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Make DIY inline preamp like Triton Fethead?
Old 21st February 2011
  #1
Gear Nut
 
🎧 10 years
Make DIY inline preamp like Triton Fethead?

Does anyone know good DIY versions of this inline preamp? A schematic with high quality components would be excellent. Like what is a transparent op amp/jfet and what capacitors are good to use. I found one cheap (around 10 dollars) product that shows the schematic but I am not sure of the quality:

Velleman UNIVERSAL MONO PREAMPLIFIER KIT: Designnotes.com

Best Regards
Old 22nd February 2011
  #2
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years


Judging by the 5% carbon resistors and the un-bypassed electrolytic caps, I'd say that the quality on that one isn't that great...

I'd look for a design that uses an integrated mic pre (like the THAT 1510 or INA217). But, for use with phantom power, a descrete BJT design is probably better.



-tINY

Old 23rd February 2011 | Show parent
  #3
Gear Nut
 
🎧 10 years
Tiny, what type of resistors are great for audio? Regards.
Old 23rd February 2011 | Show parent
  #4
Gear Guru
 
tINY's Avatar
 
1 Review written
🎧 15 years


Well matched metal film resitors



-tINY

Old 28th October 2020
  #5
Gear Head
 
2 Reviews written
🎧 10 years
Quote:
Originally Posted by ickefes ➡️
Does anyone know good DIY versions of this inline preamp? A schematic with high quality components would be excellent. Like what is a transparent op amp/jfet and what capacitors are good to use. I found one cheap (around 10 dollars) product that shows the schematic but I am not sure of the quality:

Velleman UNIVERSAL MONO PREAMPLIFIER KIT: Designnotes.com

Best Regards
Pay attention on schematics like this one:

https://groupdiy.com/index.php?topic=5743.300

Because I've made it, it works but it doesn't boost anything.
It works I mean: when you give the 48V yo get the audio signal through it.
Without 48V no signal is passing.

BUT the 25-30dB gain: not even the shadow
Old 28th October 2020
  #6
Gear Guru
Use a matched transistor pair or the CMRR will go to crap. Linear Systems makes very good ones, get a free sample.
www.linearsystems.com
Old 30th October 2020
  #7
Old 3rd November 2020
  #8
Gear Head
Pay attention that the circuit of the TritonAudio FETHEAD is changed. Now it's this one (just delivered this afternoon).
Probably they didn't find any longer LSK170BL OR they got issues with.
I attach also the pictures of the circuit.

P.S. if you ever got high noise level and low amplification by your LSK170BL, just they are fake as many ones around. I did notice that even with the correct Idss, many are really noisy.
I think that's the reason for which they use the "XG" that's a 2SK209. 2 in parallel in order to diminish the noise figure by 3dB each leg, and they are not fake stuff.
It does work fine.
Attached Thumbnails
Make DIY inline preamp like Triton Fethead?-fethead3.jpg   Make DIY inline preamp like Triton Fethead?-fethead2.jpg   Make DIY inline preamp like Triton Fethead?-fethead1.jpg   Make DIY inline preamp like Triton Fethead?-fethead.jpg  
Old 20th January 2021 | Show parent
  #9
Here for the gear
 
Quote:
Originally Posted by TORMY ➡️
I attach also the pictures of the circuit.
Can you post a better quality image of the schematic? I can't read any of the component values.

Also I think the polarity of the caps in the caps in the preamp is wrong as the positive side should be facing +48V.

Last edited by neppert; 20th January 2021 at 09:46 PM.. Reason: Addition
Old 20th January 2021 | Show parent
  #10
Gear Head
Quote:
Originally Posted by neppert ➡️
Can you post a better quality image of the schematic? I can't read any of the component values.

Also I think the polarity of the caps in the caps in the preamp is wrong as the positive side should be facing +48V.
There is not any CAP.
There are 4 FETs and 3 Resistors (and 2 diodes as protection).
Where do you see a cap up there ?

Moreover: if you click on the picture with the schematics, it appears big on the computer screen, and it's readable.
Old 21st January 2021 | Show parent
  #11
Here for the gear
 
I mean the cap in the preamp section, but never mind that, it's not important.

The picture is barely readable but it's impossible to distinguish the number 5 from a letter S.

I checked mouser for availability of components in my region and if I want to buy the 2SK209-BL, I have to buy it in bulk with a minimum quantity of 3000 pieces. That would cost me upwards of 300 euros, which is obviously not a viable option. Would it be possible to replace each pair of 2SK209-BL's with a 2N4340, 2N4341, 2N4338 or 2N4339?
Old 21st January 2021 | Show parent
  #12
Gear Head
Quote:
Originally Posted by neppert ➡️
I mean the cap in the preamp section, but never mind that, it's not important.

The picture is barely readable but it's impossible to distinguish the number 5 from a letter S.

I checked mouser for availability of components in my region and if I want to buy the 2SK209-BL, I have to buy it in bulk with a minimum quantity of 3000 pieces. That would cost me upwards of 300 euros, which is obviously not a viable option. Would it be possible to replace each pair of 2SK209-BL's with a 2N4340, 2N4341, 2N4338 or 2N4339?
Hi

The caps into the schematics are NOT necessary
Thy are there JUST for the simulation.

