[Audio-Me]

tangent, tell us how it sounds with the HD600!

 

[tangent]

Quote:

A quick question, will the LM6172 work well in this board and be stable and all?

Good question. I'd intended to try it for the heck of it, and composing this reply has helped me to think the problem through more carefully. Thanks.

We put in a place for a pair of bypass capacitors for difficult chips like the 6172. We even went to some trouble to make the path from the bypass caps to the opamp's power pins very short. (About 4mm!)

But in my brief experience with this chip, that isn't enough to make the LM6172 happy. I didn't try soldering the chip into place instead of socketing it, so perhaps that will help. In addition to the bypass cap, what I found necessary was adding a small (10pF) film cap in parallel with the feedback resistor. (Not the one that goes to ground, the other one.) This rolls off the high-frequency response of the chip, which helps keep it stable.

We had a place for such a cap in the META42 for a while, but we took it out at the last minute for a few reasons:

1) It was taking valuable board space, but it was only there for the LM6172, which is a cranky, cantankerous chip. Better alternatives exist, especially for buffered amplifiers like the META42. In an amp like the Corda, where the opamps are directly driving the load, the 100 mA current drive capability is a nice feature, but it's not helpful for us.

2) ppl said to, and heaven knows ppl's always right

I think there's still hope that the LM6172 will work anyway. If you configure the amp for multiloop, the inner loop sets the opamp for a very high gain, like 100. If you tune that gain right, you can intentionally limit the bandwidth of the opamp, which effectively rolls off the HF response of the chip. This may have the same beneficial effect as adding the feedback capacitor without the bad phase shift effects added by a capacitor.

(For those interested: To tune the chip's bandwidth, you find the chip's "gain/bandwidth product" in the datasheet and use that to find a gain that gives you enough bandwidth for audio plus a decent safety margin. 100 kHz, say. The LM6172's GBP is 100 MHz. GBP=A*B where A is amplification in dB and B is bandwidth in Hz. To limit the bandwidth of this chip to 100 kHz, you need a gain of 1000, which is probably too high to achieve. I'll have to play with this to decide if I can get enough bandwidth limiting to stabilize the chip.)

Thanks again for the prompt to try this. The results should be interesting.

EDIT: I just remembered that if a feedback capacitor turns out to be necessary, all is not lost. You can take advantage of the multiloopability of the board to add a capacitor where one of the multiloop resistors would normally go. You configure the amp for global feedback and then make use of the now-unused inner feedback loop to place the capacitor from the op-amp's output to the -IN. (R6) If you leave the output buffers out (which is reasonable with the LM6172), you've basically made a CMoy w/LM6172 and a studly power supply.

 

[tangent]

Quote:

tell us how it sounds with the HD600

Can't do that, unless I take the amp to the Headroom road show. I may in fact do that. They're coming to Denver, which is their closest approach to my house -- 6 hours away by car.

In the following week, while I'm waiting for the second set of prototypes, I'll be testing with:

Senn HD-580+Clou Red
AKG K401
Grado SR-60
Koss KSC-35

I have others I may test with, but those will be the main ones. Oh, and of course I'll use my favorite torture test phones: the HD-570. Sure they're not the best sounding phones in the world, but they sure do throw opamp circuits into a tizzy in a hurry. Perfect.

 

[eric343]

Well, IMHO the 580s are 95% of the 600s, so they're more than close enough...

 

[Audio-Me]

I also have the HD580 and Clou Red, so tell me please!

 

[jerikl]

oh man! VERY interested in hearing how the META42 sounds with the Senn, AKG and Koss phones! Are you, by chance, going to compare the sound to your Creek and Corda amps?

 

[tangent]

Quote:

VERY interested in hearing how the META42 sounds with the Senn, AKG and Koss phones! Are you, by chance, going to compare the sound to your Creek and Corda amps?

I may do some comparisons to satisfy my own curiousity, but I'm reticent to do a public comparative review.

Although I've now made at least a dozen DIY amps, I still haven't bothered to do any comparisons to the commercial amps. Maybe that lack of enthusiasm will continue, maybe I'll decide to do some tests. I've been thinking more about this, but nothing's happened yet.

Even if I do the reviews, I might choose not to publish them. Who would put much credence in the reviews, when it's clear that I'm biased? I'm not going to lie, but I may be subconsciously influenced to overstate the amp's quality. Or, maybe I'll go the opposite direction -- second guess my findings so much that the review is blasé to the point of being uninformative.

What I want is for other people to review these things -- I know what I think about the amp, but what do others think about what I have helped to bring forth? That's more interesting to me, and I think it will make for reviews more interesting to others as well.

