Extensive Corda mods
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KurtW

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I have been working for several months now on modifications to the Meier Audio Corda HA-1 headphone amp. This is already an excellent headphone amp for the price, but since I had built mine from a kit and Jan Meier has published articles about the design including schematics, it seemed like a likely candidate for upgrades. My initial changes of substituting the integrated circuits produced a nice improvement but it was not as great as I expected. After extensive modifications and comparisons to a stock Corda was well as a Sugden HeadMaster, I now feel satisfied with the modifications.

I will list the modifications roughly in the order of sonic impact, from most to least.

1. Integrated circuit swap. The National LM6171 has great specs but compared to the higher end HeadRoom amps that use the Burr Brown OPA627, the Corda seemed a little harsher and not as open and airy. It should be noted that this obviously isn’t the only difference between the Corda and HeadRoom amps, but the OPA627, although quite expensive at over $15 each, has a good reputation for very good sound. I swapped out the National chips for the OPA627 in the first stage, taking account that although they are close to pin compatible, the PCB uses some No Connect pins on the 6171 for other routing, so you have to lift up pins 1 and 5 on the 627. This swap did take the sound in the direction that I wanted it to go. The 627 isn’t well suited for driving headphones directly though, but the BB BUF634 is an excellent solution for swapping in the output stage, and its cheaper too. Again, pin 1 needs to be lifted. The BUF634 does have a larger output offset voltage, but on the two units I’ve done so far it was under 60mV which doesn’t seem to cause any problems. So far we have a solution that requires no soldering, but the 627s run a bit hot. The Corda design has a resistor from the output to V+ to put the op amp into Class A operation. This provides 10mA of bias current at idle which is way more than is needed. It’s a bit too much for the 627 and causes the chip to run hotter than I’d like to see, as well as being a fairly big load for a chip rated at 45mA output. Changing the 1.5k resistor to 5k still provides plenty of bias current to keep it in class A and solves the heat problem. Because you loose the gain in the output stage, the overall gain of the 627/637 combo is a bit lower but not a problem for many headphones. The gain can be adjusted up, and is addressed below.

2. Improved Current Source. A better although much more complicated solution to the class A bias in the first stage is to use a current source. With the stock Corda a 2 volt output signal at this stage you give a bias change of 13%. I used a cascode FET current source which regulates the current to well within 1% over this same range. There has been quite a bit posted about these already, so I’ll just comment that I used a 2N5486 and a 2N5484 with a 100 ohm isolation resistor, and selected for 3mA current. The other change is that this is connect to V- instead of V+, taking advantage of the faster N channel transistors in the output stage of the IC. I was quite surprised what a difference this made. The sound seem to open up and become much more lively.

3. Output stage improvements. Particularly with low impedance headphones, the current requred to keep the output stage biased in class A is too high to use the above current sources. However, the BUF634 has a wide band mode that sets the internal bias currents much higher. This can be implemented by connecting pin 1, which is already hanging in the air, to V-. The original design has a 1.5k ohm resistor going to V-, so once this is removed you can use this V- connection for pin 1 of the 634. The penalty you pay for this is heat. I added 16 pin dip heat sinks to the top of the 634s which weren’t needed until the wide band mode was activated. I made a couple of other changes to beef up the bass impact when using low impedance headphones. One was to double up on the 634 buffers, which just entails soldering another buffer on top of the first one. The second change was to change the 47 ohm output resistor to 10 ohms. Each of these changes seem to beef up the bass slam. I was a bit worried about stability but after checking it out with a scope, using a bunch of different headphones connected through a 6 foot extension cable made from Kimber wire, I couldn’t detect any ringing or stability problems.

