Millett "Starving Student" hybrid amp - Page 40

"The genious of Pete Millett's design is that he tied the MOSFET output stage to the heaters of the tubes. So as long as the tubes are fired up, the MOSFETs are both Class A-biased to 150ma each. Considering the almost 30V difference (~48V - 19V) at that current, it adds up to almost 4-1/2 Watts per channel of Class A power!"

Tomb you posted this in RDS's build thread and also mentioned to ask technical questions in the original thread. I am appropriately asking this question here as requested.

Is this amp really pushing out that much power? 4.5W of tube power is pretty impressive in such a small design. If it is really pumping out that kind of power couldn't it be used to run small speakers? I have small 6ohm speakers on my desktop by my computer. If I could directly power them with this amp and flip a switch for headphone listening it would be the ideal solution for me right now. I am sure that others would find this idea amazing as well if it is possible. Is there any reason why it couldn't be used to directly power speakers?

No, it's not pumping out that much power. I just used the hyperbole to illustrate that it's about as far away from NON-Class A as you can get in an amp this small. That was sort of the basis of the original question. So, that's dissipated power, not output power.

Pete's measurements say 7V RMS into 100ohms at 5% distortion. That's 490 mW, according to my calcs. He also says 3V into 32 ohms. That would be 280 mW. That's more than adequate to drive almost any headphone, I think. It's probably OK for a couple of flea speakers, too.

Sounds like a Pete question. I agree that the voltage swing is probably too great to be used as a preamp. I don't know how the gain could be adjusted, though. rds' implementation of a gain switch is more of an attenuation at the pot. I don't know if that could be used to set it up for a preamp. Maybe someone else has some better suggestions.
Check for some of the earlier posts - Jameco has linear, regulated walwarts that I think holland used in his build.
See the post above and read the rest of the response I made that caused you to post in this thread.

So at 6ohms, there is no way it would drive them properly. 1/2W of tube power would have been ok I would imagine but I am sure it is less than that based on what you said above. Too bad. I got really excited for a second over the idea of having an all in one desktop amp. I am looking into building a gainclone too and might want to use the SSMH as a preamp if it is simple enough to lower the gain to proper levels for a line out.

If one really wanted to use the SSMH as a speaker amp it'd probably be worth looking into if it would be possible to trafo-couple the output rather than cap coupled as it is now.

Trafo-couple? Can you explain further? Do you mean a transformer on the output?

I don't want it to be solely used as a speaker amp but to add something to the circuit with a switch to allow for both. It would be a great concept as a desktop amp allowing for the best of both worlds if it could be pulled off.

output transformers are used on class A speaker amps to impedence match them to low (compared to headphone) impedence speakers.

You can effectively lower the gain as much as you want by adding series resistors before the pot.
If you wanted a preamp with something around unity gain then you could have 100k resistors in series with a 10k pot.
If you want a gain of 2 to 3 so this can be used as a headphone amp as well , then you could put 50k resistors in series with a 10k pot.
This is just a voltage divided concept which allows you to attenuate the source.

This mod does not change the turn on and turn off thump. So keep that in mind. I use this amp as a pre-amp sometimes and it gives a nice low bass boom from my speakers if I turn it off when the amp is on. This could blow your speakers (see my post on the next page).

So the output transformer doesn't deliver more watts neccessarily? It just allows for the same power to be delivered to low impedence speakers?

In the case of this amp wouldn't it need a bigger power supply in order to have the amps to deliver decent sound through an OPT??

I quickly scanned this so my understanding is fairly limited:

output-trans-theory
Output Transformer Impedance

If we have a certain voltage (around 500mw with the SS) then the voltage will drop and the amps will increase in order to drive low ohm speakers. Do I have this right? Does anyone have any numbers that would show what sort of power the SS could deliver? (Running out of time to look right now).

Does this make sense to do this with this amp? Could I have a switch before the output that allows to go between a cap for the headphone output and an OPT for speaker output.

And on the practical side, don't OPTs themselves cost a fair amount?

yes i was thinking about a switch to modify the attenuation of the input signal, good to hear that it works

also thanks for the jameco suggestion, but just in case what diy choices would i have that could provide a clean 50 v output


thanks

Francisco

I never suggested the Jameco. I have no experience with it.

Don`t take this the wrong way, but if you have to ask you probably should avoid building your own power supply. It is dangerous.

Why not use the recommended Cisco power supply? It works well, is cheap and is readily available.

One quick comment, if your goal is to attenuate the signal before the pot I would use a voltage divider and not series resistance. A voltage divider would essentially be one step of a stepped attenuator and you can control both the input and output impedance of the divider so that you don't cause adverse affects on the signal.

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Well, of course not. A transformer is a passive device with no capability to create power.

I'll agree with rds here. This method will end up with a lower Zin in most cases. The relationship of the voltage divider and pot will need to be 10x which will leave either the input Z too low for some sources, or the second attenuation stage will be potentially large which can be noisy. Either way, it is two attenuators when you only need one. OTOH, putting a resistor in series with the pot essentially lengthens the resistive element of the pot and just keeps the wiper from traveling over the whole thing.

Considering the gain involved, it works pretty well on your SSMH PCB. I went conservative and just added ~50K resistors in line with the 50K pot, but it's a much better adjustment than I expected.