state amplifiers have a much lower input impedance too, with a trend to input resistances such as 10 Kohms.

In fact, even lower input impedances in Solid State amplifiers can also result in a lower DC offset at their output.

[sandyk]

1 hour 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.  

A volume control can also be used beteen 2 lower gain opamps which may result in an improved Signal to Noise Ratio.

[sandyk]

7 minutes ago, diecaster said:

 

I am sorry, but that is fallacious premier

 

You are, or should be, better than that. 

He normally is .

[sandyk]

1 hour ago, barrows said:

): the point is, the output circuit of a DAC is the SAME as the output circuit of an active preamp.

 

 A good quality preamp usually has a far better output stage than most affordable DACs, often using a discrete output stage, and quite often also uses higher voltage .+ and - supply rails (> + and -20V) than most  DACs. They often use Class A topology as well.

[sandyk]

22 minutes ago, wwaldmanfan said:

It seems to me, the better your equipment, the less you are going to hear small incremental improvements.

 

 That hasn't been my experience.

[sandyk]

3 hours ago, Panelhead said:

  All small signal circuits operate in Class A. The power and heat  generated are very low. 

 That is incorrect.

Most opamps need to have small circuit additions to force them to operate in Class A .

See Tangentsoft and DIYAudio threads for example.

 

https://www.google.com.au/search?source=hp&ei=VK6ZWqfeEoT88gWgiKb4Dw&q=biasing+an+opamp+into+class+A&oq=biasing+an+opamp+into+class+A&gs_l=psy-ab.13..0i22i30k1.2201.27259.0.33150.38.37.0.0.0.0.279.7320.0j22j15.37.0....0...1c.1.64.psy-ab..1.25.4953.0..0j35i39k1j0i131k1j0i131i46k1j46i131k1j0i10k1j0i22i10i30k1.0.tV3WKis9HAw

[sandyk]

3 minutes ago, mansr said:

Yes, but it's class A bias.

 

 So what is yours? Class AB or Class C perhaps ?

[sandyk]

3 minutes ago, diecaster said:

 

You can certainly have the cold "accuracy" of solid state preamps and amps along with their harsh odd order harmonics and hard clipping. I'll take the gradual clipping and musical even order harmonics of tubes. Even if there is more of it.

 Cold accuracy of solid state preamps and amps ?
 A well designed Solid State Preamplifier and amplifier has a natural warmth to it, just the same as the recordings/sound  it reproduces.

Well designed Vacuum Tube amplifiers can also have this attribute, although it seems that many prefer the colouration due to more even order harmonics.  Hugh Dean ( AKSA in DIY Audio) from Aspen Audio in Melbourn Au. even has some solid state amplifier designs where he deliberately tailors the remaining harmonic distortion products to add some warmth for those who prefer this kind of thing !

[sandyk]

2 minutes ago, davide256 said:

hmm. Tube transformers suck. Go Berning

 I also wonder why some worry so much about soft clipping vs. hard clipping.

 NO amplifier should ever be driven into this area if chosen to match the size of the room, loud speaker efficiency etc. AND you value the HF drivers in your speakers, although you may sometimes hear this kind of thing at certain times of the year from some neighbours parties !

[sandyk]

3 minutes ago, barrows said:

It is just not a big deal at all for the output buffer of a DAC to be the equal (or even better than) the output buffer of a preamp, it is very simple electronics, and not tricky or expensive to do.

 

Once again you are assuming that the Preamp uses simple electronics as in your example, and it's only real advantage is the ability to connect different source devices. That may be true of some cheaper Preamplifiers, but certainly not Preamplifiers that have a well established reputation.

 If they didn't add additional value and  higher performance over DACs with generic output devices such as you just quoted, they wouldn't still sell in the era of DACs with good digital attenuation. However, a Vacuum Tube based Preamp is sometimes a different matter, as some like a degree of colouration.

 I do agree however, that not all available Preamps meet this criteria.

 

A quote from  an  E.E. friend who recently revisited Valve Preamp designs.

I don't doubt though that he will be able to further refine it into a very good preamp though.

