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Old 4th August 2003, 09:56 AM   #11
Newbie is offline Newbie  Tibet
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Question before signal cable?

i've been listening to IGCs with 2-way active-XO for quite a while and being pretty satisfied with their performance. Would like to try the buffer for IGC, but it's much easier to build my buffer in the active-XO side since its' power is built for opamps. So is it ok to put the buffer before the signal cable? and how about a BUF634 to do this job?
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Old 4th August 2003, 09:56 AM   #12
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Quote:
Originally posted by Luke

I would have thought the LPF would work with out the buffer?
Of course.
You could make an LPF directly on the power op-amp chip (LM3875 in this case).

Nuuk, if you're happy with the sound (I'm sure you are ), then I would not attribute so much of the merits to the LPF.
The fact is that now you have a higher input impedance on your power amps, and a buffer.
You use a passive pot on an external box.
And, most important, you picked one of the (if not THE) best op-amps for audio applications.

I'm very happy with my pre too.
OPA627 + BUF634 is explosive.
It's finished, photos soon.
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Old 4th August 2003, 10:03 AM   #13
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Hi,

Quote:
So is it ok to put the buffer before the signal cable?
You could but it's much better to have the buffer at the input of the IGCs themselves.

A BUF634 is fine for that.

Cheers,
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Old 4th August 2003, 10:05 AM   #14
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Quote:
Originally posted by phase_accurate


I don't know how you come to this conclusion. Adding an LPF without input buffer is just a question of dimensioning.

See my post #15 within the following thread:

Gainclone...No gain, YES pain.

Regards

Charles
You state that any LPF should be well above audio... yet if you calculate what we're talking about here, it does cut into audio. Indeed typically -1.3dB @ 20KHz.

That requires a buffer.

This is a case of: Don't ask questions, just do it! Then you too will experience what this is about. This LPF was determined purely by ear. In fact, the beauty of it that anyone can vary the LPF to their hearts content. But so far this final preferred value has now been independantly verified as the desired one.

The starting point here is the standard IGC, and how we can improve it. Whatever the theory...


Quote:
Originally posted by Luke


Hi Joe,

I would have thought the LPF would work with out the buffer?
With great difficulty. Click on below link to my "The Tube Gainclone for DIY Buffs" web site and goto 'sans tube' - and the difficulties are explained in detail.

The buffer makes it straight forward as it defines a Lo-Z source that is needed to drive the LPF, otherwise its time constant is over all the place, ergo... don't use a pot. Use the pot before the buffer.

On the other hand, if you have a Lo-Z source already, for example the output of a preamp (tube preferably, sorry but I am biased) then you already have a ready made buffer. Still not ideal but usable.

Joe R.
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Old 4th August 2003, 12:17 PM   #15
Nuuk is offline Nuuk  United Kingdom
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Quote:
Good on ya Nick. We have another convert!!!!
Oh yes Joe - but is it the influence of having drunk a lot of Kalgoorlie Stout many years ago?

Let me just reiterate that this choice of buffer and opamp was purely based on the fact that I had the buffers already here. I agree, some of the other opamps that would do the job here will probably produce excellent results too.

The PSU for the buffer was another 'already have' item and I didn't have time yesterday evening to write up full details but will later.

It is a fairly standard job, 18-0-18 torroid, rectifer bridge with Schottkeys if I remember correctly, 2 x 4700uF with smaller bypass caps and one stage of regulation (317/337) in one box, then another stage of regulation (317/337) for each channel closer to the buffers.

Without wishing to get on my soapbox, may I second what Joe says. There are a lot of people on these forums expressing opinions about matters like the buffer and LPF when they have not actually tried it! I only built my GCs in the first place because I could not decide who was right - those saying they were brilliant or those saying that they couldn't be. Sometimes you just have to hear it for yourself and then you can talk with some authority!


