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Author Topic: Eddie Ciletti on slew rate, op amps and output transformers  (Read 28175 times)

arconaut

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Eddie Ciletti on slew rate, op amps and output transformers
« on: November 21, 2006, 01:07:14 PM »

Hi,

In Eddie Ciletti's column in Mix this month, he makes the following statement:

"The 2520 is slow by modern standards - 2 volts per microsecond-but you'd never know it. The transformer's 1:3 windings ratio provides more than 8dB of headroom, and that increases the slew rate. You can apply this approach to any amplifier by taking advantage of 24 bit headroom, dropping the level 6 dB under the guise of maxmizing resolution."

I'm not sure I understand this - is he saying that because the transformer increases the output voltage by a ratio of 1:3, that a slew rate is essentially faster because at the transformer the signal rises to a higher voltage in the same amount of time that it takes for the signal to rise to a lesser voltage at the op amp?

And then in his last sentence there, he is presuming that you are sending your D/A's signal into an amp that has an op amp circuit with an output transformer. But is it true? Instead of "any amp", should he say "any amp with an output transformer" and perhaps also "any amp circuit with an op amp with a slow slew rate". What if your op amp has a really good slew rate? Then you wouldn't?

Noah
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #1 on: November 21, 2006, 02:12:22 PM »

As I read this quote he's indeed saying that the 3x voltage gain in the transformer automatically translates as 3x higher slew rate as seen after the transformer. Or, otherwise put, that reduced level in an amplifier automatically reduces the effects of slew rate related distortion.

The second half of the quote a bit vague. As I interpret it, he then applies the same logic to any system where gain can be varied in several places, like an amplifier followed by an ADC. Dropping the gain before the converter and making it up in the digital domain indeed reduces slew rate (and other) distortion in the amplifier.

Actually, reducing the level in normal (class A) amplifier circuits always translates into reduced distortion, be it slew rate related or otherwise.

One rarely sees slew rate induced problems in modern op amp implementations (those aimed at audio at least). So, if slew rate is good, there's no need to be too concerned about it.

As a sideline I should note that slew rate does not relate well with distortion performance. Some op amps have fantastic slew rate specs, but slew rate induced distortion may become noticeable at much lower slew rates. Others may have a relatively slow slew rate, but don't distort much until you run into it.
Like one can have 24dBu headroom but THD starts to climb at +14 already, whereas another could only have an 18dBu headroom but still not distort discernably at +17.9.

An op amp with a degenerated bjt input pair will be much better behaved, slew-rate wise, than a jfet input op amp with the same transconductance (but no degeneration).
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Larrchild

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #2 on: November 21, 2006, 03:06:42 PM »

I think the 2520 circuits use less global negative feedback, compared to say, a 990, which slews much faster.

At 2v/us, T.I.M. would suck, otherwise.

But less feedback means more distortion rise earlier on.

Is that right, Bruno?

(And Welcome!) Cool
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #3 on: November 21, 2006, 03:50:49 PM »

When an op amp has "less global feedback" this should imply its GBW is lower. Over nearly all of its operating frequency range an op amp's gain is limited by the dominant pole (and hence gain-bandwidth product), not by its DC gain. Certainly at frequencies where slew rate limiting becomes an issue, the op amp is very firmly acting like an integrator.

An alternative interpretation of "less feedback" could be that the DC loop gain is intentionally limited to the value found at 20kHz. GBW remains the same, but loop gain at frequencies below 20kHz is lower. In circuits with non-negligible distortion, limiting the DC loop gain yields a sonic improvement (even though low-frequency distortion is clearly made worse).

Asymptotically (ie. neglecting any intentional or unintentional gain limitation at DC) the open loop gain of an op amp is determined by the internal integration capacitor (usually a feedback cap around a transistor in a miller arrangement) and the input stage's transconductance as:
A(f)=gm/(2*pi*j*f*C) (preferentially written as A(s)=gm/sC )
Solving for |A(f)|=1 yields
f=GBW=gm/(2*pi*C)

GBW is determined by gm of the input pair and the miller cap.

Slew rate limiting occurs when one of the input transistor cuts off i.e. when all of the emitter current comes out the other transistor. Slew rate works out as
SR=Ib/C

Again C is in the equation, but gm is not. Put simply, slew rate and GBW (and hence "the amount of feedback") are not directly related. Although gm of a transistor is directly proportional to emitter current, the use of degeneration (resistors in series with the emitters) allows SR and GBW to be set independently of eachother.

What makes slew rate distortion so annoying is that it occurs outside the feedback loop. When it occurs, no amount of feedback can correct it. Slew rate problems (and associated performance metrics like TIM) are not helped by the presence of more feedback. They are also not made worse.

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Larrchild

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #4 on: November 21, 2006, 04:05:09 PM »

I gotcha. So poor slew performance is as ineffective at handling the feedback signal as it is, the primary signal. Therefore, more feedback will not create more (or less) TIM in a slower opamp. Outside the loop.

Thanks!
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dcollins

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #5 on: November 22, 2006, 01:31:44 AM »

Larrchild wrote on Tue, 21 November 2006 13:05

I gotcha. So poor slew performance is as ineffective at handling the feedback signal as it is, the primary signal. Therefore, more feedback will not create more (or less) TIM in a slower opamp. Outside the loop.

