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Author Topic: Preamp suggestions needed!  (Read 20048 times)

michelterres

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Preamp suggestions needed!
« on: August 28, 2013, 10:49:42 AM »
Hi, good morning everyone, first I'd like to thank the moderators for this amazing forum, I've been reading some threads for some days and decided that it's time to participate more.
It's been a while since I've started building guitar effects, for my own use, and over an year that I've built my first amplifier, a TDA7294 with clean an dirt preamp, the preamps were both found at www.handmades.com, they call it "pré-clean" and "marshallizer", the clean channel has a good sound, nice to tune, but it really messes the tone of my pedals, I can't find a good distortion tone combined with that clean preamp, the dirt is the same, I really didn't liked it, it has a good distortion, more to overdrive, sounds like a low power marshall amp, but it's awful with muted notes, I have two guitars, one is 20 years old with weak pickups, and the other is my first guitar, with more powerfull pickups, both sound weird with the dirt preamp, I'm using a handmade Sans-Amp GT2 as a preamp for now, that one is awesome except for the fact it doesn't have a "mid" control. Now I'm having more time to work on my projects so I decided to find a good match of preamps to use on my amp, I'm looking all over for different preamps, nothing special, I just want to try a lot of them to find out what I like, for a clean I wanted something that wouldn't mess the tone of my distortion pedals when I use them, and for the dirt I wanted a more defined distortion, doesn't need to have too much gain, just enough.
Now, my point is, what I want with this thread, is to talk about it, to know what do you guys like on a preamp, what suggestions of good ones do you know, as I said on other thread, I'm willing to build a lot of them to try them out, and I'll be contributing here every time I get one working, I can do layouts, more complex circuits can take a longer time, but I'm persistent. Before I knew this forum(last week), I found out this one while looking after solid state preamps:
http://www.redcircuits.com/Page120.htm
Does anyone heard it already? I have a layout done for this one, as soon as finish building it will be my first release here.

joecool85

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Re: Preamp suggestions needed!
« Reply #1 on: August 28, 2013, 12:49:41 PM »
I've actually had good luck using a Little Gem as a preamp (even though it is designed as a small power amp).

http://runoffgroove.com/littlegem.html

**edit**
Also, I forgot I made a layout for the Dean Markley K20-X preamp here: http://www.ssguitar.com/index.php?topic=1885.msg12904#msg12904

I haven't built it myself, but a few others have and it works really well.  I have the amp it was originally designed for and love it.
« Last Edit: August 28, 2013, 12:52:06 PM by joecool85 »
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michelterres

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Re: Preamp suggestions needed!
« Reply #2 on: August 28, 2013, 12:59:46 PM »
I've actually had good luck using a Little Gem as a preamp (even though it is designed as a small power amp).

http://runoffgroove.com/littlegem.html

**edit**
Also, I forgot I made a layout for the Dean Markley K20-X preamp here: http://www.ssguitar.com/index.php?topic=1885.msg12904#msg12904

I haven't built it myself, but a few others have and it works really well.  I have the amp it was originally designed for and love it.
Thank you Joe, is that littlejem a miniamp or its a preamp and a power amp in the same board? I was planning to build a 386 based amp to have on the workbench, it's real useful, about the Dean Markley K20-X preamp, do you mind if I rearrange your layout? I more used with etching pcbs than the perfboard.

michelterres

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Re: Preamp suggestions needed!
« Reply #3 on: August 28, 2013, 01:02:01 PM »
I've actually had good luck using a Little Gem as a preamp (even though it is designed as a small power amp).

Sorry, I've got it wrong when I first read.

Roly

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Re: Preamp suggestions needed!
« Reply #4 on: August 29, 2013, 09:26:27 AM »
Welcome!   :dbtu:

handmades.com link vectors to a sales site for "MoneyFreedom.com" for me, however there is a site in Brazil http://www.handmades.com.br/forum/ which deals with "pré amplificador".

