Crate GX-80 excessive speaker excursion

[Ben Price]

I have a Crate GX-80 in good condition (along with a number of other makes/models of amps). I don't use this amp often and recently loaned it to a fiddle player. He told me that the amp makes a thumping noise when he is playing on the low strings.

So I've done some tests and I've noticed that when I lightly pat the strings of my guitar with the palm of my hand, the amp makes the thumping sound. When I touch my finger to the cone of the speaker (open back cabinet) and lightly pat the strings, it seems the excursion of the speaker is quite excessive. I believe the fiddle player is dropping his bow on the strings while playing and the amp doesn't seem to be filtering out low frequency transients.

I have the schematic and would have no issue changing discreet parts.

I am looking for your input on two questions:
1) Does anyone else have a Crate GX-80 that has the same issue (or is it just mine)?
2) Your thoughts on how to suppress the low frequency transients?

[J M Fahey]

Please post the schematic here so we all talk the same.

The amp might have some bass boost to sound "larger" , everybody wants a "4 x 12" sound" but in a compact convenient combo size so that's common.

And if your friend has a Piezo pickup with a good high impedance preamp, frequency response can go way down, well below actual violing string frequencies, he's essentially tapping a microphone diaphragm with his fingertip, in this case the bow.

Personally I'd build a small highpass filter inside a pedalbox to deal with this problem at the beginning.

I have made violin amps and always cut below 100Hz with a steep filter, you might (should)  add this in his preamp, if space allows,and if not, in a separate box.

If you dare build it, I can suggest a circuit.

[Ben Price]

J M Fahey: Thank you for your reply. I have posted the schematic to my original post.

Just to clarify. I can force the amp to make the thumping sound by patting the strings of my guitar with the palm of my hand. The amp is a solid state 80 watt combo with a 12 inch speaker. It has the standard low, mid, high controls, with a presence switch. I'm only using the clean channel. I would expect the amp to make a bit of a thumping sound when I pat the strings, but the speaker excursion seems totally out of line with the input. I would expect the amp to have some sort of transient filtering to take care of power pulses and stuff like I'm doing.

I may, or may not, end up giving the amp back to the fiddle player, but I'd like to get a handle on why this thumping is happening. My other amps don't seem to have the issue to this extent.

So, my questions are:
1) Does anyone have a Crate GX-80 with this issue?
2) Your thoughts on handling the low frequency transients.

As you suggested, I'll look into buying a used pedal on e-Bay (like a Boss GE-7), but if there is fix for the amp, I'd like to address this issue at the source.

Thank you again for your input and help.

[phatt]

Likely the signal is too hot (Big) entering the Amp.
Generally wise to use input *J2* for anything with on board preamps.

Or use my old SRV trick,, simply add a small value cap in series with the signal right at the *J2* input.

start with say ~10nF (.01uF) this in combination with J2 forms a hi-pass filter, you may have to try a few values but it's a dead simple way to circumvent the problem,, and you don't even have to open the amp.  :tu:
If the 6.5 plug housing is big enough you can squeeze a small Cap inside the plug itself.

Phil.

[Ben Price]

phatt: Thanks for the input

Here's a quick follow up question. The schematic shows a 22k followed by a .047 cap at J2. In your opinion, will putting the .01uf in front of the 22k serve to filter frequencies lower than about 100 Hz? Something that keeps coming up in my mind is that these "thumps" are transients and not really continuous low frequencies. It reminds me of the thump you sometimes hear when powering on/off an amp. So I guess what I'm asking is, will a capacitor filter out (or block) transients?

Thank you again for your input and suggestion.

Ben Price
San Antonio, Texas

[J M Fahey]

A single capacitor is a very "slow" filter, only 6dB/octave , but if you do this at different points along the signal path, they add up to very useful ones.
FWIW in my own guitar amps all coupling caps are calculated to cut around 60Hz , remember there is a transition area around the cutoff frequency; it ends up cutting below around 80Hz like a brickwall.
I use 30 or 40Hz for Bass amps, for a deep yet very tight sound.

