[Spacehead]

Output transistors oscillate... I can hear loud buzzing that varies with bias setting

2SA1011/2SC2344 are 100 Mhz

Should I increase emitter resistors to 4.7 ohm?

What about 10 ohm output resistor?

I still have 600 mV DC offset , I am testing with output capacitors. The offset comes probably from oscillation , it (buzzing) is there with and without source connected.

 

[FallenAngel]

Need some info:
1) Opamps
2) Value of emitter resistors

As the buffer is within the feedback loop of the opamp, you don't need output caps.

 

[Spacehead]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Need some info: 1) Opamps 2) Value of emitter resistors As the buffer is within the feedback loop of the opamp, you don't need output caps.

ADA4898-1 on left and right
AD8597 on ground

2SA1011/2SC2344 output transistors

Emitter resistors 2.2 ohm

Q4 J310 50 mA IDSS

R4 4.32K
R3 1K
R2 1M
R1 4.32K
R6 470K
R5 3.32K

C2 0.1 uF + 0.01 uF
C1 5x470 uF
C6 10 pF + 2 nF

other parts default

high res pics:
http://i39.tinypic.com/2gtx4js.jpg
http://i42.tinypic.com/10nazkk.jpg

 

[tangent]

Quote:

Originally Posted by Spacehead /img/forum/go_quote.gif I bought 2SA1011/2SC2344 transistors... I installed them correctly, they are base collector emitter

As far as I can tell, it looks exactly opposite the pin ordering of the recommended transistors.

If I'm right, you can try flipping them around. Maybe they weren't damaged by this mistreatment.

Are they getting hot?

 

[Spacehead]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif As far as I can tell, it looks exactly opposite the pin ordering of the recommended transistors. If I'm right, you can try flipping them around. Maybe they weren't damaged by this mistreatment. Are they getting hot?

I think they are right way, I installed them checking that emitter resistor goes with emitter
sometimes it is pretty hard to understand datasheet markings, but I found the dots on 2SA1011

They didn't get hot, only warm,
adjusting pot so that there was 44 mV voltage drop across R24 made the buzzing go away intermittently

I have read this thread and learned that what I could try is R11 to 100 ohm

And probably I didn't understand C2 right, I have 0.1 uF and 0.01 uF parallel as you can see, they were supposed to be like
first 0.01 uF , then 0.1 uF, then again 0.01 uF and so on?
So that between channels is 0.1 uF and between buffers in each channel is 0.01 uF?

 

[FallenAngel]

Transistors look to be installed correctly and if you get adjustment across output emitter resistors, they are "working" properly. 44mV is really not that much, that's only 20mA. I'd feel safe with those up to 30mA and more current should make them a little more stable.

Personally, I would go with 3R3 or 4R7 resistors (and have before) if you have any instability in the buffer.

Reducing R11 may help, but I wouldn't really expect it to. I'd try other things first.

Your implementation of C2 should not cause any problems.

 

[Spacehead]

R11 -> 100 ohm
3.3 ohm R24 and R34 didn't change DC offset at all.

It is 610 mV on right and 480 mV on left

I measure this against output ground.

How do I short the inputs exactly?

 

[FallenAngel]

Uhm... isn't ADA4898-1 bipolar? You need to use a FET-input opamp in the PPAv2.

You said there is only DC offset on left/right, not ground? Is ground using a bipolar or fet input opamp?

Measure DC offset against input ground to make things easier.

 

[Spacehead]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Uhm... isn't ADA4898-1 bipolar? You need to use a FET-input opamp in the PPAv2. You said there is only DC offset on left/right, not ground? Is ground using a bipolar or fet input opamp? Measure DC offset against input ground to make things easier.

there is not mentioned in the datasheet that ADA4898-1 would be indeed a bipolar op amp
but it's
input bias current: −0.1 μA
is high

There is AD8597 on ground which I think is JFET input because it's
input bias current: 40 nA
which would be normal for a JFET input op amp


Out of curiousity I installed OPA552 to left and right
it's input bias current: 20 pA

ok and it works. headphones connected dc offset is -2.5 mV and -1.5 mV

I now have 60 mV across 3.3 ohm R24 and R34

I need to solder ADA4627-1 to dip8 socket. I hope I don't fail it because I don't like OPA552 sound at all.

I could now restore R11 to 1K , would it reduce distortion?
I also could remove external 10 ohm resistors
And I will add the class a biasing circuit

thanks for help

 

[Spacehead]

ADA4898-1 works now when I changed R2 from 1M -> 50K
I don't have volume pot so I need to think only if RC filter goes too high , but 2.2 uF caps make it possible to lower R2 to 20K before losing any bass.
DC Offset is 20 mV and 25 mV
I could make it lower but I want to listen a little before tweaking any more

Ohms law tells me that 3.3 ohm and 60 mV would generate only 18 mA bias, is that right?


I am pretty proud of my skills
just a while ago (1-2 months) I built my first cmoy

But since then this has been all tweaking and building

 

[FallenAngel]

Keep R2 at 470K and install a 50K volume pot. In parallel, these would be around 50K anyway. For the input cap, keep it at 0.1uF.

 

[Spacehead]

I added the class a biasing circuit. I immediately set high bias, 3.7 mA for all op amps. Got DC offset to 15 mV and 19 mV , which is low enough for now.

Biasing op amps to class A really makes a difference in sound quality. It clears and opens up the sound, everything is more defined and detailed.

My PPA v2 runs now hot but its performance is very excellent. It was stable for RMAA test. Here is the testing setup:


And the results. Next to it are loopbacks with -6 dB and -1 dB , line in adjusted accordingly, Sound Blaster X-FI with OPA1642 out and LM4562 in


0.0015% THD

I guess that test should be run at -1 dB level from Sound Blaster X-FI output , so that amplifier would be driven into full volume
and the line in recording should be reduced accordingly?


Here are the pictures of completed amplifier, except volume pot and bass boost




Gain is 5 , 4.3K R4 and R3 1K

Ask if you need to know about any specific part. They are pretty generic but accurate. Matching has been done with care.

 

[jezz]

I've noticed that my PPAv2 has some hissing with some of my lower impedance (60-70ohm) headphones. I've got it configured with 3 OPA627's, a gain of 3, and 3.3ohm resistors in R24 and R34 at a bias of 20mA. Other than that, it's basically stock.

I've run through the some of the things to try when debugging hissing, however haven't found anything that works. The switch from 2.2ohm to 3.3ohm resistors on the output buffers didn't change the hissing, nor particularly did changing the gain from 11 to 6 to 3. Interestingly, the hissing does change with volume; the hissing at zero volume did not change when I changed the gain. I have installed C5. As of now, DC offset is very low at 5mV per channel.

Is there anything else I can do to try to reduce the hissing? It's not audible when I use my higher impedance headphones :-/.

 

[TzeYang]

lower the resistor between the buffer and the output of the opamp to about 100 ohms.

 

[jezz]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif lower the resistor between the buffer and the output of the opamp to about 100 ohms.

Thanks TzeYang! I don't have any 100ohm resistors on me. I tried 110ohm however the hum is still present. Given that I'm at least within the correct order of magnitude, I'm assuming that R11 isn't my issue.

Edit: As I had the case open, I pushed my buffer biases up to 100mV (~33mA per channel) just to match the bias I had before I changed R24/34. No change in hissing or DC offset.