4. OPA637 vs OPA627. The OPA637 is a higher performance version of the 627, with over double the slew rate and much faster settling time, but according to the datasheet it is not stable at gains less than 5. The Corda has the gain of first stage running around 3, but it can be set to a gain of 5 by changing the 4.7k resistor to a 2.5k for each channel. I used a 2.74k resistor in mine resulting in a gain of 4.6 and it works fine. With this value the gain is similar to the stock Corda, compensating for the gain lost in the output stage when switching to the BUF634. If you look at the graph in the data sheet you can see a gain of 1 is definitely trouble but anywhere near 5 should be okay. The 637’s improvement is like everything that is good about the 627 is just kicked up another notch. They are priced the same, so unless you already have the 627s or don’t want to make the gain change, this is the way to go.

5. Schottky Diodes. The Corda has inductors in the power supply to filter out the switching noise caused by the silicon diode bridge, but I feel the change to schottky diodes caused an improvement in imaging. There are many schottky diode options, but be sure to use one in the DO-41 format as the space is tight and the pcb holes aren’t too big. I used the fairly common NTE585.

6. Capacitor changes. The Corda uses Wima polyester box capacitors in the crossfeed circuit (the red ones). Wima makes polypropylene caps (which are green) with the same pin spacing and are only slightly bigger otherwise. It seems to be well established that polypropylene caps sound better than polyester caps, so I swapped out all of these. Although I didn’t spend a lot of time trying to verify the sonic benefits of this, I’ll have to say they weren’t real obvious. Nevertheless, I feel better about having the polypropylene caps in there. I also added a pair of 1.0uF polyester caps for the buffer decoupling on the back side of the board. I had wanted to use an even larger value but there was not enough room to do so and still keep the leads short. My Corda (an early model) came with 2200uF electrolytic caps in the power supply, but this was later changed to 3300uF. I couldn’t hear or measure any difference in performance. I didn’t try any "designer label" caps anywhere, as there is not a lot of room unless you start spreading things out, and I’m not sold on the idea that they provide a benefit worth the cost.

7. Input RCA jacks. When I was evaluating many changes with my Corda I eventually broke the internal ground connection on the stock RCA jacks. This probably wouldn’t happen under normal use with the cover on, but lots of plugging and unplugging with the cover off put a lot of stress on these and they finally gave. I found that the Cardas GRFA jacks fit nicely in the same holes, you only need to trim the insulating washers slightly. These are very nice jacks and are more fitting to the sound of the modified Corda. If you want to use another jack, be aware that the Cardas were a tight fit, and the jack needs to be insulated from the metal case.

8. The last change I made was to change the red LED to a clear lens blue one, and to adjust the resistor a bit since the blue one was really bright. There were so many changes inside, I felt one small change to the outside would be appropriate. I now have dubbed my unit "Corda Blue".

So, how does it sound? In direct comparisons to a stock Corda, using identical interconnects and power cords and using a Linn Ikemi as a source with various headphones, there is a very noticeable difference. The modified Corda is much more refined sounding, more spacious, more dynamic and better defined from top to bottom. It also seems to have much better rhythm and pace. I also compared it to the Sugden HeadMaster, which is the best solid state headphone amp I’ve heard. The Sugden still has a transparency and three dimensional airiness that even the modified Corda can’t quite match. Of course it’s bigger, much more expensive and doesn’t have crossfeed.

I did make some measurements of the two Cordas, and didn’t find a difference within my modest measurement limits for distortion and noise. Channel matching on either unit was excellent, as was frequency response.

The total parts cost for all of these mods is around $100. I took around 6 hours to implement them. This seems like a lot, but I look my time to be careful and to implement them in a fairly neat and compact way. One issue I had with a factory assembled unit was that the crossfeed switch was held to the front panel by a stator nut. Jan was kind enough to tell me that Farnell sells a stator-nut key (p/n 321-357) for this, but Farnell didn’t answer my email and I don’t know any source in the US to obtain this. If anyone does, please let me know.

Acknowledgements:
I would first like to thank Jan Meier for making the Corda such a solid design at a great price, publishing the design and being very responsive to everyone’s questions. None of these mods are my original ideas, so I’d like to thank everyone who has shared their knowledge about these things on the internet. Special thanks go to Kelly who loaned me his factory built Corda for comparisons, Chris Own whom I exchanged many emails with while we were both trying mods, and Head-Fi-wiser ppl whom I’ve never had any correspondence with but whom I have learned a lot from reading his many posts.
 
