Gain and where to attenuate

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Gain and where to attenuate

Post by chrismercurio »

split this off and moved it here... seems I also accidentially deleted you chris... sorry... dave
DowdyLama wrote:Guys:

I don't know off the top of my head exactly what a Hammond 715 is, but I'm guessing that you're exceeding the 83's voltage ratings.
Is a mu of 8 (or 5.6 with the 46) adequate? Buzz, don't tell me you're falling for Jeffrey's "we don't need no stinking gain" mantra :wink:

I was going to send Jeffery a PM but figured I would just post it here. I went to a buddy's house this morning to listen to his system and he has an Air Tight 300b amplifier with an Air Tight ATC1 preamplifier. The gain on the front of the amp was wide open and as such, the volume knob had to be turned very little to achieve a very high spl through his Altecs. The sound was thin and tinny at best. As an experiment, I turned the gain down on the amplifier to almost nothing so that the pot was nearly wide open on the preamp to set a comfortable listening level. The sonic difference was night and day with the experiment being wholly better than what we started with in every way. I thought to do this because I use a source buffer (impedance converter) or preamp with no gain at home. It can turn things down but not really up.

Can someone say why it was so much better with minimal gain from the amp and max or near max from the preamp? In the first scenario with the gain at full on the amps is the source voltage (the music?) getting choked off at the preamp?

Best regards to all,

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Location: stuck in the USA for now...

Post by JeffreyJ »

hey chris..

as you can guess form the lack of reples, this one is tough to dignos without knowledge of teh circuit...

teh difference is in teh preamp.. the amplifier is getting teh same voltage to the first tube in both cases... well, the source impedance seen by the first tube changes, but that usually affects the HF roll-off, and I bet it is designed competently...

in case one, vol down on teh preamp, it is seeing a signal that may well be down in the noise floor... when you crank this up, scenario two, the components may like seeing that higher AC level... that is my best guess... as you are on a magnetic guy's forum, let's call it Barkhausen...

not sure I helped.. but that is best guess... I'd just be glad you found the sweet spot... :^)

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Post by rf_dude »

I know this is an old thread, but it's something of interest and as a new member I'm posting my first comment.

All analog processing systems have two basic, and at times conflicting, requirements: 1) Amplify the desired signal and 2) preserve the Signal-to-Noise as best as is possible.

We tend to focus on amplifying the desired signal (i.e. - the audio) and it's easy to forget that noise is an indistinguishable part of that signal. Noise is present in many forms, but for this exercise we will focus on the broadband noise associated with the signal and ignore such things as power supply hum (which is treated separately).

The input signal to an amplifier chain consists of the desired signal (audio) and a noise-signal that is broadband. This noise is basically thermal noise, added from the physical components of the system, and can also be source noise from the original record/CD/etc.

We can do nothing about this noise - it is the basic noise floor, and sets the input Signal-to-Noise ratio of the system.

A full discussion of SNR is beyond the scope of this simple post, but basically you want to adjust the gain distribution through your amplifier chain so as to preserve the SNR as best as possible. This means, in general, to have NO attenuation ahead of the phono amp stage, no large-amplification gain stages followed by large attenuators, and overall low-noise design of the gain stages (especially the first gain stage, where overall system noise-figure is set)

The adjustments may need to be tweaked, as you have found, and at some point the optimum signal-to-noise settings will be heard. All of this discussion assumes that interactions between stages do not significantly affect the frequency characteristics (again, a major assumption but one that is "safe" as long as the stages were properly designed.

Hearing "tinny" output sound may mean that the gains were set such that the noise floor was greatly amplified and filled-in the passband of the amplifier system - the signal-to-noise was ruined, and the speakers were processing large amplitude broadband noise. By lowering the output stage gain and increasing the preamp stage gain, you were keeping the overall gain the same but lowering the noise floor in the system.

Anyway, glad to have found this forum and hope to learn lots of good things here!
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