 

Quote

I got the SC valve preamp going. It sounds quite bright, not at all soft. The tonality of instruments is really nice and organic/natural, but I find it squashes dynamics and sounds a bit compressed. It also tends to emphasise the midrange somewhat - I expect this is due to the AC coupled feedback and generally small coupling caps used everywhere (even though I increased the output coupling cap values by 20x ......

 

[sandyk]

2 minutes ago, barrows said:

 Most common high end amplifiers these days feature input impedances of 30 Kohms or more.

 These days, many designers use the findings of people like Douglas Self who is the author of several Audio design handbooks, (e.g. Audio Power Amplifier Design Handbook 5th ed - D. Self (Focal, 2009) WW) and use an input impedance of 10K, with an even lower input impedance being desirable from the point of reducing the amplifier's D.C. offset.

I didn't miss the point of Mosfets, as I have previously used them in amplifiers, but prefer more recent silicon transistors, especially well matched dual devices and Bipolar output devices with an Ft of 30MHZ or higher. ( My DIY Class A Preamp uses 200MHZ Output devices and a bias of >100mA ,  Toshiba 2SA 1930 and 2SC 5171)

As far as FETs go, they aren't so easy to obtain these days, and often  have very wide tolerances. This may be fine for large companies though where they can request closer tolerance devices.

[sandyk]

16 minutes ago, barrows said:

I am curious who these designers are.  most amplifiers I see have higher input impedances.  Although it does not take that robust an output stage to properly drive a 10K input impedance either.  Anything <~100ohms should be fine.  As mentioned there are many DACs which fit that bill (Chord, Weiss, etc)

 I am not across recent designs  from the USA, but many designers will now have taken on board the very worthwhile lessons and improvements detailed by highly respected Audio designers such as Douglas Self from the U.K. and Bob Cordell.

Many recent Power Amplifier designs published by Silicon Chip magazine, where they sometimes boast of the lowest distortion  amplifier designs ever published, are based to quite an extent on  the work of Douglas Self.

Have you seen any of the Douglas Self Audio Power Amplifier handbooks ?

[sandyk]

18 minutes ago, esldude said:

So can you explain how a DAC with a good output stage is improved by a preamp? 

 

 Perhaps your definition of a good output stage doesn't even come close to that of many other members ?

Very few affordable DACs even use low impedance buffers at their output, and are susceptible to the usual Interconnect vagaries. A friend found that even a well respected Bricasti M1 DAC was capable of a further worthwhile improvement by biasing the output I.C.s into Class A operation.

Barrows though, is usually referring to much higher priced units that many members can't  justify the expenditure on , unless they are either well heeled, or still single without any children to put through College etc.

[sandyk]

2 minutes ago, barrows said:

Well, if they did not need to purchase a very expensive preamp they could afford a much better DAC...

 

 It all depends on how many Input sources they have.

 Many members still have high quality Vinyl setups too.

[sandyk]

10 minutes ago, barrows said:

So you are only referring to theoretical designs which do not exist as commercially available products, seems quite far off topic here?

I was talking about actual high end amplifiers which audiophiles can purchase, ones which might be used with a DAC audiophiles can purchase.

 No, they are also commercial products , such as from The Signal Transfer Company .

https://www.google.com.au/search?source=hp&ei=5SaaWtXPOYqM8wXo146QAw&q=the+Signal+Transfer+Company&oq=the+Signal+Transfer+Company&gs_l=psy-ab.13..0.1440.10725.0.12872.28.20.0.7.7.0.307.4445.2-17j1.18.0....0...1c.1.64.psy-ab..3.25.4560.0..35i39k1j0i131i67k1j0i131k1j0i67k1j0i22i30k1.0.hdJr1eFS4-4

[sandyk]

1 hour ago, One and a half said:

When, not if, the DAC decides to send white noise instead of a signal, the volume of the pre amp remains the same. If the DAC creates white noise, it will be at 0db, and that signal directly into a power amplifier will most certainly fry tweeters. So why is there a discussion on using a pre amp with a DAC again?

