Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: RDV on April 05, 2006, 07:26:28 AM

Title: Should we go even simpler?
Post by: RDV on April 05, 2006, 07:26:28 AM
I've got a preamp in my initial chipamp/guitar amp I built that is from the old Thomas/Vox(USA made) SS amps like the Super Beatle. (http://fatboy.ssguitar.com/index.php?dir=schems/&file=voxtbpRDV.GIF)
 I like it, but I think we should come up with a very neutral sort of simple preamp that would have some guitar voiced EQ but little else so pedals would determine the sound a great deal sort of like what R.G. Keen has done with his Workhorse amps. I don't want to copy R.G.'s design or anything because as we know his is an all tube design.

I suppose some kind of limiting circuit might warm things up. The problem as I see with a SS front end in general is that it is too clean. I suppose what I'm trying to get at is I think we need something that is barely on the edge of breakup if you really lay into it, but still retains as much headroom as possible. I like teemuk's ideas about diodes but I don't think I want there to be much available distortion, perhaps not even a pre-volume. I'm just throwing out ideas here because I've not really been satisfied with things I've come up with in the past.

RDV
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 07:42:09 AM
I'd love to go more simple actually.  The simpler the better.  At least to start with.  What I want to see is treble, mid and bass control.  No need for distortion or reverb to start with.  I know for my amp I'd be fine having it as a "clean" amp, then I can just throw pedals in front of it and have as much distortion as I want.  I normally find that I don't like the built in distortions on amps that much anyway, with the exception of a few higher end amps.
Title: Re: Should we go even simpler?
Post by: teemuk on April 05, 2006, 08:29:53 AM
I prefer simple circuits too but i'm afraid that the words simple and Solid State really don't go together that well in this context. The transitional curve of a transistor has a quite hard knee (and opamp's has even harder) which results into a situation where the clean signal turns into a harsh distortion very quickly. I really can't take credit from what you refer to as my ideas about diodes since they aren't that uncommon. However, to voice diode clippers correctly you need a lot of diodes and/or other components. Diode clipping alone sounds quite horrible unless you're really into a 60's fuzzy bass tone. Slightly overdriven diode clipper sounds even worse. This is the most important thing and still far too overlooked: Pre- and post distortion EQ makes a big deal!

I think RDV is on the right track about limiting since guitar's dynamic range is quite tricky. I think the best way to achieve this would be to limit and perhaps even compress the signal to get rid of the most severe transients. You could experiment, for example, with LDR circuits - they are quite easy to keep simple. The limited signal could then be used to overdrive a stage that has a very soft knee in the transitional curve. There was an article somewhere in the internet about "knee shaping" with resistors and diodes. I constantly forget where since it's not on my bookmarks but it seemed far more advanced than other ideas about diode limiting i have seen so far. Even the LDR limiting alone might do the trick for some people.

Is this simple? Well, in my opinion not really - plus you also need some EQing which is different for both clean and overdriven signals. I don't say it can't be done, i just say i think it can't be done simple and that's why i prefer external pedals over complicated preamplifer too: They sound far more better than the average attempts to make analog circuits sound nice when overdriven. My ideal preamplifier would just limit the transients, have a tone control, effects loop and a spring reverb. All foot switchable on/off, of course.
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 08:36:57 AM
I'm really looking to build a decent preamp so that my lm3886 can be a stand alone amp...
Title: Re: Should we go even simpler?
Post by: RDV on April 05, 2006, 09:42:32 AM
What I'm thinking is just using a lot of diodes(LEDs perhaps for their higher threshholds) in series in the FB loop so we can get that softer breakup before the transistors or opamps do their harsher thing. It doesn't seem that would get too complicated. I've just got to get off my butt and start building.

RDV
Title: Re: Should we go even simpler?
Post by: trevize on April 05, 2006, 10:19:40 AM
i would prefer instead using jfet a la runoffgroove: fet are not like tubes but they break up (their non linear region response) is softer and very beautiful sounding.
i would probably mix the design of opamp and fet: maybe two or three fet stages with simple eq (like the big muff eq) and a simple low pass fixed filter in the end to get rid of the ear piercing frequencies.

even simpler: a mosfet stage>> simple big muff eq >>  fet stage for warming up the signal and simulating a little bit of power amp break up and maybe in the end a simple fixed low pass filter.

i hope not to be confusing, my poor english doesn't help me :)
Title: Re: Should we go even simpler?
Post by: RDV on April 05, 2006, 10:35:03 AM
I've got a few of those ROG circuits and I tried to use them as a preamp for my LM3886 amp and the problem was headroom because of the 9v supply. If I could figure out a way to power them with bipolar 15v power I'd try one again.

