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Old 27th December 2006, 06:57 PM   #1
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Default JFET Ultra Wideband Preamp PCBs for Sale

As a project for my father, I built a Class A stereo (2-channel) preamp for his specific requirements. Since it's the same price to order 1, or 20 of these boards, I happen to have quite a few. I thought about making a commercial website, but that would take all the fun out of it. So, here's a great opportunity to get a great preamp! Please contact me if you are interested in purchasing one. I need to recover my cost of production, so estimate about $25/pcb. Hopefully I'm not breaking any etiquette rules posting this here!

The primary design goals were:
1. Single input, single output. The preamp does not allow you to switch between signal sources. We did this for two reasons. First, we didn't feel it was sonically optimal to use the same preamp settings for multiple input sources (CD players, tape players, etc. Second, he primarily uses a single source (CD player), so he only wanted one input available.

2. Ultra flat frequency response. This one is a bit technical, but a good idea. Many preamps on the market have a band pass filter that limits the frequency response of the preamp to 20Hz-20kHz. This is a bad idea if you have really good speakers (Magneplanar 3.6s). You will notice that stereo imaging is dependant on frequency. So a higher pitch causes the image to change. Extremely annoying. So we design the preamp to respond from 2Hz to 200kHz, which allows it to have 100% flat phase response in addition to frequency response between 20Hz-20kHz. (See body plot on wikipedia for more information.)

3. Minimal components in the signal path. I suppose this is a common design goal, so I won't elaborate much. We chose to use discrete JFets instead of opamps to get lower noise. Difficult to tell if that worked or not. The end result is astounding, but it's hard to get a good one-to-one comparison.

4. Impedance. It never has made sense to me why manufacturers match the input impedance for components. I suppose you could make a case for reflections with impedance mismatches, but my experience has been that the output stage of many stereo components is to weak to drive the input of the power amp appropriately. So we did not match input impedance, but chose to use a high input impedance. This places a minimal load on the component (CD player) output driver. The output stage has plenty of head room to drive nearly any load.

5. Adjustable Gain. Instead of attenuating the input signal into fixed gain stages, we built adjustable gain stages. This has a disadvantage of being noisy during adjustment (you can hear the wiper in the pot turn). But it has the advantage of allow you to customize the preamp to supply the minimum gain needed for your system. This is important since the more fixed gain in a stage, the more noise it inheirantly has. So we wanted to minimize gain in each stage (has 3 gain stages), to keep noise at a minimum during standard operating conditions.

That about does it for design goals. We did pretty good meeting them. The preamp is very enjoyable, and has really increased the enjoyment of the 3.6s. In fact it was so good, that we decided to build a power amp. But I just bought PCBs from ESP Sound in Australia for that. Rod is great, and a good engineer.

There are a few things that should be considered as well. The preamp isn't extremely user friendly. It is possible to drive any stage into cut-off, or worse, distortion. This means you have to be responsible with those gain knobs. Practically, this just means you'll have to spend some time getting it setup just right. In my design I also did not use a power switch. So it's always on. Not a big deal, but may concern some.

Also the power supply for the preamp is a bit tricky. It expects 42V DC as an input. And is sensitive to noisy power supplies. The extreme voltage gives great headroom, but makes it more expensive. I built a custom power supply for this as well, which is ultra low noise (less than 0.01% ripple). I have PCBs for these as well. You could run the PCB with a lower voltage, but expect lower headroom and earlier cut-off in gain stages.

Finally, each PCB has one preamp with two stereo channels. The power supplies are separate PCBs.

It's a great preamp, and will compete with really expensive preamps. Feel free to ask questions!

May your phase always be aligned,
J
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Old 27th December 2006, 07:00 PM   #2
anatech is offline anatech  Canada
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Hi juajuara,
To help your fellow DIY builders out there, can you post the schematic and perhaps a low res layout? This will help us see what we are buying. Sight unseen is not going to be attractive.

-Chris
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Old 27th December 2006, 07:01 PM   #3
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Any schematics available?
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Old 27th December 2006, 09:21 PM   #4
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Default Re: JFET Ultra Wideband Preamp PCBs for Sale

In design objectives 2 and 4 there seem to be references to manufacturers limiting bandwidth and matching impedances. It will be great to know which are these manufacturers and designs.

