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Author Topic: Increase low end by modifying the circuit  (Read 4932 times)

longngocvu

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Increase low end by modifying the circuit
« on: July 13, 2013, 09:00:04 AM »
 Hello,

 My cheap SX Ga1065 practice amp seems to lack some bass.
 
 I'm an absolute beginner so i don't know if it's possible to increase the low end of the amp by changing some coupling caps or resistors.  If so, can you guys tell me which ones and the new values they need to be. 

 Thanks,
 
 Here is the schematic:
 

 
 
 
« Last Edit: July 13, 2013, 11:55:41 AM by longngocvu »

Roly

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Re: Increase low end by modifying the circuit
« Reply #1 on: July 13, 2013, 10:45:50 AM »
You could try increasing the value of some of the coupling caps such as C1 to double or triple its current value (0.22-0.47uF), but you may have other factors working against you, such as a small speaker/box.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Increase low end by modifying the circuit
« Reply #2 on: July 13, 2013, 12:24:50 PM »
Fully agree.
That schematic in fact looks quite good, and will sound very well ifconnected to a 4x12" .... or at least a 1x12" , but with the original 6.5"  :duh speaker, forget it.

Only result you'll get is having the little speaker fart horribly with bass it can't handle.

Make a proper hole in the back panel , add a switching jack, so the external speaker cuts the internal one off, and enjoy.

For examples on how real tiny amps sound when hooked to a proper speaker:

Here they go from a rather good 8" to a 4 x 12":

http://www.youtu.be/XzDohmkXeJs

this is a 9v battery powered, 1/2W cigarette box sized amp, into a 2x12" :

http://www.youtu.be/pdWKAAL9qco

Enzo

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Re: Increase low end by modifying the circuit
« Reply #3 on: July 13, 2013, 03:28:52 PM »
Agreed.  You can probably increase the low end response of the amp, but it probably has enough already.   The little speaker simply cannot make low notes.

If you disconnect the little speaker in there now, and connect your amp to a regular 4x12 guitar cab, you will be AMAZED at how much bigger the sound is.

Not just llittle amps like this, but any amp:  the single most effective way to change the sound of it is to change the speaker.

My approach would be to find a speaker and cab that I liked and then see if the amp lacked anything then.

Roly

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Re: Increase low end by modifying the circuit
« Reply #4 on: July 14, 2013, 05:14:10 AM »
These youtu.be links 404 for me, but these work;

http://www.youtube.com/watch?v=XzDohmkXeJs

http://www.youtube.com/watch?v=pdWKAAL9qco

It is so often overlooked that loudspeakers are subject to serious physical constraints, and you can't beat the physics.  Bass requires air displacement and small diameter speakers are just not in the race (a mountain of sales blather not withstanding).

Even some name amps fitted with larger drivers, such as the Roland Cube-series with 12-inch drivers, have amps that are pre-crippled to prevent the speaker in an undersize box from sounding terrible or even destroying itself.  You want bass?  When it comes to bass cabs, "There ain't no substitute for cubic feet".

Like JM and Enzo, I think you may be astonished at how much difference a good cab can make.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

longngocvu

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Re: Increase low end by modifying the circuit
« Reply #5 on: July 16, 2013, 05:02:57 AM »
 Thank you all! You've been really helpful.

 I replaced the C1 cap and put the amp back to back with my wooden bed to take advantage of the space under the bed. I can hear the bass increase considerably.

Roly

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Re: Increase low end by modifying the circuit
« Reply #6 on: July 16, 2013, 07:08:29 AM »
Here's another quick self education in sonics.

Put on something with a lot of bass (not doof, but real bass, just about anything by Toto, but "Africa" for example (I'm not asking you to like it!)).

Place the amp in the middle of the floor, tone controls set for 12 o'clock.  Now move the amp so its against the middle of a wall.  Now again so it is right in a corner of two walls and the floor.

Amazed?

In the first case the amp is radiating into half a sphere, against the wall into a quarter of a sphere, and finally in a corner into an eighth of a sphere. What you are doing with each step is doubling the radiation resistance that the speaker is driving, and moving between free space and horn loading.
If you say theory and practice don't agree you haven't applied enough theory.

Kaz Kylheku

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Re: Increase low end by modifying the circuit
« Reply #7 on: September 09, 2013, 01:04:41 AM »
In addition to C1, another cap to look at is C16.

(Of course, at this point I should tip my hat to the other comments: any check you write with the amp has to be cashed by the speaker!)

This capacitor is used to set up the feedback loop around the power amp such that the amp has no gain at DC.  This is done so that the amplifier has no DC offset (places no DC voltage across the speaker), but it rolls off some low frequency response.

It is possible to null the DC with other techniques and eliminate the capacitors.

