[pinkfloyd4ever]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif [*]The gain of 10 in the default BOM seems much too high to me, but it may be largely due to the reversed pot wiring. For low impedance phones, change R4 for a lower gain.

Do I want a higher value for R4 to lower the gain? Is there a formula I can use to find the gain with different R values?
Quote:

Originally Posted by error401 /img/forum/go_quote.gif [*]The LED current with the recommended 10K resistor seemed much too low at my 18V power supply. I swapped it for a 3.32K and it's still quite dim with the LED I used.

the original design calls for a high brightness LED Circuit Explanation / Part Recommendations read the note all the way at the bottom. Not sure why he didn't just spec a lower value resistor and use a normal LED, but anyway, that's prob the reason it's so dim

 

[error401]

I made my own choice on the LED resistor without looking at the LED datasheet - that was the trouble

. I tend to calculate these myself anyway.

This amp is a standard Jung Multiloop feedback design. Tangent's calculator supports this and it's probably the easiest way to determine the correct value (use the 'i know what the resistors are' option).

 

[pinkfloyd4ever]

Got my board today! Ok I really need to order those parts...working on it right now

EDIT: oh and R4G should still be a 4.32K when changing R4R and R4L for lower gain, correct?

 

[error401]

Yes, don't change any of the values in the ground channel.

 

[pinkfloyd4ever]

(from the first post of original PPAS build thread) Quote:

Originally Posted by ATAT /img/forum/go_quote.gif Warning for those using LMH6321 - The LMH6321 is a great buffer, can output nearly .7A of current and its ridiculously fast.. one problem. Pin 2 is used as current control. Before you solder the LMH6321, cut off Pin #2 (or bend it upwards). Pin 2 is connected to output on the schematic (so that opamps can be used as buffers) but this will prevent sane operation of the LMH6321.

does this still apply?

 

[error401]

Quote:

Originally Posted by pinkfloyd4ever /img/forum/go_quote.gif (from the first post of original PPAS build thread) does this still apply?

No, it's mentioned in the thread somewhere I think. This board is specifically designed for LMH6321. Originally I had a resistor hanging off pin 2 for optional current limit but I decided it wasn't worthwhile, so the pin just floats.

 

[pinkfloyd4ever]

[size=small]WWAAAAAAAHHHHHHHH!!!!![/size]

So I've got everything ready to order except my opamps, and I don't wanna build the same config someone else has already built, so AD8066/AD825 is out. Based on this Quote:

Originally Posted by NelsonVandal /img/forum/go_quote.gif Regarding opamps, for audio use, AD8620 is just expensive. It's tonality is very neutral but it's grainy and harsh and not very exciting in any way. There are better and cheaper alternatives like LM4562, AD8599 and AD8066. All three of those should be used with a warmer sounding opamp in the ground channel like AD825, NE5534, LM6171, OPA134 etc. AD8599/AD825 is a very neutral (and fine sounding) combination IMO.

,I was looking at the AD8599, but its Ib is listed as 25nA typical/180nA max, which doesn't jive with error's recommendations: Quote:

Originally Posted by error401 /img/forum/go_quote.gif Opamps with FET inputs will usually be advertised as such, since it's a quick way to identify many of their performance characteristics. If you're unsure about an opamp, look in the datasheet for its Input Bias Current (Ib) - lower means lower DC offset - anything below 25nA or so is going to be fine in this amp. FET opamps should be in the pA range.

So I looked at the LME49720/LME49710, but its 55MHz, which is more than 50MHz (duh) Quote:

Originally Posted by error401 /img/forum/go_quote.gif Also the ground channel opamp must be unity gain stable. You should also avoid fast opamps (say, over 50MHz) as they're more likely to oscillate since they aren't bypassed too well in the PPAS.

plus FallenAngel said: Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif With a buffer driving the headphones, you can also very easily use the LME49720 and LME49710 (equivalents to LM4562). Someone has built the PPAS with these opamps (and buffers for that matter) and said it was very good (at least as good as any LM4562 amp has ever sounded).

that very last part in parentheses kinda bothers me

After reading tangent's notes on opamps, I don't want to go near LM6172/LM6171..not for now anyway.

AD8058/AD8057 is way too fast (300MHz) and has a Ib of 0.5uA(!)

HELP!!! I really wanna get these parts ordered so I can get the damn thing built...Maybe I'll just stick with the good ol OPA132 or OPA227 series, but we're pretty sure those work since the circuit is pretty much the same as the original PPAS, and they work in about anything (I think?)

 

[error401]

AD8599 is probably fine. I wasn't giving hard and fast rules, just ballpark figures. With this design even 0.5uA would be okay I think, but I can't recommend it if you don't want to deal with DC offset caused by input bias.

Same story with LME49720. I've worked with this opamp quite a bit and it's always been good to me, so I don't see why it wouldn't work well here.

There are tons of opamp choices out there, and most of them will probably work fine, but if you want a relatively foolproof build you'll want to stick close to the guidelines I gave.

Still haven't found a chance to do that parts placement guide. Let me know when you receive parts to light a fire under my ass

.

 

[pinkfloyd4ever]

wow error, lightning fast response as usual. I think I'll just go with LME49720/LME49710. I guess it's prob not terrible, and since the purpose of prototyping is more to make sure things work than to get the best possible sound. They're fairly cheap too. One thing, is LME49710 unity gain stable? Is that usually in the datasheet? I'm getting really tired and looking at datasheets is starting to give me a headache

 

[error401]

Sometimes I wish I didn't have e-mail notifications on

Quote:

Originally Posted by Datasheet summary The LME49720 is unity gain stable.

 

[pinkfloyd4ever]

durr wow I'm an idiot...thanks error. I'm gonna place these orders and hit the hay

 

[FallenAngel]

I'm sure that last part would bother most, but seriously, this is considering that most think the LM4556 is a godsend in terms of opamp quality, but not everybody has heard really high-end systems. Personally, I think the LME49720 that I used in a MINT (currently functioning as my main pre-amp) sounds quite nice, so why not use it.

 

[NelsonVandal]

To pinkfloyd4ever:

AD8599 is very unproblematic. You don't have to worry about bias currents. In reality the offset is very low, just a couple of mV's.

LM4562/LME49720 is more problematic than people think. It's very sensitive to RF and care has to be taken. In spite of having bipolar input transistors (just like AD8599), the offset is very low.

There's really no offset problems with AD8058 either, but I don't think it sounds as good as the two above. It's just not "musical" or involving.

LM6171 isn't as problematic as it's rumour, at least not used as ground channel amp with low resistor values around it. As L/R amp, you have to fight the high bias currents. It has a warm and kinda nice and sweet sound. If you follow Tangent's instructions, using those extremely high resistor values, you'll get an exeptionally noisy amp.

OPA2132, I wouldn't call it the good ol'. If you use it in both positions, you'll get a rater dark and congested sound. I think it can be used with good result in combination with those opamps that lack some mid like LM4562/LME49720, AD8599, AD8066 or AD8058.

 

[pinkfloyd4ever]

Fallen, what do you plan on using in your prototypes? Did you get your boards? Anyone heard anything from grenert or tcpoint lately?

 

[tcpoint]

I just got out the dremel and cut some traces. Got all the parts together. Still deciding on what opamp (narrowed it down to ths4031/ths4032, lme49710/LME49720 or ad8610/ad8620.) I'm leaning towards the riskier ths4031 just because i haven't done an amp with that chip and hear good things about it.