Does anyone care to ask an amplifier designer a technical question? My door is open.

Roger, you are being very generous with your time and your expertise. Thank you!

You have given me a container load of practical information which only comes from practical experience. Much appreciated! As requested, here are some details of my system.

My system is 98% analogue. Phono is integrated with the preamp. Twinned MATs reduce the noise floor. All caps in the signal path are air gap or vacuum, including RIAA. Battery power, filtered.

Speakers are 5 (soon to be 6) Quad 2905's. These have been modified for the Plitron step-up, which I will revisit - but that protection circuit is not at all transparent! It desperately wants attention, if not removal. The small disgusting ceramic caps in the phase ladder are being replaced with air gap (teflon is a big improvement on ceramic, but coloured). The larger disgusting ceramic caps are replaced with a chain of RTE styrene F&F caps. Air gap caps are made in the usual way.

Amplifier power comes from a big variac driving an isolation transformer, driving power transformers. Rectifiers are Ixys, feeding LCLCLC filters. Theoretically at least, this brings ripple down to the level of intrinsic noise.

Amp topology is reminiscent of the newer Brystons, but not close to identical. Transistors are AD, Toshiba, Sanken. All resistors are nude Vishay or DIY nichrome.

Theory as well as experiment have led me to suspect that much of what we hear is dielectric-sourced distortion. Accordingly, my caps are air-gap or vacuum wherever possible, styrene when not. Wiring is insulated and protected with unshrunk shrink-wrap, except at the ends to prevent corrosion. Just my poor observations in part repayment for your generosity.

Your expertise is highly valued, so anything you have to say is welcome. Thank you!

I am puzzled why the idle current goes down rather than up as it warms. Usually its the other way round because of the negative temp coefficient of transistors.

Actually this is pretty common with bipolars. They can get into a phenomena known as 'thermal runaway' if this is not well controlled.

However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.
Your thoughts? Any advice appreciated.

There's a difference between idle and dynamic operation. Matching the hfe over the range in which the device operates is going to be a better method. Otherwise what will happen is a particular transistor can 'hog' current at higher operating points and it will thus be the thing the sets the distortion of the amplifier at power, especially full power.

with Class D switching freq over 450/500 KHz these days do you still think output filtering is an issue?. How do I educate more about this ?. If the issue of filtering not a function of switching freq please correct me. Thanks again.

There's lots of information on the web! You might want to read the papers of Bruno Putzey who has done a lot to further the art. The filter is there to filter out the switching frequency, which it cannot do completely. What is left is a sine wave called 'the residual'. Usually the filter is set to about 60-80KHz, so as to avoid phase shift within the most sensitive area of the audio passband, but otherwise get the residual down as low as possible. The higher the switching frequency the easier this is to do- the speaker inductance itself starts to play a huge role at higher frequencies.

Mac Turner brought me in to consult on his. What disturbes me about them is that they all get their sound from the IC in the front end. Perhaps some are discrete front ends but I have not seen any.

Actually you don't need any ICs in the front end prior to the encoding scheme, if you set things up right. The encoding scheme of course is going to use some sort of chip- our amp for example uses a high speed comparator chip. But the audio signal is applied directly to it.

Thank you, Ralph!

@bdp24 

Roger, here’s a question about light loading with the RM-200 Mk.2 amp: With a loudspeaker given a nominal 8 ohm load rating by it’s designer/manufacturer (it measures between 10 and 20 ohms from 20Hz to 20kHz save for the 60Hz-180Hz band, where it dips down to 7 ohms, centered at 80Hz), the RM-200 will of course put out at least it’s rated 100w/ch when connected to the amp’s 8 ohm taps. If it is instead connected to the 4 ohm taps (light loading), what will the amp’s output wattage be?

Because of the speaker’s relatively high and even impedance curve, it is not for the flattest frequency response I am interested, but rather the lowest distortion and longest tube life. I realize that with light loading those will be achieved at the cost of less power output from the amp. I have a modestly-sized room, and don’t listen at very high SPL, so am willing and able to accept that.

As I tell everyone. Light loading can be done on any tube amplifier. If you dont play above 80 dB  go to the lowest tap. If you play 90 go one higher, if you play more than 90 you may have to use the high tap to get the volume you desire. This change should be far more apparent than a cable change. 7 ohms on the 8 ohm tap is no problem for the amp but lower taps always perform better technically though how it sounds to you is more important.

4 ohms on the 4 ohm tap produces 100 watts. 8 ohms on the 4 ohm tap around 60 watts. As you go down in taps you spend more time in the class A region. The distortion and damping are much improved.

Would be nice if you had a way to measure the peak voltage you require, then much better advice can be given. You can also get at that with a SPL meter at one meter at listening level. Then with speaker sensitivity we can compute it

Everyone needs an SPL meter. For $50 its an execellent investment. People spend more than that on a poorly designed high end fuse. Please do not buy premium fuses, TuningFuses are the worst and the others I have not dissected but the people who sell them should be dissected. 

@terry9 @atmasphere 

Actually this is pretty common with bipolars. They can get into a phenomena known as 'thermal runaway' if this is not well controlled

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Ralph, did you perhaps mis-read this. He said the idle goes down not up as the amp warms. This is the opposite of thermal runaway.

Terry, do you still hold your idle current goes down as it warms?

If so I want to see your circuit. Hey, I'd like to see it anyway. This is unusual.