M³ Project Announcement - Page 11

Jim Williams ate it already.

AOS:
I see your point completely, and was in that same position myself for years.
In the interest of this, perhaps the front mounted pot would be best.

Or as Earwax mentioned, maybe those wishing to mount on the front could run leads to the pot. I'm flexible, so whatever works the best for the most people.

Or, you could allow for two places on the board to mount the pot/attenuator - front and rear. So people not willing to get shafted (heh) would install it in front - at the cost of slightly reduced performance due to longer leads between RCA input and the pot - and the others could mount it on the rear for full benefits. If you have those two places aligned, you can still use the same panel layout for both - meaning the shaft extension would still come through the same hole in the front panel. As you'd need to have a clear path for that shaft, you will already have to dedicate some space in that area to be (tall) parts free anyway so the cost of this shouldn't be too high. Or even better, use Alps Blue for front position, and attenuator for the rear - it also makes sense, as if you're willing to pay for attenuator you want to get all that you can out of it, and if you're fine with Alps Blue then the front position will also be satisfactory for you. So you could provide attenuator + shaft kit or people can just get Alps Blue (from Tangent or wherever). That way they can even upgrade later when they get into money wihtout having to redesign and rewire their panels.

Excellent response...

Thank you amb for the response... I am not an engineer, but understand the design benefits.

I'll offer a hearty thank you for this project in advance, and look forward to building one.

Will heat considerations eliminate wood as a case material?

Metal cases provide shielding that helps cut down on induced noise from external sources. Sheldon's SDS Labs MOSFET amp example has the circuit board completely exposed to the elements, which is good for heat dissipation but is susceptible to noise pick-up. If you like the looks of wood, you can always add wooden endcaps or something like that to the sides of a metal case. Whatever case material you use for this amp, it will be important to have ventilation to allow the heat to escape.

-Ti

This sounds like the most sensible layout to me. I like the sound of this idea.

M³ prototype amp preliminary test results

I spent the past couple of days testing the M³ prototype amp.

I measured the frequency response, slew rate, total harmonic distortion, intermodulation distortion and signal to noise ratio. You'll see a test summary as well as pretty graphs here:

http://www.amb.org/ti/audio/mmm/2ch_proto_20040825.html

Note that this prototype is running in two channel configuration (no ground channel), and the power supply is not as optimal as I would like it. Moreover, the prototype is built on an open breadboard, so the specs are not as good as it could be.

Despite all that, the results are really quite remarkable, as you can see. The THD and IMD are both very close to the residual distortion of the M-Audio Transit running in loopback mode, so the amp is in fact adding very little distortion of its own to the signal.

There is no measurable difference in distortion, regardless of whether the amp's outputs are loaded with 33 ohms or 330 ohms (or no load at all). This speaks volumes about the robustness of the MOSFET output stage.

I have also spent some time listening to this amp. It's already excellent.

I will be doing more testing of the circuit hopefully in three-channel mode soon, and I will also test the circuit for optimum bias settings, and tune the values of some components to see if I could achieve even better results.

Enjoy...

-Ti

Case options

OK, I know this is cheesy, but... I really Vodoochile's use of an old SCSI enclosure, and wonder would something like this work:

http://cgi.ebay.com/ws/eBayISAPI.dll...118903363&rd=1

The enclosure has switched power, ventilation, and a fan (is the fan a sonic inteference problem?). besides, most are built like tanks, and shielded well. This one is 10.25" x 9.75" x 2.0", but would that be tall enough?


Actually, being the SUN head, I'd probably go this way:

http://cgi.ebay.com/ws/eBayISAPI.dll...118212331&rd=1

If nothing else, these are cheap, indestructible, plentiful, and recycled.

And, I have this stepped attenuator that I was going to use in the SDS (to replace the Alps), but will use it here. In this case, it looks like no board pin placements will work for me, whether placed in front or back. It's Vishay, and relatively cheap (~$45).

http://home.swbell.net/pabbi/step_attenuator2.jpg

pabbi1:
You can use computer drive enclosures, but I would strongly suggest ditching the built-in swtching mode power supply (and fan). They are fine for computer equipment, but give very noisy DC output and is not suitable for a high quality audio amplifier, not to mention having the wrong voltages (typically +5V and +12V are available, sometimes also a -12 but at very low current). Tangent's TEPS would be a good choice for this amp. I would put the power supply board in a separate enclosure for maximum isolation from the amplifier. If you want it in the same enclosure, make the power transformer and any AC lines as far away from the amplifier board as possible.

