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Author Topic: Improved 'Guitar Handbook' TL071 preamp  (Read 26797 times)

lefizz

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Improved 'Guitar Handbook' TL071 preamp
« on: September 12, 2007, 07:55:09 AM »
I have had a go at redesigning the circuit using the TI single supply handbook.
It avoids most of the pitfalls of the original though i am not expert.
If anyone in the know can comment that would be good.

I will try it with both a TL071 and a OPA134.
The OPA134 should sound better since it is a very high spec opamp btu you never know!

obelix

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #1 on: September 12, 2007, 01:02:44 PM »
I can read a schematic but have no clue on how they work... just a question with your IC does this mean that nothing is connected to pins 1 and 5?

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #2 on: September 12, 2007, 02:19:08 PM »
Yeah they arent used, ethier 'no connect pins or bias ajustment pins which arent used in this application

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #3 on: September 12, 2007, 04:49:20 PM »
First impressions:

- I realize that I'm nit-picking but the gain of a non-inverting circuit is R3 / (R4+Rpot) + 1. Thus gain is adjustable between 2.6 and 11. (Yes, I delibrately ignored the capacitors to keep this simple).
- If this is an input stage then it has a rather low input impedance (220 kilo-ohms) for guitar applications.
- R1 and R2 should preferably have as low value as possible to keep the "virtual ground" as a low-impedance source. In practice, too low values will draw too much current and result in great losses so try something like 10 kohm instead.
- The circuit is not expected to drive a capacitive load and opamps are short circuit protected so R5 seems rather useless
- If this is intented to be a linear design then C3 (100 pF) seems a bit too high. It will be cutting off the higher frequencies.

Anyway, as far as I see there are no "real" problems in the circuit.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #4 on: September 12, 2007, 05:08:34 PM »
Teemuk,

Thanks so much exactly the sort of comments i was looking for, I am commpletely new to this sort of thing.

1 Your bang on i forgot to change the gain marking after i had changed the layout.

2 What sort of input impediance should i use? 470k i really am guessing any input greatfully recieved

3 thanks i with try 10k

4 fair enough

5 what sort of value would you suggest?

I have etched some board already so i will post the pcb layout when i get to work tomorrow.

Thansk for your input

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #5 on: September 12, 2007, 06:01:07 PM »
The input impedance... well...

Generally you should aim for "impedance bridging", which means that the input impedance is kept higher than the source impedance; this provides the best voltage transfer.

The source impedance of a typical guitar with magnetic pickups is several kilo-ohms (i.e. 5 kohm - 50 kohm) so at first glance it might seem that few hundred kilo-ohms is enough. Well, to make a long story short, let's just say that usually it isn't and the "rule of thumb" is to use at least 1 Meg for passive magnetic pickups. Low input impedance values generally attenuate the higher frequencies too much, thus "sucking" the guitars characteristic tone away. For "bright" single coil pickups even 1 Meg might not be enough.

In practice, all this depends on the circuitry of the guitar: Guitars with active electronics typically have a very low and fairly linear source impedance so they can be connected to circuits with quite low input impedance without considerable losses.

However, in reality you have to prepare for the "worst": A conventional pickup (and the following tone/volume control) circuitry is not a source with "linear" impedance (in fact it has a resonant character) and thus it is very much affected by input resistance and capacitance of the stage it connects to. A typical pickup is so sensitive source that even the capacitance of the guitar cable may affect its tone.

Regretfully, too little has been written about this subject. Manufacturers are quite skimp on giving meaningful data about their pickups as well. Anyway, here's some stuff that might be interesting (and which is far too often ignored):
 
http://buildyourguitar.com/resources/lemme/
http://www.terrydownsmusic.com/technotes/guitarcables/guitarcables.htm
http://www.guitar-repairs.co.uk/how_guitar_pickups_work.htm

Piezoelectric pickups are quite different from magnetic ones being practically capacitors by nature, thus connecting them to a too low input impedance will result into loss of lower frequencies. For piezoelectric pickups some manufacturers suggest using as high as 10 Megaohm input impedance. However, guitars with piezo mics often have an inboard active preamp, which can be interfaced with rather low input impedance. Anyway, this is not the case always.