Thanks!


Isn't the solution to slew-rate limiting just to LP filter the input so that no possible rate-of-change will cause the NFB to arrive "late?"

A highly over-rated parameter imho.

DC

bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #6 on: November 22, 2006, 03:24:48 AM »

Lowpass filtering works, but of course it can't cure any slew rate related problems that already occur at audio frequencies (unless you're prepared to limit the bandwidth to below 20kHz).
Also the amount of bandwidth limitation one can afford depends on how much accumulated phase shift one can tolerate.

Slew Rate as a figure of merit is indeed highly overrated. What we're interested in is what happens to the audio. An 18.5kHz+19.5kHz IMD test will tell us whether or not we've got a slew rate problem on our hands. If an amplifier performs well in such a test, we shouldn't care if slew rate is only just enough to reproduce this signal or if it's a thousand times higher. What matters is how linear the thing is when reproducing the fastest slewing audio frequency signal.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #7 on: November 26, 2006, 03:20:06 PM »

is it true that slew rate is not necessarily a fixed fenomena and that it can vary depending on the signal which is fed into the amp (waveshape and/or amplitude)??
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #8 on: November 26, 2006, 04:06:04 PM »

That depends what you mean. The slew rate of a signal is the derivative (dV/dt) or rate-of-change of that signal, and depends only on the signal. The "slew rate of an amplifier" (more correctly, the maximum slew rate of an amplifier) is the fastest dV/dt the amplifier can produce at its output. This is determined by a few design parameters of the op amp (see an earlier post in this thread) and not by the signal. Some circuits have asymmetric slew rates.
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Ruairi O'Flaherty

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #9 on: November 26, 2006, 08:32:06 PM »

Bruno Putzeys wrote on Wed, 22 November 2006 08:24

An 18.5kHz+19.5kHz IMD test will tell us whether or not we've got a slew rate problem on our hands. If an amplifier performs well in such a test, we shouldn't care if slew rate is only just enough to reproduce this signal or if it's a thousand times higher. What matters is how linear the thing is when reproducing the fastest slewing audio frequency signal.


Bruno,

could you elaborate on that test for an electronics beginner.  We are looking at how the amplifier deals with the harmonic content created by the intermodulation distortion?

thank you,
Ruairi
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #10 on: November 27, 2006, 03:31:59 AM »

In principle, slew rate related problems are perfectly measurable using a single 20kHz sine wave. More wideband signals (e.g. a TIM-100 test) stress the circuit beyond what normal audio signals can.

If we wanted to measure distortion on a 20kHz signal, the second harmonic would be at 40kHz, the third at 60kHz and so on. This makes distortion measurements at 20kHz quite a pain.

The IMD test signal mentioned is the sum of a 19.5kHz sine wave and an 18.5kHz sine wave of equal amplitudes. If the amplitude of each is 1Vrms, the peak amplitude of the sum is 2.8V. The maximum slew rate will be that of a 19kHz signal of 2Vrms.
The second order product will appear at 1kHz. The 3rd order product appears at 17.5kHz and 20.5kHz. The 4th order product at 2kHz, the 5th order at 16.5kHz and 21.5kHz.
You'll see two bunches, one around the test tone (odd products) and one near DC (even products). If the distortion is bad (clipping or hard slew rate limiting) the two regions will overlap but you can still tell them apart because the even products are at integer multiples of 1kHz and the odd products at  odd multiples of 0.5kHz.

Now nearly 40 distortion products will fit inside the audio band. If these distortion products are all very low, you can be sure the amplifier has no problems, including slew-rate wise.

This two-tone test has nothing "magical" that makes it more suitable for slew rate related problems, but it stresses the amplifier maximally and produces a lot of information. A 20kHz signal also stresses the amplifier maximally, but in a bandwidth limited system you don't get to see the result.

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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #11 on: November 27, 2006, 06:29:17 AM »

asymmetric slew rates..?

what kind of circuits exhibit the least asymmetric slew rate?
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #12 on: November 27, 2006, 07:12:44 AM »

Most normal op amp circuits have fairly symmetrical slew behaviour. Asymmetric slew rates are more commonly associated with transimpedance (often but incorrectly called current-feedback) amps.

It doesn't matter btw. What matters is how an op amp behaves when confronted with a worst case audio signal. Slew distortion sets on well before the op amp actually hits the limit. I'd rather not use an op amp with "super slew rates" if it already starts distorting at 20kHz.

In other words, don't head for the slew rate spec if you want to find out if an op amp is going to measure well or sound good. Actual THD at 20kHz says much more.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #13 on: November 27, 2006, 12:55:19 PM »

Is a low slew rate a problem in itself (audible) or does a low slew rate create problems with the functioning of the amplifier's circuit-design?


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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #14 on: November 27, 2006, 01:18:49 PM »

Slew rate is a function of particular parameters in the circuit design (emitter current of the input pair and the compensation capacitor). An amplifier that is operated close to its maximum slew rate will produce lots of distortion. A typical effect would be a sputtering noise in "s" sounds.

An op amp that produces slew rate related distortion tells you it's been incorrectly chosen for the application. Conversely, no normal audio product will have such problems, because the designer will have taken care not to select an op amp that runs out of steam below 20kHz.
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