highest gain / low part count pre-amp/clean boost?

[big bustle]

hello friends

i'm itching to learn about input distortion.

i want to find a simple, low part count pre-amp/clean boost to overload the inputs on some of my favorite circuits.

i don't want the pre-amp/circuit to provide too much coloration in terms of eq or distortion as i want the input sections of these circuits to distort.

on my bench i have a bs170/SHO bread boarded. i also have some 386, 371 and 4558 chips laying around.

what will provide the most amount of gain with some headroom to go?
which one is best for low part count fun?

should i start googling mic preamp circuits using these parts?

[trjones1]

You should search the forum as there have been bunches of threads on this topic, but the SHO is a good low-part-count choice, as is the LPB1 or the amz mosfet booster.

[raulgrell]

Really low part count, doesn't sound bad at all:

LM386

Input -> pretty small input cap -> Pin 3

Pin 1 + Pin 8 : No Connection
Pin 2 + Pin 4 : Ground
Pin 5 -> Positive end of a 10uF electrolytic -> 100k vol pot -> output
Pin 6 = 9v power supply
Pin 7: Can't remember, it's either a no connection, or a cap to ground...

If anyone knows what this is, let me know... saw a schematic ages ago, remember it sounded pretty good...

Other things to consider: The usual 1M resistor to ground at the input to prevent popping, more gain by connecting pins 1 and 8 as such: pin1 -> small resistor (500R - 5k) or a pot wired as a variable resistor -> +ve of a 10uF cap -> pin 8...

[big bustle]

thanks for the responses.

i couldnt find the ratings for the lpb but i recall it being on par with the sho somewhere around 25db of boost.

i'm now looking at the ruby and little gem schems for the lm386

something tells me these will provide far more gain than the bs170.

i'm also now realizing how badly titled this thread is. i'm sure people are passing it by thinking "not that question again"

[Ben N]

I'm sure RG has gotten sick of telling people this, so I will fill in: for maximum gain (in the true sense of amplification, not distortion) and minimum distortion (or coloration) there is no beating a non-inverting opamp. You get low parts count (9, including the opamp and the volume pot), usually far lower distortion than any single transistor can provide, far higher gain, potentially, and usually more flexibility in terms of power supply, which is really going to dictate the limits of how much clean gain you can get. If, as I gather, you are trying to hear what happens in the amp when it is pushed to the limit, without any coloration from the preamp that is doing the pushing, then this is the way to go. Your 4558 (or half of it) should do fine in this application, although there are better. Not familiar with a 371. I've never heard of anyone getting much clean headroom out of a 386, at least not without significantly attentuating the input (and using a too-small input cap certainly will cause "coloration"). The SHO will sound great, but that doesn't mean it is uncolored, especially at the high end of its range.

The main issue is power supply. To get the most headroom out of the opamp boost you will need to give it more than 9v/4.5v. Check the data sheet for what it can take. I'm sure that bipolar +/-9v would work fine, so I imagine that 18v would also be good. Then make sure your caps are rated for 25v or more.

[big bustle]

I'm sure RG has gotten sick of telling people this, so I will fill in: for maximum gain (in the true sense of amplification, not distortion) and minimum distortion (or coloration) there is no beating a non-inverting opamp. You get low parts count (9, including the opamp and the volume pot), usually far lower distortion than any single transistor can provide, far higher gain, potentially, and usually more flexibility in terms of power supply, which is really going to dictate the limits of how much clean gain you can get. If, as I gather, you are trying to hear what happens in the amp when it is pushed to the limit, without any coloration from the preamp that is doing the pushing, then this is the way to go.

thank you ben

can you point me in the right direction with some circuits or ICs to google?

[Ben N]

Microamp is a good place to start. oth GGG and Tonepad have project files and ready-made PCBs.

[CynicalMan]

The main issue is power supply. To get the most headroom out of the opamp boost you will need to give it more than 9v/4.5v. Check the data sheet for what it can take. I'm sure that bipolar +/-9v would work fine, so I imagine that 18v would also be good. Then make sure your caps are rated for 25v or more.

Using rail to rail op amps also helps with headroom.

[Minion]

I Use an onboard preamp in my guitar , it is clean , has no controlls (fixed gain , can be set with a resistor) , fits easilly in most guitar cavities (only about 1in x 1.5in) and has enough boost to give your tubes a kick in the arse , it"s just a single fet opamp (opa137) and a few cheap parts ......




It doesn"t show the wireing of the input jack (you will need a TRS input jack) but should be easy to figure out .....

[big bustle]

so the 386 claims to have 46db of boost. when cranked it's distorted and not pretty.

when set to 26db of boost there is still a fair amount of distortion. also, it's not loud enough to get any real results.

looking at the micro amp: uses tl061

i think the data sheet says it's 20db max. i am unsure of this. it's going a little over my head.

unfortunately i don't have a tl061 to try this out with. i google my ass off to find more non-inverting op amps but i can't find a single list.

i did come across an HA17358 which i think is interchangeable with an lm358. look at the lm358 datasheet i see it uses terms like "non-inverting input". so if my hunch is correct this might work.

the opa137 dayasheet shows 96db of gain. if i had one in my box i'd certainly use it.

[earthtonesaudio]

If you're using this to slam a tube amp input, those are usually very high impedance, so you could get away with using a step-up transformer to increase the voltage coming out of the effect.

Something like an LM386 which can drive low impedances could connect to the secondary of a wall-wart type transformer (i.e. 120V:12V or 120V:6V) and you'd use the primary as the output.  With the 386's gain of 20 plus a 6V:120V transformer, you could get 50dB of gain without clipping (maybe).