 Imagine what could happen with an event like this into a typical high quality amplifier such as the attached Silicon Chip ULD3 with specifications as shown, with a full power sensitivity of a tiny fraction over 1V into 4 ohm speakers. (200 Watts)

Momentarily, the amplifier would possibly deliver quite a bit higher than the rated 200W into 4 ohm speakers before blowing a fuse . (if you were lucky !)

Being white noise, the tweeters would most likely be destroyed.

Note also the fairly low input impedance of 12Kohms, which is quite a bit lower than many DACs would prefer.

Click on the image for a larger image.

[sandyk]

9 hours ago, mansr said:

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

 

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.

[sandyk]

A quick summary of some of the features of the better Preamplifiers, as published by Stereophile.

This may give some idea of why they may be more expensive

Mostly using discrete circuitry. Separate Power Supplies.
Some may even be battery operated, with a "Recharge Mode" and a total Isolation Mode, where the Total Isolation Mode usually sounds a little better.
Additional filtering of the main output stage as well as a local lower noise supply to the input differential amplifier.
The preamp's line stage may have no capacitors in the signal path and is flat to DC.
With some. a balance control is also provided, as well as balanced operation, and extensive Moving Coil Cartridge Preamp section options.
The advantage it presents over a passive control is its more powerful cable-driving ability, even over long runs.
Dual-mono, and ultimately direct-coupled signal path, intended to achieve high gain, low noise, wide bandwidth, and excellent linearity.
R-2R ladder volume control—instead of a traditional potentiometer, where sometimes relays select different combinations of resistors so that the volume can be adjusted in precise increments of 0.1dB from 0 to –70dB.

 

 

https://www.stereophile.com/category/solid-state-preamp-reviews

[sandyk]

1 hour ago, mansr said:

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

We have Farnell and others, but in many cases the components need to be sent out from the parent company, perhaps UK Farnell which has happened to me, resulting in several weeks delay, or from an Asian warehouse. They rarely carry the extensive ranges of the U.K. and U.S. parent companies due to Australia being a much smaller market,and there are often extensive lead times with many components. Quite often, there are large shipping costs, especially from the U.S.A.  unless you exceed a specified order amount. An E.E. friend has offered on a couple of occasions to include my smaller requirements with his much larger order, but I don't like doing that.

To save money I tend to use ebay quite a bit for surplus items, or https://www.utsource.net/

Neither do I like to use my Credit card for orders unless there is no other alternative, and prefer to use Paypal which companies like Farnell and RS Components do not normally accept.

[sandyk]

1 hour ago, barrows said:

The new school way of designing is illustrated by what Mr. Putzeys does with his designs:  Instead of accepting that there will be some level of (audible) distortions, Mr. Putzeys just reduces ALL distortions to levels where they cannot be perceived at all.  As long as the distortions are non perceivable, it matters not a bit what order they may be, or whether they are even or odd.

 

Which is exactly what Douglas Self is doing, and is thoroughly discussed in his Audio Handbooks.

Even then, there can be further improvements made in the PSU area , such as independently powering the front end with separate very low noise power. DC coupling is another area of audible improvement if properly implemented. It gives a "preciseness" to the LF area especially , but is not normally done with Power Amplifiers, as it also needs good associated Speaker Protection circuitry in the event of anything more than several hundred mV of DC appearing at the Speaker output. .

[sandyk]

Given  Barrows much earlier posted reports (several years ago) on the audible effects of Jitter etc. which closely corresponded to that of my own and John Kenny's reports, I will admit that Barrows does have finely tuned hearing abilities, although personally, I would be surprised to hear an NC -400 amplifier equal one of Nelson Pass's best Class A Power Amplifiers. 

I do agree with him however, that an  experienced DIY person can extract much more from many commercial designs, quite likely a little more from some of Nelson Pass's designs too.

[sandyk]

4 minutes ago, Fluffytime said:

Does anyone actually take John Kenny seriously anymore?

 

 I would expect that John Kenny is far more experienced and knowledgeable in this area than you are, although perhaps a little full of himself since he became a manufacturer.