First, a general question. Do I have to start a new subject every time I want to discuss about something? The subject that I'm discussing below must have been talked about before but do I have to find that subject and continue from there? I tried going through the thread looking for the same subject but it's just too much. So I'm starting a new subject here. Is this the right way to use this thread?

Below is my email to Nelson Pass regarding the (revisted) Zen Amp that I built quite a number of years ago. Mr. Pass responded to my email but I could not seem to locate it now. At that time, I did not know about this thread. So, hopefully someone here can provide me with some further feedback.


Couple of question about the ZEN amplifier (revisted Zen amp with the upgraded power supply) that I built many years ago.

I have always listen to it on a pair of Klipsch Forte speakers via a Denon Pre-amp with the variable loudness control set to "flat" - I guess this is the setting with the boosted bass and treble. I was able to get pretty loud sound out of it. The treble is there and the bass is there. In fact I was pretty surprise to hear such quantity and quality of bass out of 10w/ch. Basically, what I'm trying to say is that the sound is loud.

Just recently, I was trying to do something different and tried to listen to it with the variable loudness set to "max". The sound is so soft - i.e. not loud. It's like you can hardly hear much sound out of the loudspeakers. So, just to make sure that my Denon pre-amp is not defective, I hook up the ZEN amp to another Yamaha preamp that does not have bass/treble or loudness control and I got the same result - hardly any sound coming out even if I turn up the volume control to the maximum.

So, my questions:

1. Is what I just described normal? That is, without the bass and treble boost, it does sound soft - as in not loud?

2. Could it be that I did not built the amp correctly? It has been working for a long time - nothing is broken or burnt out yet?

I would appreciate to hear your response. Thanks.



Just recently, I tried another pre-amp (Kingertics KPA-1). It played loud and sound pretty good too.

1. This has to do with the gain of the pre-amp/amp, isn't it?
2. If so, can I change the input gain of the Zen amp so that it'll be more compatible with the Denon or Yamaha pre-amp?
3. And how do I change the gain of the Zen amp?

Any feedback would be very much appreciated.
For what it's worth, I don't recall this particular issue coming up before.
As for the Loudness control...
The human ear does not respond the same to soft sounds as loud ones. At high volumes, the ear is somewhat closer to what we might call flat response, but at low volumes sensitivity falls off rapidly at low and high frequencies, leaving a peak in the vicinity of 1kHz or so. Easy enough to understand from an evolution standpoint--it's exactly the frequency range of rustling grass, which is to say that it's how our ancestors heard predators coming up behind them. It was hear, or die. Strong selection pressure, there. However, it left us with a bit of an oddity in our hearing, in that we don't hear the same at low volumes as we do at high volumes.
A while back a study was done where people were tested to senstivity to various frequencies at differing volumes. The resulting data led to the Fletcher-Munsen curves. (We're going to leave out the idea that later folks complained that the Fletcher-Munsen curves weren't accurate, they became deeply ingrained in the audio world, and that's good enough for our current purposes.)
The purpose of a Loudness control, whether it's a button or a continuously variable knob, is to compensate for the ear's poor behavior at low volumes. In essence, it boosts the low and (usually) high frequencies relative to the midrange. Okay, how's this relate to your question about volume? Well, some manufacturers chose to cut the midrange, rather than boost the low and high end. The net effect is an overall reduction in volume. Same result, just a different way of getting there.
So much for the Loudness control.
Now, I'm not clear on which combinations of equipment played louder than others, but is it safe to assume that the Zen plays at a normal volume with the original (Denon) preamp, as long as the Loudness control is set to Flat?
One further point...please note that due to the behavior of a Loudness control, it's quite easy to blow drivers with it on.