Query about crossovers

[Audioguru]

The amplifier is Chinese and they do not know how to say, "input impedance". It might be 14k ohms, then your volume control should be 7k ohms or less.
In your other thread about this amplifier I cannot remember what transformer you will power it with or what speaker you have.

 

[Enzy]

I will use it to power (2) 600rms and 2(1500rms) 15" speakers also I paid somebody to wrap a E core transformer 40-0-40v and ok I think I can get 5k pots to buy for the volume control then.

Also each speaker is 8 ohms so I plan to put them in pairs for 4ohms, I would just use the 2 boards as mono so I would use the same signal wires to feed both boards and then connect 2 speakers per board, so I am expecting to get 700 watts per pair of speaker, I am wondering if I need a bass preamp after the cross over though.

 

[c_mitra]

Low value of volume control pot will load the output and is not recommended. What is more important is the input sensitivity and your volume control must not work only at one end!!!

 

[Enzy]

The amplifier is Chinese and they do not know how to say, "input impedance". It might be 14k ohms, then your volume control should be 7k ohms or less.

This is the reason why I said I could use a 5k pot then so are you saying the information isnt accurate?

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Low value of volume control pot will load the output and is not recommended. What is more important is the input sensitivity and your volume control must not work only at one end!!!

also what do you mean the volume control pot cannot only work at one end ?

 

[Audioguru]

A 5k volume control will work perfectly and is not a value too low since the output opamp in your lowpass filter circuit can easily drive it.
We do not know if the level from your signal source is high enough without adding another opamp to increase the sensitivity.

 

[Enzy]

I got some changes to do also, a friend of mine just gave me a cross over peavy 3/4 way I will use that for this amplifier but I will build the cross over just the same but to test on another amp that I will build soon.

so since I wont be building a cross over to add to this amp anymore I was thinking to just add the bass boost circuit to the amp since it was said to be a good addition, I dont know if that will be a problem though since I will be using a cross over, but if not I would add it.

I will send my input signal to the bass boost then use a 5k pot to control the volume then send the signal to the amplifier, can that work?

here is the bass boost

 

[c_mitra]

also what do you mean the volume control pot cannot only work at one end ?

If the input of the to the power amplifier is not at line level, output of the volume control, which is simply a voltage divider, may be too low (at the lower side) to drive the amplifier. You may end up using the volume control at the max end only all the time. Optimum position of the volume control is somewhere near the 1/3 to 1/2 position (angular).

Input impedance is frequency dependent (it is impedance, right?) and is most commonly specified at 1000Hz for common audio amplifier. As you are trying to amplify only the low frequencies, your amplifier will try to amplify high frequencies (that are absent) and will simply contribute to lots of noise. You need to limit the bandwidth of the amplifier too. Most likely your amplifier input is capacitance coupled and you can figure out the approx inp imp by simply looking at the circuit...

 

[Audioguru]

Why do you want a bass tone control circuit that works wrongly? I didn't calculate the frequency it boosts and the website you found does not exist anymore. It even shows an LM324 horrible opamp.
The circuit is a copy of a "mids" tone controls circuit that boosts or cuts mid frequencies but not higher or lower frequencies,. It has higher resistor and capacitor values that might just boost or cut 150Hz but not higher or lower frequencies causing a boomy sound if it is turned up.

A bass tone control is used to boost the bass of a speaker that is too small to produce good bass. The lower the frequency then the boost is more but with your big speakers you do not want that. You need just a volume control on the bass amplifier so that ALL low frequencies are adjusted to the same level.

 

[Enzy]

ok then I will use a 5k pot then send it to then amplifier I will do so tomorrow and let you know what happens.

ALso I thought you would reply to c_mitra

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also I forgot about protection circuit, If the amp is 700 watts at 40v CT that means I would use a 17 amp fuse on the transformer.

 

[Audioguru]

If the amp is 700 watts at 40v CT that means I would use a 17 amp fuse on the transformer.

40V CT is only 20VAC and 20VAC which makes plus and minus 26VDC . Then the output power into 4 ohms is only about 72W. Maybe you mean 80V CT which is 40VAC plus 40VAC which makes plus and minus 55V. Then the output power into 4 ohms is about 325W.

