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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 12 Feb 2020 at 1:25pm
Looks nice... the meters will liven up the front panel


Bruce
AT-14SA, Pickering XV-15/625, Technics SL-1600MK2, Reflex M, Lautus, Technics SH-8066, Dynaco ST120a, Eminence Beta 8A in custom cabs;; Using Majestic DAC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 12 Feb 2020 at 9:17pm
+1
Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 2020 at 1:04pm
Oliver's criterion states that RE = re (quiescent), or should do.

What if our ears told us differently?

And, what if the audio analyser told us differently?

Would Oliver be wrong?

If, through analytical testing, it were to be found that 2RE = 24mV gave the best attenuation of odd harmonics, especially the high ones, Oliver's criterion could be called into question.

Bob Cordell's book "Designing Audio Power Amplifiers" provides the answer, but requires some in-depth reading to understand. A similar explanation, again by Cordell, is offered somewhere in a DIY audio forum topic.

We must understand the component we see as being RE is only part of the real RE, and here I don't mean the inclusion of re.

The output resistance of an emitter follower includes its beta-transformed base circuit too. An exact calculation is not possible, but if the power transistor emitter current is known, it can be estimated.

Base current is being drawn off the driver emitter, and it would be folly to believe it has sufficient current that no losses can occur.

Because of those losses, some resistance must appear at the power transistor's base, this is divided by its beta and adds to RE.

That way, It is possible for the real value of RE to be considerably higher than the physical resistor's measured value.

2RE expects to have 2Vt (52mV) across it, but say 2RE is twice as big due to base source resistance. Therefore real 2RE might be 1.32 ohms, so if we set 52mV across 0R66, Vq is 104mV, which is too high, and leads to gm-doubling distortion.

Therefore, if we set half 2Vt (or simply Vt) across 2RE, we will get 2Vt across the real base-resistance-inclusive value.

24mV is just under half of 52mV. Can base-source resistance be so high to contribute greater than RE? Well, consider that this is an output-double, and with output-doubles we're asking for a lot of current gain as compared to say, an output triple.

I think 24mV might be calculated to be correct if we had an exact value for beta in the region of Ic = 30 - 40 mA. Still, as we don't, the FFT trace on the analyser gives us a reasonable picture of the effect Bob Cordell illustrates.

A possible reason for early transistor emitter-follower output stages being rated for such broad quiescent current ranges would be the lack of FFT facilities in analysis. With only THD as a measure, the range giving the best performance would be between two measured Iq's. With the advent of affordable FFT's, the actual value could be pinpointed.

We have to thank the work of Oliver, Cordell, and Self for the understanding of this phenomenon.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Feb 2020 at 4:31pm
Reading from "https://www.diyaudio.com/forums/solid-state/171159-bob-cordells-power-amplifier-book-26.html" onwards through page 29 of the DIY Audio topic, Bob Cordell gives more insight into the Vq setting.

With the aid of the AP525's FFT, I was able to make the 5th harmonic remain at a pretty constant (low) level in the range -23dB to -16dB - the sort of level we listen to domestically - at frequencies from 1kHz to 20kHz.

The 5th harmonic level didn't dominate, remaining on par with the 4th most of the time. And at levels of 0.01% and below.

The 5th gives rise to a tiring metallic sound, and then there's the 7th having a discordant sound, but the 7th is kept relatively low.

Once adjusted, I retook a look at the Vq, and it displayed 20 on the meter, which equates to 40mV between emitters (or 20mV per emitter resistor).

Bearing in mind the base contribution to RE, I think this Vq might well be correct.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Feb 2020 at 10:53am
Understanding the makeup of music should help in getting an amplifier design right:

https://en.wikipedia.org/wiki/Harmonic_seventh, for example.

If we don't understand harmonic distortion from a music point of view, we cannot design a genuinely high fidelity amplifier.

If the amp adds or subtracts harmonics audibly, it will change how the music sounds.

All amplifiers have distortion, but it is where such distortion changes the musician's intention we have to watch for.

This is where the modern FFT display can help tune-out the amplifier's influence.

Then it's found that thermal and supply fluctuations tune back-in the amplifier's influence.

One could assume that the sound will never be reproduced the same from one moment to the next, and one would be right!

One would also be right in thinking that no class-B or class-AB amplifier will be correct unless it has rigid compensation of thermal and supply fluctuations.

Therefore, a datum found through FFT examination is required, and either some complex circuitry, which itself can go wrong or some manual adjustment, regularly carried out, is essential to maintain that datum.

For this reason, I'm putting meters on the amplifier and accessible adjusters such that the user can set the datum back to where it should be as it occasionally drifts.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Feb 2020 at 10:16pm
Testing:


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Feb 2020 at 9:54am
Over the last 24 hours, there has been drift of 0.5 to 1mV on the left channel, which I put down to it being more thermally exposed. The left heatsink, being at the end of the shelf, gets all the drafts. The right one's "climate" is more constant.

Today I hope to make the accessible fine adjusters easier to use. Ultimately, these will have their slotted ends flush with the front panel, making them easily accessible for screwdriver adjustment.

Use As Monoblock


If one input receives signal 180 degrees out of phase to the other, and the loudspeaker is connected across the red terminals such that the loudspeaker plus terminal is to the channel receiving the in-phase signal, the result might well be 107 watts into 8 ohms.

Although the amplifier does not have the necessary circuitry built-in, a 180-degree phase displaced signal can be obtained using a balanced output source.

The Majestic Pre-DAC TRS outputs can be used to this effect by making up a special cable which directs tip and sleeve to one channel, and ring and sleeve to the other.

For stereo, it should appear obvious that two of the power amplifiers will be required.

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