Aleph-X builder's thread.

SGregory said:
So far 58C hasn't been a problem. In the summer 58C will become 65C which may or may not be a problem. Output per device is 32Watts per the WIKI spreadsheet.

I am using the IRFP244 chips and don't have the spec sheet in front of me. I will measure the chip temps to see if I am still safely within operatin specs with derating.

Sounds pretty conservative to me. You are probably getting something in the range of 1.2-1.5 Rθjs, and the part is rated for 150°C operation. Yours are operating at less than 120°C, which sounds high but isn't. With a decent thermal interface (e.g. Bergquist Sil-Pad K10), it will be closer to 105°C.

If you want more headroom, use the IRFP044, rated to 175°C
 
Grey,

I have an advantage, I work in the metal making industry. I have seen a few other "large" chassis here so maybe an increase in production is in order.


jwb,
Thanks for the input. I feel a little better about operating the chips at this temperature. It will be interesting to measure them to see in reality how hot they are actually running. Since it is hard to measure the actual interface temperature, how accurate is it to measure the case tempeature?

Switching Gears (Un related to improved power handling)
I think it would be interesting to listen to the difference between the 044 and the 244 chips. In the Aleph 5 circuit I found the 044 to be more on the dark side compared to the 244. Would the X circuit improve this?

Scott
 
Magura,

I like copper to but it is more difficult for me to obtain cheaply. It is also harder to cut with a table saw and router. A nice endmill would solve that problem, but, I like being married.

Where I errored in my calculations is with either the junction resistance between the spacer and plates or a lower thermal conductivity of the spacers themselves. The latter is easy to fix with either wider spacers for more surface contact area or as you said copper. Junction resistance on the other hand becomes a little more tedious as it will require better thermal compounds and/or much improved surface flatness.

If jwb is right, and I have no reason to doubt it, then I might not have a problem at all. I like this possibility.

Scott
 
Scott,
Metal working, eh? Must be nice. I'm only set up to do wood, although I can handle about anything I'll ever need to do there. Metal working isn't a common need around this town; there's no such thing as industry these days.
You can get into debates about heat vs. transconductance, but I prefer something that runs a little cooler--just 'cause I'm getting silly in my old age. For the Alephs, I went water cooled. It's cumbersome, but works like a charm. The rig I've got can dissipate enough spare heat to run an old-fashioned steam locomotive. The Aleph-Xs are air cooled.
All things being equal, I imagine that the '044 will pretty much tend to sound darker than the '244. I haven't done a direct comparison between the devices and the topologies as my Alephs use the IRF644 (a TO-220 casing) and the Aleph-Xs are filled with '044s. Shooting from the hip, I'd say that the Aleph-X might be a bit brighter than the regular Aleph assuming the same devices, and the 'pure' X a bit brighter still. Nelson is likely the only person who's had enough of these things sitting around that he could do side-by-side comparisons with different output devices.

Grey
 
The one and only
Joined 2001
Paid Member
Funny you should mention...

The 044's are doped (and who knows what else) differently than
say a 240 or 244, but we do see that they have lower Vds rating,
higher tranconductance and higher capacitance. The latter two
are the interestings specs. The higher transconductance means
that open loop gain will be higher, particularly at lower
frequencies, while the nonlinearity of the Cdg capacitance will
be likely higher.

This corresponds to the experience of sound that you mention.
The 044's tip the spectral balance toward the bottom end, where
they have more punch and lower distortion, but at the same time
they have a "darker" characteristic (darker being the opposite
of brighter, is not necessarily a negative). Conversely the higher
voltage versions (244 and such, tip the spectral balance
in the other direction and are brighter at the top end (all other
things being equal). If you are bi-amping or tri-amping (and so
on) the choices are easy.

The one way to get the best of both is to cascode an 044 with
a 244 (or even an 044). The distortion due to Cdg pretty much
disappears here, and the results are well worth the effort.

Of course one of the remaining Zen Variations will address this
issue (always saving the best for last... ) :wiz:
 
For example:

IRFP044,140: Rθjc: .83, max Tj: 175°C, max Pd @ Tc=70°C: 126W
IRFP240,244: Rθjc: .83, max Tj: 150°C, max Pd @ Tc=70°C: 96W
IRFP044N: Rθjc: 1.3, max Tj: 150°C, max Pd @ Tc=70°C: 85W

But if power dissipation is your prime design concern, there is always the unholy IRFP260N:

IRFP260N: Rθjc: .50, max Tj: 175°C, max Pd @ Tc=70°C: 210W

Yeehaw. And a nice 5000pF input capacitance, also :bigeyes:
 
jwb said:
For example:

IRFP044,140: Rθjc: .83, max Tj: 175°C, max Pd @ Tc=70°C: 126W
IRFP240,244: Rθjc: .83, max Tj: 150°C, max Pd @ Tc=70°C: 96W
IRFP044N: Rθjc: 1.3, max Tj: 150°C, max Pd @ Tc=70°C: 85W

But if power dissipation is your prime design concern, there is always the unholy IRFP260N:

IRFP260N: Rθjc: .50, max Tj: 175°C, max Pd @ Tc=70°C: 210W

Yeehaw. And a nice 5000pF input capacitance, also :bigeyes:

I intend to use the 044N for a tiny AX. Since the power rating for 044N is only 85w, what is the comfort zone for this Mosfet? I was thinking of say 37.5w, am I living too dangerously?