What you need are ONLY the components of the Fethead:
3 resistors
2 diodes
4 FETs.

PAY ATTENTION: the FETs MUST be selected.
Indeed I built the same circuit but with LSK170 (the old fashion same circuit), and it didn't worked at all.
Not only in one case (I built 3 of them) it worked fine with a mic (SM57) and not at all with a Ribbon and SM7B

Thus these FETs are quite critical.

So: pay attention to them

Abot the graphic: "5" or "S" in this case doesn't matter. You can easily distinguish them knowing what you are doing.
The graphic is not better with high res. This is the limitation of LT SPICE

I will see to remake it with kiCAD, which has better graphics
Old 21st January 2021 | Show parent
  #13
Gear Head
Quote:
Originally Posted by neppert ➡️
I mean the cap in the preamp section, but never mind that, it's not important.

The picture is barely readable but it's impossible to distinguish the number 5 from a letter S.

I checked mouser for availability of components in my region and if I want to buy the 2SK209-BL, I have to buy it in bulk with a minimum quantity of 3000 pieces. That would cost me upwards of 300 euros, which is obviously not a viable option. Would it be possible to replace each pair of 2SK209-BL's with a 2N4340, 2N4341, 2N4338 or 2N4339?

I hope this is more readable.

I rechecked the values of the components so that now it's ok.
I also already prepared PCBs.
About the FETs that you are looking for. It's possible only if they have not the required noise figure. It should be equal or less than 1nV/SQRT(Hz) and of course: polarization.
BF862 and 2sk209 are ok, for instance. But pay attention to Drain/Source that can differ from the schematics. Just a small attention to pay :-)
Attached Thumbnails
Make DIY inline preamp like Triton Fethead?-schematics.jpg  
Old 21st January 2021 | Show parent
  #14
Here for the gear
 
Quote:
Originally Posted by TormyVanCool ➡️
I hope this is more readable.

I rechecked the values of the components so that now it's ok.
I also already prepared PCBs.
About the FETs that you are looking for. It's possible only if they have not the required noise figure. It should be equal or less than 1nV/SQRT(Hz) and of course: polarization.
BF862 and 2sk209 are ok, for instance. But pay attention to Drain/Source that can differ from the schematics. Just a small attention to pay :-)
Ah yes that image is much better.

I found a retailer for the FET: https://www.tme.eu/nl/en/details/2sk...09-bl-te85l-f/

I also cooked up a schematic and PCB, would you care to take a look at it to check for any mistakes? https://oshwlab.com/zipdox/fethead
Old 22nd January 2021 | Show parent
  #15
Gear Head
Quote:
Originally Posted by neppert ➡️
Ah yes that image is much better.

I found a retailer for the FET: https://www.tme.eu/nl/en/details/2sk...09-bl-te85l-f/

I also cooked up a schematic and PCB, would you care to take a look at it to check for any mistakes? https://oshwlab.com/zipdox/fethead
Yes it's basic. To make this wrong it will be unforgivable :-) it is like to wrong to track a small line :-)

I see you use EasyEDA ... look: JLCPCB has all the components included the FETs.

5 FetHead ar costing something like 8-10 euro in total and they come with soldered component.

Even not any fatigue to sold them (they are SMD and it's not easy without the proper tools)
Old 22nd January 2021 | Show parent
  #16
Here for the gear
 
I ordered 30 PCBs and I'll order the components from TME. LCSC doesn't have the 2SK209-BL, only the 2SK209-GR. I guess I could've used the assembly service for the other components but it doesn't really matter as I have a rework station so I can do the soldering myself.
Old 22nd January 2021 | Show parent
  #17
Gear Head
Quote:
Originally Posted by neppert ➡️
I ordered 30 PCBs and I'll order the components from TME. LCSC doesn't have the 2SK209-BL, only the 2SK209-GR. I guess I could've used the assembly service for the other components but it doesn't really matter as I have a rework station so I can do the soldering myself.
Right although the only difference is that GR have less IDs current than BL. At the opposite of 2sk170GR vs BL where GR has higher current.
Moreover, if you see the picture I posted for the TritonAudio, they use the X-G FETs that from the datasheet it corresponds to GR not BL
Attached Thumbnails
Make DIY inline preamp like Triton Fethead?-2sk209.jpg  
Old 22nd January 2021 | Show parent
  #18
Here for the gear
 
What IDSS would be more desirable in this use case?
Old 22nd January 2021 | Show parent
  #19
Gear Head
Quote:
Originally Posted by neppert ➡️
What IDSS would be more desirable in this use case?
I do use the one that the designers have used.
So, considering that the 2sk170BL used previously had an Idss current of about 6mA,

and considering that they switched to the 209s because the 170s are out of production (obsolete) and what you find in commerce, often are just marked 170 but they have nothing to do with the original ones, I guess they kept the same Idss current. Hence they got the "6G" as you can clearly see form my pictures.
Thus the GR, in case of the 20.