This might work (to English);
http://translate.google.com.au/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.handmades.com.br%2Fforum%2F&sandbox=1

http://www.redcircuits.com/Page120.htm

Oh look, a FET preamp without any trimpots!    ;) 
If you say theory and practice don't agree you haven't applied enough theory.

michelterres

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Re: Preamp suggestions needed!
« Reply #5 on: August 29, 2013, 12:07:43 PM »
Welcome!   :dbtu:

handmades.com link vectors to a sales site for "MoneyFreedom.com" for me, however there is a site in Brazil http://www.handmades.com.br/forum/ which deals with "pré amplificador".

This might work (to English);
http://translate.google.com.au/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.handmades.com.br%2Fforum%2F&sandbox=1

http://www.redcircuits.com/Page120.htm

Oh look, a FET preamp without any trimpots!    ;)

Oh, I'm sorry, I actually was referring to http://www.handmades.com.br/forum/ (I always forget to type the ".br")

That clean pre-amp is kind of good, just needs to be tamed, but I'm more into trying out new pre-amps to see if I discover something I like

Oh look, a FET preamp without any trimpots!    ;)

Now that you said that, it's really weird, I didn't found much more about this circuit... guess I'll have to try it to see if it works
« Last Edit: August 29, 2013, 12:13:11 PM by michelterres »

Roly

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Re: Preamp suggestions needed!
« Reply #6 on: August 30, 2013, 06:51:43 AM »


I have just got around to modeling this in LTSpice and I can't see any obvious reason why it wouldn't work with just about any commonly available JFET.  It's fairly conventional but uses a couple of current sources, Q2 and Q4, to obtain Maximum Available Gain (MAG) from Q1 and Q3.

I think this is a bit over the top when supply voltages up to 30V are specified, and I would be inclined to argue that buffering out to prevent following stage loading reducing gain would do much the same but with the added advantage of each gain stage having an almost ideal high input and low output impedance - as I have done in my offering;

http://www.ssguitar.com/index.php?topic=2912.msg22358#msg22358

This preamp is not the Fender Blackface solid-state clone it claims.  I've modeled the first gain stage and right away there is a problem.  Because of the use of the current sources this has a stage gain of x100 and lacking Source bias it therefore starts clipping with an input of only around 100mV.

It is characterised for an input of only 6mV which is a bit of a puzzle since this is more microphone level than guitar or stomp box level.  For example I have no trouble generating over a volt peak from one pickup while chording, and a couple of hundred mV on single notes.

If we look at the real Fender "blackface" Bandmaster 1966, cct: AB763 which this preamp is supposed to emulate we can see the cathode voltage for the first stage 7025 triode sections are given as "1.9V" and "2.0V" respectively, suggesting a headroom of about this much (peak), say a comfortable volt RMS which is what I would normally expect.



The RCA databook gives a stage gain of about x52 for these conditions (Vsup=375V, Ra=100k, Rk=1k5).

I think JM has made some less than flattering remarks about Redcircuits and this particular circuit certainly seems to be an example where somebody has outsmarted themselves.

Unless you are going to keep your guitar (or stomp) volume control very well backed off, say to about 10%, you are going to overdrive the first stage FET and get clipping shred.

If you want something a bit more subtle such as a bit of characteristic curve distortion then this Redcircuits offering is not going to give it to you, and in fact misrepresents what it is - it's a fine deep shred preamp, but a Fender Blackface emulation it most certainly ain't.
If you say theory and practice don't agree you haven't applied enough theory.

michelterres

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Re: Preamp suggestions needed!
« Reply #7 on: August 30, 2013, 07:43:57 AM »
Wow, Thanks for taking time to analyse this circuit, I was searching about Redcircuits for the last few days, and surely didn't read good things about them... I will give it a try, but just because it's really cheap to build and because I already have a power supply that can handle it, about how it would sound, I was expecting something more clean, I'm used to use strong pickups, so I think my guitar will easily overdrive it, I will search for another clean preamp, for a dirt preamp I think I will try the JCM800 fet version, and the Dr boogey, I read about a guy who used dr boogey as a preamp running it with 18V.

