Hamptone gain block / mic preamp article

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Scodiddly
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Hamptone gain block / mic preamp article

Post by Scodiddly » Sat Aug 06, 2022 2:09 pm

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. :!: :!: :!:

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Re: Hamptone gain block / mic preamp article

Post by winky dinglehoffer » Sat Aug 06, 2022 4:02 pm

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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Sat Aug 06, 2022 5:48 pm

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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Sun Aug 07, 2022 1:26 pm

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.

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Re: Hamptone gain block / mic preamp article

Post by winky dinglehoffer » Mon Aug 08, 2022 1:37 pm

Scodiddly wrote:
Sat Aug 06, 2022 2:09 pm
If y'all remember way back to 2004

So I'm building a box with a four of those and a couple different varieties of transformers that I had laying around.

and then you just need mic transformers (the expensive part.
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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Mon Aug 08, 2022 7:08 pm

winky dinglehoffer wrote:
Mon Aug 08, 2022 1:37 pm
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.
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.

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.

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Re: Hamptone gain block / mic preamp article

Post by winky dinglehoffer » Mon Aug 08, 2022 8:13 pm

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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Tue Aug 09, 2022 5:44 am


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Re: Hamptone gain block / mic preamp article

Post by emrr » Tue Aug 09, 2022 6:49 am

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.
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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Mon Sep 05, 2022 12:09 pm

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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Wed Sep 14, 2022 4:44 am

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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Wed Sep 28, 2022 6:49 pm

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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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Wed Sep 28, 2022 7:36 pm

... now I just need to figure out why it's dead quiet in my workshop but picks up hum upstairs in the music room. :x

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Re: Hamptone gain block / mic preamp article

Post by Nick Sevilla » Wed Sep 28, 2022 8:34 pm

Scodiddly wrote:
Wed Sep 28, 2022 7:36 pm
... now I just need to figure out why it's dead quiet in my workshop but picks up hum upstairs in the music room. :x
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.

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Re: Hamptone gain block / mic preamp article

Post by Scodiddly » Thu Sep 29, 2022 7:44 pm

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.
Stars lamp.jpg
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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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