In my power amp, I have successfully added a bias current cancellation circuit. It simply generates a similar, but opposite bias current to what the diff amp transistors put out (and with similar temperature dependent behavior). This is not really practical to do for an op-amp, because you cannot match the device which is inside it. (Well, not strictly true! You could get another TDA amplifier chip and power it, then mirror the current ...).  Anyway, in that amp, I shorted out the equivalent of your amp's C16, and nothing bad happened: the DC offset stayed about where it was! Except, the amp suddenly had gain all the way down to DC.

A more popular way to null DC is to build a "servo": a circuit which monitors the bias point and using feedback, it applies a corrective signal at the amp's input to null the offset. A DC servo uses capacitance (it's usually based around an integrator op-amp circuit). However, because it's an active device with gain, with the capacitor in the feedback position, it can effectively simulate having a huge capacitor.  The servo can have a time constant of seconds if you like. It would not be practical to replace C16 with a capacitors large enough to have a time constant in that range. With a servo, the amp doesn't have gain down to DC, but almost.
« Last Edit: September 09, 2013, 01:07:37 AM by Kaz Kylheku »
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Kaz Kylheku

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Re: Increase low end by modifying the circuit
« Reply #8 on: September 09, 2013, 01:29:42 AM »
Now, a slightly different topic.

Part of the reason you might not be "feeling" enough "boom" from the amplifier is that it's a voltage amp.

You can add some current feedback to its feedback circuit: that is to say, make a fraction of the feedback voltage be proportional to the current that is flowing through the speaker.

Adding a modicum of current feedback will raise the output impedance of the amplifier and allow the speaker to resonate more at its resonant frequency. As well, current feedback will create a natural "mid scooped" EQ curve, using nothing but the speaker's own impedance characteristics. A lot of solid state amplifiers in guitar amp history have used current feedback: Roland JC-120's prior to 1982, Peavey Bandits from the 1980's, Marshall Valvestates, Crates like the Flexwave, ...

What current feedback does is ultimately up to the speaker. If your speaker likes to resonate at 80 Hz, you will get a thump there, et cetera. Exactly how it sounds cannot be easily described in words. It's like an EQ change, but not exactly. The tone becomes more "vibey" and "three dimensional" with a nice "chime" on top. Perhaps a custom circuit could be built which does it without current feedback, but it would have to be quite resonant compared to a run of the mill tone control, and it would have to be adjusted differently for each loudspeaker (or else it would be "fake": for instance it would try to put a 70 Hz peak on a speaker that wants it at 85 Hz.)

When adding current feedback, you have to be concerned about not upsetting the DC offset. You see those R14 and R16 resistors in the schematic? They are identical for a reason: similar bias currents flow from the - and + inputs of the amplifier, and generate similar offset voltages across these resistors. The idea is that these offset voltages at the inputs cancel each other out, which helps minimize the DC offset (along with that C16 cap which further minimizes the DC offset by reducing the gain to unity at DC, so the DC offset at the output is basically a copy of the offset at the inputs, not magnified by any gain).

Current feedback involves using a small-valued resistor (like 0.5 ohms) in series with the speaker for sensing the current and mixing that into the feedback signal. But this creates a low impedance which will upset the balance of those two resistors, and could cause a significant DC offset. For that reason, current feedback is usually AC-coupled through a capacitor. You see that time and time again in guitar amp schematics.

(If you can address the DC offset issue in the amp with, say, a DC servo, then you can DC-couple this current feedback signal, eliminating the blocking capacitor.)
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Roly

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Re: Increase low end by modifying the circuit
« Reply #9 on: September 09, 2013, 05:06:03 AM »
Quote
SX GA-1065 - 10 watts small practice guitar amp
5.5" speaker, overall dimentions: 9 3/4" wide x 10 5/8" high x 5" deep
This is never going to be a bass boomer for two reasons, the diameter of the speaker, and the small size of the enclosure.

Some time ago when I was doing a lot of stage sound there was a woman who had a Roland Cube-40 to go with her Roland synth.  The synth itself was very nice, but a bit of a play through the Cube-40 showed that output dropped quite rapidly below middle-C.

Thinking that the problem was speaker/enclosure limitation (a 12-inch, but in a minimal box), I could get the desired response if I took the amp output via a DI to the PA.

As it happened it had to come in to the workshop for some other attention, and while it was there I decided to sweep the amp just to confirm my thoughts.

WRONG!   :duh


This is the amplifier response.  (detail   Erratum; it was actually a 40)

The amp had been quite deliberately rolled off, presumably because even a 12-inch would start to frapp badly in such a small box.  While it might be satisfactory for tenor guitar it was quite wrong of the music shop to push this amp at her specifically for use with a synth (which normally has a lot of bass content).

As Kaz said...

Quote from: Kaz Kylheku
any check you write with the amp has to be cashed by the speaker!
:dbtu:
If you say theory and practice don't agree you haven't applied enough theory.