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 ?
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.
Thanks for the info, where are the feedback resistors usually located? Are they in the chain with the op amp ?
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.
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!!
Hope this comes out ok,, I really appreciate your help JM ! !
(http://i53.photobucket.com/albums/g75/hoosieroutdoorsman/opampsection.jpg)
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).
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. :)
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?
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. ;)
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.
Would that be a good place to add a LED to add a crunchier tone to the distortion?
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.
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.
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.
Would R8 be the cause of the compression sound? It is right after the pregain pot and my line of thinking is it could be loading up so to speak. Or with it being 1K is it not big enough and it is overloading the second gain stage?
Try a lower value for C3, 22uF/25V
Try 2.2uF instead.
It's a fine line between getting great OD but by then you loose the bottom end and your sound becomes tiny and lifeless.
Phil.
Correct me if I have been given wrong info, but I was told the lower a capacitors value increases bass and rolls off more treble ? Wouldn`t dropping from 22UF to 2.2 UF increase the bass at the input ? I don`t mean to question YOUR knowledge just trying to improve mine.
Hi Skynyard,
LOL, Heck even God could be wrong on the internet and no one would notice. 0:) ;D
Here in lies the problem of one line answers to complex things.
The info was correct but only in one situation.
Lowering the value of A cap that is *in series* with
the AC signal cuts bass.
Raising the value of A cap that is *in parralell*
(i.e. across the input) with the AC signal will cut more
high frequency.
Both of these conditions refer to the AC part of a circuit and guess what???
Neither one will help in this situation because C3 is part of yet another thing in a circuit called DC condition.
It's probably better if one of the more teky ones here cover that part. I'll just confuse the issue.
Meantime just keep this little rule in your head;
To build a circuit you have to set the DC conditions of a circuit first so that the AC signal will pass through in the best possible way.
AC stuff is not so hard but DC is a little more complex,,well was for Me at least. :-[
Cheers, Phil.
I have been finding out that the DC and AC circuits are confusing,,lol. I know no one can come on here and type out a complete electronics theory course that a ying yang like me could pick up and run with,,lol. But I do appreciate everyone taking the time and the patience to answer my questions so I can understand what I am looking at a little better. I know i can`t take my old Mace and make it a high gain amp,,well i probably could but that would take more than I am capable of,,lol. But with the guidance all of you are helping me with I am getting it closer and closer to the sound I am looking for.
Well I decided yesterday to really dig into this amp and see what I could do, first off the normal channel never has worked right and there was always a loud squeal when plugged into it. Had a tech tell me the norm jack was bad and went ahead and replaced it, squeal gone but the norm channel stayed on all the time. I could toggle the effects channel on and off but the norm stayed on so always figured there was a messed up switching chip in it or something. So I switched out R50 from a 33k to a 22K in reference to another forum member and put 2 LED`s in series on R11 per the same forum member and plugged her in and tried her, did not like the grungy sound the LED gave so pulled them and went back to norm. The 2 resistors I changed added a little more distortion but with the effects channel pre gain on 10 it was super bassy and compressed, on 9 it had a decent rhythm tone but the lead tone was still not edgy enough. The 2 channels on the amp are suppossed to run paralell to each other and you should be able to boost 1 channel over the other, not in this case. I don`t know why but I decided to put both channels pre gain on 10 and set the effects channel on 1 and started turning the normal channel up to see what happens, the more I turned it up the more the bassyness and compression went away. Then this sweet little crunch came out and a cool growl and tone the amp never had before emerged. So I played with the eq`s on each channeland kept raising the norm chan volume and it got better and better. So it appears the amp has been rewired with the channels in series like i wanted to do. It would have been nice if the previous owner had told me this and saved me and everyone else alot of headaches..lol.
Ok, now that Mr. Fahey`s mods made a nice change he has peaked my interest, since I have the warm up out of the way whats the next step?
Hi skynyrd,
What happened to *super bassy and compressed* you mentioned?
Did that get sorted?
Phil.
Quote from: phatt on September 12, 2010, 09:47:24 AM
Hi skynyrd,
What happened to *super bassy and compressed* you mentioned?
Did that get sorted?
Phil.
If I just use the effects channel alone with the pre gain on 10 its there,, backed off to 9 it opens up and get a better tone, with the channels being run in series like I am 99% sure it is, the normal channel feeds the effects channel. So that with the tone stoack mod I did to the norm channel is brighter and helps even out the effects channel. Now if I crank the effects channel up it will over power the norm and the bassyness returns, but for low volume like stage level being miked its a very even tone.
Well i installed a bias pot and bumped the bias up to 41 mA`s, the tubes get decently warm and not overly hot. She sound alot more full and open now, she is shaping up nicely. The tubes were on a 17mA bias before, I read where the JJ Tesla 6L6GC can be biased to 60mA`s but doubt I will push them that far, will get a small fan to pot on the back of the amp to help keep any extra heat down.
I swapped out the magnet on the bridge pick up on the Les Paul and WOW ! ! All of the compressed sound and mushy bottom end I had is gone. The pick up is a Duncan Custom and had a ceramic magnet in it, the guys on the duncan forums have been talking relentlessly about mag swapping so I figured since mags are cheaper than pups I would try it. I picked up a Alnico 4 mag off ebay for $5, took the pup apart and double checked to make sure I had the A4 going in just like the old mag and put her back together and heated the pup with a hair dryer for 20 mins to get the wax flowing and sealing everything back up. I plugged in and had the roundest fullest tone I have ever gotten out of that guitar and amp, I was in shock for about 10 min`s as all the less than likeable aspects of the tone were gone and an actual lovable tone replaced. The normal channel I left stock sounded so sweet and had the best Skynyrd tone the amp has produced, the Effects Channel I modded per Mr. Fahey had this sweet growl and really good distortion, the rythm sounds were so clear and articulate. I would have never figured that a ceramic magnet would make a change for the worst, my Peavey Heritage had an even worse compressed mushy sound and when i plugged into it with the new mag it was gone as well. If you have a Peavey amp with a solid state preamp and a pup with a ceramic magnet do yourself a favor and hit ebay and get a Alnico mag, its easy to swap and will make a HUGE difference in your sound and tone