Affect of Amp's Gain control on Input Impedance

[jstang]

Searched a little but did not find anything....

I was listening to my 4400/500 combo today - 4400 as the preamp/tuner and the model 500 as the amp...

I was messing with the Gain control of the 500 and lowered them about 33% from the max and noticed that it cleaned up the sound at higher volumes. Now I did have to raise the volume on the 4400 to make up the difference in the sound output level.

But even once I raised the 4400 volume the sound was still cleaner than with the 500's gain controls at the MAX.

So I started looking at the input impedance of the 500 vs the internal amps of the 4400. The 500 has 33k with the gain controls at the MAX and it increase the impedance with the gain reduced....

The 4400's amp have about 45K input impedance.

So I am thinking that with the gain controls at the MAX , which gives the lowest input impedance, the pre-amp outputs are being over driven....

Any thoughts? This might be important to note for those using receivers as pre-amps.


johnk

[whoaru99]

Are you sure the input attenuator/gain control changes the input impedance?

It could be just that by reducing the gain of the amp you are amplifying less of the noise floor of the preamp.

Afaik, the noise floor of the preamp is essentially fixed, so when you turn up the volume, the signal represents increasingly more of the output over the noise floor. So, from that perspective, you are better to run the pre amp hotter and the power amp with less gain because the amp amplifies more signal and less of the pre's noise.

This is called gain structuring and is very important in the pro audio world to minimize amplified noise amongst other things.

[jstang]

Understood and agree with you on the noise floor and running the preamp hotter...

But the 500's manual states the input impedance is 33k with the gain knobs at the MAX setting. So reading the schematic its pretty clear that the input impedance will go up with the gain knob at lower settings. The gain pot is a 50k pot... so as you rotated to zero gain, the impedance would track to the to value of the pot.

So I think, the lower input Z is affecting the 4400 pre-amp with the gain set to the MAX.

But you mention "amongst other things".... Cause I follow you on the noise floor, but what else can possibly happen?


Thanks.... Wisdom is always greatly appreciated....!!!!


jk

[whoaru99]

Probably more significant in pro audio when there tends to be many devices in the signal path. But, I believe another is so no device reaches clipping before any other. Of course, the benefit is not that all devices clip at the same time, the benefit is that you know where the clipping point is through the whole signal chain. Thus, stay below it thoughout.

[jstang]

This minor tuning of the gain control has made very a noticeable difference in the higher volume level range. At higher volumes the deep bass control is simply great...and harmonic decay of mid & high notes has just come alive....

The owners manual for my Soundscraftsmen MA5002 recommends to adjust the preamp volume to a 50% setting and then adjust the power amps gain control to loudest volume you will use....

This exactly where I ended with the Model 500 without reading those instructions first. The 500's owners manual has nothing about this in it.

Maybe it does pay to read the manual after all.....:D


johnk