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Wessel Lemmer 1st March 2012 12:58 AM

My first Headphone amp need expert advice
 
Hi everyone,

This is my first real audio project, Ive done a lot of research and the design below is what ive come up with. With that in mind a big thanks to tangent for his website, ive leart so much and it has been a lot of fun.

Getting to the point it is an op amp based discrete buffered amp, the outputs are two sets of 2N2222A & 2N2907A transistors controlled by an AD8066 and the v ground is another 2 sets of 2N2222A & 2N2907A controlled by one side of a TL072. the other side of the TL072 is used to set the V ground in the AD8066 (Signals)

amp | Flickr - Photo Sharing!

EDIT: i thougt adding a scematic might help, its 4 am. i need to go to bed.
EDIT: corrected some mistakes on schematic
http://www.flickr.com/photos/wessell...n/photostream/

4x 2N2222 - one per output channel 2 for Vground output
4x 2n2907 - one per output channel 2 for Vground output
4x 20k 1% - 2 in feedback loops to 8066 2 to vground signal
2x 10K 1% - 2 to vground signal from input
2x 100K 1% - for 50 50 split to TL072
2x 2uf poly - input caps
1x AD8066 - to control output stage
1x TL072 - to control V grounds
2x 270uf 25v - stabalise the v ground out
8x 1ohm 1% - share load from transistors i guess

Ive built the amp on a breadboard for now and there are a couple of issues i havent been able to sort out and cannot explain.

1. There is a minor issue of crosstalk, its very soft and not distorted so it doesnt really bother me but i cant explain why. ive tried adding another TL072 to seperate the signal V ground so each side of the 8066 has its own undistorted reference but this had the same effect. why am i getting crostalk?

2. Most of my concern lies with the way ive implemented the buffering circuits. i have to have 2 caps in serie over the V ground (out) else i get a crunchy distortion(to the right of board). two small 0.1 uf caps have little bass response and sort of flubbers. where as two larger caps say 270uf or even 2200 uf removes the crunchy distortion and it sounds good. but i start to get pops and clicks at higher volumes. from what i can tell i should have no need for caps here as the circuit should be able to handle the load by itself.

3. another clue to the above problem maybe, when i place a cap over the V+ and V- the pop and click type distortion is worse and at lower volumes?

I am driving a set of ie6(18ohm), HD205's and HD212's the 212's are really soft and distorts easily. im guessing they are 60+ ohm

Any feedback, comments, suggestions, explanations etc would really be appreciated.

jcx 1st March 2012 08:03 AM

a couple of Q drawn upside down?

free Ltspice would let you sim your circuit - easier to use included LT op amp models but others can be added

Wessel Lemmer 1st March 2012 08:32 AM

Thanks jcx, yes in the schematic the PNP's where upside down, drew that at 3 in the morning so i guess i wasnt paying attention :)

scem | Flickr - Photo Sharing!

ill go check out LTspice

sgrossklass 1st March 2012 10:00 PM

Let's hope the reversed IN+ and IN- inputs on U1a are just another result of late night schematic drawing, else you'd be in trouble...

I'm not a fan of basic class B buffer stages, they are pretty lousy in terms of high-freq distortion (real bad crossover distortion at typical headphone levels). At least add a 100..220 ohm resistor from OP out to output if nothing else.

sgrossklass 1st March 2012 11:31 PM

After some more simulation, I can only conclude that even 1 or 2 milliamps of quiescent current for the buffer are well worth it. In a class B buffer using BC337/327s driving 32 ohms at 10 kHz, I couldn't really get a lot below 1% THD at mid-high levels (a few hundred mV), with an ugly spectrum at that. Using simple diode (well, B-E junction) bias and 10k resistors plus a bypass cap drops that to little more than 0.01%, and at halved current it still is 0.03%.

Using a class B buffer stage for a headphone amp really isn't worth it; you might as well be using an opamp "barefoot" with better results. Even AB buffers at sub-1mA quiescent current levels are likely to be of limited use, since that already is what opamps are running their output stages at.

