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Author Topic: Improved 'Guitar Handbook' TL071 preamp  (Read 26790 times)

joecool85

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #15 on: September 18, 2007, 04:06:36 PM »
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...

If you need someone to proof read, I'd be glad to help.  And my fiance is an English teacher and always willing to help people with stuff like this.  Between the two of us we could probably proof the whole book lol.
Life is what you make it.
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lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #16 on: September 18, 2007, 05:12:49 PM »
Intereting to hear about your book, you have a garanteed sale here. I have been looking in many book shop in London and its all either pure valve stuff or just plain pot and cap swapping, or cable wiring stuff. Very limited all of it.

Thanks a lot for your advice again, so if the circuit will work i feel happier stuffing the board and actually starting to experiment. Tone control wise i will be trying a couple of circuits, from that useful site you linked to a while ago, one setup will be for an acoutic setup the other for an electric. Once i have values that work i will still up my pcb transfers if anyone is interested.

Do you think it is worth using film caps is these sorts of setups or would ceramic do to test?

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #17 on: September 20, 2007, 12:12:55 PM »
IF anyone is interested here is a pcb layout for the above 2 stage preamp.

Change R4 to cahnge initial gain its current set at 4 the second stage gain is variable between 2.6 and 11.

Put tone controls between Tone pad and Input2. It is currently untested but seems liek it might be useful to people building Lm3886 based chipamp, though gain have to be changed acording to your application

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #18 on: September 20, 2007, 12:13:51 PM »
Component overlay :-


lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #19 on: October 10, 2007, 08:07:51 AM »
Right i tried this setup last night and had all sorts of issues. I didnt have a guitar handy so i tried  ipod at near zero volume, gain stages set to minimum, volume minimum. Nada. After turing up the gain and volume i got a output even with the ipod at high volume, messed around for a bit and nothign helped.
New opamps, all sorts, funnily enough swappign out the OPA134 for NE5334 actually helped a bit, not sure why. I was wondering if the 2 cap bleed resistors R6 and R14 might actually drain away the signal if the impediance of the tone controls is too high.

Really not sure what is going on, tried changing the gain in the first section from 3 to 10 that didnt help. Weirdly touching the hot connection on the input jack actually increased the volume a bit, wondering if that would indicate a cap is in reversed. Tested my ground and power voltages and tehy were all fine. Any help would be recieved very very greatfully. Put loads of work in and very dissapointed.

On a brighter not the Class d power amp in the chassis sounded superb, , i have 2 cd input going into my mixer Amp board with a gain of 3 set, that then goes to a pot and into the poweramp, that really transformed the ipod sounds from this thing. Got great volumes and it sounds excellent.

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #20 on: October 10, 2007, 10:13:35 AM »
Just to let erveryone know, the PCB is shown from the "top" side (component side) so you need to mirror it.

lefizz, I'm sure you could make the layout more compact by using dual opamp and doing the mods I suggested but... Let's get to more important issues:

The layout is basically correct except,

- C2, C13 are wired in wrong polarity. The negative polarity leg must connect the ground as the "opamp side" has positive voltage in reference to it.

Another checklist besides that:

- You did rememeber that jumper from R15 to 1/2 Vcc ref, right?

- Check out that resistors are indeed correct value since color coding is sometimes hard to decipher correctly. I always measure them before fitting since I'm slightly color blind and thus telling the difference between brown-red, blue-green-purple & orange-yellow is difficult. This is one very typical problem.

- The trimmer: Their pinout varies. Make sure it's wired correctly.

- R6 and R14 indeed look pointless as they are in parallel with the tone control and input of the output stage. Their value is rather hight but still... Are you sure they are not interfering with the operation? If you remove these, make sure the polarity of the capacitors stays correct.

- General check: Capacitor polarity, solder bridges etc.

- You are sure that the tone control circuit works

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #21 on: October 10, 2007, 10:58:10 AM »
HI Teemuk,

Thanks a lot words the words of advice, as always you have given some great suggestions.
I already checked for solder bridges so that isnt the issue, i will change the 2 caps around, can't beileve i did that but still. A second more trained set off eyes really helped there. I am 90% sure i have got the jumper in place but will check and i think i will remove the r6, r14 resistors. What do you  mean about the cap polarity if i remove them? Just check everythign is as it should be or somethign else. To be honest i am sure all the cap are in the 'right' direction as in they are the same as my eagle stuff, its just was wrong!! I did actually bypass the tone controls last night and that didnt help.

Do you think as it was with caps in thewrong direction it would be possible to blow the opamps, they seem to go very strange. One minute the cicuit worked a bit other times not at all. I switched to using the cheap ones pretty soon cos i dont want to blow my opa134, they cost a lot.

