[rjkdivin]

Quote:

Originally Posted by amb The 41M ohm reading is not real meaningful except to indicate that there is no short circuit. You're essentially reading whatever small residual charge there is in the C7 capacitors interacting with your meter. This is a bit interesting. Does it measure 11.3K ohm both ways (i.e., does it read the same if you reverse the meter leads)? The only resistance shunted across the opamp supply rails is the TLE2426's internal resistor divider, although the 11.3K ohm seem too low to be from that. Are all your C5+ and C5- capacitors installed in the proper polarity? Maybe one or more of these are backwards. 0.788V is in fact about right for the drop across each of the Q5s, but then if you're only measuring 15VDC across the opamp rails, then the MOSFET rails must be at 15V + (0.788V * 2) = ~16.6V. Add the 0.8V drop across D1 and the voltage across V+ and V- should be at most ~17.4V, far short of the 24V. Something is not adding up... Check the output voltage of the STEPS again while everything is powered up and connected. Also, what do you get if you measure the voltage across the drain leads of the N and P channel MOSFET? You could simply put the test probe on the metal tabs of each pair of the MOSFETs for this measurement.

Okay, now my novice electrical skills are showing through (hey I'm a structural engineer

). I think I solved the mystery....I believe I was measuring between pins 4 and 6 on the opamps instead of pins 4 and 7....just not counting them correctly. When I measure the correct pins, I get 21.7vdc on all three....all is well.

But to answer your other questions......the 11.3kohm measurement on the opamp sockets was the same regardless of lead orientation....but of course it was apparently pins 4-6 instead of 4-7. All C5+ and C5- orientations are correct. Voltage on the MOSFET rails is 23.27vdc. Output, under load from the STEPS is 24.1vdc currently (no pun).

Sorry AMB to have wasted your time!

 

[amb]

rjkdivin, cool, I'm glad it's just a false alarm rather than an actual problem. All is well, then. Time to listen to some tunes!

 

[pabbi1]

I have posted my long threatened review of the m³ here.

Feel free to dogpile, embellish or counterpost as you see fit. I think the broader amp community needs some feedback on what Ti (amb) and Morsel have done. I for one, am digging the heck outta my OPA637 amp.

al

 

[morsel]

Thanks for your kind and gracious words. Glad you are enjoying your amp. Feel free to threaten us anytime.

 

[amb]

Hi pabbi1, that's a great review. It makes all the work we did seem worthwhile

.

 

[Akku]

A question 'bout part selection.
Is it important that the bass boost pot has an overall impedence of 50K?
I understand it is used just as a variable resitor so the resistence "seen" from the input circuit is variable too anyway.
Can I go for a cheaper pot? I don't plan to use the bass boost very much and I plan to put a switch to defeat it.
P.S.
For the volume I ordered a 10K ladder att.
P.P.S.
Sorry for my english. I hope I made my questions clear

 

[Magsy]

I recently tried some AD843's and they are quite something. They may ultimately have a little less resolving power than the 637 but they make virtually any music listenable.

I've been a little displeased that since building the M³ a lot more music sounded rough or overly harsh at the top. The AD843's have really cured that, bought some more bass and done something (good I think) to the soundstage, I can't put my finger on it exactly but its more spacious for sure. I would still like a little more vocal presence but thay maybe my cans or source, I'm opamp rolling in the 0404 to try and cure it...

There was one problem I had with them though, they need voltage! I had clipping quite easily at 23v, maybe I'm deaf as I was well past 12 o clock but it wasn't there with the 637's (or it was masked better)

I moved up to 28-29v and that has fixed it, no more clipping

Well worth a shot if you feel like a fiddle.

Also I've tried 80-140ma of bias and I cannot really hear a difference (just curious). Heat production goes up quickly though, on 140ma and 28v I couldn't touch the sinks for more than 15secs.

 

[pabbi1]

At 32v, with the 637s, I don't think there was any noticeable difference between 80ma of bias and 105ma.

 

[morsel]

Akku: The value of the bass boost pot should be in harmony with the rest of the bass boost components. 50K provides 0 to 15dB of bass boost with the default component values. Use the bass boost calculator at one of the M³ sites to play around with those values. A cheaper pot should be OK. There is no need for a bass boost defeat switch when using a pot as it is self-defeating at 0 Ohms.

 

[amb]

Quote:

Originally Posted by Akku (bass boost) Can I go for a cheaper pot?

You should use one that has good tracking between channels so that you won't have problems with uneven boost.

 

[Akku]

Thank you AMB and Morsel. I am looking forward to listen to your work (still collecting parts)

 

[WSE819]

Well I guess if I take credit for my success its only fare I live up to my screw ups. I got stuck on onother project this weekend, but late last night (2am) I had an epiphany, I had checked all components at least 2-3 times but never checked mosfets.Sure enough droped a mirror in and I had switched inner pair. Resoldered them in correctly everthing checked out fine , peformed initial setup no more than .5 millavolt offset. Pluged in a set of headphones an enjoyed some music. Good thing I am at work today and can sleep (hope our network on the west coast doesn't go down). Thanks for your help, Once cased I will do a comparison with my PPA w Phil's dyno buffers and M3 and keep the winner. My first impressions are the percussion is much more dynamic , I was concerned bass boost may color sound but it is much more subtle. Thanks for a great project, I am up to 5 headphone amps I am begining to give them rabbit names.

 

[amb]

Quote:

Originally Posted by WSE819 Sure enough droped a mirror in and I had switched inner pair. Resoldered them in correctly everthing checked out fine

I am sure you won't be the only one with this mistake, the markings on the MOSFETs are hard to read.

Quote:

Once cased I will do a comparison with my PPA w Phil's dyno buffers and M3.

That should be an interesting comparo.

Quote:

I am up to 5 headphone amps I am begining to give them rabbit names.

LOL

 

[Super-Gonzo]

In the section entitled "Notes about R3 and R4" on AMB's mmm site:
http://www.amb.org/audio/mmm/parts.html

AMB mentions that the bass boost pot can be used as a variable gain control. Would it be possible to drop the volume pot altogether, and just use variable gain to control the volume of this amp?

See PS Audio's variable gain amps as an example:
http://www.psaudio.com/products/gcc_...amplifiers.asp

If technically viable, wouldn't this result in better SNR performance, throughout the volume range?

 

[amb]

Super-Gonzo, the variable-gain method as described on my site does not allow you to completely mute the audio. Even if you set R4 = 0 (which I do not recommend, because many opamps don't work well without some resistance in the feedback loop) the minimum setting will still give you a gain of 1. For many headphones and sources, that is actually fairly loud without a volume control at the input.

As for S/N ratio, the noise floor of the M³ is so low, it shouldn't be an issue at all.