HAMPTONE jfet PREAMP

Hi all,
I've been working on the Hamptone design JFET preamp design from Tape op a year or so ago. in the schematic shown, he used a 5K taper in between the two FET gain stages. He said that you could add another taper in order to crank the input gain and lower the output gain to give the preamp a input drive type distortion. TO do this, would you put the extra taper prior to the the first stage or after the output stage?
Thanks

Thanks buttachunk,
One question about the power supply. When hooking up the ground from the wall wart power supply does it just get combined directly with the other leads that go to ground?

Hi,another question...

Is there any replacement transistor for the ztx?

Anyone?

buttachunk, what kind of transformer are you using for input? Any suggestions?

I have built a hamptone jfet stage transformerless, with a 10M resistor going to ground at the input. It already sounds pretty decent after a short test, just that the transistors get really hot, I have to check that...

I recommend NOT doing 2 cascading stages,, just one. Reason being that the distortion of one JHP stage is asymetrical... adding the second stage increases the distortion and noise, and the resulting clipping becomes symetrical (harsh/bad-sounding).

Click to expand...

What about using an interstage transformer for phase reversing, and wiring the output transformer as phase reversing also? So, the assymetrical products would sum in a even way, I think.

First gain stage -> interstage transformer reversing phase -> Second gain stage ->output transformer reversing phase...

Maybe this would solve the problem?

Also, I´ve read in another thread, in a post by chrissugar, that you should select a very low HFE transistor for the NPN transistor. Lower than 100, he said, otherwise you would have lots of distortion.

Mine works like a dream. No noise and very musical coloured sound.
Also with the added local feedback controll ( my mod ) you can have the whole palete of sound from clean to coloured.

chrissugar

For cheap iron, you can always use EDCOR. Buy from them and keep it alive! They do nice iron, cheaper than chinese stuff...

Chrissugar, are you using two stages or just a tranformer + one gain stage?

Would be very interested in knowing a little more about this loca feedback MOD...

Rafa,

Transformer plus two stages. Tried it with Lundahl and OEP. Sounds great. Now I have OEPs in the preamps (the ones used in the G9 at the input).
About the feedback mod you can read at the other thread and daArry posted for me a pic with it:
http://www.groupdiy.com/index.php?topic=1184&postdays=0&postorder=asc&start=30

chrissugar

Thanks!

I've been playing around with a few of these for the last couple of weeks, chrissugar has been very helpful. His feedback mod really makes these things come alive.

I'm having a hell of a time getting the intrument input to play nice in this circuit. It's giving me a ton of hum when nothings plugged into it.
Can anybody confirm that it's drawn correctly in the magazine? I've got the sleeve going to ground, the tipsleeve going to the center pole of my spdt input pad switch, and the tip going to the first jfp stage, and then to a 10M resistor which is connect to ground.

some info to consider if you're building based on the original article:

http://www.groupdiy.com/index.php?topic=1184&postorder=asc&start=44

ed

[quote author="E-money"]
I'm having a hell of a time getting the intrument input to play nice in this circuit. It's giving me a ton of hum when nothings plugged into it.
[/quote]

Beeing a very high Z input it is normal the to colect humm if it is open, but the trick is to wire the DI input connector in a way that will not let the jfet input open until you introduce a jack into the DI in. When there is no jack in the DI in, the jfet in is always conected to the input transformer.
Also look at Jakob's G9, it is the same trick.

chrissugar

So if using a single gain stage for a mic pre, should the taper come after the gain stage or before?

thanks for all your help. I got a Cinimag tranformer for the input.

which type did you get? any suggestions?

Coldsnow, the pot comes after. Then I like to put the transformer after that.

Click to expand...

I have no experience with the ham-tone but from a headroom- and distortion-point of view, this is surely not the ideal configuration.

On the other hand, this is a colour-toolbox only and anyway, so forget my interruption!

Samuel

I am currently in the beginning process of building a hamptone DI. A year or so ago I bought Scott's kit and have been very pleased with the sound of the DI and preamp, so I would like to make a portable box. I am missing a few parts, so I have some questions about replacements. Can I use a regular old XICON cap in place of the more expensive Sprague 470uf 50v? I also lack the 10uf, 100v sprague cap for the output of the card, but I do have a 1 uf, 100v Sprague cap. I have a feeling that 1uf will not work properly, but can someone tell me why and what the result will be? Hopefully the answer will help me understand capacitance a little better. I have a similiar question regarding resistance. What value resistor do I need to use to send to ground between the DI jack and the gain stage input? How do changes in value of that resistor affect the signal? I currently have a 2.2m 1/4w and a 3m 1/2w.
Thanks in advance,

jon

using an interstage transformers is indeed an interesting subject