I will finish off the design by doing as you say and using a dual opamp and removing the reduntant second set off biasing resistors, i just didnt have any OPA2134 at the time and wanted somethign to together quickly. Looking back that might not have been the best option.

Anyway thanks a million you have given me some great ideas about how to go foward.

Thanks a lot,you have really cheered me up.

teemuk

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #22 on: October 10, 2007, 01:38:18 PM »
The polarity issue: Most capacitors "tolerate" incorrect polarity at low voltages (read: they do not explode). However, they can work very strangely, for example, their capacitance may be lower, they may have significant series resistance, etc.

When you remove the ground referencing resistors from the other side of the caps you must make sure that the preceding stage has lower DC level than the feeding opamp stage. It's just a question maintaining the proper polarity.

It would help if I saw the input stages of the power amp and the tone control circuit but I assume that both basically consist of a resistor to ground that ties the DC level to zero volts. If there's some additional AC coupling involved you can likely remove it since you need just one capacitor for that purpose. That is the case even if those stages have a DC offset: Just connect the capacitor so that it retains proper polarity. Really simple.

If the circuit worked correctly now and then it also may be just an issue with a poor solder joint (contact) somewhere. Instead of just disconnecting the current path these may also show up as "resistors" that attenuate the signal. You say you swapped the opamps and this helped a little: Maybe this lowered the resistance of the cold solder joint (or some other contact) a bit?

Anyway, first things first: Correct the issue with wrong capacitor polarity and see if you still have some problems.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #23 on: October 10, 2007, 04:24:33 PM »
OK well the caps seemed to sort it out to a ceratin degree. No distortion and everythign appears near enough normal. I think i need to change the gain pot from log to lin since as it stands it offers very little control with all the action in the last 1/5 of a turn. My Marshal tone tones dont seem to work very well ethier i think i might need to do a bit of rewiring.

One thing that does confuse me however is the lack of gain. Even with the tone controls removed from the circuit the difference between the gain pot at 47k and at 0 ( I measure it and it was wired correctly) thus changing the gain from 1 to 11 seem really not as much as i would have expected, i mean i can easily force the cirucit into distortion but this is from an ipod at 80% volume. i would have thought much less would have been needed.

I did measure the ac voltage from it using a multimeter and that said i was getting about 0.250 volts, well if that went into the cicuit with 10x gain first stage surely with my t-amp set at a input sensitivy of 2v and a 1.5 gain in the mixer amp i should be well over full volume but this just isnt the case. I might try swapping the opamps once more to see what happen but i find it quite confusing what is going on.

Phil

joecool85

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #24 on: October 11, 2007, 10:26:35 PM »
You will find that an iPod will not put out as much voltage as a guitar pickup, as well as the impedance will be significantly different.
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lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #25 on: October 12, 2007, 07:10:09 AM »
I just read somewhere that a guitar can put out up to 2.5v peak to peak, surely would means that the preamp  actually need very little gain, in my setup at the moment its got up to 100X gain which seems liek it would be total over kill. I think i need to lower it, this should help the noise floor a lot and reduce the hiss i currently get and higher gain settings.

Actually my gain is even higher because the mixer amp also has 3x gain in it, this doesnt of course take into account the losses for the tone controls which will be significant.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #26 on: October 29, 2007, 07:35:51 AM »
Hello all,

I have been workign on my preamp again this weekend with less than stella results.
I wanted to change the setup so i had a fixed gain of 2 in the first section, then a volume pot and tone tones between it and the final fixed 11 x gain section. To this end i took the schematic from 'Tone stack generator' software and built this with a pot on the front for gain control.

I put this back in and the only thign that works is the volume/gain control none of the tone controls are doing anything.

Any ideas?


LJ King

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #27 on: October 29, 2007, 10:32:23 AM »

The simple cause of those symtoms is not having a ground connection at the 1A lug of the mid pot. Validate that it is actually grounded.

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #28 on: October 29, 2007, 12:08:03 PM »
Well it is defientely connected, however i did have to bend some of the pins on the pot back into place after bending them 90 degrees back to wire point to point. This is probably the issue, i will check it out tonight.

Thanks a lot.
 
Oh also i set the input impedance of the first section to 1m, the second section is set the same. I imagine this isnt needed since the issue with the guitar pick has already been dealt with. What would a suitable value be for the second input impedance settign resistor? 100k?

Phil

lefizz

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Re: Improved 'Guitar Handbook' TL071 preamp
« Reply #29 on: October 30, 2007, 06:27:14 AM »
LJ King, i found that a couple of the pot were broken, replacing them fixed the issue, thanks a lot.


 

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