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Old 7th August 2003, 10:31 AM   #1
Raka is offline Raka  Europe
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Default Low pass filter

Hi,

I'm building a new GC with a pre stage (buf634 and OPA627 a la Carlos) and would like to include the low pass filter as suggested by Rassmusen.
Should I locate the LPF attached to the very chip? I'm planning the inverted Thorsten one, but because now will be buffered and the pot (10K) will be at the input of the OPA I don't know which the values for the filter will be.
Will be ok with the following values?
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Old 7th August 2003, 12:39 PM   #2
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.....why include the buf634.....?
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Old 7th August 2003, 01:39 PM   #3
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The BUF634 makes sense on a standalone pre.
You can drive your power amps meters away from the pre with no problems.
Raka, the filter should be inside your GC, not the pre.
But if you're going to put everyting inside the same box, the pre doesn't make sense anymore, just use the OPA627s with the filter, as on Nuuk's thread.
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Old 8th August 2003, 02:50 AM   #4
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Default Inverting or not?

Hi guys,

there is something that i do not quite follow here:

The desition to build an inverter GC is for the sonic benefits of the inverter configuration, right?
But the problem associated with this config is low input impedance, right?

So then you compensate the low impendance with a buffer. But the buffer is NON INVERTNG!.So say good bye to the advantage of the inverting GC config.

So are we going on circles here or what?

Wouldn't be more practical to just build the GC as original (non inverting) if you need the high input impedance?
To put a filter on the non inverting input would be may be easier.
I say this because I build 9 different GC with inverting and non inv, config and the sound in not bad at all in either config.

cheers

Ric
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Old 8th August 2003, 04:24 AM   #5
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You see, that's something I also don't understand. If you need a filter on the GC, it means that something went wrong when building it. The whole advantage of that amp is simplicity and minimized number of components, which in the end produce spectacular results. I've built few of those GC and it never crossed my mind to add a filter because highs were no good, actually they are better than in all other amps I tried so far.

When I hear people commenting that the buffer improves the sound, I take it with a big dose of sceptisism, because I'm still afraid that the cure might be worse than the disease. That's also one reason I still didn't try a buffer

That's also a reason that I shouldn't be really commenting on that either
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Old 8th August 2003, 04:51 AM   #6
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....i think you should try both buffer (opa627) and filter - try also to invert the 627 and use a current source....a vishay j511.......
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Old 8th August 2003, 07:36 AM   #7
moamps is offline moamps  Croatia
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Default Re: Inverting or not?

Quote:
Originally posted by Ricren
The desition to build an inverter GC is for the sonic benefits of the inverter configuration, right?
But the problem associated with this config is low input impedance, right?
So then you compensate the low impendance with a buffer. But the buffer is NON INVERTNG!.So say good bye to the advantage of the inverting GC config.
So are we going on circles here or what?
Hi,

I agree with you. We obviously need some optimization here.

Quote:
I say this because I build 9 different GC with inverting and non inv, config and the sound in not bad at all in either config.
Not bad?
Try buffer.

Regards
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Old 8th August 2003, 07:47 AM   #8
vuki is offline vuki  Croatia
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I also built few GCs (5-6) in inv. and noninv. configurations and to my ears there is much greater difference in sound between various IC's than between inv. and noninv. configuration.
Also, I tried JFET buffered lowpass in front of inverted LM3875 GC and it really improves sound, but moamps' tubed version is even better.
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Old 8th August 2003, 07:50 AM   #9
moamps is offline moamps  Croatia
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Quote:
Originally posted by Peter Daniel
[B]You see, that's something I also don't understand. If you need a filter on the GC, it means that something went wrong when building it.
Hi,
I don't think so. Something is wrong with chip itself. Chip isn't ideal. Pease read
http://www.national.com/ms/LB/LB-19.pdf (only two pages)
There you can find good explanation of slew rate limiting.

Regards
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Old 8th August 2003, 07:54 AM   #10
Nuuk is offline Nuuk  United Kingdom
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First may I suggest that you read and understand the signature line to this post.

Sometimes we seem to get into ruts on this forum where a topic is raised, dealt with and then brought up in another thread, may be in the hopes that a different answer will be arrived at!

I don't know how many have actually tried the buffered version of the IGC but I do know what I hear and we are not talking a minor difference, we are talking about a big improvement. This week a friend came round and listened to my buffered ICG, having listened to the same amp but unbuffered last week. He immediately noticed the improvement for himself.

The recurring theme that I talk about is this business of putting forward a theory to say that something shouldn't sound better, therefore it won't.

When I came to this forum originally and started reading about chip amps, I thought to myself that they couldn't possibly sound as good as the reports. BUT I didn't start posting to that effect because I hadn't built one for myself, or heard one that somebody else had built.

I don't want to pick on anybody in particular but I have to say that I am surprised at one contributor to this thread who has generally put his mouth where his soldering iron is and spoken from experience rather than conjecture, making a statement about the buffered ICG instead of trying it for himself!

End of lecture
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