I have this amp: https://www.aliexpress.us/item/3256...t_main.69.3f611802EMT5Qe&gatewayAdapt=glo2usa
I am using an old PC PSU going into this DC boost: https://www.aliexpress.us/item/3256...t_main.63.3f611802EMT5Qe&gatewayAdapt=glo2usa
I have the booster set to 50V as tested with my multimeter. The amp is in PBTL mode (side note. there is no difference in SPL as measured with my iphone app in BTL or PBTL mode. DOes that sound right) The most I can pull in terms of amps is 1.1A. Shouldn't be be able to pull 150w @8ohms? and if i understand ohms law that would be 3A right?
I have tried this with a different power supply as well.
My goal is to see what the loudest usable volume is out of this amp before it sounds bad or maxes out my power supply or itself. I am using it as a bass guitar amp with a 4x12 8ohm cab.
I am using a looper pedal fed into a graphic eq pedal with everything cranked and there's no insane distortion. I threw another preamp in front of the graphic EQ. As i turned everything up I got more distortion. My SPL stayed the same (around 124ish) and yet my current draw was still 1,1A.
I am using an old PC PSU going into this DC boost: https://www.aliexpress.us/item/3256...t_main.63.3f611802EMT5Qe&gatewayAdapt=glo2usa
I have the booster set to 50V as tested with my multimeter. The amp is in PBTL mode (side note. there is no difference in SPL as measured with my iphone app in BTL or PBTL mode. DOes that sound right) The most I can pull in terms of amps is 1.1A. Shouldn't be be able to pull 150w @8ohms? and if i understand ohms law that would be 3A right?
I have tried this with a different power supply as well.
My goal is to see what the loudest usable volume is out of this amp before it sounds bad or maxes out my power supply or itself. I am using it as a bass guitar amp with a 4x12 8ohm cab.
I am using a looper pedal fed into a graphic eq pedal with everything cranked and there's no insane distortion. I threw another preamp in front of the graphic EQ. As i turned everything up I got more distortion. My SPL stayed the same (around 124ish) and yet my current draw was still 1,1A.
Get a scope and look at what it puts out. The correct term for amps is current. And yes, 1.1 A @50 V is way too low. I use an 48 V/8 A SMPS (400 W) with mine and it cranks out 250 W into 4 ohms in PBTL mode. Just before clipping. It is frikkin' loud. I measured the SPL at some time but I forgot what it was. Loud. Definitely more than 124 dB(C).
You just reminded me I have an old digital scope. I completely forgot how to use it. It's one of these: https://www.seeedstudio.com/DSO-Nano-v2-p-681.html
Do i put it in series with the speaker? Or parallel across the speaker outputs? Exactly what should I be measuring?
I was wondering if maybe I have my multimeter in the wrong spot? I'm measuring amps between the booster and the power into the amp. If I had a scope where would I measure? Do I clamp on the speaker outputs?
Regarding measuring current: If the amp can pull more than the power supply wouldn't the power supply shutoff? I'm wondering where my limit is. COuld it be the booster? I spun the current adjusting screw many times to the right while measuring the output and nothing changed. Maybe I need to spin it many times more?
Do i put it in series with the speaker? Or parallel across the speaker outputs? Exactly what should I be measuring?
I was wondering if maybe I have my multimeter in the wrong spot? I'm measuring amps between the booster and the power into the amp. If I had a scope where would I measure? Do I clamp on the speaker outputs?
Regarding measuring current: If the amp can pull more than the power supply wouldn't the power supply shutoff? I'm wondering where my limit is. COuld it be the booster? I spun the current adjusting screw many times to the right while measuring the output and nothing changed. Maybe I need to spin it many times more?
You want to see the output voltage of the amplifier - so measure parallel across the speaker outputs.
Assuming you are using a sine wave generator then look for when the output gets distorted and not looks like a nice sine wave anymore - usually seen by the top and bottom being "flat" (clipping).
Do be carefull and use a 10x probe - the DSO-nano can only manage 80Vpp without a 10x probe according to your link - and with a PBTL coupled amplifier with a 50V supply it might (when working correctly) give almost 100Vpp.
But let's do a little math here:
Voltage:
150W/8Ohm will require sqrt(2*150*8)V peak - around 49Vpeak.
With the amp coupled in PBTL and supplied with 50Vdc this is almost achievable (theoretical that is) - but rather optimistic.
So far so good.
Current:
Peak current will be Vp / 8Ohm = 49Vpeak/8 Ohm - or a little more than 6A.
And I am sure that this will be a problem for your PC PSU unless it is very sturdy.
Because (and I assume that the PC PSU delivers 12Vdc):
Input current (from PC PSU) will be 50Vdc/12Vdc * Output current (to amplifier)
At peak current this will then be 25.5A (actually a little bit more as the booster can not be ideal).
PC PSUs are known for being very fast at limiting output current (and will react to the peak current delivered) - so in other words, what you are attempting would require that the PC PSU was labelled for more than 300W on the 12V output.
There is more to be said on this - in particular that your multimeter will likely measure the DC-current and this will be significantly smaller than the peak current.
But start with the oscilloscope and measure how much output voltage you can get from the amplifier before it starts to clip.
Cheers, Martin
Assuming you are using a sine wave generator then look for when the output gets distorted and not looks like a nice sine wave anymore - usually seen by the top and bottom being "flat" (clipping).
Do be carefull and use a 10x probe - the DSO-nano can only manage 80Vpp without a 10x probe according to your link - and with a PBTL coupled amplifier with a 50V supply it might (when working correctly) give almost 100Vpp.
But let's do a little math here:
Voltage:
150W/8Ohm will require sqrt(2*150*8)V peak - around 49Vpeak.
