[marvda1]

what determines an amp to be of high gain?  is there one spec to look at to determine this?

[amplifierguru]

Hi Marvda1,

Amplifier gain is generally set by the feedback network and can be the pro de facto standard of 20 times (26dB) or other. I have my own standard of 28 times (or 29dB) for my amps which gives an approximate sensitivity of 1V rms for 100W rms into 8 ohms.

This would be fairly average gain so I would consider an amp to be high gain if it was 6dB greater. This would mean sensitivity would be 500mV for 100W.

Hope that helps.
Greg

[marvda1]

thanks greg, so would a 150 watt amp that takes 1.6 -1.9v for full output be average or high gain and would there be a mismatch with a preamp that can output 17v ?

thanks,
marv

[amplifierguru]

Hi Marvda1,

Your 150W amp that needs ~ 1.75V has a gain of only 20 times (or 26dB). It is quite low gain.

Your preamp max output capability tells me that it should have no trouble providing that 1.6-1.9 V needed to drive the 150W amp to full power, assuming the amp does not have a low input impedance. Most amps are > 10K. Mine are 28K.

Greg

[WEEZ]

..mind if I butt in here?....

I don't understand why today's equipment has so much gain...my god, some pre-amps have 20db of gain. Add another 26 or 29 db in the power amp and the volume control is barely cracked open before you blow yourself out of the room 

It's no wonder that passive contollers are so popular these days 

Example: My amp has 26db of gain. My pre-amp has 12db. My speakers are 87db@1w@1m (8 ohm). With my softest recording I am filling the room (and my ears) @ -28 on the volume control 

I read somewhere that 32db total is more than enough for most average speakers in most rooms. (and I believe that's overstated)

So where am I wrong?

WEEZ?

[Daryl]

Hi Weez,

Total gain for the system is not really the angle you want to approach the issue from.

A CD player typicly has an output of 2v with a signal recorded at 0db (clipping level).

Designing your poweramp with it's gain chosen so that it reaches it's maximum output with an input of 2v is a good idea for the following reasons.

You want the poweramps gain as low as possible to get the most immunity from noise at it's input as possible.

Problem is if the poweramps gain is too low the preamp might clip before the power amp and since componets are mix and match you don't know what the design parameters of the other componets in the chain will be so you must be at least a little conservative.

Almost any preamp can easily achieve 2v at it's output so the power amp will probably clip before the preamp does if it reaches it's maximum output level with an input of 2v.

If your poweramp is to be capable of 200w program at 8ohm (40v) then a gain of 26db will turn 2v input into 40v output.

Now your preamps job is to pass the input signal to the power amp.

A preamplifier with a gain of 0db will take CD players 2v (typical) output and send 2v to the power amplifer and all is good.

However since audio componets are mix and match you don't know the specifics of the other componets in the chain and you will need the flexibility to be able to accomadate other possible design choices.

More importantly you rarely listen at full output and in fact you will listen to range of output levels which constantly change.

Thus your preamp will always have a gain control that adjusts your preamp to a range of specific gains.

The dynamic range of your ears is very high so to accomadate night listening while others are sleeping you might need your gain control to be adjustable down to say -60db or more.

On the other end of the scale you don't know what voltage the poweramp might require to reach full output or what the output voltage of individual source componets will be.

Also a particular recording might be recorded at a very low level and not make use of the full output voltage capability of the source componet.

Because of these variables you will want the maximum level of your preamps gain control to be say 10db or more so that you can drive your power amp to it's full level over a range of possible circumstances.

Those are the considerations for choosing poweramp/preamp gain.

Daryl

[WEEZ]

In my case:

CD Player rated 2.2V output; 140ohm output z

Pre-amp input z 50k; output 3.5V rated (7.5V max.); output z 150 ohms; 12.5 db gain

Amplifier input z 24k; not sure of sensitivity but approx. 1 V, I think; 75 watts rated power; 26db gain

Sounds fine, but half or more of my usable volume control is wasted. 

WEEZ

[Steve]

I know and agree with you Weez.
 
Amps could probably eliminate a whole gainstage, with its associated problems, and still have enough gain, unless one is using some super high gain stage, which probably has limited bandwidth anyway.

I cannot think of any descent gain preamp that could not produce enough output voltage to drive a lower gain amp. My current 2 stage test tube amps have only 12db of gain, 3.5 vrms input for 25 watts rms output. Practically any gainstage preamp can drive them to full power with today's players. Of course it does cost more, so it is up to the consumer if one wants separates. But I think it is well worth it or I would be selling integrateds.

One could get rid of the poor quality analog stage found in CD players and one could also see an improvement. Using both ideas would be best, but that is not going to happen.

[dado5]

I cannot think of any descent gain preamp that could not produce enough output voltage to drive a lower gain amp. My current 2 stage test tube amps have only 12db of gain, 3.5 vrms input for 25 watts rms output......

Good on ya Steve!

I made mention in another post that you are one of a very few producers who engineer their gear with gain in mind. I natter the fellows at Rogue Audio every time I visit about building a pre-amp with over 20db gain to drive amps with over 30. That is just silly.  A 300B SE circuit with a typical 3Kish primary will only need around 40 Vrms at the grid to make 8 watts at the speaker. There are many tubes out there that can swing this easily from 2 Vrms by themselves, yet nearly every commercial product uses some multi-stage input circuit that only forces the squashing of the music signal. I really wish more tube producers took your approach instead of rote rehashing 1950's circuits or using inappropriate tubes.