Practically there are only few things I consider as drawbacks in using a very high input impedance. 1) Resistors larger than 1 Meg are difficult to source (you can connect those in series though), 2) using high value resistors inreases noise (not a very good for "feature" for an input stage) and 3) high impedances are more vulnerable to capacitive effects (tone loss due to "parasitic" capacitances, instability etc.)


About capacitor C3: This is highly subjective issue but I would rather try values closer to 10 - 47 pF. In fact, I might even omit the capacitor completely assuming the circuit stays stable without it. Anyway, if audible low-pass filtering is sought after then ignore my comments.

LJ King

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #6 on: September 12, 2007, 07:11:20 PM »
About capacitor C3: This is highly subjective issue but I would rather try values closer to 10 - 47 pF. In fact, I might even omit the capacitor completely assuming the circuit stays stable without it. Anyway, if audible low-pass filtering is sought after then ignore my comments.

Wouldn't the -3db frequency provided by the value of C3 be determined by the series value of the 50K gain pot and R4?  :-\

I calculate a 100pf cap and 60Kohms to be at 26.5KHz. Your ears (and speakers) must be much better than mine to consider that as audible.  ;D  ;D

It is good to hear someone talk about pickup impedance rather than "DC resistance".  :tu:

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #7 on: September 12, 2007, 08:06:49 PM »
I do  know the guy has very expensive guitars and his main acoustic and electric both have active electronics in them. Does this mean i should leave the 220k value alone or should i still go with something more like a 1M?

Thanks a lot by the way a am learning a lot and very quickly.

Do you think this circuit on its own would be enough to drive a passive set of tone controls and then on into the class d power amp or should a put a buffer after the tone controls?

phil

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #8 on: September 12, 2007, 11:00:03 PM »
220 kilo-ohms would be ok for active guitars but you make the circuit more "universal" if you use 1M.

As is, the circuit has a gain between about 8.5 dB and 20.8 dB. At least on the maximum setting the insertion losses of most tone control circuit types should be compensated and you still would have some gain (instead of attenuation). At lowest gain the gain stage and the tone control will quite surely have an attenuating effect in total.

Typical tone control circuits like to see a fairly high impedance at the input of the following stage. Likely the input impedance of the class-D amp is not very high? So yes, I would separate the tone controls from the power amplifier with another gain stage that has a rather high input impedance. I guess that is considered buffering. With resistor R5 the source resistance of the gain stage is about 1K so it will happily feed most tone control circuits without considerable losses.

Anyway, so far I have only seen a schematic of a gain stage so its quite impossible to answer anything definite to your questions. You must note that you are the one having the details of the design and therefore you should answer most questions by yourself:

Do you know the input sensitivity of the class-D power amplifier (voltage required for maximum output)? You must know it in order to rate the preamplifier's total gain accordingly. Guitars have a varying range of output signal levels (active ones usually have a "hotter" signal) but a safe bet is to assume that the amplitude of a pickup's output signal is approximately few millivolts (i.e. 20 mV). If the output sensitivity is, say, 1V you definitely need more gain than 11x because 11 x 0.02V equals only 0.22V. And that example didn't even include the rather considerable losses introduced by the passive tone control!  ...More reasons to include another gain stage.

It seems to me that at this point you still have a mere sketch of the final circuit and perhaps it might be worthwhile to think about the design in its entirety for a while. It would be unfair to tell you how you to design your amplifier but what I can do is to provide some basic suggestions:

- A fairly typical configuration is a design consisting of two of those opamp gain stages (or similar) with the tone control in between them.
- In such circuit, typical options for volume control's position are between tone control and the second gain stage or between the second gain stage and power amplifier (assuming you want a volume control feature)
- Gain control is likely needed in only one of the opamp stages
- The first gain stage should preferably have enough gain to compensate nearly all losses of the tone stack because this improves signal-to-noise ratio
- The second stage needs enough gain to amplify the input signal to the level of the power amplifier's input sensitivity (or preferably a bit higher)
- Unless you are building a distortion effect keep the gain per stage at a reasonable level (i.e. 3 - 10 x). It's better to have two series gain stages with a gain of 10 than one with gain of 100.

That should get you started...

