carlosfm said:
Why don't you put the chip on the bottom side and use single layer, as you said?
Don't bother too much about a ground plane, specially around the signal pins, it creates capacitance.
Also, if you rotate the bypass caps by 90º (side by side) you will get the electrolythics much closer.
Your use of V+ for heatsinking is correct. But even then, the SMD chip gets quite warm.
You have space on that board for some poplyprop output dc coupling caps. These are optional. If you have input coupling caps on the power amp, no caps on the pre. Or vice-versa.
PS: Just before I hit 'submit' I've noticed that you are using that chip as a normal op-amp.
It won't work.
Have you seen my schematic?
You need a way to fine-tune DC-offset, that circuit I made also deals with the input dc-offset.
Otherwise you would have to use input dc-coupling caps to protect the volume pot.
Then you will muck up the performance of the pre by over-capping.
Carlos, I thought that only applied if you are using current output?
Re: Eagle file...
Stan,
You have a keen eye...but that is not an error. I named both grounds for V+ and V- the same...there is no physical connection between them...hence the 'air wire'. If you want you can easily jumper between the two, the best place is probably between the two big filter caps. If you check with a multimeter you will see that you have 0V between the two grounds...as it should be.
G.
sklimek said:
Stan,
You have a keen eye...but that is not an error. I named both grounds for V+ and V- the same...there is no physical connection between them...hence the 'air wire'. If you want you can easily jumper between the two, the best place is probably between the two big filter caps. If you check with a multimeter you will see that you have 0V between the two grounds...as it should be.
G.
carlosfm said:
Carlos,
Did you ever experiment any further into methods for reducing the DC offset?? Also for what reason did you choose the 7809 regulator...do you really need 9V to null the offset???
Any help would be appreciated...also maybe we should move this discussion to another thread...as it is a bit off topic.
Thanks
G.
carlosfm said:
I hadn't thought of that...it's a good enough reason for me!
I can't wait to try this out! I may research a bit for some different techniques to control the offset, but I will definately try out the regulator idea too. Gotta love the pinout on the 815, why can't BB follow Analogs example!
Thanks Again.
G.
Gcollier said:
Notice that it also minimizes input DC-offset, which is high for current-feedback op-amps.
You would have to use an input dc coupling cap, or you may kill the volume pot.
Whatever you try you will, in my experience, compromise the performance of the pre.
I use my pre without input and output coupling caps.
That's because I use input caps on my power amp.
Otherwise I would use output caps on the pre and nothing on the power amp.
Both is not recommended at all, it veils the sound.
The pre is so transparent that whatever you do, you will notice right away.
carlosfm said:
I made a similar option with my "pre"(I use AD ssm2142) and amp(LM3886).
But in my case I don't use cap's in the amp input, instead I control the DC offset in the pre.
I added a multi-turn trimmer to the ground pin of the ssm2142 chip.
For commercial applications this is not possible because it needs to tuned once in a while.
carlosfm said:
Carlos, did you try a FET input? Similar to the schematic shown below? National has a nice application note on current feedback op amps (OA-07.PDF) They also show a simple offset adjustment using a pair of 6.2V zeners, a pot and a handfull of resistors. I might give these a try too, Although I am really thinking of using an ultra low noise voltage reference like a REF01 to do the same job as your 78L09.
Cheers.
G.
Attachments
Gcollier said:
No, I didn't want to use any input stage, that would be the easy way. But not the best way in terms of performance.
My goal was to use the AD815 alone.
Gcollier said:
That's up to you, I don't think zeners would be the way to go, but a good regulator yes.
Notice that it is double regulation I used, the 78L09 is after an LM317.
I used caps after the 15K resistors (1uf MKT, if I remember), for extra filtering, but then I didn't update the schematic...
Too busy now, I have a completely dismantled Teac VRDS-T1 on my bench.
XELB said:Let me see.....
Changing DAC and improving the PSU
What dac?
This is a CD transport.
It had a mechanical problem, that I've just fixed some days ago.
3 months waiting for a small plastic grear from Japan.
Yes, PSU mods, clock with dedicated PSU and reclocked S/PDIF output, etc... etc... etc...
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