Hamptone gain block / mic preamp article
- Scodiddly
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Hamptone gain block / mic preamp article
If y'all remember way back to 2004 or so when Scott Hampton wrote an article about a DIY J-FET mic preamp design, including schematics and application notes... I finally built mine last year. Turns out it has maybe a little too much gain for most people these days, as we're all mostly driving the mic preamps in our interfaces instead of big iron tape machines.
However, I figured out that the basic gain block circuit, which does something like 23dB of fixed gain, is a pretty nice alternative to the Cloud Lifter type devices for dynamic mics with the standard mic pre nowadays. So I'm building a box with a four of those and a couple different varieties of transformers that I had laying around.
I'm having circuit boards made, and I'll have extras. Anyone interested? The actual board and components will be pretty cheap, just a few bucks. A really good modern switching supply is under $15, and then you just need mic transformers (the expensive part). Unbalanced output probably equivalent to a decent condensor mic when a dynamic is plugged in. No provision for phantom power but that would be pretty easy to add with another (+48vdc) switching supply.
I've based the layout on some 2N5457 FETs that I've got from something else, but aren't those kind of rare nowadays? I could do it for something reasonably equivalent as the exact FET isn't critical.
However, I figured out that the basic gain block circuit, which does something like 23dB of fixed gain, is a pretty nice alternative to the Cloud Lifter type devices for dynamic mics with the standard mic pre nowadays. So I'm building a box with a four of those and a couple different varieties of transformers that I had laying around.
I'm having circuit boards made, and I'll have extras. Anyone interested? The actual board and components will be pretty cheap, just a few bucks. A really good modern switching supply is under $15, and then you just need mic transformers (the expensive part). Unbalanced output probably equivalent to a decent condensor mic when a dynamic is plugged in. No provision for phantom power but that would be pretty easy to add with another (+48vdc) switching supply.
I've based the layout on some 2N5457 FETs that I've got from something else, but aren't those kind of rare nowadays? I could do it for something reasonably equivalent as the exact FET isn't critical.
- winky dinglehoffer
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Re: Hamptone gain block / mic preamp article
I'd be in for a couple of channels (or is it 4 channels on one board? In that case I'd be in for 1 pcb). I have 2N5457s and a few other FETs, so no worries for me in that regard. I might even have extras to share if anyone is in need. EDIT: I've got at least 20 2N5457 that are surplus to my needs & would donate to anyone who's working on this project & needs them.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
It does look like somebody is actually making through-hole 2N5457 again these days. So I'm not going to worry about that - minimum order is only 10 boards, and so far more than half already have a home.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
PCB files uploaded to Chinese website and paid via PayPal - probably 3-4 weeks since I'm being cheap on the delivery speed.
Still freaks me out a little bit every time I do this. Finish my design, upload the files, pick a PCB color (they'll be yellow!) and a couple other options, and then just wait.
Still freaks me out a little bit every time I do this. Finish my design, upload the files, pick a PCB color (they'll be yellow!) and a couple other options, and then just wait.
- winky dinglehoffer
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Re: Hamptone gain block / mic preamp article
Any thoughts on transformer ratios for this project? I have some smallish 1:3 and 1:4 lying around—that’s the cheap option. I have a few others of different ratios, bu it’d be nice to put these to good use if they’ll work here.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
Not a clue. I've got a pair of Cinemag CM-75101APC transformers that I got in a group buy years ago, also a pair of old DuKane 3A55 transformers. I recently happened into an appropriate rack chassis, so I'm going to put in four channels with those transformers and see how it sounds. The input impedance of the J-FET should be 1M, so no worries about having to "drive" a strong signal into it.winky dinglehoffer wrote: ↑Mon Aug 08, 2022 1:37 pmAny thoughts on transformer ratios for this project? I have some smallish 1:3 and 1:4 lying around—that’s the cheap option. I have a few others of different ratios, bu it’d be nice to put these to good use if they’ll work here.
I might even build a "fun" version with a doorbell transformer and a stack of 9v batteries to power it, just to see if it's good or bad.
- winky dinglehoffer
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Re: Hamptone gain block / mic preamp article
I didn’t know if levelwise there might be a ballpark ratio that would give best results. I have a variety of options available, so I guess I could just experiment till it sounds great.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
Link to the original article:
https://tapeop.com/interviews/btg/37/diy-jfet-mic-pre/
https://tapeop.com/interviews/btg/37/diy-jfet-mic-pre/
Re: Hamptone gain block / mic preamp article
It’s a hi-Z input so a higher ratio transformer gives the lowest noise. Anything 10k or higher worth a try. Tradeoff is higher ratio, higher gain, though the way transformers work a 10K isn’t as much less as a 50k might appear, in the neighborhood of 6dB.
Doug Williams
ElectroMagnetic Radiation Recorders
Tape Op issue 73
ElectroMagnetic Radiation Recorders
Tape Op issue 73
- Scodiddly
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Re: Hamptone gain block / mic preamp article
Got my cards in last week, finally had a chance to stuff one and make a rats nest of test leads and temporary wires. It works! There are a couple things to tweak, but I think it will be a success.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
Another update - I have one channel working, still with a few test leads but waiting on parts to fill out the project (4 channels in one chassis). It took a bit of work to stop noise from the PSU, which was manifesting itself as a fairly quiet ~2k tone. With that solved, it turns out this works great as a preamp for a dynamic mic into a phone/computer mic input. Last couple of Zoom calls I've used a Sennheiser 441 at a distance of several inches, through a primitive little iOS "interface" which pretends to be a headset.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
I built my four channel "Dynapusher", and it works great! Nice clean gain, and because of the transformer it's actually got a very high gain going on. Definitely ribbon friendly.
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- Scodiddly
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Re: Hamptone gain block / mic preamp article
... now I just need to figure out why it's dead quiet in my workshop but picks up hum upstairs in the music room.
- Nick Sevilla
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Re: Hamptone gain block / mic preamp article
If it isn't some other equipment's hum, you might just need to shield that power supply inside your, I might say impressive, unit.
Good job!
Howling at the neighbors. Hoping they have more mic cables.
- Scodiddly
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Re: Hamptone gain block / mic preamp article
Found the culprit, at least for the higher frequency stuff that was showing up in the Beyer M500. Guess I need a cleaner PSU than an old laptop brick, or at least throw some capacitors into the LED strips behind there.
Still some 180Hz hum showing up in the Shure 333, which might be the neighborhood rather than my house. Maybe replacing that old aluminum siding with vinyl was a bad idea? I can position the 333 to minimize that hum and notch it out too.
EDIT: Also I need to remember to wipe off the lens on my phone before taking a photo.
Still some 180Hz hum showing up in the Shure 333, which might be the neighborhood rather than my house. Maybe replacing that old aluminum siding with vinyl was a bad idea? I can position the 333 to minimize that hum and notch it out too.
EDIT: Also I need to remember to wipe off the lens on my phone before taking a photo.
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