Cheers
Hock Hua
 
I was basing my guesstimations as to sound quality on the capacitance alone. I had neglected the transconductance, although it seems that it simply urges the sound along in the same direction. One of these days, I'll do a SOZ or something and sit down for a comparison listen.
Choice of devices is an arcane art. Assuming that you want to stick to IRF devices instead of Toshiba et. al., there are only two TO-247 P-channel parts: the IRFP9140 and the IRFP9240. So if you want to go complementary, you've got a limited playing field. If you're content to use N devices, you've got a whole basket-full to choose from. Then you get into capacitance, transconductance, and so forth. The thermal characteristics are important, of course, especially for a class A amp where you're going to be flogging them within an inch of their lives, but at least there you have the option of choosing to parallel devices that have other characteristics that you want. Of course, the capacitance adds up when you do that, but then...
Cascoding comes in the back door and negates some of the capacitance, in addition to decreasing some of the heat load. This is something that I've been playing with, but with different intentions from what Nelson has planned for the Zen series. I've got a one-dead-channel Hafler DH-200 (all right, let's have a show of hands...how many of you have one of those sitting around?) and it's beginning to annoy me. I'm hip deep in scrawled topologies, some of which have survived the "think about it while driving to work" test. The problem--besides money and time, the usual demons--is that I've got about five or six projects queued up ahead of that one...but it's the Hafler's heat sinks that bite my ankles as I walk by.
So many devices, so little time.

Grey

P.S.: I'd even considered using a tube front end for the Hafler rebuild, which would multiply the device choices further. Decided not to go that route, as hybrid circuits generally combine the worst of their characteristics, not the best. Also I wanted to keep it compact, and the high voltage supply was going to be hard to fit inside the chassis.
 
Well still no avail w/ my ax... i've been working on a volume preamp using a TL08 (Crappy op amp...), but this project is definately not as important asg etting the amp to work...

one of my buddys in my town said that he 'didnt think enough voltage was getting to the fets', but he hasnt trouble shooted it, just talked about it...

Any ideas on where to start?...

Thanks alot, and thanks chad for the email :)
 
KoolScooby,
Since no one else here sems to be of any help to you for what ever reason I'd be more than happy to troubleshoot it for you if you sent me just the board. I can use my own supply as its still in breadboard stage. Let me know if this would work out for you.

Or.... If you can supply me with a complete set of G, S ,D voltage readings on all the devices fomr one channel like I posted in a post previous to this I may also be able to help this way. I'd also like to know what type of tranny you are using and the VA rating of it. I wonder if your rails are collapsing too far for the amp to bias on and operate properly.


Grey,
You can already buy a Hafler with a tubed front end... just go visit Norm at Smart devices. He's a big Hafler fan.
Mark
 
Mark A. Gulbrandsen said:
KoolScooby,
Since no one else here sems to be of any help to you for what ever reason I'd be more than happy to troubleshoot it for you if you sent me just the board. I can use my own supply as its still in breadboard stage. Let me know if this would work out for you.

Or.... If you can supply me with a complete set of G, S ,D voltage readings on all the devices fomr one channel like I posted in a post previous to this I may also be able to help this way. I'd also like to know what type of tranny you are using and the VA rating of it. I wonder if your rails are collapsing too far for the amp to bias on and operate properly.


Grey,
You can already buy a Hafler with a tubed front end... just go visit Norm at Smart devices. He's a big Hafler fan.
Mark


Mark, Thanks very much for the offer ... that would be great, but i want to learn these troubleshooting steps on my own...

BDP told me he thinks my problem is my PS, and i believe it, because i'm just using a Variable DC PSU taht goes 0 to 25 and 1amp, so i'm pretty sure thats not adequate... he suggested me to work on a PS then the AX...

I'll try and get you a Set of GDS readings from my fets like you had earlier maybe tonight, if not tonight, then it might be as late as next thursday through next weekend, i'm going to a conference tomorrow thru wednesday... Thanks much mark and everyone else
 
Actually its not necessary to get the readings yet. That PSU is definately way inadaquate to run this amp as the rails run around 3 amps each if you set the bias for .5 volts across the source resistors. I would highly reccomend what I'm using when I breadboard stuff and that is a 680 VA toroid powered through a 20 amp variac. I've got my primaries in series on the toroid so its 220 volts and then the secondary is about right when the variac is at 95% of full AC voltage. If I hook the primaries as 120 volts than the variac is about midway and the primary current is about 6.8 amps for a stereo pair!, when wired for 220 volts the AC current is half that.

Mark
 
...i mentioned the Power Supply in post #198...

i was trying my hardest to help! :)

Mark is on the right track though... we need some good readings
at certain points... the first being verifying a good PSU. It may be
that your test PSU just isn't up to the task.

maybe if this had a thread of it's own it would get more looks...
but extracting all the posts may be a bit of a pain.

but it looks like you're getting competant help now! ;)
 
Mark A. Gulbrandsen said:
Actually its not necessary to get the readings yet. That PSU is definately way inadaquate to run this amp as the rails run around 3 amps each if you set the bias for .5 volts across the source resistors. I would highly reccomend what I'm using when I breadboard stuff and that is a 680 VA toroid powered through a 20 amp variac. I've got my primaries in series on the toroid so its 220 volts and then the secondary is about right when the variac is at 95% of full AC voltage. If I hook the primaries as 120 volts than the variac is about midway and the primary current is about 6.8 amps for a stereo pair!, when wired for 220 volts the AC current is half that.

Mark


Indeed.. my PSU is definately not up to par... BDP has emailed me suggesting some specs as well... how much did your 680VA Toroid go for ? and the 20 amp variac?

thanks much everyone :)