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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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Old 12th May 2012, 08:00 AM   #31
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Richard Murdey
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For reasons explained in my blog post, I'm fairly confident that of the changes made (3) above made a very large improvement. For some reason, adding ceramic bypass capacitors sweetened up the treble, removed glare, and generally improved the musicality.

According to the simulation the diamond buffer bandwidth is about 4 MHz, clearly higher than the usable range of the electrolytics, which extends to perhaps 1 MHz. On the other hand there is no signal energy up there, and even if there was it would be inaudible, and furthermore the circuit is not in any kind of feedback loop, so I wouldn't have expected any instability or indeed great effect on the sound. But there you are, if I had expected it I would have built it that way in the first place.
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Old 12th May 2012, 08:31 AM   #32
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Will you have any more boards available for this ? After a simple pot buffer for a preamp I'm building - this looks ideal :-)
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Old 12th May 2012, 12:42 PM   #33
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Ray, I have kits available but not the individual boards right now. I will definitely be making a new batch of boards and kits, but not for a few months at least.
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Old 13th May 2012, 02:20 PM   #34
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I just got some new speakers, and I am flirting with the idea of biamping them.

Do you think we could use 2 buffers per channel in parallel after an attenuator to drive the four total channels?
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Old 13th May 2012, 11:27 PM   #35
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Yes, this presents no problems at all.
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Old 14th May 2012, 08:49 AM   #36
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So just two of them in parallel?
Do you think R1 and R2 will have to be matched or something? On all 4 channels?
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Old 14th May 2012, 02:09 PM   #37
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By the way, I just received the b-boards. I will try to start working on them today (as output stage of lightspeed) and get back to you.
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Old 14th May 2012, 11:50 PM   #38
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OK just finished soldering and cleaning the boards.

Once again, nice job Richard. I m too tired to do any tests now, but I will definetely do tomorrow.

My only remark is again the size of the pads. Especially with the carbon resistors and the big ground planes. It can easily get messy and you cannot see if the solder has flown on the pad or has created a blob on the wire.
(I m using 0,46mm multicore solder and an Ersa station with a 2mm chisel tip @450 Celcius)
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Old 15th May 2012, 12:21 AM   #39
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Sorry about the pad size. It's the default in Eagle, and changing it is non-trivial.

Re. parallel, yes, just feed the attenuation output into as many B-boards as you need. No need to match anything, or any other special precautions. What you are asking for is exactly the job buffers are designed to do.

See circuit in post 23 for the details of configuring the B-board with a volume control. The coupling capacitor may not seem like its necessary, and in many ways its not, but what it does do is make the output DC offset of the B-board independent of the volume setting. Without it you may hear "clicks" when changing volume if you are using a stepped attenuator.
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Old 15th May 2012, 01:47 AM   #40
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I have some mundorfs supremes left (2,2uF). I will try using those until I get some 0,47uF ones.
About the 100k resistor, any suggestions on the type most suitable? I think I have some dales @1/4W
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