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #9 on: September 13, 2007, 06:18:48 AM »
Hi Teemuk,

Yet again you provide a reallu useful post. I guess i should explain the basics of the amp i wish to build.
The amp will be used by a friend who is a professional busker on the London Tube, the real issue for such muscians is the fact that while busking is now legal and controlled the 'stages' have no power points therefore the amps have to run off batteries.

The only available one for electric guitars is the Crate Limo /Taxi series which while fine for messing about are far to fragile for day to day work.
All the buskers i know have at least 2 since they break down so often.

The amp i wish to make will be based on a tripath class-D or Class-T as they like to call it TA2021B, they make about 13 watts per channel into 4 Ohms clean.
I will be using a ready made kit from www.41hz.com, Amp 32 which is intended for hifi use.
I built several of these for hifi use an found them bo be excellent amps period let alone for the money, the bonus being the 80+% effiencey which should make the battery in the unit last a decent time. I do not know the input sensitivity of these kits however i believe that hifi typically works at around 2v, I may be wrong. This would mean that i would need a second stage with a gain of 10 judging by what you tell me.

To explain what i want in the amp, it will have two guitar preamp channels. Each with tone control, one with a Baxneldale setup for acoustic guitar, the other with a fender\mashal style dip in the middle range for Electric. I have these tone stack example you posted in my other thread.The elecric channel will have a bypassable fx unit (The English Channel) on the front.

Both these channel them go into a summing amps which will also have an input for a ipod/cd player before finalling going into the Tripath based power amp module.

From your post the it would seem to me the best option would be to have a fixed 10 x gain stage initially by removing the pot with a 1M input impedance. This should help drive the following tone controls after which we would have the opamp setup we have previously been talkign about with a pot and variable gain between 2.6 an 11 which should take use up to near 2v depending on the attenuation in the tone controls.

Temuuk, once again thanks for your help, you have been utterly invaluable in explaining these sorts of cicuits and the thing to consider.
You really have moved this project on leaps and bounds.

Phil

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #10 on: September 18, 2007, 05:44:02 AM »
Hi,

Here is where i have got to thus far. Pad labled Tone goes out to the tone controls and then come back into the second stage at the point labelled Input 2. The thing I am confused about is what i do on the input of the second stage, at the moment it is a near enough a replication of the first stage but i imagine i should change the input impedance (r15) and the input cap (c8).

As always any comments greatfully recieved.

Phil

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #11 on: September 18, 2007, 07:39:52 AM »
Input resistors R7 and R15 can share a common "bias" voltage divider, you don't need two separate ones.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #12 on: September 18, 2007, 08:39:19 AM »
OK that seems fair enough. What about those input impediance resistors and caps. Should i decrease the  value of r15 and increase the value of the input cap (c8) surely this would reduce noise in the second stage and since it isnt dealign with the guitar pickup this shouldnt matter?

Also i have fixed the gain in the first stage at 10, the more i think about this it seems too high. wouldnt something like 3 be better.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #13 on: September 18, 2007, 08:40:41 AM »
Teemuk i read you are producing a book on guitar amp design is this in print yet, I think it would be very useful to me.

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #14 on: September 18, 2007, 02:53:44 PM »
All of your questions are so much related to the way you want your design to perform - and more importantly to sound - that I feel you will never get answers. Basically, what you have already is a working circuit so the rest is up to highly subjective tonal issues.

If I didn't mention it already, I usually like to have less low frequency coupling at the first stages, thus bass frequencies have less total gain and consequently will clip less - thus the "blocking" effect (caused by low-freq clipping) is reduced. Seems to be what you are aiming for. Anyway, this kind of stuff that deals with tone of the circuit is something you should design yourself.

You said you have the tone control in the middle of the stages. The input impedance of the second stage will define how much the circuit in question [tone control] is loaded. This will affect it's frequency response and a too small load will mainly increase insertion losses. Of course that very much depends on what kind of tone control you have. Have you tried Duncan Munro's "Tone Stack Calculator" software or something similar? It would help you in designing the tone control circuit and defining the proper impedances for it's source and load.

The book is not in print as I'm still writing it. It's in english language, which is not my native tongue so there's a lot of work in correcting the grammar issues and stuff written confusingly. Also, I always seem to find more stuff that I feel should be included. I think the time when I'm satisfied to its content will come soon, though. However, the rest is still open...

 

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