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Nick Dangerous

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The big question is: Will Jan offer a stripped down "Corda basic" kit? Once these tweaks become somewhat standardized, I predict a lot of people will want their Cordas modified.

I don't own a Corda yet, but this is something that greatly interests me (along with adding a DACT stepped attenuator).
 
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Jan Meier

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Dear Kurt,

Thanks for your great description of the various mods. It sure makes make me rethink a number of design-decisions made in the past :)

I'm presently working on a model above the HA-1 and your comments certainly are very helpful. I just wanted to add one remark:

You noted a bias change of 13% using resistors. However, electrically spoken the use of a resistor is the same as having a constant bias current and a lowering of the load impedance. The LM6171 is able to drive loads of 100 Ohm. I don't feel it will sound substantially different when a ohmic load of 1500 Ohm is added in parallel to the load presented by the crossfeed filter (input stage) or by the headphone (output stage). To connect the output stage to -V theoretically indeed is better than to connect it to +V. I simply wasn't aware of this yet at the time I designed the HA-1.

People might wonder whether all the modifications made by Kurt will also be implemented in the standard HA-1. However, please note that, as Kurt indicated, the parts cost increase considerably. The CORDA HA-1 has not been designed to give you the best sound but to provide an optimal price-performance ratio.

Enjoy your music :)

Jan
 
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Nick Dangerous

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That was quick... gotta love the internet!

I agree. The existing Corda HA-1 is already an excellent product. Rather than upgrade the components and charge more, a HA-2 would be a better solution for those of us determined to spend more.

BTW, Kelly is freaking over his modded Corda!
 
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braillediver

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Has anyone used the National Semi LM7171 it's supposed to be a faster LM6171? If so what needs to be modified if anything?
 
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kelly

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Thanks for your comments, Jan. I'm thankful that you're a good sport about this. In its base configuration, I've had many kind words to say about the Corda HA-1 because I do think it acheives an excellent price/performance ratio. I'm glad to hear you're of this persuasion also and aren't offended that we thought there may have been ways to make it better.

Nick, I think freaking may be a strong word... or maybe not.

I'm eager to have people other than me listen to it before I state some hyperboles and lose the thin shred of credibility I have around here. Let's just say... it's very good.

If I had not mentioned this before, Kurt is an incredibly cool person and one of the people on HeadFi who I've come to admire and respect. This experience has been quite educational for me, killer amp aside. Kurt also seems to have pretty cool taste in jazz--something that needs further exploring.
 
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KurtW

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I would like to clarify a couple things about my comments on the current source. It does provide a much more constant bias current to the output transistors of the IC, but as Jan states, it doesn't change the load impedance much. My comparisons of current source vs load resistor were done using values so that each were giving around 3mA of current. It was also done with a OPA627/637 and not a LM6171, and it was comparing the resistor going to V+ (as the first stage of the Corda is wired) vs the current source going to V-. Others have claimed that a current source to V- sounds better than a resistor to V-, but I haven't not done this experiment myself.
 
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Nezer

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Well... I did some of these mods.

I replaced the first stage LM6171s with OPA627s. I also removed the biasing resistors and replaced it with 4.7ks (as close to 5k that I had). Instead of popping the old one out and popping in the new ones I left the old socket blank and tied pins 6 and 4 together through the 4.7k thus tieing the resistor to V- instead of V+.

While I was in there I replaced two of the 1000uF 35V Cerafines that I installed about a month ago with 2200uF 16V Cerafines. Naturally these were the ones *after* the regulators. I figure the ones before the regulators are seeing somewhere around 20 or so volts so 16V there isn't really a good match unless one likes the sound of botique caps popping. I for one do not. :wink:

I may replace the second stage at a later date with the buffers listed above.

