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Old 20th Oct 2020, 12:27 pm   #1
Al (astral highway)
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Default Inductors in output stage of quad 405

Snippet attached.

This theoretical question came up for me after following the recent thread about output transistors in a Quad 405

I don’t currently own a pair of these and I’m not working on one. But my mind is restless over this as I understand there are both controversial( the Crowbar) and excellent design aspects of these amps ..
This q is focussed on an LR pair and also L2.


In the collectors of both Q9 and Q10 (BDY77 — manufacturer ??) is an LR that has very low impedance at audio frequencies and somewhat more, clearly, at RF, suggesting a tendency for spurious oscillation that needs a stopper.

I used an applet to scope out some complex impedances...

E.g at 1MHz impedance Z is: 17.57Ω + j 8.87 Ω = 26.5 R approx

At 500 KHz the result is slightly lower at nearly 22R

Note, the inductor in this position is only 6.9uH!

Some BDY77 clones I’ve seen have Ft around 800KHz but I have no idea what the original device specs were.

Inductor L2 is only 3uH.

This looks all very subtle. What’s going on in this delicately calibrated arrangement?
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Old 20th Oct 2020, 2:06 pm   #2
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Default Re: Inductors in output stage of quad 405

My first instinct would be to say they're part of the Zobel network to keep the amplifier stable. C12/R39 are the other section of the network. However, L1/L3 don't look they're in quite the right place for that, and I'm not familiar with the details of the 405 design so I'm not really qualified to comment.

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Old 20th Oct 2020, 2:07 pm   #3
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Default Re: Inductors in output stage of quad 405

Scrap that - clearly L2 is part of the Zobel network. <slaps head>

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Old 20th Oct 2020, 2:09 pm   #4
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Default Re: Inductors in output stage of quad 405

My understanding of the current dumping design (which is far from complete) is that these inductors are critical to the correct operation of the bridge and that the maths behind them is far from simple.
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Old 20th Oct 2020, 3:15 pm   #5
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Default Re: Inductors in output stage of quad 405

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Originally Posted by cmjones01 View Post
My first instinct would be to say they're part of the Zobel network to keep the amplifier stable. C12/R39 are the other section of the network. However, L1/L3 don't look they're in quite the right place for that..
Thanks for pointing out C12/R39, Chris, I overlooked them.

So we've got our 6.9uH/22R in parallel, our 3uH in series, then 0.1uF and 10R in series in the Zobel network?
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Old 20th Oct 2020, 3:16 pm   #6
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Default Re: Inductors in output stage of quad 405

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Originally Posted by barrymagrec View Post
My understanding ... is that these inductors are critical to the correct operation of the bridge and that the maths behind them is far from simple.

But I'm intrigued if anyone can shed a light on how the maths works here!
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Old 20th Oct 2020, 4:40 pm   #7
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Default Re: Inductors in output stage of quad 405

No, it's not a zobel network.

The quad 405 is a seriously weird amplifier. It uses a small class A amplifier and a riugh brute-force stage combined in a bridge arrangement to for a feed-forward error (=distortion) cancelling system. For best efficiency t lower frequencies, one of the elements of that bridge is an inductor... a surprisingly small inductor.

No point writing a description here, it's already been done at length. Look up "Current Dumping" amplifier in wireless world about 40 years ago and Peter Walker and Peter Baxendall will take you through it.

The idea is that the crude heavy-mover transistors do the bulk of the work in a rough way, and that a fast, agile class A stage corrects the errors.

That L and R sum the outputs of these two amplifiers in a complex fashion that is also part of the cancellation control scheme.

It's very clever, but it makes assumptions about how much umph the class A stage needs to do to fill in the errors. It gets a bit marginal at the high frequency into low-Z load corner of things. It was the latest and whizziest thing when I did my power amp design, and I was unsure of it, so I didn't go down that route. So far, only Quad have used it in any consumer product to my knowledge.

Some people rather like them, and within reasonable limitations on power and load impedance, I think they're OK. They're quite well thought of on the forum. The BBC integrated them into some legendary active-crossover loudspeakers, though they later switched to some more conventional power amps.

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Old 20th Oct 2020, 4:43 pm   #8
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Default Re: Inductors in output stage of quad 405

The R//L in the collectors was part of making the amp stable. When they changed power transistor types they disappeared, but other parts were added, and then changed value.

Quad knew that there was a problem in making the amp unconditionally stable under all loads. So the design was tweaked a lot over the multiple changes in power transistor over the years.

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Old 20th Oct 2020, 4:48 pm   #9
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Default Re: Inductors in output stage of quad 405

They continue to use dumpers it to this day in their latest products https://www.quad-hifi.co.uk/artera-stereo/

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Old 20th Oct 2020, 5:43 pm   #10
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Default Re: Inductors in output stage of quad 405

Quote:
Originally Posted by Radio Wrangler View Post

No point writing a description here, it's already been done at length. Look up "Current Dumping" amplifier in wireless world about 40 years ago and Peter Walker and Peter Baxendall will take you through it.
Thanks, David!

Here's a link to an article in case anyone is in the mood for some very convoluted algebra. I'll put that on hold!


https://www.keith-snook.info/wireles...DCD.pdfhttp://
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Old 20th Oct 2020, 5:44 pm   #11
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Default Re: Inductors in output stage of quad 405

Quote:
Originally Posted by Craig Sawyers View Post
They continue to use dumpers it to this day in their latest products https://www.quad-hifi.co.uk/artera-stereo/
Interesting, Craig! Thanks
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Old 20th Oct 2020, 11:16 pm   #12
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Default Re: Inductors in output stage of quad 405

Duh! where I said Baxandall above, It should be Mike Albinson. The perils of also reading a thread on the 33!

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Old 21st Oct 2020, 10:46 am   #13
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Default Re: Inductors in output stage of quad 405

http://http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1975/Current%20Dumping%20Audio%20Amplifer%20DCD.pdfAnd I inadvetently poster to Keith Snooks’s home page. Much interesting stuff there but here is the paper by Baxandall and Albinson
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Last edited by Radio Wrangler; 21st Oct 2020 at 12:22 pm. Reason: too many http//'s for it to work, so trimmed!
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