looking for recommendations: preamp a perfboard - EDIT: FETBOY

[rock soderstrom]

will a DMM that measure  hfe be enough for selecting the fets?

i have ~6x 2N5457 and ~20x J201, hopefully i can use J201 instead. do you select the 5457s according to hfe? or Voltage?

No, DMM with HFE measurement function do not help here. The whole topic is well explained in the internet, check google.

e.g. http://stompville.co.uk/?p=112

An easy way to select the right Fet for your application is to build up one gain stage of the Hamptone preamp and check in circuit(with  a socket). Than you can select easily for the right drain voltage.

 

[aviel]

Thanks! 
will have a look.

i saw here on the forum that if i dont find a good FET (which i wont, since i have only 6) is to add another trimmer just to get the voltage down  (the base of the darlington shoulf be V+/2, right?)

i am bit confused by the original hamptone article, stating: "The drain voltage (of the 5457) is slightly above the supply." how is that even possible?

 

[rock soderstrom]

Trimmers are good for your testrig, I do not like them (in this case) in final preamps. I prefer to substitute them with static resistors at the end.

Yes, it is right, the drain voltage of the FET (or base of the darlington) should be somewhere around V+/2. Of course, if you alter the drain and source resistors to reach that goal, you will alter the other parameters (gain, etc..) of your FET, too. In my opinion, its ok to slightly adjust with a trimmer, but some of the FETs will never work right in this circuit. Thats one of reasons experts do not like this circuit. They say this design is flawed (FETs need selecting and "Kindergarten" current source, I guess you have read this in the old threads..) and I think they are right, if you judge this from pro-circuit-designer perspective. Nobody would start a industrial production project with a circuit like that, but we are different.  We can do it.

I can not find "The drain voltage (of the 5457) is slightly above the supply." in the original article. Makes no sense. I guess he is talking about V+/2.

 

[aviel]

started building this, but i currently get the base of the darlington at ~20Volts (or V-2v), am i missing something?
i tried to caclulate Ids that is required, according to the attached image.
turns out i need ~0.7mA for Vds = 11V, Vgs = 0. according to the data sheets, at Vds=11V and Vgs=0, i will get TYPICAL 4.5 mA, much higher than needed... how does this work then?

 

[volker]

JFETs have big tolerances. That's why there is a range given for I_DSS and the cut-off voltage, not a fixed value. Also, you don't operate at V_GS = 0. The source is elevated by the voltage across the 1,8k resistor.

 

[rock soderstrom]

started building this, but i currently get the base of the darlington at ~20Volts (or V-2v), am i missing something?

Thats what I am talking about, the drain voltage of your JFET is to high. You need the right one or you have to adjust this. Datasheets will not help you a lot here.

 

[aviel]

JFETs have big tolerances. That's why there is a range given for I_DSS and the cut-off voltage, not a fixed value. Also, you don't operate at V_GS = 0. The source is elevated by the voltage across the 1,8k resistor.

forgot the VGs is indeed different from zero

Thats what I am talking about, the drain voltage of your JFET is to high. You need the right one or you have to adjust this. Datasheets will not help you a lot here.

got it . i was able to achieve v/2 for the jfet drain (for me V is 18V). i used 2 BD 139G instead of the darlingron (emitter to base). everything seems to work, but i dont get symetrical clipping, seems that the low part of the sine clips before the top one..i am not sure what cause this though, did this happen to you?

 

[rock soderstrom]

Cool! I am not sure about the symetrical clipping. I finalized my preamps with a bass loop at the inputs by ear.  8)

 

[aviel]

Hey!
would be glad to get some advice,
here is my build so far (see picture).

i was thiking of building opamps modules as building blocks and being able to change them to get some different flavours.
originl plan was to use pins to plug them into my main pcb. will this intorduce too much noise? should i just build the 2 modules as one unit?

regarding capcitor seletion, which one did you fiddle with? the jfet drain capacitor?

Thanks!

 

[rock soderstrom]

Yes, you can build up modular gain blocks. This was done in the past and with a careful layout the noise would be manageable. I am not a big fan of such builds, I prefer dedicated setups, neat and tight designs with all the functions i need on a self etched PCB. This would be the next step for me, transfer you experience with this circuit  into a final PCB design. (use your perfboard build as your test bed further on)

Choose the coupling capacitors to your taste and whats on your bench. Every quality MKP etc. will do fine, some have even used old Tantalum caps. Everything is posibble due to the low B+.

As mentioned before, I made the source capacitors on the FETs switchable . (without C / 1uF MKP / Elco + 1uF MKP)  This will give you a variable gain/ bass boost function, pretty handy for finding the right sound for your needs. For nearly pop-free switching, implement this capacitors with a huge serial resistor. (check google for "cathode capacitor switching" for further information)

 

[aviel]

Thanks for the helpful pointers! will definitely have a look.
i got some old 22 uf Tantlum that i plan to use, wasnt hard to find 10uF poly as suggested.
i am still not into self etching, i do most of it on veroboard or just plain perfboard..

Can i ask what you mean by careful layout? i know there are some do's and don'ts when planing a PCB (not crossing power and signal line for example).
but in this case all i can think of is  short as possible distance between transformer and first gain stage.  i will have to pull out wires from the gain block to the gain pot, then back to the second stage.

 

[rock soderstrom]

I am by no means an expert for PCB (and perfboard) design, but I think you have mentioned the most important points. Keep outputs away from inputs, keep all high impendance traces as short as possible and use shielded wires for the pots.

Perfboard and veroboard are not bad at all.  I still use them for very small circuits in guitar pedals and I have seen people doing successfully crazy complex stuff with them. On the other side I hate searching errors on a perfboard, this is really a pain in the ass.
Thats why I have taught myself to produce PCBs at home. Its easy and I love etching PCBs now! Thats magical and makes me feel like a wizard ;-)

Give it at try, this could bring you on a whole new level. I use the toner transfer method with a flat iron and I etch with ferric chlorid.

 

[aviel]

Thanks for all amazing insight and patience..!!