700W into 4 ohms is a signal that is 150V p-p and needs a power supply that is plus 80VDC and minus 80VDC. Then the transformer should be 116V CT. The power supply filter capacitors on the amplifier are rated for only 80V so they might blow up soon. You are gambling with a Chinese amplifier.

 

[Enzy]

It 40-0-40v I have a toroid thats 44-0-44 but when I test it its like 94v. but wouldnt that be less than 50v going through each capacitor? Also my aim was to build a class D amplifier on my own but I cannot find a schematic that works and I ask here but no one seems to have any willing to share I wanted something like 700w or even higher.

I know its a gamble with this amplifier but I had no choice the only good amp I know how to build is using the tda7293 or tda7294 and what I want to learn to do now is put those chips in parallel.

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[c_mitra]

Your transformer is rated at 225VA and perhaps it can deliver 500W for a short time. But how can it run a 700W amplifier? It will happily run a 250W amplifier and if you are conservative, do not use it more than 200W. Going by looks (and looks are known to be deceptive), the transformer on the left (40-0-40) appears to be still lower in power capacity.

Unless you mean 700W of peak power (whatever that may mean)...

 

[Enzy]

I actually wanted RMS wattage but I will have to get another transformer at a later date or build an amplifier thats more powerful.

This is an amp using the same chip how does this circuit look.



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I says this is rated at 3000 watts.

 

[Audioguru]

The voltage of a transformer is measured at its rated load current because its resistance causes a voltage drop with current. A good transformer voltage increases only half as much as your transformer but maybe your mains voltage was high when you measured it. 40VAC has a peak of 56.6V and a rectifier will drop the peaks of its ripple to 55.6V. The Chinese amplifier is clipping at 700W into 4 ohms with a 50VAC-0V-50VAC (100V CT) transformer.

 

[Enzy]

I see a tl081 chip under it, I assume thats a preamp I tried it lastnight at idle it was pulling 17 watts from the wattmeter I plugged it into but the gain was low im not sure because of me using a bad jack to test it. But I realize it has a soft start but the jack I had seemed to have a broken wire so when I tested it to allow it to play it kept on flickering the light on the amplifier and at one point i saw the wattage go up to 200 watts for a while then I unplugged it, It fried the speaker I had connected to it.

While playing the voltage across it was 89.2 volts

 

[Enzy]

I got a new speaker to test it just now ( peavey pro 10 neo) 200 watts continuous, 8 ohm and I have a 8 ohm horn I didnt yyet try a volume control so I just paralled both coils and it sounded much better it actually had good gain, I disconnected the speaker and used a 18hz test tone to test it but I realize I that after a certain point the led on the board started to flicker and the volume which it flickers is lower than the higher frequency tones. What I find strange is that I expected to see a square wave output but instead isnt a sine wave, so that means the amp isnt actually a class D?

I used a DSO nano v3 to test the output wave I just got it, its a little pocket scope so not so sure how much I can rely on the wave but other people used it and it seemed good enough for simple tasks like this.

 

[c_mitra]

While playing the voltage across it was 89.2 volts

You mean across the speaker terminals? With full input? This appears to be very high, I think.

 

[Audioguru]

A class-D audio amplifier switches its output transistors on and off at 800kHz with PWM. An LC filter at its output smooths the pulses so they are gone and only the audio that modulated the PWM remains.
A tweeter is fragile and should never be fed a continuous powerful tone.
A horn causes the sound to have many peak frequencies and cancelled frequencies which causes the "horn" sound that I hate to hear from a speaker.

 

[Enzy]

You mean across the speaker terminals? With full input? This appears to be very high, I think.

No not across speaker terminals, just referring to the input power from the transformer at the board, across the speaker terminals the voltage was low I was getting like 4v to 8v rms.

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What can I use to protect speakers?

 

[Audioguru]

The power transformer is center-tapped so its total voltage might be 89.2VAC. Then the DC voltages will be plus and minus 62VDC at full power and a little higher with reduced power.
To protect the speakers then keep the volume control turned down.