Since I have a bunch of parts sitting around and I love my Corda, I figure I may as well build another one from Jan's schematic. Some quick questions though:

1. What are good replacements for the 10 pF Polystyrol Film caps in the schematic. Jan has the 10 pF Polystyrol Film caps connected between the output and feedback pin of the LM6171. I can't find any such caps on digikey, the only 10pF caps I find are 500V Mica Film or some ceramic discs. Would either of these work?

2. What about the Electrolytic Caps for the power supply. Digikey has about 10 different Electrolytic series from Panasonic alone. Which series is best? Smallest? How do you find these things out?

3. I don't want to do a real power supply like the Corda has, so I'll be using a regulated 24V wall-wart or a +-12V linear power supply. Is is worth the extra bucks and effort to use a linear power supply? Wouldn't I need to get an enclosure for it, and jacks/plugs too? Wouldn't I have to deal with the mains power with a linear power supply, and have the risk of zapping myself?

4. What kind of current rating would the wall-wart or linear power supply need? Is 400mA sufficient for the 4 LM6171 chips driving both low and high impedance headphones?

Thanks!

--
Scott

2. I use the Panasonic Z series. Again, not too critical.

3. Make sure the wall-wart or other supply is NOT a switching supply - often found with laptops, for example. Switching supplies are very noisy. Anything that is not a switching supply is a linear supply (regulated or not).

4. I think that 200mA is the minimum supply current. Ask Jan to confirm.

As Chu mentioned the choice of the electrolytic capacitors is not critical, provided you also bypass them with some foil capacitors. (the impedance of electrolytic capacitors tends to rise at high frequencies)

Chu is also right with the powersupply. Never use a switching device!! A regulated supply is strongly recommended. +/- 12 Volts will do fine. Note that wall-warts normally have +12 Volts only which can be converted into a +/- 6 Volts dual line (see the project pages). Although 6 Volts is a little bit low the amp will work fine.

At idle at +/- 15 Volts the amp draws approximately 50 mA of current. If you want a portable the 1.5 kOhm resistors that drive the outputstages of the opamps into class-A can be removed and the current consumption is reduced below 10 mA.

Have fun,

Jan

--
Scott

Imagine you had a very wealthy client who said: "I want you to build me a premium Corda with the very best components available." How would you build it? What premium components come to your mind with that kind of request?

For example, you might replace the Alps with a DACT which is much more transparent and much more expensive. Actually, a DACT's price almost equals the price of the Corda. However, since price is no object, your wealthy client would approve immediately.

Really, I very curious as to how you would handle this kind of order. So thanks in advance for your answer!!!

BTW, the Sennheiser HD600 I bought from you are fantastic! Thanks again for including me in the lucky group!

To be truthful, I never tried any tweakings with so-called premium components. With the CORDA I simply put in components of very decent quality within a reasonable price range.

In my opinion the concept of this amplifier is much more important than the exact components used. For instance certain audiophiles highly believe in extremely expensive electrolytic capacitors in the power supply. The advantage of these capacitors is a very low impedance at high frequencies and thereby damping of high frequency noise that enters the amp from the mains voltage.

However, In my opinion this noise should not even enter the amp at all and therefore the CORDA has an extensive power supply filtering. I use cheap electrolytic caps but the filtering and the bypassing with (cheap) foil-capacitors is a much more fundamental and effective approach than using expensive components.

A very similar approach can be found with the layout. Star-grounding and component-placement are important for optimal results. Very often it's much more effective to improve the "working-conditions" of the components than to use better components.

The concept of the CORDA headamp has been optimized for a specific price-tag. To make a better headamp for a reasonable price would require a different and more extensive concept. (Actually, I'm presently working on such a concept, but that's another story for the future). Replacing components might change the sound of the amp, but differences will not be very large and not necessarily better.

Cheers and enjoy your HD600,

Jan

Good luck with your new project. It sounds interesting!!!

Hey Jan (or anyone), a question: in your notes about the Corda HA1 design, you mentioned that the op-amps could be substituted with several others, including the BurrBrown OPA627. This is the same one as in the Max, and I've heard several amps that use it, and love the sound. Okay, don't laugh, I'm a newbie when it comes to circuit/amp designs...Could I simply replace the opamps in your design with the OPA627 and expect good results, or did you design the amp to work well specifically with the LM opamps you use....basically what I'm trying to ask is: can I just drop the BurrBrown opamps right in, or would I have to change anything else?

Thanks,
Brian

PS - Jan, I heard the Corda in action at the Bay Area Headwize/fi meeting last weekend, man you did a great job! You definately have the best crossfeed out there, or at least out of what I've heard. Also, much respect for putting out the schematics and work notes for the amp you are selling! If I had a little more money and less of an obsession with tweaking/DIY stuff, I'd definately buy your HA1 right now!

[Edited by Brian on 08-30-2001 at 11:17 PM.]

***
Could I simply replace the opamps in your design with the OPA627 and expect good results, or did you design the amp to work well specifically with the LM opamps you use....basically what I'm trying to ask is: can I just drop the BurrBrown opamps right in, or would I have to change anything else?
***

In principle you can use any opamp. However, note that the endstages of the opamps in my design are biased with a 10 mA current. This is not a problem with the LM6171 who was a maximum current output of 150 mA. However, most opamps have lower outputs and the biasing current should be reduced.

Have fun.

The non-identified parts are zero Ohm bridges.

In my originial design I had a filter between mains and trafo to dampen noise on the power line. This filter has been replaced in the design of the Corda by the LC-filtering after the rectification. I guess Chu Moy and I forgot to remove the filter from the schematics.

Have fun