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

20 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?



Big heat sinks: 250 in2 , plus similar sized black anodized aluminum end pieces, per mono block.

I tried to indicate 0.08 ohm to 0.12 ohm. Mostly tighter around 0.1.

(Blush) Because it’s easier (blush)

Idle current diminishes from 1600mA to 1200 mA, in the course of one hour. Rails are +25V and -25V, so dissipation 80W reducing to 60W.

@blueranger

Thanks for your expertise. I have a question about damping. You said a heavy gauge speaker wire would actually decrease damping. So a speaker gauge around 16 might exert more control over the woofers for deeper bass?

What I wanted to convey that a damping factor of 1000 will never reach or be appreciated by your speaker. Above 10 is generally enough, hard enough to obtain on a tube amp. SS amps will have higher factors but to no avail at the listening end.

You want low resistance cables. For a few meters 16 ga is fine. Double the length and you should subtract 3 from the ga to get the same resistance, therefore 13 ga. Isn’t it interesting that every 3 ga doubles the resistance over the entire range of wire...Bet people dont know that one. You can only learn that at a traditional school like UVA. Any alumni here?

But here is the worst news to many is; Damping does not really control a woofer. Im sorry as this will disturb many. Paul Klipsch spent his lifetime trying to put this across. I dont know why it ever started being called damping. Perhaps someone here will do a little research. My clue would be to see when pentodes replaced the 45 triode in radios of the late 1930s.

We have to keep in mind that these radios and phonographs had single 8-12 inch speakers and an open back cabinet. In that case one could say we are damping the woofer by providing it a low impedance so that the resonant peak will not be so big.

Modern speakers dont require damping, they are already damped in their cabinet. They really are. Go tap on the woofer, you will hear the resonant bass frequency. Short the terminals and see how much your tap changes in level. I expect not much. You can even do this with your speaker connected. Tap it with the amp off then tap it with the amp on. The amp will be the short if it is high damping. its not quite fair to do this with a tube amp as the transformer may damp the speaker more with the amp off than on. HUM, isnt that interesting.

The resonant frequency of the woofer determines the low frequency cutoff and will not change with wire, damping or anyone's good wishes. It will however go a bit lower with time as the woofer loosens up, but only a few Hz. 

What modern speakers need is a low impedance drive to deal with their often widely varying impedance. A 8 ohm speaker can easily go from 50 ohms down to 4 or 3 or 2 ohms. That’s quite a range. Then damping makes a difference because it provides constant voltage to the varying impedance.

Of course some speaker makers keep their impedance rather constant, which is a very good thing if you want to play with a wide variety of amplifiers.

Maybe an open minded university, interested in electonics history, would let me do PhD research on such an arcane topic. Offers accepted. Though I hardly have the time for anything so rigorous.

@terry9  Idle current diminishes from 1600mA to 1200 mA, in the course of one hour. Rails are +25V and -25V, so dissipation 80W reducing to 60W.

Sounds like a nice amp. High idle, reasonable rails. Congrats.  Its for more sensible than some 200-1000 watt monster or a bridged amp.

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.

Thanks for the nod ... I guess the takeaway is, "Don't be so lazy. Match HFE at operating temperature and current." Thanks!

In case you are wondering, the 25V rails are to accommodate Quad ESL's. With 75:1 step up transformers (Plitron), the amps can only punch the panels to 3750V, exactly half the level which engages the protection circuit. So I can bypass the protection circuit with impunity, and get a remarably refined ESL sound.