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I would really recommend between 10K and 20K.
But well, 25K is not that far off.
You may need to adjust R2 and R3 (after the 9V reg), so that you can null DC offset.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

Thank you for the suggestion. I'll give it a try and reffer later...

Filip

Hola Seniors,

Carlos, I want to thank you for this design - it is very special. I turned it on for a quick listen (not being done yet) - and it scared me! I've never heard drums like that. The intensity and attack were so much closer to real. It really rocks!

Now I just have to finish it.

So here is the problem - the same as a lot of other people. I can't get the DC offset on the output below about 80mV. First - there is no pot connected but there is a 3k dummy load (my dac's i/v resistor value) on the input and 20k on the output. My plan is to use a stepped attenuator on the output, going to a GC. I've changed the 15k resistors on the output of the 78l09 to 4700ohm, and can get just about 1v offset from the nulling circuit - but still no dice. The output refuses to go to zero (I'm using the surface mount AD815ARBZ). I can easily null the input offset to 0.0mV - it takes about 200mV from the nulling circuit (which is just what your 15k resistors deliver).

Any tips?

-Patrick

Hi Patrick,

I recommend that you try a volume pot on the input of the pre, it will help you identify the problem.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

Hello Carlos-

I added a 10k pot in front of either channel - and things stayed just about the same. No input offset, an output offset of about 100mV...

any ideas?

Are you using the SIP or the SMD chip?
I've heard reports that some SIP AD815 chips (probably a whole series) behave strangely, with high output offset.
The SMD (DDPAK) AD815 chip is much better, provides consistent results and has a much better pinout.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

SMD version...

A little update -

I haven't been able to get rid of the DC offset on the output but it doesn't matter - my amp doesn't have any input blocking caps, so I have to use the ones after the AD815. Anyone got cap suggestions?

Also, there is a little hum to the system now. I believe it is electromagnetic, from the transformers nearby, because the hum doesn't change with the volume, and the PS looks quite clean. I've got some thin steel sheathing somewhere, so I'm going to play with that.

Adding the AD815 to my NI gainclone has been a definite addition, thanks Carlos!

As you can't get ride of that offset and want to use caps, I suggest trying the ones I'm using in all my stuff: ClarityCap.
I think 5.6uF is quite enough.
PX Series for abour 5 Euro (+/- 6.5 USD) and
SA Series for abour 10 Euro (+/- 13 USD) the best ones
These toys sounds fantastic.

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Pedro Martins

Playing with Canon EOS 5D mkII...

Thanks for the suggestion, I'm gonna try it.

cuibono, are you sure those trimmers are ok?
Check them out.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

Hi Carlos,

I used these pots (new, from digikey): P2U4103-ND

They are Panasonic ECG series, 10k logarithmic stereo pots. There is one still in the circuit, working fine as far as I can tell. I couldn't null the output offset, but could easily null the input offset. The output just wont go past -85mV offset. Occasionally, I could trim the output to zero, but then it would slowly climb back to -85mV or so, but usually it just refused to go past -85mV, even if I applied 1V of offset (!). Strange..

I mean the trimmers, R4 and R5. 500 ohms.
You may also need to change R2 and R3 (15K) for smaller values.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

The trimers seem to be working fine - I can vary the voltage from 0 to 1v, as i changed r2 and r3 to about 480 ohms (somewhere around there). It only takes about .2V from the trimmers to null the input offset...

Hi Carlos,

It's been quite some time but I have finally made a preamp using your schematic. I have just used a bit of stripboard and a chip from BG micro AD815ays variety.

I am not using any caps in the signal path or the nulling circuit at the moment. I can't turn it up too loud or I do get some dc. However, at normal listening levels (fairly quiet for me) it is working great. Dynamics are fantastic. I am hoping to put the nulling circuit in soon though.

I'm using a 20K Alps pot at present, but am wondering whether you would recommend using a Joshua Tree from Twisted Pear? The minimum impedance of the JT is around to 2.5K.

cheers