Hamptone JFET mic pre help!

Hello! I have recently built a single channel Microphone pre amp from a schematic by Scott Hampton (hamptone.com). The schem was printed in an old issue of TapeOp. I had never built anything electronic before that was not a kit, and I really need help in laying out the PCB (actually I used perf board). I want to re-build the first channel of pre amp and then build a second for a nice two channel box. My first channel is rather noisy (ground hum) above 75% gain. I did not use an output transformer. It seemed like the output tx was not necessary.

Could anyone help me lay out this circuit? I have a link to the schem at the bottom. I have changed the LM78M24 to a LM78M20 after reading some things on Scott's website (please disregard the other notes I have scribbled next to the PSU!). I had alot of fun building this pre!

Please help me figure out where to put the components to lower the noise!
Thanks alot!

here is the schematic from TapeOp, designed by Scott Hampton -
http://rockstudio.com/jpg/JFPsmall.jpg
 
Hello Rockstudio--
First, let me ask how you've grounded everything here, and what sort of input transformer are you using?

Second, let me recommend the Edcor WSM600/600 as an output transformer--it's inexpensive (about $8), and actually sounds ridiculously good, especially at that price. I think that, at that level of gain, output impedance might be a bit high, although upon second glance it looks as though the two NPN's are acting as buffers...
 
Thanks for your help! I am using a jensen JT-115K-E input tx. The schematic recommends a 600:600 output tx. I couldn't afford to buy the jensen model I found.
As for grounding, I have the power supply on its own small board with its own grounding lug. The pre amp has two grounding lugs, and I basically put straight wires to ground (for every cluster of components). I had never built anything from a schematic only, so I just used my imagination as to how the grounds should be connected.
The input and output jacks have short leads to ground as does the gain pot.
Is there a rule to follow when placing the transformer on the board?
Thanks Again!
 
I'm just going over simple things in my head, so forgive me if I suggest something you've already tried.

Are you using twisted pairs for your power/audio leads?

Is your AC line in the chassis near any audio lines?

Do you have LED's or power switches located near any of the audio stages?

Am I correct in my interpretation that the input, output, gain pot, and audio stages all have separate lines to ground? I was reading and interesting bit of information today about layout and grounding. It talked about how, if you're running several wires to ground within the audio stage, the circuit will see all of them as very small value resistors. By returning all of these stages to a common ground point, there is negligible resistance that can cause voltage drops that appear in series with the signal. Then, what happens is those voltage drops get amplified along with the audio signal, and it shows up as voltage noise. Try running the audio stage grounding along a single bus wire, and then run that to ground at one point with a thick grounding cable, such as a piece of solder wick.

The reason I asked about the type of input transformer is because mic input transformers are susceptible to RF buzz if not shielded properly. Did you connect the transformer's shield to ground as well?

Hopefully some of these things will help, and you'll learn some stuff that will benefit you the next time you build one.

Speaking of which, I never realized how incredibly simple this circuit is! I think I'll build one myself, just for the sake of experimentation!
 
mnewport, thanks for your tips. I will try all of the above suggestions before I build the second channel. I was careful not too put anything AC near the pre amp board, and all of my lines are twisted together. I really want to make sure that I don't cause (inductance?) if I put something too close to a JFET or transformer. Thanks again! I think you should build on of these yourself, I have been using mine in the studio and loving it. I have to add some batteries for phantom power now...
 
Feedback for the JFET preamp and question...

Hello,
I just built one channel (so far) including a power supply for this Hamptone JFP design from that TapeOp mag article, too.

I have feedback for you as well as question(s) for anyone reading this...

First off, the most important thing I implemented was a solid "star ground" configuration. Star grounding is when you run all of your grounds back to a common point. Keeping them as short as possible and as large a conductor as possible, too. See my pictures as an example (link at the end of this message).

I used solder wick as the main ground-buss area that all grounds are tied to, which is physically located very close to the power supply.

