[tfarney]

Those amps that have variable gain switches...is it like a very simple version of a stepped attenuator, with the maximum gained stepped back through resistors for the various settings, or is it more complicated than that?

Tim

 

[nikongod]

it depends on the amp, but usually like you say, it is a simple 2 through 5 position switch adjusting the feedback resistors.

edited/added:
there was significant debate a while back whether to short across resistors, put more resistors in series, or in parallel... like so many things everyone has an opinion. I suppose you could also use single resistors depending how you set it up.

 

[cotdt]

yeah they are attenuators and don't actually change the gain of the amp modules themselves.

 

[nikongod]

Quote:

Originally Posted by cotdt /img/forum/go_quote.gif yeah they are attenuators and don't actually change the gain of the amp modules themselves.

they do when they are located in the feedback loops of the amp.

this is what I was refering to.

 

[cotdt]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif they do when they are located in the feedback loops of the amp.

some amps do that but it's a rather unelegant solution, as changing the gain can potentially change the sound signature of the amp

 

[FallenAngel]

Quote:

Originally Posted by cotdt /img/forum/go_quote.gif some amps do that but it's a rather unelegant solution, as changing the gain can potentially change the sound signature of the amp

Changing the gain via feedback loop really shouldn't change the sound signature, it's when you put input (and especially output) resistors that it will, not might, change the sound signature.

Of course this only happens in SS amps with feedback loops; I don't think there are many commercial amps that offer "gain switches" that are either tube based or discrete without feedback.

 

[richardu]

Input resistance should be OK if the amp is well designed. Output resistance will affect the sound signature. Changing feedback resistance is not advisable in a true hi-fi amp. Personally I would refuse to buy an amplifier with variable feedback.

 

[FallenAngel]

Quote:

Originally Posted by richardu /img/forum/go_quote.gif Input resistance should be OK if the amp is well designed. Output resistance will affect the sound signature. Changing feedback resistance is not advisable in a true hi-fi amp. Personally I would refuse to buy an amplifier with variable feedback.

A voltage divider on the input is easy enough to implement. Considering how much feedback we really change for gain, as compared to open loop gain, it really shouldn't change the sound at all. This is definitely more true in buffered amps vs opamps driving headphones (like most portables, but I don't use those).

 

[nikongod]

In the case of an SS amp, adjusting the gain does not generally have that large of an effect on sound if the open-loop gain of the circuit is very high. If you uhave a circuit that runs open loop at gain of say 1000, and you adjust from 8 to 4 there will be very few if any differences aside from voltage output for a given input. If the circuit runs open loop at voltage gain of say 10, and you implement feedback to drop it to 8 or 4, there will likley be differences.

This is also VERY VERY true with tube amps. the character of a singlepower with "texture knob" (adjustable feedback) changes drastically depending where the knob is set. I personally like this because it allows you to tune the character of the amp very quickly.
Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Of course this only happens in SS amps with feedback loops; I don't think there are many commercial amps that offer "gain switches" that are either tube based or discrete without feedback.

there are a couple.

Headamp offers the gilmore reference with a gain switch.
The ray-samuels amps use a proper gain switch where indicated.
There may be others.

You could probably get mikhail to put a texture knob in anything if you asked nicely, but its only a real option on the PPX3 and MPX3 (unless im mistaken).

Quote:

Originally Posted by richardu /img/forum/go_quote.gif Input resistance should be OK if the amp is well designed. Output resistance will affect the sound signature. Changing feedback resistance is not advisable in a true hi-fi amp. Personally I would refuse to buy an amplifier with variable feedback.

Why?
If you are going with feedback at all, making it adjustable offers many advantages.
There is of course the "no global-feedback at all" camp, and they make excelent points, but we are talking about once you have decided to have it.

in certain places, it is a problem, but in a preamp or headphone amp it only brings advantages.

 

[richardu]

There are many complex distortions which can only realistically be optimised at a single feedback setting (if at all). This goes back to some of the arguments against multi-stage feedback. These effects are generally less prominent in individual stage feedback. I am willing to accept multi-stage feedback but only on the basis that it is properly designed. Allowing adjustable feedback to me indicates a design which has not paid sufficient attention to these distortions. I guess some people like these, as per the mention of a 'texture' knob.

 

[nikongod]

We are talking about preamps and headphone amps still. I would not advocate adjustable gain on a power amp, too many places to go wrong.
Quote:

Originally Posted by richardu /img/forum/go_quote.gif There are many complex distortions which can only realistically be optimised at a single feedback setting (if at all). This goes back to some of the arguments against multi-stage feedback. These effects are generally less prominent in individual stage feedback.

you dont seem to say how this has any bearing on the discussion regarding the adjustability of feedback and gain in a given amp. this borders on an unimportant issue here.
Quote:

I am willing to accept multi-stage feedback but only on the basis that it is properly designed. Allowing adjustable feedback to me indicates a design which has not paid sufficient attention to these distortions.

is the distortion from an opamp different when changing from voltage gain of 10 to 5?
Quote:

I guess some people like these, as per the mention of a 'texture' knob.

aah indeed, but all you do with feedback is trade one distortion for a different one... whether its better to have a strong second harmonic as is genearlly the case with a tube amp without feedback or lower output impedance and harmonics which will be lower in magnitude but higher in order could be called a question of preferences. why not make a more versatile amp and give the user more options?