I've got a Fender Squier SP-10 amp that was given to me along with several pieces of test equipment.
I don't play instruments so I built a simple detector and use that to receive the signal from my part 15 AM transmitter so I can monitor it.
I discovered the bass response seems better with the back off and the bass control full.
The tone control circuit is weird.
It's like an individual volume control for the bass and the treble.
If both controls are fully CCW, there's no audio output.
If the bass control is turned up, the volume level of the bass changes.
If the treble control is turned up the volume level of the treble changes.
If the tone controls are set to the middle, does that provide a flat response?
Had thought I'd need to up the values of the audio coupling caps, but using the values of the capacitors and whatever resistance there is to ground after the cap then plugging them in to an online calculator I see the frequency response seems flat to 10Hz.
Also I don't quite understand the low pass filters on the OP-AMPs. What's throwing me off is the 100pF caps between the + and - OP-AMP inputs. I'm assuming maybe that is to reduce unwanted oscillations, but am not sure.
I also do not quite understand the 1uF caps between the resistor on the - OP-AMP input and ground.
I don't play instruments so I built a simple detector and use that to receive the signal from my part 15 AM transmitter so I can monitor it.
I discovered the bass response seems better with the back off and the bass control full.
The tone control circuit is weird.
It's like an individual volume control for the bass and the treble.
If both controls are fully CCW, there's no audio output.
If the bass control is turned up, the volume level of the bass changes.
If the treble control is turned up the volume level of the treble changes.
If the tone controls are set to the middle, does that provide a flat response?
Had thought I'd need to up the values of the audio coupling caps, but using the values of the capacitors and whatever resistance there is to ground after the cap then plugging them in to an online calculator I see the frequency response seems flat to 10Hz.
Also I don't quite understand the low pass filters on the OP-AMPs. What's throwing me off is the 100pF caps between the + and - OP-AMP inputs. I'm assuming maybe that is to reduce unwanted oscillations, but am not sure.
I also do not quite understand the 1uF caps between the resistor on the - OP-AMP input and ground.
So the 1uF cap serves as a high pass filter, right?
If so how do I calculate the frequency?
So far the amp sounds decent enough for the frequency range I have the processing set to for my part 15 transmitter which is 30Hz-5kHz, however I want to be sure the amp starts to roll off the bass between 40-50Hz.
Also is there a way to modify the tone controls to make them better?
Looking up the amp chip it seems to have lower distortion with a 4 ohm load and the amp has an 8 ohm speaker.
The power transformer is 10.5-0-10.5 Vrms at .5A at least that's what's on the transformer label.
I have a power transformer that is 10-0-10V at maybe 2A. I may swap that one in before trying a 4 ohm speaker just to ensure the supply has enough oomph for the amp chip to properly drive a 4 ohm load. May also up the value of the 2200uF 16V caps as well to make the supply stiffer.
This amp doesn't seem too well regarded and doesn't sell for much used.
What would be a good four ohm 6" speaker to use with the amp?
I may also mod the headphone jack to serve as an external speaker jack.
If so how do I calculate the frequency?
So far the amp sounds decent enough for the frequency range I have the processing set to for my part 15 transmitter which is 30Hz-5kHz, however I want to be sure the amp starts to roll off the bass between 40-50Hz.
Also is there a way to modify the tone controls to make them better?
Looking up the amp chip it seems to have lower distortion with a 4 ohm load and the amp has an 8 ohm speaker.
The power transformer is 10.5-0-10.5 Vrms at .5A at least that's what's on the transformer label.
I have a power transformer that is 10-0-10V at maybe 2A. I may swap that one in before trying a 4 ohm speaker just to ensure the supply has enough oomph for the amp chip to properly drive a 4 ohm load. May also up the value of the 2200uF 16V caps as well to make the supply stiffer.
This amp doesn't seem too well regarded and doesn't sell for much used.
What would be a good four ohm 6" speaker to use with the amp?
I may also mod the headphone jack to serve as an external speaker jack.
The 1uF in the feedback networks reduce the bass. But to unity, not to zero, so they are not exactly a high pass filter.
If you don't need the tone controls, there would be flatter response and more gain if they were bypassed.
For the coupling capacitors, f = 1 / ( 2 x Pi x R x C )
If C = 1uF and R = 100k, then f = 1.6 Hz bandwidth.