The stepped attenuator you have is definitely a "run wires to it" affair .

-Ti

Sorry, I just meant the power recepticle... and not the P/s itself. The main question was the fan. I just always seem to find yet another SUN exclosure (with an Ultra 2 or Ultra10 attached to it).


While talking P/s, Tangent, how far a step down is the Velleman K1823, and conversely, how much better would the Wellbourne PS1 be? I only ask since there are several Velleman dsitributors within 10 minutes from me. And, if we're going for the better solution, is the $150 Wellbourne price worth the difference over the ~$80 TEPS?

In this amp design, will the P/s be a super critical consideration, or is separating the P/s from amp board more critical? Or both? :-)

Will someone take pity on us lesser mortals here and add further instructions (later) about configuration and connecting the TEPS to the M³?

Sorry to contunually define the bottom feeding segmant of the thread,

Al

If you could power the fan from a separate power circuit than the amp, then it would help keep the amp's power clean. Having a fan motor on the same power rail as the amp would be a bad idea, as the fan produces back-EMF pulses and pollutes the power.

I never liked the idea of a fan in home audio gear, though. Even the quietest fan makes noise and draws dust into the enclosure. I prefer passive (heat sink) cooling as much as possible.

I'll let Tangent answer your question about TEPS vs. other options (hope he's reading this thread).

The M³ has good PSRR, so it's not too critical on the power supply but the PSU should have high enough output current to handle the ~300mA to 500mA continuous draw and not burn itself up. Good clean power is "motherhood and apple pie" for amps in general, so I would advocate getting the best power supply you could afford.

We are going the route of not putting the PSU on the same board as the amp to give DIYers more choices, and the ability to put the PSU in a separate case for best isolation.

Absolutely. That's what this community is all about. As I said in an earlier post, the power supply for this amp is an area that is still subject to change. Assuming it doesn't change drastically (i.e., we retain the TLE2426 rail splitter design), then the STEPS (single supply version of the TEPS) will be a good match. It would have to use the 24VCT version of the Amveco toroidal transformer instead of the 30VCT variant to prevent the regulator IC from overheating. At this time, though, I think that level of detail is a bit premature, and will be forthcoming when we're ready to release the official version of the M³.

-Ti

any news here?I am eager finished version:-)

Whew, what an amazing thread! I just blundered in here looking for a volume pot for a different project. Now I am hooked, and I need an M³ (I wouldn't mind an M3 as well, but that is a bigger project).

I can only offer comments from a thermal and mechanical perspective. On the topic of heat sinking, is it feasible to mount the devices to the chassis? Morsel alluded to this in the first post, but I didn't see much discussion on the subject. External heat sinking will provide far superior thermal rejection than internal sinking with secondary convection to the chassis (the btus have to sneak out somewhere). If not, I would be tempted to provide for enhanced circulation. If the case of choice already has vents, nevermind. Their server seems to be down just now, so I haven't seen it yet.

I am all for the back mounted attenuator (with driveshaft). I have a few commercial examples of this approach (preamps mostly). They work great and are very durable. Somehow the forward bearing gives a more solid feel to the control as well.

For a great example of both approaches peek inside an AKG K1000 (SAC) amp.


gerG

The resistors were added to the PPA to shorten the high frequency current loop to keep it confined within the smalest area around the op amp power rails. Do not confuse power supply decoupling as we are doing hear with the simple Bypassing used in PPA version 1.0. with just simple Bypassing the actual PCB foils can become inductors and actualy hearting stability by creating a high impedence L/c Tank circuit tuned well up into the RF range and probaly lower in frequency than the unity gain bandwidth of your chosen op amp thus allowing the entire circuit to oscilate at the Tuned L/c frequency. Simply adding this small resistor kills the "q" Factor of any possible L/c tank and maintains stability. Granted quality RF grade capacitors of at least 0.1uF is min and bigger would be evean better. The Wima and BC component polypropylene types have less than 10 nH of inductence and this is added to any PCB foil's inductence.

True the JFET isolation technique is more effective up to the point that Criss takes over however this is still farly high in frequency. My Battery operated reference trravaler that the PPA style isolation technique was first used on employed all three methods of isolation the two used on PPA v 1.1 in addition to a Capacitence multiplier.