As for my initial impressions I find the 627 on the first stage to be a much more synergistic match with the K501s. I'm not so sure I'd get this much benefit from the buffer. The sound has warmed-up a bit (which is a good thing with the K501s IMHO) as well as tightened-up. Though I haven't tested throughly it appears everything is more detailed.
 
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kelly

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Nezer,
I really begin to wonder if the opa627's strength is bass and its weakness is virtually everything else on the top side. I know you can get flat response test tones with both opamps but many amps sound a lot better than the HeadRoom's in the higher frequencies and the amps sound more extended. Meanwhile, no solid state I've heard can match the 2001 Max in bass performance. Makes you wonder.
 
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Nezer

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Altough I'm not about to pop-in 637s and go through the hassle of adjusting the gain up (my source is so hot the Corda is too almost too loud at 10:00 with the 501s and the Koss need 240 Ohms before 9:30 gets too loud), I think the 637 might have better extension on the top end. I'm quite happy with the Corda and I'm thinking it is going to be moved to the office as soon as I build-up a Meta42.

There is a lot of thought that the AD8610s are miles ahead of the 627/637s and it mates well with an EL2002 so I will probably end-up building the Meta42 with those and keeping the 627 in the Corda. I suppose I could replace the regulators with 7[89]12s (if they exist) in the Corda and drop-in the 8610s on an adapter and see for myself, but that's just too much trouble.

What is clear is the 627 is light-years ahead of the stock 6171!

If the 627 suffers from the high-end extensions as you have stated (and I agree that it really appears to) this is porbably why I find it so pleasing when mated with the 501s which , *IMHO*, is harsh on the top-end with the 6171 driver.

I so can't wait for that damned Cardas cable to show-up so I can get my freaking HD600s back in service!
 
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kelly

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Nezer
I wonder how many people have REALLY heard the OPA637. Kurt's didn't really come alive until he also added the FET.

Also, the gain was adjusted on mine such that it's not a problem at all. In fact, the volume now rides a little higher for normal listening volume which is preferable to me since I listen at low volumes and the pot performs better toward the middle.

I would agree though--from what I've heard I'd suspect the K501 is better suited to the opa627. But then again, I don't like the K501 much.
 
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Nezer

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I put-in some 637s into the Corda a while back and I was impressed. I might pop them in but I really don't want to muck with the gain. Maybe once I adjust to the 627 I'll throw the 637s in and just see...

You know, if you have only heard 501s through the Corda it might do you well to listen to it again with the 627 in-place. The 627 does round-out the 501s nicely and when you add-in a cable swap on the 501s and an Analouger's bass-boost it really balances-out well. Though for that kind of money (for the Analoguer) you could get HD600s.

I think this BB-modded Corda will work out nicely at the office. Now I need three more rigs to round out all my listening areas (upstairs, downstairs, office and portable). I'm thinking variations of the Meta42 for the others. I have a Porta Corda for the portable setup now. I wonder how the LM7171s would do in it?
 
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kelly

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I first heard the K501 via the 2001 HeadRoom Max at the Dallas meet. I would gladly take the stock Corda HA-1 and HD600 over the K501/Max combo. The modded Corda with either my ER-4S or HD600 is another thing entirely and it's quite nice.
 
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Nezer

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In that case you have heard the 501s at thier best.
 
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Okay guys I'm gonna try KurtW's Corda Blue mods so help me out if you can ya?

1) There are four LM6171 op-amps in the Corda HA-1 (MkI). Two of them near the volume pot. And two behind the headphone jacks. The ones near the volume pot are where the OPA627 or 637 go right? (with pin 1 and 5 lifted of course). And the ones behind the headphone jacks are where the buf634 go. Am I correct in this?

2) Also, when stacking the buffers ie 2 or 4 per channel, should the lifted pin 1 be soldered together so that the pins 1 on the stacked buffers can be connected to V-? Or does only one of the buffers need to be connected (say the bottom one).

Thanking you in advance.
 
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