For the feedback networks, the calculation is similar, using the feedback resistor that is in series with each capacitor.
Note there are different resistor values used.
If you don't need the tone controls, there would be flatter response and more gain if they were bypassed.
For the coupling capacitors, f = 1 / ( 2 x Pi x R x C )
If C = 1uF and R = 100k, then f = 1.6 Hz bandwidth.
For the feedback networks, the calculation is similar, using the feedback resistor that is in series with each capacitor.
Note there are different resistor values used.
I did swap the power transformer for the one with more amperage. It was a tight squeeze, but the amp still fit the cabinet properly.
Seemed to improve the amp slightly.
I fed an AF generator to the amp and put my scope between ground and the + of C3 and then C11. Those caps do definitely cause a reduction in gain of the bass above 60Hz. So they will get changed for larger values.
I may see what new caps I have on hand and select a value close to 10uF.
For the tone control circuit, here's what I've thought about doing.
Remove C12, R16 and R17.
Put a wire across the terminals where C12 connected.
I'd then have a gain control between the second and third stages.
With the tone control circuit as it is, I can see why this amp isn't very highly regarded.
Seemed to improve the amp slightly.
I fed an AF generator to the amp and put my scope between ground and the + of C3 and then C11. Those caps do definitely cause a reduction in gain of the bass above 60Hz. So they will get changed for larger values.
I may see what new caps I have on hand and select a value close to 10uF.
For the tone control circuit, here's what I've thought about doing.
Remove C12, R16 and R17.
Put a wire across the terminals where C12 connected.
I'd then have a gain control between the second and third stages.
With the tone control circuit as it is, I can see why this amp isn't very highly regarded.
Instead of using a cap for C3 and C11, I just shorted across them.
Doing the mod to the tone controls now allows me to drive IC2B into distortion fairly easily.
For someone who plays guitar, they might like that feature.
The amp does sound tons better now. Even has decent bass response.
At some point this week I may do a frequency response test on the amp just to see what it's capable of.
If for some reason (highly doubtful, but never say never) the amp is flat to 20KHz, I may remove one of the ports and install a dome tweeter with a proper two way crossover just to see how it sounds.
I also tested the amp with the back on and it has deeper sounding bass with the back off.
EDIT:
Here's the updated schematic.
Doing the mod to the tone controls now allows me to drive IC2B into distortion fairly easily.
For someone who plays guitar, they might like that feature.
The amp does sound tons better now. Even has decent bass response.
At some point this week I may do a frequency response test on the amp just to see what it's capable of.
If for some reason (highly doubtful, but never say never) the amp is flat to 20KHz, I may remove one of the ports and install a dome tweeter with a proper two way crossover just to see how it sounds.
I also tested the amp with the back on and it has deeper sounding bass with the back off.
EDIT:
Here's the updated schematic.
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VR3 isn't needed for sure, however it does provide a more fine adjustment over the gain than VR2 does. I also basically left it in there as I don't like having a control on a device that does nothing.
Had both pots been the same value, I could have used a better tone control circuit, but given I desired flat frequency response, I doubt tone controls would have been beneficial.
The .1uF caps, do they go across the + and - supply pins at each OP-AMP or do they go between each supply pin and ground?
Due to the speaker, the output gradually decreases starting about 60Hz and has decent enough output at 40Hz. That's basically the response I was wanting to achieve.
Here's a thought. Why don't I do this?
Had both pots been the same value, I could have used a better tone control circuit, but given I desired flat frequency response, I doubt tone controls would have been beneficial.
The .1uF caps, do they go across the + and - supply pins at each OP-AMP or do they go between each supply pin and ground?
Due to the speaker, the output gradually decreases starting about 60Hz and has decent enough output at 40Hz. That's basically the response I was wanting to achieve.
Here's a thought. Why don't I do this?
VR2 is now the resistance to ground for IC2A + input.
My schematic is a little different in that SW1 is a SPDT switch and merely selects between the regular and overdrive which causes a problem as with the volume controls set to 0, there's still a barely distorted audio in the speaker.
Either mine was an early version and the schematic is a later revision where they fixed the problem, or mine is a later version and they cheapened things by using the SPST switch.
I disconnected R9 and now there's no audio in the speaker when the controls are set to 0.
Overall, I am well satisfied as I took something basically useless to me and made it very useful.