I have certainly never seen a commercial preamp matching it's input impedance to the output impedance of a source. The two are usually orders of magnitude apart.

An unusual post indeed. Seeing that it's your first do you really expect anyone here would be as desparate as to buy a preamp concept based on such a dubious design brief. And a circuit sight unseen?!
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Old 27th December 2006, 10:51 PM   #5
steenoe is offline steenoe  Denmark
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Quote:
As a project for my father,
Your DAD actually asked for all that? I mean, my dad would have at the most, asked for something that sounds good Any chance to see that PCB?
One input is always an option!!! No matter what preamp.
Quote:
Many preamps on the market have a band pass filter that limits the frequency response of the preamp to 20Hz-20kHz.
I have yet to see one that has that kind of limitation!

Steen
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Old 28th December 2006, 08:46 AM   #6
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Default PCB Bottom Picture (Terrible with a camera!)

All the traces are on the bottom. The most difficult joint is the .100" header. I do want to specifically mention that these PCBs are professionally manufactured.

I'll post a picture of the top in a minute. I do have schematics, but will not distribute them to keep some control over my intellectual property. It's basically a cascaded common collector configuration, but using JFETs instead of BJTs. I used typical radio frequency design principles and scaled the bandwidth to audio frequencies. That was the trick to provide ultra-linear amplification with correct phasing.

Many folks seem to be incredulous about preamps that claim 20Hz-200kHz frequency response. I appreciate the comments, but I don't understand where they are coming from. Obviously Conrad Johnson or Matisse have enough engineering and practical experience to extend that a little, although I notice the revered Matisse reference preamp claims a -3dB at 175kHz, which means it may be possible hear the phase shifting above 17.5kHz. But for any nay sayers, here's the first preamp I pulled from google: http://www.audioholics.com/productre...AVR5805p12.php

Notice the 10-22kHz bandwidth.

If you want a long list of people using impedence matching, run a google on "phono preamp transformer." That's a lot of people sold on matching input impedence. My opinion is that you want the voltage to float as freely as possible. By matching impedence you've created maximum power transfer, but I stress that it's a voltage you want out of the input device. That's the reason nearly all preamps are voltage amplifiers. But everyone has an opinion! ;-)

j

PS. Keep the fight against Apartheid rocking!
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Old 28th December 2006, 08:46 AM   #7
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Default PCB Top (A little better with the camera!)

Here's the top of the PCB. Pretty straight forward. The header is used to provide easy management of cabling.

Any more questions?

Thanks to all for responses, I should have thought to include pictures in the first post. I can also take some pictures of the end product. As long as you promise not to laugh at my soldering skills, or lack thereof! Let me know if there's interest in that.

j
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Old 28th December 2006, 11:12 AM   #8
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I think you really should post the schematic here. Some of your text in your first posts sounds, well, interesting, to some of us here. For instance, a preamp sensitive to supply noise is not a very good design.
Then again, there may be some circuit features that would offset that disadvantage.

Bottom line: show us the beef! Your first customer can anyway post the schematic. But if you don't do it first, there may be no first customer...

Jan Didden
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Old 28th December 2006, 11:29 AM   #9
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Default Re: PCB Bottom Picture (Terrible with a camera!)

Quote:
Originally posted by juajuara
http://www.audioholics.com/productre...AVR5805p12.php

Notice the 10-22kHz bandwidth.




If you want a long list of people using impedence matching, run a google on "phono preamp transformer."



What i do notice from the link is that a 10-22kHz bandwidth was chosen to perform S/N measurements. What's that got to do with the preamp frequency response?


And how is loading a phono cartridge relevant to your design??? Can it actually be used as a phono amp too? Or is that an optional extra?






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With the oil of Aphrodite, and the dust of the Grand Wazoo
He said "You might not believe this, little fella
But it'll cure your asthma too""

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Old 28th December 2006, 12:35 PM   #10
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I, for one, don't find 200kHz bandwidth in a low level circuit all that astounding. I've been getting 350-400kHz--and that with only minimal NFB (<10dB).
I'd also be interested in the identities of any current manufacture (or any in the last twenty years, for that matter) high end preamps that bandwidth limit to 20kHz. Even tube amps usually manage 30-50kHz.

Grey
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