TOPIC:

mdsimon2 wrote:

All of this is a bit of simplification as these voltages assume 0 dB input signals which is almost never the case

Just on that. Nothing on CD/digital source can be over 0db input signal (unless I suppose deliberately boosted). I think that’s correct. So assuming the sound mixer didn’t have any peaks go above -2db and then the peaks and transients could be 15-20db above the average level of the music so even if the MV was at 0db would we typically be outputting a signal to the amp of ~ -20db?

By the way the OEM of the amp has told me that only the early models were 0.45v input sensitivity. My model is 0.6v input sensitivity.

asx77 wrote:

Just on that. Nothing on CD/digital source can be over 0db input signal (unless I suppose deliberately boosted).

The first bit is correct. There is no possibility of a boost.

I'm really not sure of the value of all of this discussion to be honest though due to the variability between recordings, and also between different sources if you use more than one.

Audiophiles have coped for decades without trying to figure out a maximum volume control position for a peak input signal not not reach a hard clipping limit of their amp.

Hi @ultrasonic, you’re correct of course.

When this started out I was concerned about running out of juice but since then using an online SPL calculator ( myhometheater.homestead.com/splcalculator.html )and by working through some of the above calcs I have realised I’ve like you say NOTHING to worry about:

For SPL 75db at my listening seat I only need 0.12W !!

I remember reading somewhere that “the first Watt is the most important Watt” and yep they’re right! Stick an extra 18db on top of the 75db for dynamics and I still only need about 8 Watts! So as you say I don’t need to worry and should just enjoy the music but it’s also nice to understand these things.

Hi!
Despite having no concerns over lack of power any more, I am still curious to measure the amps. I plan to borrow an oscilloscope from work at some point when I've a free day, and find out where my amps clip and using the spectrum analyser or FFT, have a look at how much and what type of distortion they produce at different levels. For nothing more now than interest!

I've got myself a 4 Ohm 50W resistor for a dummy load, connected it and measured Vout for a number of Vins.
Interestingly the voltage gain varies depending on the Vin.

For examples
Vin = 0.009 Vout = 0.258 VGain = 28.6 dbGain = 29.1
Vin = 0.066 Vout = 1.810 VGain = 27.4 dbGain = 28.8
Vin = 0.210 Vout = 5.490 VGain = 26.2 dbGain = 28.4
Vin = 0.599 Vout = 10.830 VGain = 18.1 dbGain = 25.1

Amp input sensitivity is 0.6v. I've been told that valve amps tend to have their ratings set at the point at which distortion has risen to 1%. I think that means I can increase the Vin further but it will result in more distortion. I decided not to try. Dummy load or not, it didn't feel necessary.

Can anyone explain though why the calculated gain varies for different Vins?

Thanks

asx77 wrote:

Hi!
Can anyone explain though why the calculated gain varies for different Vins?

I imagine you are actually reaching some amount of clipping and that is why the voltage is not increasing as you expect. As an example here is a scope shot of a signal just on the verge of clipping and then one of the same signal boosted +3 dB and driven well in to clipping. If the signal was not clipped I would have expected an increase from 2.07V to 2.92V but because of clipping I only get 2.42V.

That being said I am very unfamiliar with how tube amps behave so maybe there is something else going on. Will definitely be interesting to see what you get with a spectrum analyzer. I recently purchased a MOTU M4 interface and have been using it for all sorts of measurements including amplifiers, it has been a lot of fun.

Michael