Defectice Output Power Devices at Integr. Amps with Tone Control inside the NFB Loop

[tiefbassuebertr]

In modern and vintage integrated amps most equalizers operates according the famous inventor and developer Peter James Baxandall under
Dois pré-amplificadores de Baxandall
resp. post 15 (pdf) about
http://www.diyaudio.com/forums/soli...based-james-baxandalls-topology-wanted-2.html

But there are two groups from the view of operating:

1) Equalizer network lies inside of an extra line stage (both as serial network and NFB network from the separate line stage itself) - not critical in order to unwanted oscillations.

2) Equalizer network lies inside of the NFB loop from the power amp section - very critical in order to unwanted oscillations, particularly in such cases, where the high frequency knob goes in direction minimal (treble adjust - some guys call it "trouble adjust")

Previously I don't know the reasons for sudden unexpected shorted output BjT's (and sometimes burned speaker voice coils in cases without DC protect).

In the meantime I know, that are always to expect defective output stages in such cases, where the treble adjust is in "min" position and the power amp section (actually an operational amplifier with power buffer in all cases) is not stable for unity gain mode (as long the tone control network works in the NFB loop from the power amp section).
An example is Luxman's L210 - go to the circuit diagram about
LUXMAN L-210 SCH Service Manual free download, schematics, eeprom, repair info for electronics

Now the questions from me:

1) How do I know at the circuit diagram from the tone control network in the NFB loop, whether the unity gain condition for the power amp section is necessary?

2) In which books and diyaudio threads is this issue discussed in detail ?
Unfortunately some threads are to large for finding some advices - e. g.:
http://www.diyaudio.com/forums/solid-state/171159-bob-cordells-power-amplifier-book.html
http://www.diyaudio.com/forums/lounge/146693-john-curls-blowtorch-preamplifier-part-ii.html

Thank you for advices.

The follow threads don't provide any advices:
http://www.diyaudio.com/forums/soli...k-based-james-baxandalls-topology-wanted.html
http://www.diyaudio.com/forums/analog-line-level/194119-high-end-tone-control.html
http://www.diyaudio.com/forums/analog-line-level/246221-baxandall-tone-topologies.html
http://www.diyaudio.com/forums/solid-state/153755-baxandall-tone-control-circuit.html
http://www.diyaudio.com/forums/solid-state/92121-need-equation-3-band-baxandall-equalizer.html
http://www.diyaudio.com/forums/tubes-valves/52982-baxandall-tone-control.html
http://www.diyaudio.com/forums/analogue-source/40902-baxandall-eq.html
http://www.diyaudio.com/forums/solid-state/164885-tone-control-feedback-loop.html
http://www.diyaudio.com/forums/analog-line-level/221235-ne5532-tone-control.html
http://www.diyaudio.com/forums/chip-amps/227593-lm3886-gc-lm4562-tone-control.html

 

[Ian Finch]

Difficult to track down your very specific topic.
Most hits were to Self's writings from either "Small Signal Audio Design" or "Self on Audio" when I Googled "unity gain tone control stability" There are book extracts to read with Google, so at least have a look.

 

[DF96]

There is no point in putting tone controls inside a feedback loop. If this appears to be the case then one of the following is true:
1. you have misunderstood the circuit diagram
2. the circuit designer is incompetent

 

[tiefbassuebertr]

Difficult to track down your very specific topic.
Most hits were to Self's writings from either "Small Signal Audio Design" or "Self on Audio" when I Googled "unity gain tone control stability" There are book extracts to read with Google, so at least have a look.

Very specific topic? I don't think so. A wide range of commercial products uses this topology to avoid additional line stage in their integrated amp models. And by very many devices I observe unwanted oscillations at the power amp output in case of closed treble adjust (this oscillation gradually disappears while turning the treble knob in direction of maximum).

This links I have found:
http://home.comcast.net/~stphkeri/NegativeFeedbackTone.pdf
Patent US4633189 - NF tone control circuit - Google Patenten
to get genuine paper go to PAT2PDF - Free PDF copies of patents: Download and print!
and enter this number: US4633189

There is no point in putting tone controls inside a feedback loop.
If this appears to be the case then one of the following is true:
1. you have misunderstood the circuit diagram
2. the circuit designer is incompetent

I don't understand the marked term.
How does that help me further ??
Mean you in case of the mentioned luxman L210 in post #1, I do not understand the circuit?
Or do you think, the developer of the circuit was incompetent ?

 

[nironiro]

Old thread, but no answers.
Many commercial amplifiers use that type of tone controls, for example Marantz PM310, Pioneer SA 510 and 610, SX 600L, SX 700L, JVC RK10L ...
Yes, that was not high end amps, but they sound pretty good, and me too don't think that their designers was incompetent.
Here is one description from Pioneer SA 610 service manual: