2. amps-preamps
  3. 1427794264

Input sensitivity, Gain and Preamp matching

Friends,
I am considering to buy an SET amp which has an input sensitivity of 2.5 volts (rather high) and gain is only 14 db (probably low ?). The output power is 16 watts. My choice of preamp was going to be a pure passive using a Dave Slagle autoformer or at most a single stage tube preamp with only 6db gain. But with the specs of the power amp I am not sure if a low gain or zero gain preamp would be a compromise or not. Can anyone throw some light on this subject ?

The input impedance of the power amp is a healthy 47kohms.
pani
pani OP
1,231 posts
 
Thanks everyone for responding on this one.

My source is mostly analog. A Naim Superline phonostage. I am sure it puts out around 3 volts because my cartridge has an output of 0.5 mv fed into full MC gain of the phonostage.

My speakers are Tannoy Turnberry SE which are only 93 db sensitive (not 103 db). Currently I use a Wavac 300B SET which produces only 9 watts. On 70% of the music I do not hear compression in my room (150 sqft). But there is still 30% of the music which I cannot listen at my desired volume levels. The amp I am considering now has parallel 300B and produces 16 watts (conservatively) per channel. I know I will better with more power but if this is going to solve the power issue with say 95% of my music I am still happy because the combination of Tannoy and 300B SET is amazing.

BTW, isnt it true that a power amp should ideally run at full gain (not full power) all the time ? That is how one can get the best dynamics at any given volume level IMO. All studio amps have a gain control which are always kept at max level. My understanding was that the input sensitivity decides the ease with which one can get the amp to full gain.

Secondly there is this concept of voltage gain amplifiers and current gain amplifiers. Does that have anything to do in this discussion ?
almarg's avatar
almarg
9,670 posts
 
Pani, the Naim Superline has a specified gain of 64 db. That corresponds to a voltage multiplication of 1585. (20 x logarithm 1585 = 64). So the 0.5 mv output of your cartridge under the standard test conditions would be raised by the phono stage to 0.5 mv x 1585 = 0.79 volts. However the maximum levels of some recordings may exceed those standard test conditions several-fold, I believe by something like a factor of 5 in at least a few cases. So there may be SOME recordings on which the max level of the phono stage output might reach 3 or even 4 volts. But as far as I am aware that won't be the case with most recordings.

Also, keep in mind that the difference between 16 watts and 9 watts is just 2.5 db, which subjectively is not all that much of a difference. If you presently cannot listen to 30% of the music at desired volume levels, I would not expect that a 2.5 db increase in power would represent a solution. And even more so considering that on many recordings you may not be able to utilize much or any of that added power if you do go with a low or no gain preamp, because of the amp's low sensitivity (corresponding to a high sensitivity number, as was pointed out).

Re your question about running power amps at full gain, if the amp has a volume control and is used with a preamp having a volume control, yes, chances are that in general it would be preferable to run the power amp's control at max, which in effect takes it (and any adverse sonic effects it might contribute) out of the signal path. On the other hand, though, there may be some cases where that benefit would be outweighed by other considerations. One example perhaps being a need to increase the signal level at the output of the preamp in order to minimize the audibility of groundloop-related noise, and another example perhaps being a need to avoid running the preamp's volume control undesirably close to the bottom of its range.

I believe that your last question relates to what Ralph (Atmasphere) often refers to as power paradigm vs. voltage paradigm amplifiers, which is a separate subject relating to matching amplifiers and speakers in a manner that will result in proper tonal balance.

Good luck, however you decide to proceed. Regards,
-- Al
atmasphere's avatar
atmasphere
12,116 posts
 
Pani, If you really are serious about this and plan to keep your loudspeakers, do yourself a favor and get an amplifier with significantly more power. At a minimum you are going to need 60 watts (my room is very similar dimensions FWIW).

IOW in your environment an SET is impractical. The more power an SET makes, the more limited its bandwidth- much more so than with push-pull amps!

If you really are stuck on that SET sound, get rid of the speakers and get something more efficient. The problem I think you will quickly see is there is no free lunch- as speakers become more efficient, getting full bass response gets harder and harder.

Now one of the secrets of the SET 'magic' is that they have a distortion character where as you reduce power, the distortion linearly decreases to zero (unmeasurable). Most push pull amps don't do this, but that is not to say all are like that (for example our amps share this quality with SETs). To do this generally the amp should not have a dedicated phase splitter circuit, which is one aspect that can cause distortion to rise as power is decreased below a certain level.

Secondly there is this concept of voltage gain amplifiers and current gain amplifiers. Does that have anything to do in this discussion ?

The quick answer is 'maybe'.

There are a couple of ways of interpreting this comment. Here is the first one:

Almost all amplifiers express voltage gain, whether tube, solid state or class D. Current gain amps do exist, but not in the context of audio. All speakers are driven by both voltage and current at the same time (voltage and current together are called 'power'), so if you don't have voltage gain you can't make any power.

The second interpretation:

In audio there are two paradigms of loudspeaker drive, the Voltage Paradigm which is the most common, and the Power Paradigm, which is the older of the two. See http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php for more information. However there is a 3rd means known as current drive where the amplifier has significantly higher output impedance than the speaker. This third system never developed into a set of rules for driving speakers and remains experimental. The Power Paradigm (whether people know it or not) is fairly well accepted in the high end audio community; the Quad ESL63 is a good example as are most horn speakers, SET amplifiers and for that matter, any tube amp that does not employ negative feedback to control output impedance.
pani OP
1,231 posts
 
Thank Al and Atmasphere. Yes, I am darn serious about keeping the speakers and your suggestion about getting more powerful amps is well taken. My hunt is on. And thanks everyone for the caution about matching passive preamp with the low sensitive power amp. It was an important tip at the right time.
More to discover