RDV
Title: Re: Should we go even simpler?
Post by: teemuk on April 05, 2006, 02:18:47 PM
I've got a few of those ROG circuits and I tried to use them as a preamp for my LM3886 amp and the problem was headroom because of the 9v supply. If I could figure out a way to power them with bipolar 15v power I'd try one again.

Jfets are one option but i think they are highly overrated for what they really are. The problem, that is quite evident when you further examine all Runoffgroove circuits, is that most of them only provide a semi clean gain stage that distorts on higher signal amplitudes. Listen to the samples on the page: How many of these circuits you can honestly call "clean". Actually, a totally clean fet amplifier would have a very poor distortion tone: I built a clean and simple jfet booster few months ago. It is basically a simple common source amplifier circuit with some source bypassing to make up a variable gain. Honestly it doesn't sound any good when it distorts probably because of the lack of any EQing besides the low decoupling that provides a high pass filter. ...But as a clean booster it's really awesome! 95% of the "fet-tone magic" lies in the surrounding circuitry. This is why i would rather go with having two channels: Clean and dirty.

Well, enough about my opinions on jfet tone - i still prefer them over opamps in simple circuits hands down.

Anyway RDV, converting to higher voltage bipolar supply shouldn't be that difficult: Just connect drain resistor to positive supply and gate and source resistors to negative supply. As a downside, you probably have to bias the gain stages again and this could mean almost complete rebuild. While you're at it you might as well extend past the +- 15 V limit for even more headroom. I'm currently planning to build an amplifier that has a preamp which swings to almost +- 25 V (50Vpp) on a 2V input signal. Both power amplifier and the preamp run from a 60 V supply. Now, that starts to be some serious headroom. This one doesn't have fets: I couldn't find ones that could take this high voltage.
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 02:22:58 PM
This sounds like something that could work with a lm3886.
Title: Re: Should we go even simpler?
Post by: teemuk on April 05, 2006, 03:25:28 PM
Here is how the preamp looks so far:
http://img465.imageshack.us/img465/8698/preamplifier6od.gif

I really can't take credit for the design since it's mostly based on Flavio Dellapione´s 60W guitar amplifier's preamp found from Red Free Circuit Designs. http://www.redcircuits.com/
Main mods are a higher input impedance and a simpler tonestack that replaces the way too complicated tonestack/harmonic modifier of the original circuit. The power amp circuit that i will use will swing to max voltage with a 200 mV output signal from the preamp - (i don't know if such is the case with your LM3886 PA) - so the headroom of about +-24 V with a +-1.2 V input is quite overkill for the design. Anyway, i wanted a clean preamp. I don't know whether i will implement any more features since i'm afraid that they would ruin the simplicity.

A word of warning: Though the circuit seems very simple it operates on a "high" single supply voltage and relies mostly on a Sziklai pair for gain - which doesn't have a very impressive SNRR figure when compared to, say, an opamp. Unless the power supply has huge capacitance or is well regulated you will definitely get a lot of hum with this circuit.
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 03:34:07 PM
Huge caps are fine with me!  haha, I love the way they look anyway.  I like the looks of this circuit.  If you build it and like it, I'd like to hear about it.  I think if we just changed the tone stack to a marshall style one I would be all set to use it for my lm3886 amp.  From my research, it seems the lm3886 circuit wants around +/-25v input.
Title: Re: Should we go even simpler?
Post by: RDV on April 05, 2006, 04:50:28 PM
Big caps I got! The ones I bought for chipamp supplies are 10,000µF @ 50v. No too expensive either, less than $4.00 USD each.

RDV
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 05:02:14 PM
Thats what my lm3886 uses, two 10,000uF 50v caps.
Title: Re: Should we go even simpler?
Post by: teemuk on April 05, 2006, 05:54:52 PM
The standard marshall style stack should fit in as is and there shouldn't be notable frequency losses due to output/input impedance mismatches. If you build this preamp be sure to have enough capacitance in order to minimize the ripple voltage. The circuit is quite unforgiving in amplifying it.
Title: Re: Should we go even simpler?
Post by: joecool85 on April 05, 2006, 06:04:53 PM
How much is ample?  Like...47,000uF caps? or are we talking 1500uF caps?  I'm ok with either.
Title: Re: Should we go even simpler?
Post by: R.G. on April 05, 2006, 08:50:45 PM
Do yourself a favor - regulate your preamp voltages, and separage preamp and signal grounds from power amp and power grounds. That's more effective than the biggest capacitors you can buy for filtering the preamp supply.

ALL single ended circuits should be assumed to be intolerant of power supply ripple and ground hum until proven otherwise. It's the nature of the beast.