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Old 15th February 2008, 02:21 AM   #31
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I use Vac as a synonym for Vrms and think that all other people also do, but that does not mean it isn't correct. Writing Vrms, Vpk and Vpp does not hurt.
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Old 15th February 2008, 09:00 AM   #32
AndrewT is online now AndrewT  Scotland
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Hi,
let's start at the speaker end.
60W into 4r0 requires sqrt[60*4]=15.5Vac=21.9Vpk=43.8Vpp
Gain is set to x33 (+30.4db).
Input sensitivity=15.5/33=470mVac
I suggest the pre-amp maximum output overhead should be between 10db and 20db. This requires a maximum output of between 0.47V *3 to 0.47*10 i.e between 1.4Vac and 4.7Vac.

Your maximum peak output of 13Vpp (=6.5Vpk=4.6Vac) is very close to the +20db overhead figure. Perfect!!!!!

Next, how quiet is the preamp stage?
If the noise output from the pre-amp after being amplified by your x33(+30.4db) gain power-amp still leaves the hiss from the speaker as inaudible in a quiet room, then again, perfect.
But it may be audible if you go close to the speaker. Assess this one after you have built it. Tell us the result.

Now, back to the beginning and look at PSU requirement based on that 60W into 4r0.
National seems to suggest that 4.1V is lost between Chipamp supply input and Chipamp output.
Your target output is ~20.9Vpk.
PSU should be about 20.9+4.1~=+-25Vdc. Perfect

Think it through in little bites, it does start to make sense and soon becomes the big picture.

To complete the big picture you need an attenuator (volume control) as part of your pre-amp input. This is what stops your system playing too loud or distorting due to clipping.
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Old 15th February 2008, 11:02 AM   #33
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Quote:
Originally posted by johan851
Well, the amp is supposed to sound better at a gain of around 30, and the kit comes with parts to set the gain at 33, so I'll just stick with that. As I look at the numbers, I'm going to need some kind of input attenuator no matter what I do.
If your kit has NFB (gain loop) of 22k with 680R, then you could change that 680R for 1K (Ri) without harming its sound.
In fact, its usually lower DC output that way.

In addition, you could change its input-side in-series resistor up to 2.2k (Rb), which is also a tiny little change.

Then, you should be able to set input loader (Rin) to 18k for a bit stronger load (corresponds to 17% stronger load than NFB).

Those are the only three "gain drop" changes that I have documented of benefit.

P.S. The amp that sounds better at a gain of 30 is the one with the two very large caps on the amplifier board. But, this general advice of 30 gain doesn't apply to the chipamp.com kit, so you may use lower gain.
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Old 15th February 2008, 04:57 PM   #34
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Thanks for looking over that AndrewT. It looks like my numbers were right, but the thing I missed was this:
Quote:
I suggest the pre-amp maximum output overhead should be between 10db and 20db. This requires a maximum output of between 0.47V *3 to 0.47*10 i.e between 1.4Vac and 4.7Vac.
It seems to me that at max preamp volume (it does have a volume control) we'd be getting 4.7VAC out, resulting in an attempted 155VAC chipamp out, which would cause all kinds of clipping. So I don't understand how you got the preamp maximum output overhead' figure.

I believe you, though, so I'm just going to build it. But I'd like to understand that part of your calculation.
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Old 15th February 2008, 05:11 PM   #35
AndrewT is online now AndrewT  Scotland
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Hi,
if a pre-amp can only just manage to pass the full signal voltage then it is in the region of high distortion since many are single ended ClassA. Many bad recordings are outputting near maximum all the time.

It is usual to restrict the normal maximum signal to well below the clipping limit of the pre-amp and similarly in any preceding stages.

There is a further benefit in having this overhead. If any very fast transient interference signals contaminate your wanted signal, these will be slightly attenuated by the RF filters built into the amp stages. But some gets through. If these fast transients are clipped they are more audible.

I have seen many design their pre-amps to try to achieve that +20db (x10) simply to get the alledged better sound that ensues.

Think back to the days of vinyl replay.
~20db of overhead was theoretically sufficient to pass the highest recorded velocities on the recording. It sounded terrible. +30db sounded better. Some strove for +35db and I believe that exceptionally +40db was achieved. There is that spare 10 to 20db range again to lessen the audibility of the snap, crackle and pop that folk thought they were getting rid of when they adopted CD.
Quote:
resulting in an attempted 155VAC
you never intentionally use the overhead. It is there to ensure the signal is never clipped and to keep the wanted signal in the most linear amplification range but above the noise.
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Old 15th February 2008, 06:33 PM   #36
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That makes complete sense. Thanks for being patient!
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