Listening again today, I notice that the chips seem to get slightly warmer with the buffer. Is there any reason for this or am I just playing it louder because it sounds so good?
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Old 4th August 2003, 12:19 PM   #16
Nuuk is offline Nuuk  United Kingdom
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Good day to you Carlos. If you let me have your wife's email address, I will write to her and ask if you talk about OPA627's in your sleep.
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Old 4th August 2003, 01:05 PM   #17
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Quote:
Originally posted by Nuuk


Oh yes Joe - but is it the influence of having drunk a lot of Kalgoorlie Stout many years ago?


You lucky sod! The rest must wonder where a place like Kalgoorlie is? Like places called Wagga Wagga or Woy Woy (where Spike Milligan's brother still lives to this day) - it's in the land of Oz, the place Down Under, the land of Tassie Devils, Platypusses, Wallabies, Koalas and Kangaroos. If they haven't cotton on... then we can't help you!

Quote:
Let me just reiterate that this choice of buffer and opamp was purely based on the fact that I had the buffers already here. I agree, some of the other opamps that would do the job here will probably produce excellent results too.

The PSU for the buffer was another 'already have' item and I didn't have time yesterday evening to write up full details but will later.

It is a fairly standard job, 18-0-18 torroid, rectifer bridge with Schottkeys if I remember correctly, 2 x 4700uF with smaller bypass caps and one stage of regulation (317/337) in one box, then another stage of regulation (317/337) for each channel closer to the buffers.


Hey, no sweat! We got interesting feedback!

Quote:

Without wishing to get on my soapbox, may I second what Joe says. There are a lot of people on these forums expressing opinions about matters like the buffer and LPF when they have not actually tried it! I only built my GCs in the first place because I could not decide who was right - those saying they were brilliant or those saying that they couldn't be. Sometimes you just have to hear it for yourself and then you can talk with some authority!


Hear, hear!

Quote:
Listening again today, I notice that the chips seem to get slightly warmer with the buffer. Is there any reason for this or am I just playing it louder because it sounds so good?
Hey, only you can answer that!

Mind you, if it got very warm with no music playing, then I'd worry. But if it gets hotter as you crank the volume up, then I'd consider that normal. Stereophile did a half-rated-power continous test on their original Gaincard and it got uncomfortably hot, even to touch.

Joe R.
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Old 4th August 2003, 02:35 PM   #18
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Talking Medical help

Quote:
Originally posted by Nuuk
Good day to you Carlos. If you let me have your wife's email address, I will write to her and ask if you talk about OPA627's in your sleep.
Actually some of my friends say I need medical help.
Then I show them some of the things I've done and then THEY need medical help.
The last one who almost called the doctor was my brother.
My ultra-small GC (pictures soon) blowed his big Vincent amp into dust. At his room, with his speakers.
BTW, that ultra-small GC (world record?) I made with OPA548s.
I'm impressed.
They're very good too.

Oh, this week I'm gonna test my Pre and GC power amp seriously.
A friend of mine who works with high-end is going to prepare a room for us.
Let me just tell you that the source will be a LINN CD12!!!
Speakers will be several, from the same cathegory.
Hahahahaha!
Seriously!
That guy has been at my home a week ago and got out needing medical help.
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Old 4th August 2003, 02:42 PM   #19
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Joe R wrote:

Quote:
You state that any LPF should be well above audio... yet if you calculate what we're talking about here, it does cut into audio. Indeed typically -1.3dB @ 20KHz.
Back then when I mentioned this LPF thingie in the other thread, my intent was different from what you are doing now.
I just want to point out the possibility of having an LPF without any buffer, which should be a piece of cake.

I don't want to claim that it sounds the same (and I don't want to try it out by now, but maybe someone else wants to do so) but the LTF would be the same if you had the 10k resistor first and the 680 Ohms resistor second with still the same capacitor from the node between them to ground.


Regards

Charles

BTW: AFAIK I was the first one mentioning the use of an LPF at the input of a GC.
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Old 4th August 2003, 03:05 PM   #20
moamps is offline moamps  Croatia
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Quote:
Originally posted by phase_accurate
AFAIK I was the first one mentioning the use of an LPF at the input of a GC.
Hi,

IMHO, National Semiconductors was first.

Regards
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