GC Preamp Suggestions

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#522
tobias_svensk said:
What? have you found another problem or are you correcting me? 🙂

I don't see any problems with those two connectors just maybee a little overkill because it's possible to power the opa and buf separate?
Click to expand...

Oh, connectors?
So you made provision for independent PSU for each chip?

For a moment I though those were regulators and you messed with the pinout.:clown:

Forget it, I'm too snubberized today.
 
#524
Do you have any updates for me Russ?

Have you tried soldering a capacitor to the +/- pins of the opamp, for added decoupling.

Other than this suggestion I'm a little lost now, as I dont have much experince of PCB design.

It could also be some of the traces are too close together and
are creating a trace capacitance, but i think thats clutching at straws, and is highly unlikley.
 
#525
Hi noober. I am waiting for some L7815 L7915 to get here today to build a new +/- 15V PS for these circuits.

The good news is that the 627s are still alive, I tested them in another circuit. I am not sure of the buf634s as I did not want to desolder them.

I highly suspect my supply is too hot (over 18V). As I cannot find a single short to ground or any other trace. Also, because both boards are effected in exactly the same way.

I hope to get the regulators today, monday at the latest.

Any new tweaks to the PCB design N00ber?
 
#526
Very weird...
Did you mount the BUF634s on the correct direction?

This may look like a stupid question, but nothing that has been pointed for you to check out seems to be the problem for me.

The circuit must work fine at +/-18V. Mine does.
Use some heatsinks on the BUF634s and that's it.
 
#527
Russ White said:
Hi noober. I am waiting for some L7815 L7915 to get here today to build a new +/- 15V PS for these circuits.

The good news is that the 627s are still alive, I tested them in another circuit. I am not sure of the buf634s as I did not want to desolder them.

I highly suspect my supply is too hot (over 18V). As I cannot find a single short to ground or any other trace. Also, because both boards are effected in exactly the same way.

I hope to get the regulators today, monday at the latest.

Any new tweaks to the PCB design N00ber?
Click to expand...


I was holding back untill we can see if my board has far bigger problems than I could have imagined.

I will start tweaking, and adding the suggestions that you posted earlier, in the hope that it is something simple that could be easily resolved.

I'll get back to work
Good luck with your PSU Russ

n00beR
 
#528
carlosfm said:
Very weird...
Did you mount the BUF634s on the correct direction?

This may look like a stupid question, but nothing that has been pointed for you to check out seems to be the problem for me.

The circuit must work fine at +/-18V. Mine does.
Use some heatsinks on the BUF634s and that's it.
Click to expand...




photo

The above link is a photo of one of the complete boards. The bUF634 looks right doesn't it?


I Am considering two theories right now...

1) That there is a loose/faulty power supply wire to one of the 4 V+/- connections.
2) That the audio gods are quite angry with me and nothing in holy hades can help me out. Somebody must sacrifice a chicken. Quick. 😀

Thanks,
Russ
 
#529
Russ White said:
photo

The above link is a photo of one of the complete boards. The bUF634 looks right doesn't it?
Click to expand...

Yes, it does.
You just need a heatsink on the BUF634, but that's not the problem here...

Russ White said:
1) That there is a loose/faulty power supply wire to one of the 4 V+/- connections.
Click to expand...

Well, why don't you check the voltage directly at each chip's PSU pin?
I always avoid connectors, anyway. There's nothing like solder.😉

Russ White said:
2) That the audio gods are quite angry with me and nothing in holy hades can help me out. Somebody must sacrifice a chicken. Quick. 😀
Click to expand...

Gear is always right, and the parts are non-believers.😀
The problem can't be solved with a pray. 😎
 
#532
carlosfm said:
Do you have a resistor to ground on the input?
Or at least a volume pot?
Do you have a resistor to ground on the output?
Click to expand...


I do have a volume pot, and a 120K resistor to ground to set input inpedance.

I have no resistor to ground at output, There are pads for it, should I add it? If so what value would be good?

Another note. I tested resistance between the V+ pin and the output pin on the buf634 and I get something a little over 22K. I expected infinity. That with the buffer on the board.
 
#537
Russ,

Are you using capacitor for opamp out to opamp -in and at the same time using diode to bias, as seen in Noober pcb design?

I thought we should try either one of the method.

In one of your post, you use RF shunt cap between opamp out and inverting input = 22uf ceramic. Is this value too high? Ceramic a good choice?

I am going to try out this pre, hope to have feedback from you.
 
#538
munster68 said:
Russ,

Are you using capacitor for opamp out to opamp -in and at the same time using diode to bias, as seen in Noober pcb design?

I thought we should try either one of the method.

In one of your post, you use RF shunt cap between opamp out and inverting input = 22uf ceramic. Is this value too high? Ceramic a good choice?

I am going to try out this pre, hope to have feedback from you.
Click to expand...

If I said 22uf I meant 22pf. 22uf is certainly way too high.

According to my calculations for the RF shunt cap (from opamp out to opamp inv. in) 22pf should work well. Also this is what is recommending in many of the sources I have read online. I just wouldn't go much over 100pf

I have tried the circuit with and without that cap. It works fine either way so long as you do not have a lot of RF interference.

Oddly, when I ommit that cap I get some nasty oscillation with my portable gear (FLAC player and CDP) but not with my normal gear (AC). So I will be leaving it in. I can not detect any detrimental effects.

I think my earlier problems stemmed from my PS being too hot. I think the higher volatge fried my 627s.

Either that, or I had a loose PS connection(there are 2 V+ and 2 V- per channel) and that some how fried that chips.

In any case I fried 3 627s. 🙂 Luckily I have 5 left.

I drill a new hole for ground at the center(roughly) of the board intead of using the provided ones. I have Joined signal ground to power ground at the PS and by bridging the traces on the board. Both work. I will probably just leave the bridged the traces on the board as it did not introduce any noise and it two less wires.

One thing I noticed. The 277s sound nearly as good as the 627s!!! 🙂
 
#539
I love this thing. 🙂

N00ber your board design was just fine! 🙂 :up:

The problems appear now to have been caused by my own inexperience/stupidity.

This is one great sounding pre now. And quite flexible.

Chips tested so far:

opa627
opa637
opa227
opa228
opa277
ad8610
LT1115

All of the above sound excellent!!! Though some better than others. What a difference the buffer makes!

Here is a picture of how it looks this AM. I will be adding RCA jacks later and a rotary switch for input selection, for now in and out are 1/8 stereo jacks.

BTW, this an awesome headphone amp!!!

No hum at all, no hiss from any source (even my notoriously hissy FLAC/MP3 player). Very good RF rejection! :up:

Thanks so much to everyone, especially Brian and N00ber who graciously arranged for me to have the boards.

Now lets see rev. 2.0 😀

An externally hosted image should be here but it was not working when we last tested it.
 
#540
Hi Russ

Glad to hear that the design is working good afterall.
Also the fact that it's a compact thing looks very atractive.
Now if there would be somebody kind enough to have some boards manufactured! I'm the first in line to buy a couple🙂

Regards
 
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