Determine the non-linear function of valves with op-amps

[MuLdEr]

I have a few questions for You. Because I want to build the simulator of valve amplifier with op-amps and I have trouble with determine the non-linearity. How limiter should I choice? The simply circuit with zener diodes in feedback loop it's not enought to do it. I have computed some simulations. And what are You thinking about the jfet transistors? I mean, it have a quite similar DC characteristic like valve.

Look at the my pics. I included results of DC sweep simulation of VOX AC30 - first and second stage on 12AX7. And BF245C to match the characteristic. I use LTSpice.

I want to know what You think about my idea and I'll be glad if You have any comments.

 

[Russlk]

This site is a good tutotial on modeling vacuum tubes and if you scroll down to "Appendix A" and click on it, there is a model of 12AX7.
http://www.normankoren.com/Audio/Tubemodspice_article.html

 

[MuLdEr]

Reply

Russlk. You missunderstood me :cry:

I wrote:
I want to build the simulator of valve amplifier with op-amps.

Is that right? No?

I simulated the DC transfer function of first stage of VOX AC30 preamplifier. That shows my pics. I use LTSpice and tube models from Norman Koren site who's not actually into PSpice simulations.

My intention is to build an op-amp circuit which simulates the non-linearities of tubes. Especially VOX AC30 amplifier. So I must build op-amp circuit with feedback loop. And in that feedback I must put diodes or/and transistors with some resistors to copy the DC transfer function and in that way simulate the non-linearity of any tube. Do You understand me? The sound from original valve amplifier and my simulator should be the same. And I hope will be

Is here anybody who understand me and could help me :?:

 

[Nigel Goodwin]

There was a VOX AC30 simulator design on Elliot Sound Products website, designed by Stephan Möller in Germany. However, it was removed last September as Stephan has now sold it as a commercial design (or something like that).

I've still got a copy of the original files, but the crucial component values are missing - you had to email Stephen to get them, and sign a non-disclosure agreement.

 

[MuLdEr]

Reply

Thanks for Your reply.

I will be very happy and thankful If You send me all those origin files from Stephan's project.

It's not important that the some values are not shown. Because the most important to me is how to do it and why some things was done in this way.
Thus I will be able to built simulator of Marshall amplifier or another.

If You can please send it at mdl4891@eastwest.com.pl

all the best

MuLdEr.

 

[ljcox]

Simulating valves

see the latest Elector magazine, they have a simulated valve amp using (from memory) MOSFETs

 

[MuLdEr]

Reply

see the latest Elector magazine, they have a simulated valve amp using (from memory) MOSFETs

Dear Mr. ljcox

I come from Poland, not Australia. So how I would get latest Elector magazine?

Does magazine have any web page and pdf files with thier articles?
If yes give me an address. If no and You have this number, send on to me schematic(s) and description. You can scan it or write it down

It can be a very interesting article.

However, please write back with answer.

Regards

MuLdEr.

 

[Nigel Goodwin]

Re: Reply

see the latest Elector magazine, they have a simulated valve amp using (from memory) MOSFETs

Dear Mr. ljcox

I come from Poland, not Australia. So how I would get latest Elector magazine?

Does magazine have any web page and pdf files with thier articles?

It's actually Elektor, and it's not Australian - it's European, based in Holland.

The URL for it's UK site is http://www.elektor-electronics.co.uk. According to the previous issue, the current one (which I haven't got yet) includes an RIAA preamp, simulating a 'valve' sound.

If you want an AC30 'sound', why not just build an AC30 type amplifier?, as they are only low power the transformers aren't too expensive - and as you already know, the circuits are freely available.

 

[MuLdEr]

Reply

Thanks for URL.

I've planned to build VOX AC30 TB (probably in summer). I won't buy any transformers because I have a lot of it.

I just want to build the simulator because it's interesting project and some alternative for valves which are expensive as well as have some time of work given by producent in defined environments, loses void which cause fat sound and many other...

 

[Russlk]

Many years ago I wrote a BASIC program to design a non-linear amplifier as below. I don't know if I can find it now, but if you are interested, I will look for it. As I recall, there is a program on my website that designs a non-linear network, not using an op amp. It works into a high Z load.

 

[ljcox]

Simulating Valve amp

The Elektor magazine you need is the November 2003 issue, see page 47.

Len

 

[MuLdEr]

Reply

Elektor-Electronics originally come from Holland as write Nigel and it's publishing only in four countries Not in Poland. I don't want to subscribtion to the UK edition of magazine. It is non-sens. Because I will have to wait very long for each issue and pay a lot of money. Besides I've just wondered about JFet simulating valve. I no need all articles. I think its a quite understandable.

However, thank You all for any help.