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Discrete folded cascode headamp

post #1 of 22
Thread Starter 
I just got my boards from GoldPhoenix in China. 11 boards for 129 USD incl shipment to Sweden. From gerbers to delivery in one week, that's fast. I also got very fast replies via e-mail. I've seen nicer boards, but they're OK.

It's intended for (semi)portable use in a Hammond 1455J1201 with 8 AAA batteries. When powered by wallwart the supply is regulated to 15 V.

I choose a folded casode input/diamond buffer output design because I like Analogs AD829, AD797, ADA4841 and ADA4899. LT1469 is also claimed to sound good. It simulates very well and seems stable in LTSpice. "All" good discrete DIY headphone amps seem to be symmetrical, so I thought I'd try something totally different, but it would have been safer to go for something like a Gilmore lite clone.

Folded cascode headphone amplifier eSnips Folder

You're welcome to comment on how to make it better. The VR (offset trimpot) will be something like 500R and RH/RI 68R instead for more precision, otherwise I think this will be it to start with. The 22 uF input capacitor is just to simulate the DAP. The amp is DC-coupled. Everything is star grounded. The two ground planes are used as shield.

I wonder if 2SK170GR is OK (diff pair), or should I get 2SK170BL instead? Can I parallel two GR's?

Is anyone willing to help me with measurements, oscilloscope and distortion analysis?

Anyone willing to take a risk and buy a board?
post #2 of 22
Quote:
Originally Posted by NelsonVandal View Post
Anyone willing to take a risk and buy a board?
Sure mate, bring it on Got a BOM available? Im ordering other stuff from Mouser really soon.
post #3 of 22
Thread Starter 
I just ordered the parts I need from ELFA, I'll make a BOM tonight.
post #4 of 22
Quote:
Originally Posted by NelsonVandal View Post
I just ordered the parts I need from ELFA, I'll make a BOM tonight.
I would like to keep my shirt and pants, so i'll avoid Elfa if I can
post #5 of 22
Thread Starter 
Quote:
Originally Posted by MrMajestic2 View Post
I would like to keep my shirt and pants, so i'll avoid Elfa if I can
I know what you mean. They charge 5 SEK plus VAT for one resistor, and they only stock those huge 0207 0,6W. I have ELFA just a couple of km's from where I live, so I go there by bike and avoid shipping charge. I've ordered from DigiKey and Newark before, but with shipping and taxes they're not cheap either. Besides, there are too many parts to choose from, and I always order more than I really want. I wish Farnell.se or RS sold to us privateers.
post #6 of 22
Quote:
Originally Posted by NelsonVandal View Post
I know what you mean. They charge 5 SEK plus VAT for one resistor, and they only stock those huge 0207 0,6W. I have ELFA just a couple of km's from where I live, so I go there by bike and avoid shipping charge. I've ordered from DigiKey and Newark before, but with shipping and taxes they're not cheap either. Besides, there are too many parts to choose from, and I always order more than I really want. I wish Farnell.se or RS sold to us privateers.
Yeah, I dont have that far to Elfa either, so I go there to pick up stuff I need right away. If you need something from Mouser just let me know, Im ordering really soon. Where do you live by the way?
post #7 of 22
I'd like 2 boards.
post #8 of 22
Thread Starter 
Quote:
Originally Posted by MrMajestic2 View Post
Yeah, I dont have that far to Elfa either, so I go there to pick up stuff I need right away. If you need something from Mouser just let me know, Im ordering really soon. Where do you live by the way?
Ok, that might come in handy. I live in Malmö.
post #9 of 22
Thread Starter 
Quote:
Originally Posted by FallenAngel View Post
I'd like 2 boards.
Are you sure? This is an intermediate risk business. I haven't built it yet, not even on perfboard, and I don't own an oscilloscope or distortion analyser. I can just use my ears and a DMM. I think I can get at least one channel to play by the end of this week. There'll be some work matching JFETs and bipolars.

I don't want to sell anything that doesn't work, but I'll let you know as soon as I'm sure everything seems to be OK.
post #10 of 22
Quote:
Originally Posted by NelsonVandal View Post
Ok, that might come in handy. I live in Malmö.
I live in Stockholm, but it would probably still be cheaper to send it in a padded envelope if its not too many components. Elfa has sick prices on a lot of stuff. Im ordering really really soon, just so you know
post #11 of 22
Trying to talk me out of it already?

I've seen your project before, it looks interesting. I don't have the equipment you're looking for but wouldn't mind building one of these. Calculated risk - best case scenario, I get my hands on a wonderful portable, worst case, I lose a little cash, seems like a good deal to me.

Now lets just convince somebody with a scope to build one as well
post #12 of 22
Thread Starter 
Here's a BOM, I hope I got it right. I changed the polyester cap to 0,33 uF. It simulates better and is small enough to fit below the board. This cap will be a "bug" in the ground channel, since I forgot it in the layout. There are two holes marked "8" for it. I also changed the 0R22 rail resistors to 0R47, but I don't really know why.

The resistors RG, RJ, RK, RL have to be tuned together with the JFETS forming the CCS's to give appropriate currents, and are only there as examples.

RM and RN should be choosen to give the desired bias to the output transistors. Since I use HD650, 20 mA will be enough for me. I don't think they should be biased to more than 30 mA or so since there's no room for heatsinks.

RE and RF will probably also have to be tuned depending on the Hfe of the cascode transistors.

To FallenAngel, sure I can send you the boards, but I'd suggest you wait a couple of days, so I'm sure they're OK. The price will be 12 USD each plus shipping. I don't know if you can find the right 3,5 mm connectors, if you like I can get them for you from ELFA. They're 1.75 USD each.

To Mr Majestic, do you want to wait a couple of days, or are you willing to take the risk?
LL
post #13 of 22
Quote:
Originally Posted by NelsonVandal View Post
To Mr Majestic, do you want to wait a couple of days, or are you willing to take the risk?
I can wait a few days. Gives you time to think about if you need anything from Mouser as well.
post #14 of 22
Oh snap, I didnt realise that we were talking SMD stuff here. Ah wait, I see that thru hole is ok too. Is there any part that is SMD only?
post #15 of 22
Thread Starter 
Quote:
Originally Posted by MrMajestic2 View Post
Oh snap, I didnt realise that we were talking SMD stuff here. Ah wait, I see that thru hole is ok too. Is there any part that is SMD only?
Yes, the Zobel was added late and there were no room for TH. There are also small ceramic capacitors to parallel the electrolytics on the bottom of the board. All are 0805 parts, so they're fairly small.

I'm not sure the Zobel is needed, nor the ceramics paralleling the electrolytics.

I don't think SMD is hard to solder. I think the density is more of a problem. I think it will take some bending and fiddling. Everything is very tight, except the SMD parts. If you don't want to solder SMD, I can solder them to the board.
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