Which component responsible for compression?

[johnnysmoke]

Long time follower, first time question asker. I've built a lot of pedals with the layouts from this site, THANK YOU! While I have honed my soldering skills over the past 4 years, I've not had much luck with learning what does exactly what.

Which component might be responsible for "compression/limiting"??

This is from a Crush 25BX bass amp - it has a nasty compression/limiter for the first few milliseconds of every note played. It gets annoying. It doesn't matter if you use the speaker, headphones, or if I use the line out direct into my PC and record, the compression is there, and it sounds like crap. Otherwise, the tone is sweet, and my guitar through a Meathead clone with the dark mod sound very baddass through this thing...except for the compression. I've seen a few other people mention/complain about the compression, but no one has a "fix". When I listen to people demoing this amp on YouTube, I can hear it; funny that they never mention it.

[johnnysmoke]

Oh, and any help is very appreciated. I can't tell you how grateful I would be. I know this isn't a pedal...but the circuitry seems simple, and I think a couple of you guys could most likely answer my question in your sleep.

Thank you once again.

Cheers!

[PMowdes]

Here's a useful breakdown of the MXR dynacomp

http://www.electrosmash.com/mxr-dyna-comp-analysis

Should help.

[johnnysmoke]

I appreciate the help, but I think I have more questions now than before. Yikes. Forgive me, maybe I'm oversimplifying this, I was hoping a cap swap or removal would "cure" this issue of this amp clipping the beginning of every single note played, no matter how hard or soft, no matter how the volume on the guitar or bass is turned up or down, nada. First few milliseconds of every note has what I would describe as a "lower volume" then blooms into a fuller volume.

At this point (and I am fairly ignorant of electronics at this point) I'm assuming that the TL072 is the opamp responsible for the unwanted limiting? But..there's something telling the opamp to squish the beginning of every now, then release.

The TL072 doesn't have an input bias like the 3080 (and I'm not going to pretend how that works, LOL) so I'm not sure how to apply this rather confusing knowledge.

Also, I'm running my own business, and going to school full-time working on my master's degree. My hour is up, I have a paper to finish for today, and begin the one that is due Monday morning. Sigh.

[PMowdes]

Are you sure it's compressed or limited......??

Could it be "Gated"??  which is often caused by mis-biasing.

Measure the voltages at the legs of each of the ICs and transistors.  Usually gives you an idea if something is faulty

[johnnysmoke]

Ok, will do.

Whether it is compressed or limited or gated...I don't know. I've seen a few people complain/inquire about this "feature", and they called it compression, so I just borrowed the terminology.  Apparently, Orange thought this was a good idea to protect the speaker, I suppose, to take the initial "thump" out of the bass when a note is struck, according to some other forums I've read. This isn't a "fault" - this is by Orange's design. It was meant to do this. >:-(

As far as "limit" or "compress", I'm not sure which, sorry. I can only describe it as the volume is a bit lower for a few milliseconds, then "releases" to a fuller sound/volume. If I play a note/chord and let it ring out, the volume will be "lower" or "compressed" (limited??)  for just a little bit, then release, and the note will ring out like normal.

This issue is really annoying when playing fast, but it's not an "issue" in that there is nothing faulty or broken - it was meant to be that way straight from the factory. It is even more "in your face" when I pug the line out into my PC and record...I think most people, like my wife, wouldn't notice it, but me, it is driving me nuts.

What I want to do is mod this thing, so it doesn't do that any more.

Urg..I've spent all morning on this instead of my paper...LOL. I gotta hit my homework for a while, and come back to this.

Thanks again for your input.

J

[Travis]

Unfortunately that photo isn't going to help with anything. Without a schematic, your guess is as good as ours. If you posted good photos of the trace side somebody could potentially make a schematic, though I can't promise that somebody will actually be interested in taking the time to do that.

The Dynacomp style compressors use an OTA for the compression. The TL072 and 4558 are not OTAs, this is something different.

Personally I would just get rid of this Orange and build or buy a better sounding amp. Orange has really destroyed their reputation in recent years by putting out amps like this, but they're probably making more money than ever. Kinda weird how that works

[rocket88]

I second Travis's statement about modern oranges. It's the main reason I've made a turretboard layout for one of the vintage amps I've always wanted. I did a quick search online and found the schematic for the crush 20 guitar amp, while this is a bass amp if I'm not mistaken,, but it gives some indication of the possible component layout. I don't see anything that makes me think there's a compressor built it, could be wrong though th. If you're hitting the limiter what's happening is you've got too much output from your instrument or pedal and it's distorting the amp. It sounds to me that basically the amp is getting overloaded so it's sorta shutting down so to speak, then when the signal is at a level it can handle it's allowing it to pass undistorted. Is there a gain knob? Cause that could be a control for the amount of signal hitting the front end of the amp.

[motterpaul]

out of curiosity, where is your gain and volume setting? I would roll down the gain and turn up the volume and see if that helps. Also - turn the bass control down to where it is not booming - and turn up the mids to where you hear that attack you want.

If there is any compression in the circuit, it is just typical clipping in the opamp or transistor caused by too much gain going in - which in the end will sound like compression with distortion.

[johnnysmoke]

My gain is as far down as it will go, volume up, bass down - yeah, I've tried that already. Seriously, its a built in feature - I've learned more, and apparently the limiter was not such a great idea; Orange changed it when they released newer versions of this amp (as according to an email from Orange themselves). They also told me (as if I didn't know) that it is there to protect the speaker, as well as circuitry in the output area.

It seems that the two JFETs (K30A) near the input (and part of the input buffer) are part of the limiter, we/I will one day figure out the other components, but with school (working on my master's) killing me with homework, it is hard to do. We are having a discussion about it at a closed group on Facebook - Guitar/amp help tips and fixes

"Follow the -ve arm of the circuit down from the opamp through the electrolytic cap. The gain of the stage is set by: ( the series resistor ) + ( the top half of the Gain pot in parallel with the resistance of the JFET ). The control voltage is generated later in the circuit from the signal. It is just a slowly varying DC voltage which turns the JFET harder on or off. The larger the signal is, the more the control voltage turns off the JFET and increases its resistance. This in turn cuts down the gain of the opamp stage reducing the signal through it. If we had an idea of the values involved we could perhaps replace that JFET with just a resistor in parallel with the Gain pot. That said, I can see why they have put this circuit in a small bass amp with a very small speaker and it is a shame to have to remove it as it provides genuine safety for the speaker."

 

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From: motterpaul [via Guitar FX Layouts] <[hidden email]>
To: johnnysmoke <[hidden email]>
Sent: Wednesday, May 3, 2017 5:06 PM
Subject: Re: Which component responsible for compression?

out of curiosity, where is your gain and volume setting? I would roll down the gain and turn up the volume and see if that helps. Also - turn the bass control down to where it is not booming - and turn up the mids to where you hear that attack you want.

If there is any compression in the circuit, it is just typical clipping in the opamp or transistor caused by too much gain going in - which in the end will sound like compression with distortion.

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NAML

[dovadid]

Hello

I have been trying to repair that same amp and have searched the diagrams with no success.

Could you tell me what are the colors of the resistor R5?

[motterpaul]

I can't find R5 in the picture above unless it is that one hidden by the cap on the right. Try zooming in on that picture in your browser.

On 2/22/2021 12:00:44 PM, dovadid [via Guitar FX Layouts] <[hidden email]> wrote:

Hello

I have been trying to repair that same amp and have searched the diagrams with no success.

Could you tell me what are the colors of the resistor R5?

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[dovadid]

R5 is a resistence but is a presision resistence have 5 bars of colors