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Old 13th September 2004, 07:27 PM   #11
carlosfm is offline carlosfm  Portugal
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Thumbs up AD815

The "idea" I was talking about was to find a way to have no DC on the output, with direct output (no caps, anything).
It's in practice now.
I'm feeding (double-regulated) voltage to the non-inverting inputs, with independent multi-turn trimmers to null DC-offset.


Now... you have to grab me because I'm listening to music compulsively.
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Old 13th September 2004, 07:40 PM   #12
Devil_H@ck is offline Devil_H@ck  Belgium
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That's a creative idea . Nice!
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Old 14th September 2004, 01:07 AM   #13
carlosfm is offline carlosfm  Portugal
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I removed the input caps too, because now I have no DC-offset on the input. Zero. Nill. Rien.
I nulled DC-offset on the output without a pot, just the input resistors to ground.
But...
DC at the output varies with the position of the pot.
Whith my settings now it changes from ~ -30mv to around +40mv at max volume.
At a little pass mid-volume there's 0mv.
There's a little issue.
In a scale from 1 to 10 as volume setting, from 9.5 to 10 there's a jump in DC to around 400mv.
As I use input caps on my power amp and on any amp I make, there's no problem for me. Also, I would be mad to listen to music at this volume.

I may consider using two outputs on the pre: one with caps, the other one without.
It would be easy for me to use input and output coupling caps.
But this chip deserves all the attention, all the efford.
The AD815 is used on the best preamp I've ever heard: the Jeff Rowland Coherence.
I had to try it.
My way.
It seamps the the JR uses input and output signal trafos, although I don't know exactly the circuit they use.

Fiddling with current-feedback op-amps is not easy, and these are made for video applications, not audio.
This has been a challenge for me.
But it pays, definitelly.
I was getting bored to just use VF 8-pin op-amps.
I'm now gonna le it -in for some days and listen to some music.
Carol Kidd on HDCD is playin' on my room right now.
Gotta go, she's calling me!
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Old 15th September 2004, 02:25 AM   #14
cwteoh is offline cwteoh  Malaysia
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Anyone know where can I get BUF634T from Online shop? I am from Malaysia. And I can't find this component from Farnell. RS show the price each is USD800++. I am not sure this BUF634T cost that price!!!
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Old 18th September 2004, 07:05 PM   #15
cwteoh is offline cwteoh  Malaysia
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sorry for the previous post... I was looking at the wrong item and giving the wrong info...

I got the actual info and correct part from RS, it is around USD11 per piece.
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Old 22nd September 2004, 09:46 PM   #16
hihopes is offline hihopes  South Africa
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Well Carlos Filipe, don't leave us all hanging in the air. Which revs your motor more - the TPA or the AD - and how do they compare to your OPA627/BUF634 setup?

I am thinking about using OPA1632's to give me a differential input into the TPA so as to make best use of its CMR capabilities, but I am a bit inexperienced so perhaps I will try a simpler design first. (The OPA1632 also looks like a bit of a cranky beast)
Anyone had any experience along these lines yet?
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Old 22nd September 2004, 11:03 PM   #17
carlosfm is offline carlosfm  Portugal
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Quote:
Originally posted by hihopes
Well Carlos Filipe, don't leave us all hanging in the air. Which revs your motor more - the TPA or the AD - and how do they compare to your OPA627/BUF634 setup?
I thought this thread didn't have much interest?
Most of the posts is me talking alone...


Ladies and gentlemen, the winner:

AD815

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Old 23rd September 2004, 12:01 AM   #18
darkmoebius is offline darkmoebius  United States
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Quote:
Originally posted by carlosfm


I thought this thread didn't have much interest?
Most of the posts is me talking alone...

Nah, there's an awful lot of us out here lurking in the shadows, my friend.

Quote:
Ladies and gentlemen, the winner:

AD815

Let's here what made you change favorites.

Are you using the Alpha mini-pots that Joe Ras suggested in that other thread?
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Old 23rd September 2004, 01:18 AM   #19
carlosfm is offline carlosfm  Portugal
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Quote:
Originally posted by darkmoebius
Let's here what made you change favorites.
Are you using the Alpha mini-pots that Joe Ras suggested in that other thread?
I tested them, yes.
My intention is to make a new pre, new case, new board, and use an Alpha pot.
Meanwhile, I opened my OPA627+BUF637 pre, made some internal changes and changed the OPA+BUF board for the AD815 board.
Volume pot on my pre is a 20k log. Alps Blue.

The AD815 is better, but not night and day.
The main difference is in the treble.
Gorgeous.
"Round", not the smallest hint of "splash", very very detailed.
That sort of detail that a drummer could recognize a Zildjan cymbal.

The TPA6120 is very good, but ultimately a little too smooth and laid back for my taste.

So, here's my conclusion:
1. AD815
2. OPA627+BUF634
3. TPA6120

Any of these makes a very good pre.

But the AD815 is in another league of it's own.
Like the Jeff Rowland Coherence.
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Old 23rd September 2004, 01:58 AM   #20
darkmoebius is offline darkmoebius  United States
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Quote:
Originally posted by carlosfm
Volume pot on my pre is a 20k log. Alps Blue.
I can' wait to hear your impressions regardig the Alps vs. Alphas.

Quote:
The AD815 is better, but not night and day.
The main difference is in the treble.
Gorgeous.
"Round", not the smallest hint of "splash", very very detailed.
That sort of detail that a drummer could recognize a Zildjan cymbal.
Sold!!! I played drums as a teen. I've always wanted a system that could recreate the shimmer and decay of a light brush stroke across the ridges on the top of a cymbal. It's right up there with the deep woody resonance of a cello as my favorite tones.

Quote:
But the AD815 is in another league of it's own.
Like the Jeff Rowland Coherence.
This may sound like an "amateur hour" question, but will there be a need for the buffer once you use the AD815 pre?
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