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

I was measuring HFE with a handheld, not warm, not at operating current, and only using the result as a first cut to cull outliers.

Emitter resistors are 0R1 +/- 0R02, so not much.

4 x PNP and 4 x NPN.

No VBE multiplier, but then I leave my amps on most of the time, and they stabilize below 50C.

Thanks very much for taking an interest, Roger.

Thanks for the balanced info Roger. 4 resistors > transformer! I've been running the RM-200 with a balanced pre-amp, but if connected to an unbalanced source (First Watt B4 x/o), what is your recommended method of converting the RM-200's XLR jack for use with an RCA plug, or put another way, connecting an RCA plug to the XLR jack? I've seen a cable with XLR's at one end, RCA's at the other. Acceptable?

@geek101    Hi Roger, thanks for the great offer. I would like to know what are all the measurable variables does one consider when manufacturing an amplifier.

I am guessing this list is most exhaustive then the spec sheet typically posted?. If not correct me.

For most of the lesser informed people buyers like me we only have spec sheet and most amps almost are always good with stated specs. So it is really hard to differentiate and choose.

Good Question. One thing to do is follow John Atkinson's measurements of amplifiers. If you dont get Stereophile its a good mag and only $12-15 a year. If I get 10 new subscribers I get a toaster... :).. Just kidding

I find most amplifier specs to be missing something that should be there. Perhaps because that particular spec is not so good. So here is the list.

Distortion from 0.25 watts to full power over 20-20,000 cycles. This spec came from the FTC.

Damping factor can be most important thing if it is below 8-10. However much more than 10 doesnt make much difference. In other words, some SS amps have damping over 100. Thats ok, but at what sonic cost.  Damping factor will have a great effect on frequency response if your speaker has a widely varying impedance curve. Dont be impressed by super high damping. Its not going to matter.

-3dB power bandwidth. Jadis did not publish this because the spec would be horrible. Heres a 4 chassis amp that can do 200 watts but not to 20Khz. The spec 20-20Khz is more like 5 watts. I have measured it.

Noise often specified in db below full output. Not a good way to spec this. I would prefer a simple statement in noise in mV both wideband and A weighted. Good luck on that one. 

Power consumption will give you some idea of how much this thing is going to heat your room. 

That pretty much covers it. MTBF (mean time between failures) is speced on all parts of your computer, industrial power supplies and many things. Unfortunately I have never seen this spec on an amplifier. The number for most amps would be very low. MTBF for things like hard drives is 300,000 to 1,200,000 hours

 http://knowledge.seagate.com/articles/en_US/FAQ/174791en

The MTBF of amplifiers varies widely and is only known by those of us who fix them. I am very proud of the MTBF for my amplifiers. I can only state it in years and its at least 30 years. 

If you want to buy an amplifier that will not trouble you, use the internet and find what people have experienced.

Roger, thank you again for your response. My ears have required long warm up times for almost all of the gear I have had over the years. Solid State as well as tubes, dacs, tuners, tape machines, etc. I suppose R. V. knew what I was talking about, the warm up problems. Is the forming switch the same as an amp having a stand by mode ? I assume it is this capacitor thing that I have been responding to. I am glad you are here posting, as I am greatly enjoying the information. Enjoy ! MrD.

@terry9 

I was measuring HFE with a handheld, not warm, not at operating current, and only using the result as a first cut to cull outliers.

Emitter resistors are 0R1 +/- 0R02, so not much.

4 x PNP and 4 x NPN.

No VBE multiplier, but then I leave my amps on most of the time, and they stabilize below 50C.

50 C/122F is nicely cool, only 25 C above ambient in a standard room. You either have big heat sinks or not so much idle current.

Do you mean resistors from 0.1 ohms to 0.02 ohms? I like the American system of resistors.  That is a large range. Do your account for this and make the idle currents equal in each transistor?

HFE for a power transistor at low current is rather meaningless. Why not do all of it at operating current?

A VBE multiplier is a very good thing and easy to add. How much does your total idle current vary from cold to hot and how long does it take to get there? What is the idle dissipation?