With the amp coupled in PBTL and supplied with 50Vdc this is almost achievable (theoretical that is) - but rather optimistic.
So far so good.
Current:
Peak current will be Vp / 8Ohm = 49Vpeak/8 Ohm - or a little more than 6A.
And I am sure that this will be a problem for your PC PSU unless it is very sturdy.
Because (and I assume that the PC PSU delivers 12Vdc):
Input current (from PC PSU) will be 50Vdc/12Vdc * Output current (to amplifier)
At peak current this will then be 25.5A (actually a little bit more as the booster can not be ideal).
PC PSUs are known for being very fast at limiting output current (and will react to the peak current delivered) - so in other words, what you are attempting would require that the PC PSU was labelled for more than 300W on the 12V output.
There is more to be said on this - in particular that your multimeter will likely measure the DC-current and this will be significantly smaller than the peak current.
But start with the oscilloscope and measure how much output voltage you can get from the amplifier before it starts to clip.
Cheers, Martin
Do NOT measure across the speaker outputs because the negative connection is also hot. This is a BTL amp that drives the speaker with opposite signals. On one speaker terminal to ground you should get a 50 Vpp sine wave and on the other as well but inverted. So the speaker gets 100 Vpp and that is 35 V(RMS) and that is 314 W into 4 ohms. Martin's calculations are pretty much identical albeit for an 8 ohm load.
I don't have a current limiting probe for my scope so I presume this is no longer an option.
Can I use my multimeter to measure the output of the amp? I have this model: https://www.amazon.com/dp/B07SBL39D5?ref=emc_s_m_5_i_atc Can't I use the AC voltage mode to see what is coming out of the amp? Can I still use my bass loop as my audio source or should I use a sine wave? If so does it matter what frequency?
The PC PSU i have is rated for 12 amps. So shouldn't this be able to put out enough to go above 1.1A?
i also have a 24v 3.6A power supply (it's from a dymo printer) which I connected directly to the amp, skipping the booster. Measuring current in series between the amp and 24V supply was still no more than 1.1A. So wouldn't this suggest It's the amp not pulling enough power?
Ok - I would not call a 10x probe a current limiting probe, but it seems that the oscilloscope is out of reach for now (there are ways to get around it, but it would require quite a bit of explanation and some resistors to build a voltage divider).
But without an oscilloscope it will be difficult to determine what goes on with the output.
Furthermore I can only support the post from bucks bunny - use a dummy load and not a speaker.
Measuring how much power an amplifier can deliver with a presumably sensitive speaker can easily cause damage to you ears.
However without an oscilloscope it will then be impossible to identify when the output distorts.
Using a looper - and I assume a loop being played on a bass guitar - will result in a very varying signal.
Even if you bring the amplifier to clipping the average DC current will be far less than the DC current at steady state full power.
Instead use a sine wave generator (and some serious ear protection if you continue using the speaker as load!).
Somewhere between 100 and 200Hz (just above expected port and speaker resonances)
When you think the amplifier has come to its maximum output then measure the supply voltage - is it still 50V?
If so - then measure the supply current.
If not you are having an issue with the power supply dropping the output
NOTE: make the measurements short - to protect the speakers, your ear and your hapless neighbours
Cheers, Martin
But without an oscilloscope it will be difficult to determine what goes on with the output.
Furthermore I can only support the post from bucks bunny - use a dummy load and not a speaker.
Measuring how much power an amplifier can deliver with a presumably sensitive speaker can easily cause damage to you ears.
However without an oscilloscope it will then be impossible to identify when the output distorts.
Using a looper - and I assume a loop being played on a bass guitar - will result in a very varying signal.
Even if you bring the amplifier to clipping the average DC current will be far less than the DC current at steady state full power.
Instead use a sine wave generator (and some serious ear protection if you continue using the speaker as load!).
Somewhere between 100 and 200Hz (just above expected port and speaker resonances)
When you think the amplifier has come to its maximum output then measure the supply voltage - is it still 50V?
If so - then measure the supply current.
If not you are having an issue with the power supply dropping the output
NOTE: make the measurements short - to protect the speakers, your ear and your hapless neighbours
Cheers, Martin
I'm looking at DIY dummy loads and realize I don't have the resistors to make one of these. I will have to buy some and wait for them to be delivered. Would one 8ohm 100w resistor be enough?
As far as testing goes. My 4x10 cab is rated for 800w so wouldn't this be enough of a dummy load? My main goal is to find out what the max output of this amp is. I am trying to understand why the gear I have isn't enough. And the sub question to that is why isn't it drawing more than 1.1A?
If I were to feed a 100hz sine wave into the amp and my multimeter set to AC (i would measure this in parallel across the speaker correct?) this would show what? My peak to peak voltage?
1) is my testing set up correct?
2) what data would this give me? How would this help diagnose why the amp isn't hitting it's peak?
As far as testing goes. My 4x10 cab is rated for 800w so wouldn't this be enough of a dummy load? My main goal is to find out what the max output of this amp is. I am trying to understand why the gear I have isn't enough. And the sub question to that is why isn't it drawing more than 1.1A?
If I were to feed a 100hz sine wave into the amp and my multimeter set to AC (i would measure this in parallel across the speaker correct?) this would show what? My peak to peak voltage?
1) is my testing set up correct?
2) what data would this give me? How would this help diagnose why the amp isn't hitting it's peak?
If you put it in water, it will be enough.
The multimeter displays the rms voltage. This is convenient if you need to find the average power knowing the resistance; the voltmeter reading is simply squared and the result is divided by the resistance value. P=(Vrms*Vrms)/R
Unfortunately, the voltmeter does not see the shape of a sine wave and you will not be able to determine whether the output of the amplifier is distorted by a sine wave or not.