DIY Amp & Pedal Squeals

[exztinct01]

posted this on another forum but still waiting for answers

first, thanks to all your help for finishing my first guitar amp. It's not yet in a chassis since I don't have the tools  for building it but when I get some money, I will immediately go to a metalwork shop to order a custom build one.

My take on my first build (again, it consists of P27 preamp using TL072 connected via shielded wire to LM1875 single channel where the shield connects only to the power amp, common connection is through the PS)
- Noise is very low, I barely hear it when not playing, even when both Volume and Master Volume are at maximum.
- We actually used it (uncased) in a praise and worship setting since our guitarist was so excited about the amp. Honestly, our drummer plays so loud that our soundman finds it hard to bring the vocals and keyboard up to its level, but the guitar in my DIY amp sounded so clear throughout the hall, with leads singing 
- After about 45 minutes of playing (our guitarist was not using any effect pedals that time, just the amp and guitar at maximum volume both), I touched the heatsink and the IC and HONESTLY, it did not even get warm (being out in the open air of course helped).
- Our guitarist even asks me why there is no gain pot and I don't know what to answer him. All I know is that the preamp already has two volume controls and the Master Volume at max produces a little bit distortion. But he asks for more, I don't know how to add a circuit that will integrate a gain control there.
- After our service concluded, I put my DIY MXR Distortion Plus in front of the preamp, set it with maximum gain and Volume at about 3 then played a song with distorted guitar. Turning the pedal on makes a dramatic increase in volume compared with the pedal off so I don't know how to make the transition from clean to distorted using the DS+ sound good. Some suggested that I try different diodes than the 1N4148's.
- Also, when I tried to increase the pedal's volume (I think it was at 7 and above), the speaker suddenly squealed. That high pitch sound disappears when I lowered the volume. So, how do I remove that squeal? Should I maintain my pedal at low gain or low volume? I avoid having my pedal at low gain and volume coz I don't think I hear distortion at those settings. I believe I hear those distorted sounds clearer at high volume.
- Also, after playing with the pedal, my LM1875 IC and heatsink got warm. Not too hot, just warm. 

Any suggestions, opinions, recommendations, mad reactions are accepted  :trouble

[GibsonGM]

I think it might not be really clear what you're looking for?

For the Dist+, why can't he turn down the volume pot, before the dist is turned on??  Find a unity level, and away you go...I tend to use MORE than 1 distortion pedal, and have a "nearly clean" for quiet/picked parts, and a 'b@llz to the wall' level, just slightly louder and much dirtier. You can even set the switch up to go from one to the other, not have to turn one off then the other on.

Getting "Squeal" when you turn on a high-gain preamp and feed it to a preamp..yup, that's normal   The Dist+ IS a preamp, you know...amp feeding amp.   You're causing things to oscillate and couple into each other - ANY noise at all in the system is amplified to great levels because of the nature of what you're doing!
   
- You can try moving the offending items farther away from each other (move pedal away from preamp).
- turn down the gain on one, the other, or both until you find a sweet spot that sounds good AND doesn't howl.
- turn on the Dist+ only when beginning to play, turn off immediately when done.  This is a common technique...I don't leave ANY boosts or dist's on when I'm not playing due to annoying hiss, normal due to massive amplification and room noise.

If you want to run a Dist wide open, you need to clean up the amp quite a bit, I find.  Trying to have the pedal AND dirt - bad idea.  Just my experience...that, and turn it off if not playing, or get a noise gate! 

Hope this was in some way helpful, bud...it's actually hard to "police your noise" if you're using high gain stuff at high volumes.

[exztinct01]

  I guess I made the wrong idea of playing with all volume and gain pots at maximum. So you mean I had too much gain in my overall system 

Anyway, I was not able to use the LO input of the P27 preamp because I don't have a switching Jack to create the voltage divider.
I'll just learn how to setup my amp and pedals to avoid that much gain. I think I just did that because my distortion plus tend to lose its dirt when played at low volumes. I don't know if it's just my ear or it really does that. I used 100k pot, for volume and 50k for gain coz I can't find a reverse log pot here.

How about for smooth transition from clean to dirty sound? Why does turning the pedal on makes a dramatic increase in volume? How do I adjust that so that clean to dirty is smooth volumewise?

[GibsonGM]

YES, LOL, at max, you are asking for trouble (unless you've taken some other precautions somewhere, with your levels).   Too much gain - yes!   They purposely limit how much gain any given stage of amplification should have, to avoid just this...

I agree, many pedals sound 'less than great' at low levels.   I'd go ahead and use ANY 1Meg pot instead of the 50k...that'll give you more sweep into the clean range, some more options, and may also affect the tone a bit.       Same for the volume - get the recommended pot...10k.   Just set it for unity gain.      If you lose too much high freq. by turning it down, come back and we'll tailor that for you    

The cause of 'lack of punch' is really more to do with the ear, and how we perceive things at low vs. higher volumes...the ear itself acts like a filter.  I love the tone of a tube amp cranked with a booster out in front, Rangemaster or something.   At low volume, I just want to play quiet blues with the SAME setup...   speakers also act differently when pushed, as does the power output section of the amp...it's an entire system, we have to try to find a balance.    Can be a huge pain, man   

[exztinct01]

But learning it all would be worth the effort.

I'll try the suggested 1 MegOhm for gain, and 10k for volume. But, what is unity gain? 

Is that 10k pot what will make the transition from clean to dirty smooth in terms of volume?

[exztinct01]

 
after waiting a reply for hours, I realized why don't I just search google about unity gain. Voila, now I understand!
But, I thought the recommended volume pot is 100k log accdg to tonepad? So that's what I used.
Why 10k then?

[GibsonGM]

I have the vol. pot as 10K, based on R.G. Keene's and Jack Orman's schematic from 1998, but may have used a 100k myself!  It works well (well, DID, because I scrapped it to build other things, ha ha).   IMO, it won't matter much...if you like the 100k, keep it.  I do believe that the closer you stay to the design, the better off you are, tho - they put those things there on purpose...

There is a cap off the diodes in the circuit that will act as a filter with the volume control...the value of that pot will determine how this simple filter works.  May not be anything really worth considering.

The vol. pot is all about volume, not dirt, so it won't change gain - but it WILL affect how hard your signal hits the amp or whatever is downstream from the pedal.  It's not a 'fine tuning' kind of thing, but the diff. between 10k and 100K will be noticeable.  As you sweep the pot around in volume, the R that is there is working with the .001u cap, looks like it affects how bright the output will be.    THAT can make your dist. sound better at low volume (more high frequency).

Sorry for the garble, in a rush to go to work, LOL...

[dschwartz]

Don't  forget the guitar pickups..cheap, vintage or even standard pickups will squeal like a dying pig with high gain at high volumes..

[exztinct01]

is the 27db gain of the P27 preamp too much if I only need it for clean tones?
Well I could reduce it so that I could use the MXR distortion plus in front of it for dirty sounds without reaching too much gain.