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OP Amp question

Started by skynyrd, August 26, 2010, 03:51:12 AM

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skynyrd

I just picked up a old Duece VT series to use as a guinnea pig for some mods. It has TL072 op amps in it, what would be a good op amp to replace them with to get a higher gain sound ?

J M Fahey

No one worth using.
TL072 Op Amps are excellent and gain does not depend on the part but on the negative feedback used.
As a comparison: tubes are usually used "open loop" , so in a particular circuit a certain 12AX7 might give 45x gain, and another 55x, it might pay to swap a few and pick the best ; but in an Op Amp gain stage, which is used "closed loop",  if you have fedback resistors of , say, 100K and 4k7, the gain for the non inverting configuration will be : (100/4.7)+1=22.3x no matter what the Op Amp.
That's the purpose of Op Amps !
A "better" one will provide less hiss, lower distortion (we are talking "internal" distortion, not "external" one such as achieved by putting diodes in feedback loops or any other distortion circuit) or better high frequency response.
It might pay to replace old LM741 types which were barely usable with modern ones though.

skynyrd

Thanks for the info, where are the feedback resistors usually located? Are they in the chain with the op amp ?

J M Fahey

Post any preamp schematic which uses op amps , (or part of it as to not to make it too big) and I'll tell you on any specific one there what parts make the feedback loop a, the gain I calculate there and what happens if you mod the value of any of them.
Sort of Op Amps 101 using a practical example.
Anyway google "operational amplifier gain block" and you should findmore than one tutorial.
Start with the simplest one and work up from there.

EDWARDEFFECT1

hi sounds like this thread is right up my alley for a question that is simular. i was fixing a dead channel on an 808s mackie pa.i replace one op amp and it still didn't work so i replaced a second one in the signal chain and the channel now works, but you can't turn up the channel volume much because the channel is extremely hot now.you probably would blow out the speakers with distortion as it is really a mess.is this a feedback problem???...thanks!!

skynyrd

Hope this comes out ok,, I really appreciate your help JM ! !

teemuk

#6
The feedback resistors in that one are R5, R4 and the potentiometer. In most cases the feedback is a basic, (dominantly resistive) voltage divider (like in this design) in rarer cases a combination of several of them. Without the divider attenuating the feedback signal you would have 100% negative feedback resulting into gain ratio of unity. Basically, in this design you have resistance of R5 divided by the resistance formed by adding the potentiometer's resistance "clockwise" from wiper (let's call that "Rpot") to resistance of R4. The gain is thus about R5 / (R4 + "Rpot") +1 - as the equation goes for a basic non-inverting opamp gain stage.

Because this is an AC circuit the gain at different frequencies will also be altered a little by the capacitances in the circuit. For example, C4 bypasses R5 and the low impedance of that capacitor at high frequencies will effectively lower gain at such. Similarly, impedance of C4 is high at low frequencies and this will decrease the gain at such. You can use the RC filter math to calculate the -3dB points.

The "other half" of the "pre-gain" potentiometer R6 is not part of the feedback loop because of the grounded wiper. However, it will still affect the total gain because the potentiometer is forming a voltage divider with R7 that consequently attenuates the output signal.

The gain of the following stage is defined by fixed component values but calculating it is a rather complex task because the divider in the feedback loop has two parallel RC elements. The branch with resistor R9 in it is pretty much a dominant element because of the high series capacitance of C6. That parameter makes this branch effect on everything but very lowest frequencies (at which the imepdance of the capacitor increases resulting into decrease in gain). The branch with capacitor C7 in it will increase the gain at higher frequencies. At low frequencies C7 will have a considerably high impedance and thus this branch will not be as effective in the divider as the parallel one. Again RC filter math can calculate the -3dB points but the equation for entire stage gain is getting rather complex; the gain is very much a function of frequency in this one. Personally, for this kind of stuff I rather just calculate the approximate gain(s) and if I need to get into finer details I do it with a circuit simulator. The key idea for me is knowing how it works and what affects what, not wasting half an hour doing the math and drawing gain vs. frequency curves by hand.

Edit: Fixed R3 to R5. (I read that one wrong the first time).

DJPhil

I was following along and meant to send this out last night, sorry if it's useless now.  xP
I've been drugged up lately from some recent medical work so I'm not all here.

Just wanted to repost the schematic with a bit of touch up for readability, hope this helps. :)

skynyrd

So basically to get more gain I would need a higher ohm pot and lower the value of R8 ? That way I keep the frequency response of C4 in the gain stage even?

J M Fahey

#9
No. The above explanations were technically *excellent*, but I'll write you a simpler practical one.
Not yet because weekends are very busy for me (these crazy musicians who insist on actually playing !!! ;)  ) but probably on Sunday night or Monday.
Anyway, as a warmup, increase R11 to 47K or, even better yet, lift one leg, put another 22K from the free leg to the old hole, and wire a switch across one of them, so your feedback resistor will be either 22K or 44K; you'll get a very noticeable 6dB increase in gain. :tu:
Post your impressions.
PS: R8 will provide no gain adjustment, it's there as an anti-RF filter.
Don't worry, go slow and sure.
Did you google something about basic Op Amp gain blocks?
I thought so ... do it.  ;)

skynyrd

Thanks for the clarification, yes I did google the op amp gain blocks but 99% of what I found kept going through mathmatic equasions and I just don`t have the education of electronics theory to really understand what they are even talking about,,lol. Yes dumbing it down will be greatly appreciated,,lol.

skynyrd

Would that be a good place to add a LED to add a crunchier tone to the distortion?

phatt

Hi skynyrd,
               Yep tricky at first and does mess with the brain a bit but just keep sucking up stuff.
Oh try this page; http://www.geofex.com/article_folders/TStech/tsxfram.htm

Covers some basics that a guitar player who wishes to tinker might find helpful. 8|

Cheers, Phil.

skynyrd

Thanks Phil, I saved that to my favs for reading. I do appreciate the help you guys are offering, this stuff is really intrigueing to dive into. I played for 20 years with the idea that the amp had to stay stock to get the sound, I have learned a BUNCH that I wish I knew back then.

skynyrd

Well I got it switched out with a 47K resistor, with the pre gain on 10 its very bassy,mushy,compressed sounding, with the pregain backed off to 9 it has a great tone and decent distortion. Would like to be able to get rid of the mush and compression on 10 and gain some good crunch.