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Old 1st August 2011, 03:11 PM   #2111
Eric is offline Eric  United States
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Hi Anthony,

A good starting place might be to read through my Aleph-X web page that is linked in my signature. It walks you through all of the basics of output power, power supply voltage and bias levels, and heatsinks.

Eric
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Old 2nd August 2011, 05:10 AM   #2112
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A good, cheap heatsink is a junkyard motorcycle engine. The heads an be used for "small" heatsinks, the engine block is an excellent large heatsink. Alternatively, look for any aircooled car or airplane engine. VV bug comes to mind. Plenty of thermal mass and lots of fins. Should be very cheap, too. Late Bob Pease had such a heatsink in a photo somewhere, that's what inspired me to look for one. If that's not enough, then simply get an aluminum head from any watercooled car, and use water cooling
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Old 10th October 2011, 05:32 PM   #2113
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Default A bit late but....

I finally got to fire up my aleph-X today.

It is the "standard" 45w version running 15v rails with 10k macmillan resistors.

After warming up & adjustment of V1, V2, V3 the relative offset measures just a few millivolts, same for absolute DC offset from either speaker terminal to ground.

The problem is that the nominal 0.5 volts I started with across the source resistors is now scewed so that I have 4.6v across one and 5.4v across the other. Is this actually a problem (if so any advice gratefully received) or can I just ignore it?
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Old 10th October 2011, 09:18 PM   #2114
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Default Oops...

"4.6v across one and 5.4v across the other" - Sorry, those values are of course 10 x too big - should be 0.46v and 0.54v across the source resistors.

If I even out the voltages over the source resistors to 0.5v each then the relative offset increases to 0.054v - should I just leave it like that?

What are acceptable offset values?
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Old 10th October 2011, 10:56 PM   #2115
Zen Mod is offline Zen Mod  Serbia
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you're good with all values
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Old 11th October 2011, 12:42 PM   #2116
Eric is offline Eric  United States
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Hi Jason,

I agree, your numbers look good. Offset will vary as things warm up and keeping it within the few milivolts of zero that you have is fine. I have a number of adjustment details on my website that you can check out.

Eric
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Old 11th October 2011, 07:33 PM   #2117
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Default Playing music

Thank you for your prompt replies.

I have reset the voltage across the source resistors back to 0.5v, relative offset is running at 56mv.

I plugged in some speakers and as expected there is nothing but silence at the speaker cone (I've built this amplifier with a regulated power supply).

I used a cheap portable radio/cd player as a signal source for now and played a CD through the amplifier - all works as expected

I still need to check a few things out with a scope before i'm completely happy but things are looking good so far.

And Eric - Thankyou for your informative web site!

Just 3 more to test now.....

Last edited by Jason Hubbard; 11th October 2011 at 07:35 PM.
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Old 25th October 2011, 09:42 PM   #2118
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Default Clipping

I have connected the amplifier to a signal generator and have noticed a couple of problems;

With just over 3 volts input the output starts to clip - the bottom half of a sine wave is clipping first (but i'm probably swinging 26 volts pk-pk, just 4 volts shy of the rail voltages) is this about right or should it clip symetrically?

I don't have 3v coming out of my preamp - 0.5v is all i have, so i'm not going to be able to drive the amplifier properly without changing the gain (or introducing another gain stage somewhere). Which resistor values can be changed and what is the penalty for doing so (i'm not sure what can be changed without affecting the balance of things).
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Old 26th October 2011, 12:02 AM   #2119
Eric is offline Eric  United States
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Hi Jason,

The problem you are experiencing with input clipping is not really a "problem" per se. I have a short write up about this on my web page in the Power Bandwidth and Input Sensitivity sub-section of Output Testing. In order to get greater input sensitivity, try doubling R16 and R30. Doing this gave me 5dB more output for the same input level, but it comes at the cost of reducing the power bandwidth. I have a small hum at the speaker that I think is coming from my power supply, but I haven't spent the time to track it down. Increasing my input sensitivity increased the amount of hum I was experiencing at the speaker, so I think I ended up changing it back soon after. My speakers are not very sensitive, so this hasn't really been an issue for me.

I have also noticed my clipping does not occur quite symmetrically. One side clips a little bit before the other side, though not by much. Input levels before clipping occurs are also on my web page.

Hope this helps a little...

Eric
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Old 26th October 2011, 01:10 AM   #2120
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Default Newbie With Some Questions

Been into audio equipment for a long while (got cables older than some of the builders here, and am just considering DIY for the first time. A few weeks ago I heard a friend's Pass Labs Int-30A sounding really beautiful. I'd been thinking about saving up $1.5-3K for a used Spectral amp, but was pretty knocked out after researching the Pass Labs equipment to find that Nelson has so many designs available to DIYers willing to put in some time, thought, and effort. That seems like a really good bargain to me.

- I loved the sound of the Int-30A, so whatever DIY amp has the closest "family resemblance" to that is what I'd like to point toward. Am I correct in supposing the Aleph-X is the closest available DIY design to the Pass XA series? (I'm aware this isn't normally what one would consider a starter project, but I'm prepared to take things slowly and carefully, do lots of research, and ask what are hopefully very occasional, appropriate questions.)

- In order to keep costs and numbers of boxes and power cords down, I'd prefer to build a stereo design rather than two monoboxes. Speakers are a pair of Vandersteen 2Ces, early 90s vintage. Dimensions of the bay in a wooden cabinet in which a stereo amp would sit (open front, 2" high opening cut across the lower back for cable and power connections) are 22" wide, 23" deep, 8" high. Current amp is a late-80s PSE Studio IV, 100wpc into 8 ohms, nearly double that into 4 ohms. Do I have a prayer of building a stereo amp with reasonable power output for the Vandersteens that would fit into the available space and have room for sufficient heat dissipation? If such a stereo amp is possible, what sort of wpc output range should I look at?

Otherwise, guess I'll be looking at a couple of mono amps.

Thanks very much in advance for your thoughts and advice.
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