 

[eric343]

Tangent: I agree on the review issue; I feel much the same with posting my comments on it. I'm quite frankly rather worried that when Audio&Me gets the amp I'm doing for him, he won't find it at all to his liking, especially since I've noticed my ears hear music a bit differently than others (read my reviews of the Team Head Six Pack and compare it to the others for an example)...

However, regardless of whether or not Audio&Me likes it, it will be a relatively independent review, which is by all accounts a good thing to have.

 

[Audio-Me]

Quote:

Originally posted by eric343 I'm quite frankly rather worried that when Audio&Me gets the amp I'm doing for him, he won't find it at all to his liking...

person,

. I'm quite optimistic, as you should be as well! Say, eric and tangent, if you don't mind, could you two please pm me your full thoughts on all aspects about the meta42? Oh and eric, I expect thorough testing of mine and brief review prior to shipping to make sure nothing drastic needs to be changed.

Don't worry, I won't burn you if I don't like it, I'll just sell it to somebody that might.

 

[ppl]

Tangent:
Thanks for your kind words regarding the capacitor around the Opamp. I try to post only the Things i have verrified so as not to leed any one down the wrong path. The capacitor around the opamp to reduce the open-loop bandwidth of the opamp is called a phase leed capacitor and that is what it dose it reduces the Phaseshift of the opamp by trading band-width for Phase margin. If an Amp is unstable without the cap and the cap makes the circuit stable this comes under my clasification of a Band-Aid fix rather than curing the sickness. Like you stated there are alot of stable wide-bandwidth opamps that will be stable in this circuit. I for one have no paitence for hard to use circuit components and that includes opamps that require the measures above to be stable. any unity gain stable opamp should be plenty stable when operated at higher gains or it IMHO is not an ideal device for the circuit. The phase leed cap also throws away bandwidth and slew rate that the Chip maker designed in and you paid for. To quote Jim Williams~ "A slower opamp gives Compensation for free"

What the Phase leed cap dose to the sound is again IMHO horrible. it strips any and all life out of the Music this is most noticable on headphones. the music becoms Dull! boring! and lacks the Air it should have it for some strange reason it evean effects the Bass. This is for small capacitors less than 10 pF. Now the effect is not somthing instantly notiable but somthing that is noteced as more time is spent listening to the Amp. These test were used on circuits that were stable without the cap so as to remove any factors that might arise from high frequency Peaking. this is also quite Noticeable

 

[slindeman]

I can just solder the cap on the bottom of the board anyway so it is no big deal. I love the sound of the LM6172 w/band-aid and that's all that matters to me.

It's also nice that it is so inexpensive and can drive headphones directly.

 

[morsel]

The 6172 puts out 100mA which is nice if you are using the opamp as your output stage. Since the META42 does not need a high current output opamp for the voltage gain stage due to the Elantec buffers in the output stage, it is a waste to not use a more refined, well-behaved FET input opamp. There are many to choose from. If you are set on the 6172, perhaps a PortaCorda or a DIY version of the Corda would be more to your taste?

 

[ppl]

Morsel is right i think that if your hart is content on the national Opamp this is not the Project for you. BTW yes bottom of the board will work but Directly soldered to the feedback resistor is better.

 

[tangent]

I just tried the LM6172: as hoped, multilooping it fixes its stability problem without having to add a cap to the feedback path! I didn't even populate the high-speed bypass cap position. I do have some 1uF film caps 1 inch from each power pin, though. Traditional bypass caps are like 0.01 to 0.1 uF and are closer to the chip. These 1uF caps are more like high-speed reservoir caps, but they're doing the job.

Unfortunately, I got about 1.2V of DC offset out of the thing. I had to place a Cmoy amp downstream to test the META42 w/ LM6172 amp. (The CMoy has input caps, which strip the DC out of the audio signal.) This is no doubt due to imbalanced input offset currents, though I thought the resistors I used in the META42 were configured pretty well for this already. I guess the combination of high gain and the high input offset currents of the LM6172 means you have to be extra-picky when setting these resistor values.

So yes, it works, but no, the META42 doesn't make the LM6172 any easier to use. They're still PITA opamps.

 

[slindeman]

Morsel and ppl,

Say you are planning to build an inexpensive portable amp with LM6172, panasonic pot, and the pactec dual 9V battery portable case. Along comes a pre-fabbed board that could possibly do everything you want so you don't have to use generic protoboard -- why shouldn't you be a little excited about using that board for a different purpose than it was designed?

Especially since I could now pcb-mount the potentiometer (big plus with the panasonic ones), have a great power supply, etc. I don't quite see what the problem is. Why would I be better off building a project I don't need or want, like the Corda (I own one), or porta-corda (single 9V). I already plan to also build a 42 amp with the board anyway.

Tangent,

Thanks for going the extra mile and trying these things out for me. You didn't really have to do that, and I do appreciate it a lot.