Hamptone

[adason]

Any experiences with this little circuit?

 

[adason]

More about circuit i found.

 

[adason]

I will try it on small breadboard and report back.

 

[adason]

It's allive! Well, i found 2N4392, so thats what i used.
Could not located mpsa18, so i used 2xBC550 as darlington.
Used C2705 for lower bjt as current source, since i have plenty from yamaha pre repair.
I found small pcb from other pre project, so i improvised.
It sounds juicy, musical, with nice second harmonic sweetness to it.

 

[adason]

Had to ground the pot case, as it was buzzing when touched. Now its quiet.
Definitely worth exploring further.
It works from 15 to 25 vdc, i use 18 vdc regulated.

 

[Walor]

nice second harmonic sweetness

This is how THD can be declared unimportant 😎
Remove C3/C5 (or C503) and it won't be as sweet anymore 😉

 

[adason]

You know EEs here would call that blasphemy!

I would say right amount of right distortion is important.

Next i will build clones of these circuits and will experiment with various input jfets, since i have quite a few.

 

[adason]

Ok, i have just located 2n5457, together with its complementary 2n5460, not bad. I will try it next.

 

[lineup]

I will also try.
Maybe with IRF610 at the output.

 

[adason]

Ok, please do, if it looks ok, i will definitely try it.

I made a typo in previous post, i was looking for mpsa14, darlington, not mpsa18...those i found plenty, as well as mpsa06, 42, 92 and others. None darlington. Hence i made my darlington from two bc550. But i am certain i have mpsa14 somewhere.
If irf610 works, i have plenty too.

 

[adason]

Since its raining, i made some progress. Made second set with 2N4392. Both perform well. Due to variations in jfets, there is some disbalance between channels, so i recomment matching.
Both sound great. Thd is ~ 0.06% to 0.08%. Most importantly, pure second harmonic, very little 3rd and up.
Here are some measurements from rightmark.

 

[adason]

Noise level and fr response.

 

[adason]

And most importantly spectrum, it overlay of both circuits, even they are slightly different, they perform in the same manner.

 

[adason]

Made one circuit with mpf102, since its so common. Now, would you look at that! Pure 2nd harmonic bliss! Can any of you show preamp which has only 2nd and no other harmonic? Reminds me Schiit Valhalla.

 

[adason]

Building more clones, will be testing various input jfets. First i will try 2N5457, originally used jfet in the hamptone. Than this is what else i found in my stash. I am aware that some jfets might need some bias adjustments, i am not going to individually deal with it for now.
2SK246
PF5102 (i think this is replacement for mpf102)
J112
J113
LSK170
2SK30A
Then i found some, which are not jfets, but small depletion mos, not sure if they would work here, but it may be worth the try.
DN2535 vfet
BSS129 dmos

 

[Citizen124032]

The 2N5457 - 2N5460 family is an outstanding league, but as with all J-fets, abundant spread in specs within, hence the R12/R18 pot.
In it's simplicity, it's a kind of a 'pass-amp"!

 

[adason]

Just for fun i did what Walor suggested, removed c3/c5.
Measured before and after.
THD dropped in general, 2nd harmonic went down, but small amount of 3rd appeared.
However, gain dropped significantly too, about 12dB.
So its totally modable. You can make it any way you like it.

 

[adason]

Originally suggested 2n5457 had highest amount of pure 2nd harmonic. No other harmonics present. But it sounded too mushy on main system.

 

[adason]

So i replaced it with dn2535, now it has lowest distortion so far! Very clean. Definitely will give it a listen.

Btw, distortion of 2n5457 was ~0.2%.
With dn2535 its 0.02%.

 

[adason]

Well, back from vacation, i am going to compare three working prototypes of hamptone preamps. With and without the caps.
All will be measured at same pretty high level, to utilize full scale of sound card and satisfy rightmark software. At 388mV rms, 1.12V pkpk.