Wessel Lemmer 3rd March 2012 12:44 AM

So ive been playing with LTspice program and barely got any sleep. Please dont laugh but ive come to the realisation that im trying to build a mini class B amp and that i have one as a v ground buffer too. Running some tests on the program showed strong spikes in vground until i added a filter cap on the feedback loop of the vgrounds opamp. I did that on my test board and tada click and pop distortions at high volume went away. also i could now add caps over +9 and -9 and no popclicks. really is helping me a great deal to see what is going on, and yet still so much to learn. ive also seen this method of buffering is kind of OK for use as ground but for signal its not going to help at all.

i have ordered 3 LME49600 buffers wich i intend to use in a carryaround amp. But hey, im enjoy playing with the discrete stuff so i was wondering how to do a class A buffer if at all?

Thanks sgrossklass, it seems you went to quite some effort to help. honestly i didnt follow you too well, but im getting there.

sgrossklass 4th March 2012 02:26 PM

I've been doing a lot of simulation lately anyway, so it was little more than a refresh on buffer stages.

A class A buffer (able to transition into AB if higher currents are required) can be as simple as in the Eaton amp. (I'd add a 10-22 bootstrap cap across the LED though, and I'd rather use 4k7s instead of 2k2s, besides you could leave the top one out for opamp class A biasing.) Aside from current draw and heatsinking requirements, they are near ideal for giving a good, well-implemented opamp the ability to effortlessly drive headphones. The currents required easily allow the use of TO-126 medium power devices (typically BD139/140 or the like) without much need to slow down the opamp, if any, and the typically large emitter degeneration resistors (3.3+, often 10..15 ohms) give good temperature stability.

I suggest you study NwAvGuy's blog, Douglas Self's various articles (and books) and Samuel Groner's opamp measurements.

Here's a little schematic to get you started. I think the BD models were from a BJT library in the LTspice group's files section that I merged into cmp\standard.bjt, as was the UniversalOpamp2 symbol. (You can also obtain BD139/140 models from Bob Cordell's website as an extra to his power amp book.)

Wessel Lemmer 8th March 2012 04:38 PM

Thanks for tolerating another "my first amp" thread. I can now see i have made quite a few mistakes and those just happen to be where i have made assumptions, funny how that is.

I got my LME49600's YAY, built a little test circuit with 3 of them, one for buffering v-ground and the other as output stages... and for the first time in my life i was smiling :P

ill continue with my discrete endeavor, and someday soon i could have a face off :)

here is a list of some more assumptions that Ive made, please correct me if needed.

true or false? Its better to have a faster transistor i chose 2n2222 and 2n2907 because they are 300MHz and 200MHz and i chose ug06d diodes for the same reason as they are "ultra fast recovery"

the 2n2222 and 2n2907 can handle a max of 0.6 amps, is this not enough? i thought one could get 200ma easily. "why use TO-126 medium power devices"

true false? input caps should be as large as possible and shouldn't be electrolytic. Then resistors that bring the incoming signal up should be as small as possible, this is to minimize a high pass filter effect. i got 10uf and 200 ohms, is there some flaw here?

is the AD8066 a good op-amp? I like the sound of it but thats compared to a TL072. will something more expensive be much better.

is there any benefit in using separate single channel op amps

is there any benefit to have separate op amps to create V-Ground references for each channel, in other words having 2 op amps per channel?

Maybe i should stop here for now :)

Wessel Lemmer 9th March 2012 12:20 AM

Quote:

Originally Posted by Wessel Lemmer (Post 2938276)

true or false? Its better to have a faster transistor i chose 2n2222 and 2n2907 because they are 300MHz and 200MHz

OK, Im guessing this is actually Bandwith. The question then is pretty much the same. is it better to have more?

counter culture 9th March 2012 01:52 AM

Quote:

Originally Posted by Wessel Lemmer (Post 2938760)
OK, Im guessing this is actually Bandwith. The question then is pretty much the same. is it better to have more?

The problem with excess bandwidth is that it increases the likelihood of HF instability (oscillation) and you can't always detect it audibly, although it can be subtly degrading the sound quality. Sometimes it's obvious, the active devices self-destruct due to overheating. A headphone amp realistically only needs a slew rate of ~1V/uS.

Without entering into a mathematical treatment, input caps should be dimensioned according to the input (shunt) resistance. The low frequency cutoff occurs at the frequency where the capacitive reactance equals the resistance.

Read the nwavguys blog with reference to virtual grounds. It's a topology that's better avoided.

I've left a few terms hanging here, look up the things you don't understand.


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