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Old 15th February 2002, 06:30 PM   #1
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Question Mic Preamp from X preamp circuit?

Hello,
I've recently purchased the BZLS circuit boards from AudioXpress, and I'd planned to use them for my home hifi system, when it occurred to me:
Has anyone tried using Pass' topologies for balanced Microphone preamps?
Certainly, balanced operation, simple circuit design, good sound and good noise specs are big factors in choosing a mic pre. Plus, no vacuum tubes to get finnicky just at the wrong time during a recording, seems like a bonus. My only concern is over the limited gain of the Balanced Line Stage. The typical mic preamp has from 50dB-70dB of gain.
The natural next step was to look for greater efficiency in a similar circuit; the Aleph P 1.7.
That wasn't enough either. I decided to go for the "Biiiiig Salad" and try building an X generation preamp stage/stages and take advantage of supersymmetry for even better efficiency, noise and distortion characteristics.
Everyone else here has focused on backengineering the X series power amps from what I can tell. Much has been discussed on drift, tweaks, offsets, etc... which all seem to have to do with the output stages for the most part (please somebody correct me if I am wrong).
It would seem to be a simple task to only build the x-amp input stage based almost entirely upon the first sketches in Mr. Pass' patent file. I am willing to try the variations cooked up on this forum as well.
Would anyone have any suggestions to make about going about such a project? I am willing to use trial and error, but it'd sure be nice to learn from all of your experiences instead of reinventing the wheel.
Any thoughts would be greatly appreciated.
Thanks.
-Erik.
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Old 15th February 2002, 07:39 PM   #2
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Hi,

If you are looking for that much gain, I think you should consider constructing a 2 stage X-series out of JFET's a la the Pearl. I wanted to make an XPearl, but chickened out not knowing how to handle the filter so I just made some changes to the original circuit instead.

If your voltage levels are really low, it seems to me that you might want to place the first stage in the microphone itself. If you use a transformer as many mics do, you don't have to use balanced topology, especially if the distance to the diaphragm is very small.

As far as stability goes, I have been quite vocal on current sources in the X-stage but it does not seem to be a problem with my latest circuit. Without the need to set up voltage bias for the output stage, the design is pretty simple and quite well documented in the reverse engineering thread.

Please consider posting something on voltage leves etc., so we can all take a look at it! dB gain is challenging since everybody usually gets confused about voltages and power, so absolute values are the best (input signal mV, output signal V)


A JFET based unit with limited output voltage requirements could be made quite small.

Are you planning to use a transformer on the mike end (I would probably try to do without it if the plan is to place the XPhone out there, but bear in mind i have no experience with microphone requirements, and galvanic separation might be required)?

Petter
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Old 13th November 2002, 02:01 PM   #3
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Exclamation Multi Stage X Preamp for microphone

Hi Petter,
It's been a while since I've been able to devote any time to electronics, instead trying to concentrate on finishing my audio engineering courses and finally getting my degree so I can strut around like I know something . I was roused from my electronics slumber by another forumite here who sent me an email asking my progress on the mic pre. I think his name was bommels... Darned if it didn't put a bug in my brain. So, I'm back to finish my mission.

Regarding your suggestions for the two stage design. I think that is the way I will have to go. Getting that much gain from a single stage is probably impossible. Two might cut the mustard as they say, especially considering the folded cascodes involved in each.
I have been particularly interested in the AlephX and XBOSOZ discussions here lately, and I'm sure that there are tips and modifications that will help me in my quest to build a good mic pre.

In answer to your suggestions: Unfortunatlely, it would be impractical to build the first stage of the mic pre into the body of the mic itself. A typical studio microphone preamp must be able to operate with any microphone plugged into it via three pin XLR jacks. Many people like to bring their own gear to a session and swear that their mic is mystically special and will make them sound better. Thus, I probably wouldn't have time or get permission to install extra electronics in their equipment
Regarding a transformer, it wouldn't be necessary in an X preamp since the noise cancellation of the design, properly tweaked would be superior and much more cost effective than the transformer. It would also avoid the phase shift of the transformer, so I'd consider it a plus all around.
Perhaps for proper current gain, I might try a power MOSFET output stage to compliment the JFET input stage. It's the way tube preamps are built using low noise triodes for input and something like medium MU 12AU7s for output to drive any subsequent inputs on recording devices, power amps etc...

While I haven't done any actual new work on this project, I'll do some preliminary research including building the BOSOZ in it's original form, then using my second board, I'll do various mods using a breadboard "sidecar" and see the results with current sources, complimentary symmetry stages, folded cascodes and supersymmetry. I think the progression from simpler to more complex will teach me a great deal. Though, as I said I don't necessarily want to reinvent the wheel, so I'll take a heaping helping of cues from the aforementioned discussions on the X style Aleph and BOSOZ amps and see if I can't get a good head start.
Stay tuned, I'm going to make this puppy work. Ultimately, it's going to be the basis for a 16 channel homebrew mixing board with all the bells and whistles. The question becomes, how small could I get the mic pre? Could I cram the circuit minus power supply into a something in the neighborhood of a 2"x2" pcb?

I'll post a dB to voltage scale when I dig one up from my textbooks, but basically 0 dB on a VU meter = .776V. That's the "zone" you want your output gain to occupy, adding more depending on the headroom of the device you're sending the signal to.

Thanks again. I'll post again with more details later.
-Erik.
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