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Old 30th September 2001, 07:38 AM   #1
jteef is offline jteef  United States
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first is there any standard? What kind of RMS output voltage can I expect to see from CD players?

I am asking because the amp I built begins to clip with right around 2 VRMS on the input and I am considering how much gain I should use in the pre amp I am going to build.

Also, somebody has told me that reducing the gain of a circuit makes it more prone to oscillation, but in rod elliots high speed op amps article, he claims the opposite. While I believe RE more than somebody from a web board at this point, I would still like to know for sure.

I am somewhat new to this so please bear with me. The circuit I am generally referring to is basically a burr brown OPA134 op amp operating as basically an input buffer with a gain of two. This would be directly coupled to a TI THS6012 CFB op amp with a gain of 5.5 to start with. (this is the value obtained from Rod Elliots circuit using a 1k ohm as RF and a 220 ohm for RG. This provides an overall circuit gain of 21dB(11.0)

My audio book explains the stability criterion of a single pole amplifier through the use of phase margin. A small phase margin is ideal...nearing 180 degrees would then be more and more unstable.

The equations my book lists are: loop gain transfer function:

G(s)H(s) = bK/(1+s/w1)

where b is the feedback ratio and K is the gain constant, and w1 is the pole frequency.
The book is unclear about what exactly K is in the real world. is it just the open loop gain? I didn't see anything in the data sheet about open loop gain. This leads me to believe that I am looking too deeply into something that doesn't matter.

in any case, with a constant K and a varying b...a larger b = smaller stability margin, but with a high value of K, both are going to be approximately 90 degrees anyway(see end of last paragraph ). So I guess my question is, what else is affected if RE's statement is true. Again, I do not know if I am way off base here or not...If I am please correct me as a lot of this stuff can get very confusing.

thanks

jt
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Old 30th September 2001, 10:40 PM   #2
hifiZen is offline hifiZen  Canada
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In theory, line-level is supposed to be standardized at 0dB = 0.775 Vrms. This is a very old standard from the days when studio equipment was mostly transformer coupled, and low output impedence was difficult to acheive. It was decided to standardize all studio equipment at 600 ohm source and load impedences to maximize power transfer. 0.775 Vrms gives 1 mW into a 600 ohm load. Cool eh?

Okay, so then we had all these consumer equipment manufacturers trying to play tricks on customers by increasing the output levels on their equipment, so that at the same volume setting on your preamp, their stuff sounds louder. The same thing is happening in video. Increasingly, I see products from various manufacturers whose video signals do not meet industry specs, because they're trying to pull some stunts to make white look brighter or colours more saturated (in an effort to look "different" in the showroom)... be especially wary of this when you're shopping for video equipment. High levels are dangerous because of the possibility of clipping (which in video can cause some serious problems), or worse yet, equipment damage (usually a slow progression of subtle performance degredation). Unfortunately no-one's policed the standards with audio and therefore we now have 2 V outputs from various equipment now. Just for reference, I build all my preamps as unity gain buffers following the attenuator. You should choose whatever gain suits your sources and power amp sensetivities. From the sounds of it, unity gain might not be a bad way for you to go.

On to stability...

First off, what we really want is a <i>large</i> phase margin. Phase margin defines how far away we are from that magic 180 degrees at the frequency where the open loop gain falls below unity (1). The larger the phase margin, the more stable the circuit. An important factor you may have to take into account is the load. It is very important not to forget that the load is part of G(s)!! This is because any load impedence will interact with the amp circuit's open-loop output impedence, possibly forming another pole, hence more phase shift. This is why capacitive loads can de-stabilize an amp, and just one reason I design all my amplifiers for low open-loop output impedence. In theory, with a zero open-loop output impedence, the amplifier's operation will be totally independent of the load.

Now, the formula you have shows the loop function (and you guessed right - K is open-loop gain)...
argh... i wanted to do a little ASCII diagram here, but it's not working. Well, you're probably looking at a similar diagram anyway.

OK, so we have G(s) in the forward path and H(s) in the feedback path... so we get G(s)H(s) around the loop, from which we can determine phase margin. You can see that if we increase the <i>open-loop</i> gain K, the unity-gain frequency increases, and the phase margin narrows. So, increasing the open-loop gain can have the effect of decreasing stability.

Our closed-loop transfer function will look like this:
G(s)/[1+G(s)H(s)] As you can see, the larger G(s) is, the closer we get to 1/H(s), which is what we want. Now, if we increase the closed-loop gain (by decreasing H(s)) then looking at the loop function G(s)H(s) reveals that our phase margin is improved. So, increasing closed-loop gain can have the effect of increasing stability.

There you have it, increasing the gain will either increase or decrease amplifier stability, depending on <i>which</i> gain you're talking about.

[Edited by hifiZen on 09-30-2001 at 05:47 PM]
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Old 1st October 2001, 12:22 AM   #3
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Every CD player I have come across has 2V standard output, but as mentioned most put out more than that so they compare favourably in A/B comparisons.

Cheers,

Pete
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