I did a frequency response test using the internal speaker and my ears and it sounds like the treble starts to roll off at about 13kHz. It's not the speaker causing it as that normally results in the audio just decreasing in volume. What it does is at 13kHz I can still hear the signal some, but it sounds different.
My schematic is a little different in that SW1 is a SPDT switch and merely selects between the regular and overdrive which causes a problem as with the volume controls set to 0, there's still a barely distorted audio in the speaker.
Either mine was an early version and the schematic is a later revision where they fixed the problem, or mine is a later version and they cheapened things by using the SPST switch.
I disconnected R9 and now there's no audio in the speaker when the controls are set to 0.
Overall, I am well satisfied as I took something basically useless to me and made it very useful.
I did a frequency response test using the internal speaker and my ears and it sounds like the treble starts to roll off at about 13kHz. It's not the speaker causing it as that normally results in the audio just decreasing in volume. What it does is at 13kHz I can still hear the signal some, but it sounds different.
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So while looking around for a better schematic I ran into a post on a forum that states this amp sounds better with an external speaker.
I decided to go ahead and rewire the phones jack to make it work for an external speaker. It was easier to remove the stock jack and use a different jack as I would then not have to cut PC board traces (already had to cut two to install VR2 where it is) to accomplish the mod. Sure I could have shorted across the 180 ohm resistor, but I would have been limited to using a stereo 1/4" cable.
I used a Neutrik stereo switched 1/4" jack which looks just like the amp could have come from the factory that way. The input jack was replaced by the previous owner (probably failed if it looked as cheap as the phones jack does) using a regular 1/4" mono jack which does not look good. If I can find another Neutrik jack I'll go ahead and replace that one with it. If I don't have one I'll add one onto my next Mouser order.
For a little 10 watt amp that thing gets loud.
I see one problem though. The chassis is steel which doesn't conduct heat as well as aluminum and the output amp chip bolts right to the chassis. I may see if I have a heatsink that can fit on the chassis and mount the chip to that.
The last thing I need to do is up the value of the 2200uF filter caps. I'm thinking maybe 10,000uF will work. That will make the supply much stiffer and better able to handle audio transients.
Here's a couple pictures. Had to shoehorn the power transformer in there.
I decided to go ahead and rewire the phones jack to make it work for an external speaker. It was easier to remove the stock jack and use a different jack as I would then not have to cut PC board traces (already had to cut two to install VR2 where it is) to accomplish the mod. Sure I could have shorted across the 180 ohm resistor, but I would have been limited to using a stereo 1/4" cable.
I used a Neutrik stereo switched 1/4" jack which looks just like the amp could have come from the factory that way. The input jack was replaced by the previous owner (probably failed if it looked as cheap as the phones jack does) using a regular 1/4" mono jack which does not look good. If I can find another Neutrik jack I'll go ahead and replace that one with it. If I don't have one I'll add one onto my next Mouser order.
For a little 10 watt amp that thing gets loud.
I see one problem though. The chassis is steel which doesn't conduct heat as well as aluminum and the output amp chip bolts right to the chassis. I may see if I have a heatsink that can fit on the chassis and mount the chip to that.
The last thing I need to do is up the value of the 2200uF filter caps. I'm thinking maybe 10,000uF will work. That will make the supply much stiffer and better able to handle audio transients.
Here's a couple pictures. Had to shoehorn the power transformer in there.
The no load voltage is +13.56 Vdc and - 13.60Vdc.
R27 and R28 would be removed and I'd put the regulators there. C23 and C24 will be replaced with 1uF caps.
The two OP-AMPs draw very little power so the regulators may not even need a heatsink. If they do need a heatsink it will likely be a small one that is required and if so I have some stick on heatsinks I can use.
Here's the schematic showing the regulators.
While updating the schematic I noticed it's the 230 volt version so perhaps that could also explain the slight difference in the way the clean/overdrive switch is done.
R27 and R28 would be removed and I'd put the regulators there. C23 and C24 will be replaced with 1uF caps.
The two OP-AMPs draw very little power so the regulators may not even need a heatsink. If they do need a heatsink it will likely be a small one that is required and if so I have some stick on heatsinks I can use.
Here's the schematic showing the regulators.
While updating the schematic I noticed it's the 230 volt version so perhaps that could also explain the slight difference in the way the clean/overdrive switch is done.