Balanced Line Stage vs. ad811 based preamp - diyAudio
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Old 27th February 2002, 02:06 AM   #1
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Default Balanced Line Stage vs. ad811 based preamp

I am using a preamp which is based on the AD811 video amp chip. The preamp employs a very solid power supply with power regulation at the chip. Audio Amateur magazine had a preamp project called the Valkyrie line stage pre-amp based on this chip. According to the article, the AD744/AD811 combination is supposed to be almost as good as using descrete components.

My question is, how would the Pass Balanced Line stage preamp compare in sound quality to the AD744/AD811 combination.
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Old 2nd March 2002, 03:09 PM   #2
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I canīt tell 100 % because I havenīt heard these opamps. But after making the zen preamp I never used opamps again.
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Old 2nd March 2002, 08:42 PM   #3
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Default balnanced line stage vs ad811 based preamp

I recall Walt Jung has published a number of articles along the lines of this preamp you are referring to and the idea is quite interesting.

But your question is a a bit difficult to answer and I would suggest that from practical experience the opamp combination may sound detailed in the highs and fast, may also be dry and lifeless, a processed sound.

Some time ago I conducted some subjective comparisons for a dac buffer and line stage with gain using a variety of opamps.

The fact that they all sounded different in some way suggested that none of them were correct!

Why this is is even more interesting?

The Balanced Line Stage and chip opamps are from two difference schools of thought.

Opamps are usually very complex circuits with many tranistors/ fets etc, and have all sorts of funny topologies to provide whopping open loop gain , low noise and maybe low distortion.*

They nearly all require feedback to contol gain and reduce distortion and need external stability capacitors in some cases.

However, to the horror of many audio users, they are surpised to find most chips only provide piddley levels of true class A drive, only a fraction of a milliamp in many cases and they then operate in class B (this is a fact). Yuck!

Some recent chips do however have nifty distortion cancelling topologies, but they are not class A devices!

The better applications like Headrooms opamp headphone driver use Case A buffers to counter this , or you can try and bias the chip into class A with a current source.

The problem here is most chips will go up in flames over 0.5 watts disipation.

It is reasonable to assume SE class A operation is preferable for at least a preamplifers, so why bother with chips in the first place.

Manufacturers use them because its cheap and easy, and Mr Average punter may never know better.

The Pass Balanced Line Stage, as you are now doubt aware is almost the direct opposite in approach.

The circuit is simple, the signal passes through few elements, there is no feedback, Class A operation is continuous, distortion is independant of the load (only the output swing will reduce), and the circuit can to optimised or signal level and performance.

I suggest you try the balanced line stage, I'm sure the effort will be very worth while.

My own experience with the Aleph 5 thus far indicates (with credit to Mr Pass) it delivers where others fail and I expect a similar outcome with the Balanced Line Stage.



* (Please note I am not a Pass salesman , but this would be an enjoyable and rewarding occupation- perhaps in my next reincarnnation)
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Old 3rd March 2002, 03:28 PM   #4
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Thanks so much for your detailed reply...The AD 811 is a class A buffer, and in all of my readings, a highly regarded chip for audio use. That said, I am coming around to your thinking that I should build the Pass balanced line stage, or wait for the Pass Hi-Lo X-Over, as my system is Bi-amped.
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Old 3rd March 2002, 07:50 PM   #5
SteveG is offline SteveG  United States
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Default I would (and will) go for the Pass circuit

Macka made a very interesting point in saying that these two designs are from two different schools of thought. I tend to fall somewhere in between the subjective and objective camps (I believe the truth is usually found in a balanced, no pun intended, approach).
What do you want out of your equipment? I think that is the question to ask before you build anything.
For me, the answer would have to be musicality. I have not found musicality in op amp designs.
The pass circuit is said to lean towards the warm side of the balance. I find this to sound more like real live music.
Choose what SOUNDS best to YOU (or what you think will sound best to you)! A little bit simplistic, I know, but you will be the one listening, right?
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Old 3rd March 2002, 09:31 PM   #6
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Location: Dallas,Texas
Smile AD811

The AD811 is not a buffer, it is a current feedback op amp. It has a very high gain bandwidth and is pretty twitchy to get decoupling right. Not really a device for any but the experienced.

Almost all preamps operate Class A unless you are driving headphones or 600 ohm loads. A few volts into a 10k or greater load will not push even modest opamps into Class B operation.

The Pass preamp circuits sound good and are easy to build and trouble shoot. They are simple enough to build with point to point wiring on vector board instead of a PC board if desired.

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Old 3rd March 2002, 11:23 PM   #7
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Harry, thanks for your reply.

When you say the AD 811 is twitchy, does that mean the op-amp might oscillate if not properly installed?

How would I know if the Op-amp was misbehaving? Would I put a sine wave or a square wave through the Op amp and view it on an oscilliscope?
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