More Output devices...A75

FBJ

Member
2002-03-12 7:46 am
USA
DRUM ROLL PLEASE........ I have searched the Fourm but nothing!
Another Drum Roll please....... Well has anyone added more output FETs to the PASS A75 amplifier? yes I know " more matching and expense" but someone has to ask this question right. I remember in one of the old TAA issuses Nelson Pass answered a reader who wanted to increase the number of output devices of the PASS A40 and NP said just beef up the power supply. So I wonder................:D
 

FBJ

Member
2002-03-12 7:46 am
USA
Well that's a good question..... and...... deserves a good answer.
a good answer?
a good answer?
Oh, all my PASS projects have been low power versions and I like to do one last (well maybe) project so I can get back to a few important things like.....eating better and buying some new clothes, sleeping longer hours instead of working on audio projects to the late hours of the night, new car too.
And I like to devote my time to building speakers. I have no power tools yet. And I won't have the time or money to do both.:cool:
 
Actually, I would like to second this question....

I am considering building a larger Aleph as an experiment. My design is targeted toward 48 fets per monoblock and the design output will be somewhere around 300 watts... I'm figuring on 4 kva transformers with rail voltages set for a 300 watt target...

My line of thinking goes like this.... If I add more outputs, I can achieve better damping, better performance into low impedance loads, overall better transient current carrying capability, and lower junction temperature per device... Therefore, if I use efficient enough cooling, I should be able to push the amount of bias current up a bit higher while not stressing the devices too much....

Yes, managing the heat will be a factor...

I am fully expecting that the universe will be experiencing "heat death" at some point so I figured what the heck I may as well contribute... In all seriousness, I think the idea of eventually marrying the X front end with the Aleph back end would be pretty cool. I figure this will give me a very solid base to spring from...

Hence, when adding additional output devices what other factors should be considered? At what point will the drivers be unable to have enough muscle to push the output stage?

Any thoughts would be appreciated!!!

Thanks,

Steve
 
While I'm inquiring...

In the case of the larger Aleph, I have been considering two scenarios regarding mounting the outputs:

Scenario #1 - outputs mounted directly to heatsinks.

Scenario #2 - outputs mounted to 1/2 thick plate, which heatsinks are then attached to using either thermal grease or thermal epoxy.

When using the modeling software to calculate dissipation, none of the software allows for the scenario #2 configuration.

Does anyone have experience in this area? I am assuming that by mounting to the plate it actually increases the C/W calc slightly because of the inefficiency of the gap between the plate and the heatsinks. But then there's the increase in mass from the plate.... :confused: Am I correct in assuming that by mounting to the plates it will create a more even temperature across each bank of outputs and therefore will maintain more even current sharing due to all devices being at closer temperatures? And yes, all devices will be matched. Does it matter? Thoughts on this???

My Threshold amp has the outputs mounted on aluminum plates which, are then bolted (using grease) to the heatsinks. I have seen enough amps built using either carrier plates or "L" brackets to mount the outputs that I'm curious if it's done strictly for aesthetic or assembly efficiency, or if there is some other reason for it. Clearly, when using an "L" bracket it raises the junction temp so my question is why do the manufacturers do it?

Finally, has anyone actually looked inside the X-Series at how the outputs are mounted? Did NP use plates to carry the outputs or are they mounted right to the heatsinks?

I figured that I'd put this out there as it directly deals with increasing the number of outputs of any circuit (and how to mount them).

Thanks in advance for you thoughts,

Steve
 
I would think since the bias is adjustable up to the set point, you could keep adding sets till you run out of adjustment.I'm sure you could reach a limit. How many do you want to add. Your already driving 12 pair per channel. If you beef up the heatsink you could match and bias at a higher current and maintain Class A into lower impedance loads. If that is what you want to do this for. If your just into spending money go for it. It cost me about 300 bucks for output devices and I got two matched sets (out of 100 mosfets). I later sold the set I didn't use in the trading post. Hopefully Nelson will be weighing in here soon and we'll find out about driving the extra capacitance or whatever limitations you may have to look for.:)
 

FBJ

Member
2002-03-12 7:46 am
USA
Sorry for joking around, I was in a happy mood when I made my last two posts. My BOZ was just in the passdiy gallery and I couldn't help but simle about that.

The reason I am interested in more output devices in the A75 is that amp could be my last DIY project. Cause I have so many incompleted projects that if I didn't bulid any thing new it would take me 5 or 6 years to finish what I got. And the amplifier that I always wanted is a Threshold SA/12e. I know the SA/4e had 56 output devices total and I think the SA/12e had 80 mosfets total.

I am not a new DIYer. I have been messing around since the early 80's. And I have been making it to all the Stereophile shows on the west coast and the CES for the last 14 years. I really feel that the audio companies only have the edge on us DIYer with better looking enclosures and testing facilities. But as for sound wise we (DIYer) are very close. So a few years back I decide to put all my projects in very nice looking enclosures and fix all my projects just the way I want them. Now all my six Borbely amps and four preamps don't have homes. As well as my Pass A40, Curico Daniel tube preamp and head amp. And Curico electrostatic headphone amp. Also I have been buying parts upgrades for all my projects too over the years. And I probably got other half to three quarters together projects that I don't remember cause I have been staying with relatives and I got all my stuff in a small corner of the garage. Also this doesn't count the six Nelson Pass amps and four preamps that I have almost all the parts and PCBs for.
Even though I would like to start building speakers I really don't think I'll ever do it cause I am better at electronics than woodworking.
Bye,
FBJ
 
One thing to consider when adding mosfet output device to an existing design is the added gate capacitance and whether the driver circuitry is capable of dealing with it. I'm not the person who could answer that question however, at least in this instance.

I have to wonder if the benefits of class A circuitry are audible at 300W, however. To me the first 5 or 10 watts are the most important.