Misunderstanding Signal Path Purity
April 2005 by Steve Deckert

I run into the following question all the time:  "Why if you are running a preamp would you want a volume control on an amplifier?"

The short answer to that is why have a gain control on any amplifier?  The longer answer requires some insight first.  Let's look at signal path purity for a minute.  We all know or have heard that simpler is generally better when it comes to sound quality and circuits.  This is generally a safe assumption.  I mean look at the Zen Triode amps with one of the shortest signal paths an amplifier can have...  No one will dispute how great they sound.  Given such an amplifier it becomes counter productive to handicap it with a lower quality preamp, patch cords, source, etc. etc.  

In looking back over the years our web site has always stressed the beauty of the Zen Triodes super simple signal path and I think many have become convinced that is why they sound so good.  News flash:  That's only a small part of the reason.  Nevertheless many people look for ways to simplify it even further.  An example of this is removing the gain control and even the input / output jacks in favor of directly soldered connections.

Now in theory this would improve the sound every time but here on Earth it's never that simple.  For a long time I never gave this concept of simplicity much thought since making a business of it.  In fact I used it to make some pretty good points along the way.  For this reason people are failing to understand why I sometimes promote using a preamp with an amplifier such as ours that can easily work just fine without one.  They will conclude that it makes little sense to own an amplifier with just a couple parts in the signal path if you're going to double or triple that parts count by adding another component.

HERE IS WHAT YOU ARE MISSING or NOT UNDERSTANDING ...

Purity.  

Lets look at it for a minute.  Take a clear flat bottomed glass and place it over a very clear photograph.  If you've chosen a good glass you should not be able to see it, or at least the photograph should remain as clear as before.

Now lets add 1/4 inch of pure crystal clear water.  Again you should not be able to see any reduction in the fidelity of the photograph.  Obviously if the water was full of impurities you might detect a slight haze over the picture.  In that case adding a second 1/4 inch layer of impure water would increase the haze.  You could continue this until you reached a point where the picture was no longer worth looking at.

You can think of each 1/4 inch layer of water as a component.  Based on the example above using impure water as an analogy it becomes quite obvious that the more components you add the less transparency you get.  But lets look at the analogy again using pure water with minimal impurities.  You can add 1/4 inch layers of water to the glass until the glass is full and amazingly the picture is still clear with no haze.

This is the difference between average audio gear and exceptional audio gear. When you have gear that is close to pure, you can have several components in the signal path without significant negative side effects.

THE NEXT THING to grasp is why you would want to add an extra component in the first place... and the answer in a word is CONTROL.

Lets look at the Zen amp with it's gain control being fed directly from a good source. The sound doesn't suck.  In fact we usually would prefer everyone try it that way first because if you add a preamp with impurities you will get a haze over the music that was not there before.

Having said that, if you add a preamp that is like the water with minimal impurities you suddenly have two volume or gain controls!  And that is the reason why you would want one in the first place - at least most of the time.  You see, when you have a volume control on the preamp you can associate that control with your source.  You have a way to raise and lower the level of your source.  When you have a gain control on the amplifier you have a way to raise and lower how much level the amp requires for full output, or just good sound.  Having the ability to find the magic ratio between these two volume controls allows you to tailor the bloom, dynamics and weight of the presentation the natural way - without tone controls.

I find that most of the time when I use a good preamp with a Zen Triode amp the amps gain control responds best at somewhere between 2/3 and 3/4 up.  Sometime it's all the way up.  Sometimes it's not.  Sometimes the recording will determine which way is best, sometimes it's the components and sometimes it's both.

Eliminating the volume control on the amp in favor of a simpler signal path would also eliminate the real advantage of having a preamp in at least a healthy percentage of scenarios.  The way to know is to listen for several weeks and experiment with the ratio between the two volume controls.  If you find that in every single case the best sound is with the gain control on the amp pegged, AND you never intend to try a different preamp or source in the foreseeable future, you are well justified to remove it.

Interestingly enough when the volume control is near or at the full on position, the carbon wiper has little or no resistance and becomes nothing more than a set of contacts not unlike a switch.  This means you don't really have anything in the signal path other then a switch when this is the case.

The reasons why almost every amp I've made have gain controls should now be clear. The reason why adding another component with little or no impurities doesn't screw up the sound quality should also be clear.  The advantages of having two volume controls in the signal path in my experience usually outweigh any potential drawbacks.

Please read this over and over.  Having two over's in the sentence adds dynamics to the point.

Cheers,

Steve Deckert

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