Hamptone gain block / mic preamp article

[Scodiddly]

It was still a little weird how the position of the mic itself didn't matter, the noise was very consistent.

Finally put three and one together and found the culprit:

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[winky dinglehoffer]

Twinsies (mas o menos).

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[floid]

Great looking builds.

Did you stick to the original schematic? I ask because at one point the folks at groupDiy were talking about one of the transistors getting overly hot, i think maybe the ztx653, and also about either individually biasing each fet or sorting them so that uniform values could be used. It's where I got stuck on this project when I tried it few years ago, overthinkng

[Scodiddly]

Did you stick to the original schematic? I ask because at one point the folks at groupDiy were talking about one of the transistors getting overly hot, i think maybe the ztx653, and also about either individually biasing each fet or sorting them so that uniform values could be used. It's where I got stuck on this project when I tried it few years ago, overthinkng

No reason not to share the schematic! I did get rid of that hot transistor, it was there in case the output was needed as an arc welder power supply. Massive overkill. Without it, the total power consumption of my 4-channel version is 1.5 watts.

R5 is 1M for a DI input - for an input transformer it'll be somewhere in the 10k's of resistance, depending on the transformer. My DuKane transformers, in the original tube schematics, wanted 68K, the Cinemags more like 100K.

[winky dinglehoffer]

Great looking builds.

individually biasing each fet or sorting them so that uniform values could be used.

As the schematic notes, adjust the value of R1 for 12 or 13 volts. I socketed R1, but Scodiddly says you can just drop a resistor in place without soldering and measure the voltage (I have different value R1 in each of mine). If you've got a suitable transformer on hand, this is a really simple and inexpensive build. And it works great.

[Scodiddly]

DSCN9304.JPG

Love the enclosures!

Now while I don't like being competitive in this kind of thing... oh, who the hell am I kidding?
It's the top unit - the one right below it is the full TapeOp article preamp I built last year.

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One of the local libraries has a laser CNC thing, like a small industrial unit!

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Quarter-sawn oak, from an old oak 2x4 I found in the crawlspace of my house.

[winky dinglehoffer]

Wood panel(ing) is far classier than my spraypaint and waterslide decals. Nice work!

[floid]

I did get rid of that hot resistor, it was there in case the output was needed as an arc welder power supply. Massive overkill. Without it, the total power consumption of my 4-channel version is 1.5 watts.

R5 is 1M for a DI input - for an input transformer it'll be somewhere in the 10k's of resistance, depending on the transformer. My DuKane transformers, in the original tube schematics, wanted 68K, the Cinemags more like 100K.

Great, thanks! I assume you mean hot transistor. That darlington sitting atop a current source in the original is remarkably similar to a cheap stereo I fixed for a friend earlier this year, which used dinky little to-92 transistors in thatconfiguration to drive an 8" speaker. They like to burn up though, even with heatsinking .

Great looking builds.

individually biasing each fet or sorting them so that uniform values could be used.

As the schematic notes, adjust the value of R1 for 12 or 13 volts. I socketed R1, but Scodiddly says you can just drop a resistor in place without soldering and measure the voltage (I have different value R1 in each of mine). If you've got a suitable transformer on hand, this is a really simple and inexpensive build. And it works great.

Does changing this resistor affect the gain?

[Scodiddly]

Great, thanks! I assume you mean hot transistor.

Fixed my post, thank you.

Does changing this resistor affect the gain?

I don't think it would change the gain (much), the biggest issue is setting the zero point of the waveform for symmetrical clipping (if any). Basically not wasting any headroom by having the zero point be too close to one rail or the other.

[Scodiddly]

Wood panel(ing) is far classier than my spraypaint and waterslide decals. Nice work!

Thanks!

I am planning on eventually building one like yours, since I've still got one octal DuKane transformer left. Nice industrial vibe.

[trodden]

Somebody help my dumbass. Why a fixed gain mic preamp? Do you have to use a compressor afterwards to set your levels? What if the output is too hot for whatever is next in line?

[winky dinglehoffer]

Why a fixed gain mic preamp?

A fixed gain preamp is not at all uncommon. Lots of classic designs consist of one or more fixed gain amps with some form of resistive attenuation (pad, pot, or fader) before, between, or after the fixed gain stage(s). Adding an input pad to this design is nothing more than calculating resistor values & wiring up a switch.

There are other options for attenuating signal--pads built into mics, inline pads, that sort of thing. For me, my A to D has line level inputs with gain adjustable from 0 to 20 dB. So if I need a little boost, that takes care of me. If I need less level, I'll use an inline pad.

For me, I like the simplicity, small size, and sound of this design. I may do one with a pad on the input, but not having it is not the biggest deal in the world, IMO.

[Scodiddly]

Fixed gain also means a simpler and possibly quieter circuit.

[Scodiddly]

Of course the main reason was that I had a pair of CineMag transformers burning a hole in my pocket. Apparently some group buy like 20 years ago.

[trodden]

So is these DIY gain blocks the same idea as this?

https://soyuzmicrophones.com/launcher-d ... PZEALw_wcB

Seems this is run before a preamp as well as a fixed gain.