This would only work with a high impedance input following the boost.  Other input types might cause pedal death.

[Lurco]

how high will my signal of 100mV go with 96dB of gain? what if I get 1V out of my PU? will 2 cascaded opas get me 192dB?  

[cpm]

back to topic:

i want to find a simple, low part count pre-amp/clean boost to overload the inputs on some of my favorite circuits.

First thing to consider is that the maximum output swing from the boost will be the power rails.
And, if you are planing to boost the input of other pedals, which will probably run off 9v, its not usually a good idea to feed them with a bigger input signal that its supply. You may fry opamps, mosfets, etc...

[anchovie]

how high will my signal of 100mV go with 96dB of gain? what if I get 1V out of my PU? will 2 cascaded opas get me 192dB?  

http://www.muzique.com/schem/gain.htm

According to this, 100mv with 96dB of gain would give you about 6.3kV, assuming you can find a 6.3kV DC supply and a rail-to-rail op-amp that will take it.

I think you need to step back a bit and study more about what the dB scale actually means. Also, the gain specs quoted in op-amp datasheets aren't really relevant as the circuits we tend to use in effects use feedback resistors to set a fixed gain.

You say you want to overdrive the inputs on some of your favourite circuits, perhaps you could say what they are - chances are they will have a variety of input stages between them so there can't simply be a single phenomena known as "input distortion".

[petemoore]

  Start with the power supply.
  Decide say 18v is more than enough ? or push the voltage up to just under what your components can take. [18's enough?]
  Once you get that done, buy a big chunk-o perf and either breadboard or perf-bread something with a Mosfet, something with an opamp...something with whatever active seems to fit for ya'.
  There's some high gain options.
  I just finished a Strato-blaster again ! 
  This time 3m3 at the input, high impedance, you too might equate the HF response as ''clean'', it's very responsive on the HF's, glassy even.   

[Minion]

If you're using this to slam a tube amp input, those are usually very high impedance, so you could get away with using a step-up transformer to increase the voltage coming out of the effect.

Something like an LM386 which can drive low impedances could connect to the secondary of a wall-wart type transformer (i.e. 120V:12V or 120V:6V) and you'd use the primary as the output.  With the 386's gain of 20 plus a 6V:120V transformer, you could get 50dB of gain without clipping (maybe).


This would only work with a high impedance input following the boost.  Other input types might cause pedal death.

A Low impedance signal into a high impedance input isn"t a problem , All guitars that have active electronics have a low impedance output and many poeple use actives with a tube amp , the problem is when you do the reverse , My amp also has a Low impedance input and I find it actually sounds better in the high impedance input ......

[PRR]

> 100mv with 96dB of gain would give you about 6.3kV, assuming you can find a 6.3kV DC supply and a rail-to-rail op-amp that will take it.

If the "100mV" is measured conventionally, RMS/Average, that is 6,310V RMS/Ave but that requires (assume Sine) 17,846V power for a perfect rail-rail amplifier.

> will 2 cascaded opas get me 192dB?

If they did, then your 100mV needs 398,107,171V ave, 112,5847,078V peak-peak.

It may be difficult to build a stomp-box around a 100 Million Volt battery.

Taking our tongues out of our cheeks: few guitar amps will accept over 1V-2V without distorting. The 3V possible with a really good 9V battery ought to smack most inputs senseless. For very severe abuse, two 9V batts on a TL071 at gain of 50 (34dB), trimmable down to 10 (20dB) in case your "100mV" turns out to be more.

[Ben N]

so the 386 claims to have 46db of boost. when cranked it's distorted and not pretty.

True. But the suggestion to use it in conjunction with a transformer is interesting. Look up John Hollis's "Titan Boost."

looking at the micro amp: uses tl061

Yeah, but the TL061 has the same pinout as a lot of other single 8-pin opamps, including its very close cousins the TL071 and TL081. You can use a dual (such as your 4558) as well, as long as you lay it out accordingly, either to ground the iinput of the second opamp in the package, or perhaps to parallel the two opamps, or just to use it as a 2-channel microamp. Lots of opamps would do.

i think the data sheet says it's 20db max. i am unsure of this. it's going a little over my head.

You're right, you are. As long as you can provide sufficient power, you can tweak this to provide all the boost you can use.

unfortunately i don't have a tl061 to try this out with. i google my ass off to find more non-inverting op amps but i can't find a single list.

You need to get a handle on some basic concepts. "Inverting" and "non-inverting" are opamp configurations, not different types of opamp. Every opamp in the world AFAIK can be set up to run as inverting or non-inverting. There are lots of tutorials on this on the web, including, I think, one by our very own RG Keen, but I can't find that at the moment. Instead, I found this, which covers the very basics.

[petemoore]

  If you plug a preamp into a preamp...you know what happens, it sounds louder and all, may distort and sound like...who knows Ritchie and the other cats used an RTR's preamp to boost their preamp, probably tends to just sound rough IME, super-boosting, certainly depends on what it is, and.
  Super-boosting an amp can sound similar to preamp, or very good.
  The lines is squiggly and a fine one, between big boost and preamp.
  The only time I wanted more than what an OA or a Mosfet could output was driving a Dyna tube power amp.
  Anyway, try two gain stages or maybe 2GS's with a buffer/tone control or whatever, more than one stage, like preamp.
If the stage recieving a boosted input starts to distort, de-boost the input or raise the headroom at the power supply [especially of course more important to gain stages recieving a boosted input] to clean it up again.