Roly

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Re: Preamp suggestions needed!
« Reply #8 on: August 30, 2013, 08:27:16 AM »
If you follow the link I give above to a different thread here you will find my FET preamp design.  This should be pretty clean, and also give a bit of characteristic curve distortion at high drive levels.  This circuit shows a tonestack of my own design, but there is no reason why you couldn't substitute the Fender or Marshall tonestacks from the Redcircuits design.  While a power supply is shown any clean supply from 18 to 30 volts should work okay.  @Stormbringer now has one of these near completion.
If you say theory and practice don't agree you haven't applied enough theory.

michelterres

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Re: Preamp suggestions needed!
« Reply #9 on: August 30, 2013, 08:46:05 AM »
If you follow the link I give above to a different thread here you will find my FET preamp design.  This should be pretty clean, and also give a bit of characteristic curve distortion at high drive levels.  This circuit shows a tonestack of my own design, but there is no reason why you couldn't substitute the Fender or Marshall tonestacks from the Redcircuits design.  While a power supply is shown any clean supply from 18 to 30 volts should work okay.  @Stormbringer now has one of these near completion.

I think I will use your circuit the way you posted it, the magic is in testing original, leaving the modifications for latter, I don't have much time left for this week, but i think I can do a PCB layout for it next week, then I can see if I have all the parts at home to build it, thanks for the help :tu:

Kaz Kylheku

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Re: Preamp suggestions needed!
« Reply #10 on: August 30, 2013, 05:31:04 PM »


I have just got around to modeling this in LTSpice and I can't see any obvious reason why it wouldn't work with just about any commonly available JFET.  It's fairly conventional but uses a couple of current sources, Q2 and Q4, to obtain Maximum Available Gain (MAG) from Q1 and Q3.

I was looking at this yesterday and also came to the conclusion that it is not cascoded stages; but a simple case of active loading for more gain.

The circuit description claims that it is cascoding, but in cascoding, the "upper" half of the stage is a common base/gate/grid circuit, and so there is a load resistor above where the output is taken, not like here, where it is between the transistors.

Maybe an interesting way to modify this circuit might be to actually convert one or both of the stages to an actual cascoding topology.

Could you post the LTSpice files somewhere?
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Davelectro

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Re: Preamp suggestions needed!
« Reply #11 on: August 30, 2013, 09:16:10 PM »
Look no further. This is what you need:



Try to use fets with Vp ~-2V for best performance.

Roly

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Re: Preamp suggestions needed!
« Reply #12 on: August 31, 2013, 06:33:40 AM »
I didn't read the blerb Kaz, so I didn't notice the claim that it is a cascode, which it clearly isn't.  (I get the impression that Redcircuits knows just enough to be dangerous - since when has Miller feedback been a limiting factor in guitar amps?).


A cascode with grounded Source followed by grounded Gate

@Davelectro - well it actually has some Source resistors, but again with the active loads - why?  This is not a situation where we need to go flat out for MAG; 24-30 volts and a simple resistor will do almost the same job.


The LTSpice model for the first stage of the Redcircuits pre is appended.

Also appended is a screenshot of (an earlier version using a Fender tonestack) my pre with a high level input showing the asymmetric distortion, flattening above/peaking below, prior to actual clipping.  The Redcircuit simply clips.
If you say theory and practice don't agree you haven't applied enough theory.

joecool85

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Re: Preamp suggestions needed!
« Reply #13 on: September 01, 2013, 08:33:01 PM »
Wow michel, I knew that people here would help you out, but holy cow!  That should give you plenty of ideas!
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michelterres

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Re: Preamp suggestions needed!
« Reply #14 on: September 04, 2013, 11:36:41 AM »
Wow michel, I knew that people here would help you out, but holy cow!  That should give you plenty of ideas!

Yeah, I surely have work for the next 3* months.

*Or more, everything depends on how much money I can save :cheesy: