|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
20th Oct 2020, 12:27 pm | #1 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
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?
__________________
Al Last edited by Al (astral highway); 20th Oct 2020 at 12:57 pm. Reason: On the hoof with a smartphone ! |
20th Oct 2020, 2:06 pm | #2 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
|
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.
Chris
__________________
What's going on in the workshop? http://martin-jones.com/ |
20th Oct 2020, 2:07 pm | #3 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
|
Re: Inductors in output stage of quad 405
Scrap that - clearly L2 is part of the Zobel network. <slaps head>
Chris
__________________
What's going on in the workshop? http://martin-jones.com/ |
20th Oct 2020, 2:09 pm | #4 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
|
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.
|
20th Oct 2020, 3:15 pm | #5 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Inductors in output stage of quad 405
Quote:
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?
__________________
Al |
|
20th Oct 2020, 3:16 pm | #6 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Inductors in output stage of quad 405
Quote:
But I'm intrigued if anyone can shed a light on how the maths works here!
__________________
Al |
|
20th Oct 2020, 4:40 pm | #7 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
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. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
20th Oct 2020, 4:43 pm | #8 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
|
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. Craig |
20th Oct 2020, 4:48 pm | #9 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
|
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/
Craig |
20th Oct 2020, 5:43 pm | #10 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Inductors in output stage of quad 405
Quote:
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://
__________________
Al |
|
20th Oct 2020, 5:44 pm | #11 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Inductors in output stage of quad 405
Quote:
__________________
Al |
|
20th Oct 2020, 11:16 pm | #12 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
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!
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
21st Oct 2020, 10:46 am | #13 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
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
__________________
Al Last edited by Radio Wrangler; 21st Oct 2020 at 12:22 pm. Reason: too many http//'s for it to work, so trimmed! |