Aside this, I bought a bunch of 170BLs from Banzaimusic.com,and measuring them, I noticed that only 2 of them where into the BL specs. All the other ones were into the GR specs (related to the datasheet of 2sk/lsk170) and the circuit didn't work at all. Using the 170BL but they were not matched, it worked only in few cases but not in others.

Hence: it's critical design.

I just only hope that with the 209GR (that are equivalent to the 170BL) things will be better.
Old 22nd January 2021
  #20
Gear Guru
Unless you have a curve tracer don't waste time with unmatched jfets. Even then, temperature variances will cause them to drift apart. The solution is easy, the matched ultra low noise LSK389A from:
www.linearsystems.com

If you are nice they will send you a free sample.
Old 22nd January 2021 | Show parent
  #21
Here for the gear
 
Quote:
Originally Posted by Jim Williams ➡️
Unless you have a curve tracer don't waste time with unmatched jfets. Even then, temperature variances will cause them to drift apart. The solution is easy, the matched ultra low noise LSK389A from:
www.linearsystems.com

If you are nice they will send you a free sample.
Won't the fact that there are 2 FETs in parallel average out some of the discrepancies?
Old 23rd January 2021
  #22
Gear Guru
Only if matched. Placing both packages into a thermal sharing clamp with heat sink compound ajoining them will cause the thermal drifts to cancel out somewhat. dbx did this in the old days to keep VCA gain control matching transistors from drifting.
Old 23rd January 2021 | Show parent
  #23
Here for the gear
 
Quote:
Originally Posted by Jim Williams ➡️
Only if matched. Placing both packages into a thermal sharing clamp with heat sink compound ajoining them will cause the thermal drifts to cancel out somewhat. dbx did this in the old days to keep VCA gain control matching transistors from drifting.
Is the drifting really that bad? What will it cause? How will it influence the sound quality? What's are the worst case and best case scenarios?
Old 24th January 2021
  #24
Gear Guru
Build some and measure it? I used to measure the drift while breathing on the DUT. Raise the temperature and watch.
Old 24th January 2021
  #25
Here for the gear
I just had to have a look what "DUT" meant then,

Device under test...

I thought for a second it was part of the FET or the circuit, or maybe the probes on your meter...
Old 24th January 2021 | Show parent
  #26
Here for the gear
 
Quote:
Originally Posted by Jim Williams ➡️
Build some and measure it? I used to measure the drift while breathing on the DUT. Raise the temperature and watch.
You just verbally spat in my face. Unless you know what you're talking about and can tell me the actual implications of "drift", leave this thread. I asked a simple question and you essentially told me to go find out myself.
Old 25th January 2021 | Show parent
  #27
Here for the gear
Quote:
Originally Posted by Jim Williams ➡️
Placing both packages into a thermal sharing clamp with heat sink compound ajoining them will cause the thermal drifts to cancel out somewhat. dbx did this in the old days to keep VCA gain control matching transistors from drifting.
I'm loving this, was thinking of doing some of these on a little strip of paxolin board, point to point, and thought could I make a little plastic cradle for these 209's and just use a silver paste of somekind to not damage when soldering and re-use in the future if they become obsolete again and someone needs them,

But of course heat sink compound!
Old 25th January 2021 | Show parent
  #28
Here for the gear
Quote:
Originally Posted by neppert ➡️
You just verbally spat in my face. Unless you know what you're talking about and can tell me the actual implications of "drift", leave this thread. I asked a simple question and you essentially told me to go find out myself.
I'm sure Jim just meant what he referred to earlier, the CMRR will always be bad with unmatched things, I've just got into building a phantom power box and its just the same with the 6.8k resistor matching to help with the balancing and noise etc

I'm researching this myself at the moment, I don't understand drift but I imagine both lines on your XLR must have to be as exact as possible, almost like timing on a engine, so both signal lines arrive at the same time in "realtime" and in the same phase correlation etc

I've just been reading these articles, so maybe it could be relevant?

Flicker Noise and Current Noise in FET's

https://en.wikipedia.org/wiki/Flicker_noise

https://www.analog.com/en/analog-dia...ssue-174.html#
Old 25th January 2021 | Show parent
  #29
Gear Guru
Quote:
Originally Posted by neppert ➡️
You just verbally spat in my face. Unless you know what you're talking about and can tell me the actual implications of "drift", leave this thread. I asked a simple question and you essentially told me to go find out myself.
The subject is far to difficult to explain here in a on-line forum. That's why they teach this stuff in college, it takes a bit of time to absorb and process the information.

I was often challenged in college to "figure it out for yourself". If that is too taxing, do easier things with your time.
Old 1st February 2021
  #30
Here for the gear
 
I made it, albeit I made a mistake in the PCB layout by using the wrong XLR connector footprint, so I had to cut a trace and bodge one of the connectors. It's fixed now, the oshwlab link I posted earlier should show the fixed PCB.

Anyway, this is what is sounds like. It's connected to a Behringer U-Phoria UMC22. The mic is some generic dynamic capsule with solid copper wires on it shoved straight into the end of an XLR cable. There is no noticeable noise unless you turn the gain up very high, at which point even whispering almost clips. I haven't shielded it yet though, so there is room for improvement. Obviously a better mic would improve things too.
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