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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 25 Oct 2020 at 7:02pm
I got lost about 115 pages ago! How close do you think you are to finalising the design? Do I remember correctly that the plan is to release it as a DAK?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Oct 2020 at 10:23pm
Originally posted by ICL1P ICL1P wrote:

Do I remember correctly that the plan is to release it as a DAK?


Yes, it would be nice for that to happen.

However, it would seem that I might have been better calling this "1970s Design Fantasy". The reason seems to be my memory, in that I was so sure this type of configuration used to sound good.

115 pages plus have passed by, trying everything I could think of, plus just as many ideas from the gurus of commercial amplifier design.

Maybe it's my ears? Perhaps it's my expectations? Perhaps it was an unmentionable substance?

The last idea featured in this topic, increasing loop-gain by shelving down the post-audio frequencies to increase the high-frequency negative feedback, has some effect. It is not the wow moment I've been patiently working for, but for now, it sounds like it is heading in the right direction.

Fewer of the "tinkly" highs sound overemphasized, which means some still do. This causes the distraction away from the "correct" tone, resulting in the collapse of reality into the false brightness some consider to be hi-fi.

At this point, it might merely be "leading edge distortion" (my terminology) due to me removing the local NFB from the voltage stage. It had to be done to hear the differences other things caused, but now, perhaps it needs to go back, slightly differently, to match the increased HF NFB.

Incidentally, the increase in HF NFB is a bit of a cheat, but a clever one, even though I say it myself ;-)

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 9:39am
Good results with the "HF NFB expander network". In everything I have read over many years, I have never seen feedback EQ used in that way.

It is getting close to curve tracking, where the phase of the closed-loop curve is identical to the phase of the open loop curve. Incidentally, the Accession patent works in a similar way.

The difficulty in getting both curves to track identically is that, 1. either the closed loop frequency response is cut early, or 2. the amount of negative feedback is reduced to push the open-loop cut frequency higher.

After doing the expander network, I reduced the open-loop gain by using a 330k local NFB resistor, and to my ears, this led to an improvement.

How can this be measured? Well, I think I've found the answer, and to do it, means operating the amplifier near clipping. In the area where the phases depart from one another, the distortion sweep becomes jagged. It starts just above where the phases change, grows larger, and then peters out where the phases come together again.

The frequency span centres on the upper mids to treble, and this happens to coincide with where the music doesn't seem quite right.

The HF NFB expander network obviously helps by reducing HF distortion, but the real effect might be in better alignment of phases. The downside to obtaining exact alignment is the reduction in global negative feedback, which is shown by simulation, to fall to just 14dB. The audible phase alignment distortion might be reduced, but overall THD might increase sufficiently to nullify any improvement. The only way to find out is to try it (yet again).

Edited by Graham Slee - Yesterday at 9:41am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 1:19pm
I include this diagram to help with the understanding of the above post.

loop gain illustration
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 7:20pm
"However, it would seem that I might have been better calling this "1970s Design Fantasy". The reason seems to be my memory, in that I was so sure this type of configuration used to sound good."

The 1970s power amps did sound good, even great!

 They used E-I laminated transformers. E-I transformers have been around long enough to be design-maximized... a lot longer than toroids. 
E-I transformers with separate bobbins for primary and secondary have no need for a shield between windings.

 Toroids, with their higher efficiency, do pass more of the noise from the mains right through... 
Even with a shield between the primary and secondary windings.

 Do you think a proper E-I transformer gives better sound quality than a toroid?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 8:02pm
Originally posted by BAK BAK wrote:

Do you think a proper E-I transformer gives better sound quality than a toroid?




After trying several toroids, mostly well-made by reputable and experienced manufacturers, and really wanting them to be the better type, having spent hundreds of £s buying them, they rest in the toroidal graveyard.

Think? I know E-I transformers result in a better sound. How some audio manufacturers can say otherwise is beyond my understanding.
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