Other than the grounding, of course keeping your audio paths as short as possible and shielded is important. Your chassis should be earth grounded (the third prong on your AC line cord coming in to the unit, if your powere supply is internal, that is) and isolate your signal ground from your chassis with a switch to lift or connect sig. gnd. to earth gnd. This includes the 1/4" jack for post input transformer connection, aka "DI".

Also, I did not use a PCB for all audio paths. More reliable than a PCB, more room for the parts to breath, easier to modifier, fix, revise, etc.

Shield your signal wires that travel more than a few inches.

Only use high quality coupling capacitors. Try the Solen Fast caps found at Antique Electronics > http://www.tubesandmore.com/

The problem that I am having IS THIS > My bias for the JFET is a little off. For example, as per the original design that Hampton issued, the JFET is biased in a manner that sets the drain voltage to ~17.5 volts. I don't know why, but it is off enough to cause severe asyemmetrical clipping. This is obviously unacceptable.
Anyone have any ideas??

Thanks~
 

chvario

Member
2015-08-05 4:46 pm
There hasn't been any activity on this thread for a while.
I have been working on my version of the Tape-Op Hamptone JFP mic preamp for a while. I thought I was pretty much done, but then started seeing issues. First with noise (which I think I can solve with right grounding and putting distance between my audio lines and the AC)

Where I am stuck though is with the non-Jfet transistors: I am seeing is that the Darlington and NPN transistors get VERY hot. Is that the case for others who have built their version of the pre-amp as well? What are you doing about it? You can't really put a heatsink on those to-92 devices.

Thank you very much, I appreciate your advice.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
How hot is very hot ? Can you keep the top of your middle finger on them for 5 to 10 seconds without them burning you ? If so then its probably OK.

You can actually get T092 clip-on heatsinks, they used to be very common.

(This thread is so old that the circuit linked to in post #1 is a dead link)
 

chvario

Member
2015-08-05 4:46 pm
Thank you for the answer.
Lets say I could will myself to have my finger on the transistor for 8 seconds, but it starts hurting around 3-4 seconds in, when you want to pull it away. Is that too hot? :)

I will leave the circuit on for an hour, and will check whether everything still works. And I will put heatsinks onto the transistors. There are instructables on how to make heatsinks for to92 yourself.

I found a copy of the article on the preamp here:
http://www.tangible-technology.com/ipr/AE230/html/wk_6/hamptone_fet_scan.pdf

Cheers, Chris
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
That makes more sense seeing the circuit... yes, I think a clip on heatsink might be advisable.

There is a quick check you can do that could be useful. If you measure the voltage across the 100 ohm resistor, then you can calculate the current flowing.

Ohms law I=V/R. The diagram shows approx 40 milliamps which means you would see 0.4 volts.

Knowing the current we can calculate the power in each transistor by measuring the voltage across each device and multiplying that by the current. So 40ma and 12 volts across the top darlington would equal 0.48 watts (0.040*12). That is right up at the maximum for the quoted device (its rated 0.5 watt).

So I would say you do need small clip on heatsinks for reliability.
 

chvario

Member
2015-08-05 4:46 pm
Thank you for talking me through the measurements, and to point my attention to the max wattage of the devices, that's immensely useful. I will measure/calculate that later today.
The transistors didn't die during the hour I left the circuit running, so I am hopeful I should be fine with clip on heatsinks. I already ordered them.
 

chvario

Member
2015-08-05 4:46 pm
Thank you, that's good to know about the kit having clip-on heatsinks. Without the heatsinks, the darlington seems to be breaking consistently.

While doing the measurements suggested by Mooly, I realized that it broke again:
Voltage across the 100 Ohm resistor is 3.5 Volt, which gets us close enough to the expected 40 milliamps.
But:
Voltage between collector and emitter of the Darlington is around 22V.
Voltage at the base of the Darlington is only around 1.5V, which which throws the biasing of the Jfet way off. I measured that voltage previously, when the circuit was working, and was around 13V.
Once the heatsinks get here, I will switch out the transistors and try again.

Do you agree that this could be caused by the Darlington overheating and breaking? Or the NPN?

Thanks again, Chris