I suggest you go for "60 Hz" too, since the amp will still be used with a guitar.
Use this online calculator:
http://www.sengpielaudio.com/calculator-RCpad.htm
for example, at the input of the power amp, we have C45+R79 (what matters most is the resistor to ground, not the series one) , so .22uF and 27k
Calculator shows that as 27 Hz cutoff, I'd use a .1uF cap which yields 59Hz  :dbtu:
And so on.
At the input C1 (.047uF) and R3 (220k) give you 16 Hz  :loco
While Phatt's suggested .01uF would give you very reasonable 72 Hz .
Why doesn't this surprise me?   
And so on.
Just mod series pass caps, do not touch feedback or tone control ones which although might seem to work way too low, need those values for other reasons.
But stacking 2 or 3 (or more) 60-80 Hz hipass filters along the signal chain will quickly clean your sound.

That said, when heavy distorting you might still see the cone move, that's something else: guitar signals are very asymmetric, and the cone will follow the average, imagine that if the signal has, say, +2V / -5V peaks, or is positive 70% of the time and negative 30% , the cone "will stay more time towards one side than the other" , you won't see the (way too fast)  back and forth vibration at audio frequencies but the average position.

To boot, Crate amps have very assymetric (on purpose)  signal clipping *and*  variable duty cycle, again on purpose.
That's a trick invented by the Engineer Murphy at Carvin for their SX amplifiers, who forgot to patent it on time, so now others use it without paying.

They didn't use it in this early amp (1989)  but practically in all others afterwards.

[phatt]

Hi JMF,,,So my amateur guesswork was close ay? 
Maybe I should patent my guesstamation calculator idea.  :lmao:

Meantime,, thanks for the clue about the circuit,, interesting. :tu:

Oh and thanks for the calculator link
Phil.

[J M Fahey]

You can't patent your guesstimator because it's already known and used for ages, under the name of "experience"  :dbtu:

That helped you suggest a very reasonable and working cap value  :cheesy:

[phatt]

Dear Mr Fahey,,
Without derailing this ,, can I jump in a quick Q?

Does this "Variable duty cycle" thing have anything to do with that rather odd looking Diode and resistor combination on the Emitter of Q15?
That D29 and R112 (a 5 Ohm resistor) in that position would certainly alter the performance of the output,, obviously in a good way.
Can you explain how that works,, please. My guesstermator calculator just blew a fuse.
I'm thinking,,Some kind of interaction between the diode and resistor as signal increases,,, scratching my head on this and As I'm in the middle of designing a power amplifier I'd like to understand some of the tricks used.
Phil.

[J M Fahey]

It wasn't used in that old amp, but in most later ones.
Here's the patent and a simplified explanation.
Teemu also explains it in his wonderful book.


In fact, the patent explanation is not very accurate, and does not mention variable duty cycle, truth is the idea was stolen from the real inventor, Mr Murphy from Carvin who missed patenting it ... big mistake.
In this Crate amp, clipping is by identical back to back diodes, so always symmetrical, and when heavily overdriven you get a 50% dity squarewave (both halves are the same).
In this invention, clipping starts unsymmetrical because both diodes are different,
, 40 is a 6.8V Zener, while 38 is 3.3V , so negative half clips earlier.
So far, not that big deal, some pedals use 2 diodes one way, one diode the other, or germanium/silicon mixed or at least Red/Green Leds , but that is always the same.
In this patent, notice that there is an extra diode rectifying audio, which is smoothed and varies Bias, so clipping threshold changes with guitar pick pressure: it starts clipping negative side more, ends up clipping the top side more, and goes backward when note volume dies, plus time constant is calculated to do so during a typical chord.
End result is a varying sound timbre depending on pick pressure ... call it "tubey" if you wish.
Truth is overdriven tube preamps do something similar, because grids rectify when overdriven and also change bias, waveform and duty cycle, while typical "boring" diode clipping does not.
eavey transtube does something similar, they add "unneeded" diodes in almost every stage to rectify and mess with bias and, more importgant, waveform and duty cycle.
Overdriven FETs also do the same as tubes.
So as you see there's many ways to skin a cat.

Read about the John Murphy 1983 invention:
https://www.trueaudio.com/at_eetjlm.htm
unfortunately he was working for Carvin, so *he*  couldn't patent it and Carvin didn't care.


his is the main invention and 90% of the sound, Crate added the time constant to further "swirl" the sound.

EDIT: by the way, Murphy also added a "crud cutting"(his words)  mid range notch after distortion; Crate added a pot to make it variable and patented "Contour" ......  :trouble