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LJM Audio

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#1,061
Mikett said:
I have an extra 500VA toroid transformer with 30,000ufx2 capacitors. I have approximately +- 50VDC rails. I need to build a rock solid 2-4ohm capable subwoofer mono amplifier.

Which amp is the best solution. It is strictly subwoofer duty and needs to be very current capable.
Any suggestions which is the best board to use?
Click to expand...
You won't be able to put too many watts into 2 ohms speakers from a 500VA +-50v supply...You'd need at least 1500VA at +-50v into 2 ohms in class d and 2000 w in class AB or you choose to rewind the transformer for lower voltage.
 
#1,062
dreamth said:
You won't be able to put too many watts into 2 ohms speakers from a 500VA +-50v supply...You'd need at least 1500VA at +-50v into 2 ohms in class d and 2000 w in class AB or you choose to rewind the transformer for lower voltage.
Click to expand...
yes. Theoretically the same power. The voltage required for 4 ohms is 1 / 1.4 of 8 ohms
But this is not important. Just use it directly.
Because power amplifier is not a mathematical problem. It's an art question.
It has no strict requirements.
In fact, most speakers do not require much power. About 2-10 W power.
 
#1,064
Last edited:
#1,065
ljm_ljm said:
You can actually use L15. It is better than L7.

You mean you need high pitch adjustment. Bass adjustment function.
I don't like amplifiers with high and low tone adjustment functions. So I can't recommend it.
If the high tone is necessary, the low tone is adjusted. I recommend using EQ software in the computer.
Click to expand...
I'm shopping 2 L15 boards from Aliexpress and looking for a preamp without tone compensations following you idea. Do you have any model to suggest-me?
I'm using a power supply with toroidal transformer with 20000uf for neg supply and 20000uf for positive supply. I think that those capacitors are sufficient.
 
#1,066
Romero2018 said:
I'm shopping 2 L15 boards from Aliexpress and looking for a preamp without tone compensations following you idea. Do you have any model to suggest-me?
I'm using a power supply with toroidal transformer with 20000uf for neg supply and 20000uf for positive supply. I think that those capacitors are sufficient.
Click to expand...
P7MINI pre amplifier is recommended.
 

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#1,068
Few months ago I bought L7 power amplifier and supply it with 2x42 transformer. Of course, BJT buffer transistors got in fire immediately after I switch on. Some final conclusion in this thread about L7 is that the maximum safe supply is +-40V DC. If my math is good, with such supply, maximum output power is 200W on 4Ω load?

I dont care about THD and quality of the music, I doing some research and making some experiments, so I just wondered what is the maximum safe output power of L7 power amplifier? Of course, with using proper heatsink on output transistors.
 
#1,070
masoivy said:
Few months ago I bought L7 power amplifier and supply it with 2x42 transformer. Of course, BJT buffer transistors got in fire immediately after I switch on. Some final conclusion in this thread about L7 is that the maximum safe supply is +-40V DC. If my math is good, with such supply, maximum output power is 200W on 4Ω load?

I dont care about THD and quality of the music, I doing some research and making some experiments, so I just wondered what is the maximum safe output power of L7 power amplifier? Of course, with using proper heatsink on output transistors.
Click to expand...
Don't buy fake goods. L7+- 60V is OK.
 
#1,072
Romero2018 said:
I found on Aliexpress this item I Think is correct. Below is a link for the product:
https://pt.aliexpress.com/item/1005002369425322.html?spm=a2g0o.productlist.0.0.77a726fdncwgZu&algo_pvid=a31a5148-4847-43ba-89b9-5e97a64f55ed&algo_exp_id=a31a5148-4847-43ba-89b9-5e97a64f55ed-8&pdp_ext_f={"sku_id":"12000020375357379"}&pdp_npi=2@dis!BRL!88.1!70.48!!!!!@2101e9d416641419321514773e40b8!12000020375357379!sea&curPageLogUid=v0LaFbEg3wf6
Regards,
Click to expand...
Yes, there is no problem.
We need to believe in operational amplifiers. That is to say, the technology of semiconductor manufacturers is OK.
We do not need to use complex preamplifiers.
 
#1,074
Hello LJM,

Question regarding the Quiescent Current (Iq) of MX50X2. PCB photos included here. I purchased 4 sets of the PCB about 2 years ago with the Kit, components included. I source the NPN output transistors with MOUSER, the Toshiba TTC5200.

I measure the voltage (as V1, V2) between Output and TTC5200 Collector, and the voltage (as V3, V4) between Output and the TTC5200 Emitter. V1= 5.3mV, V2=5.8mV, V3=6.6mV and V4=4.8mV respectively, after working 30 minutes, input connect to ground while measuring. Before sold to PCB, all Toshiba TTC5200 tested with small current (Ic=6.4mA), hFE seems matched pretty well, 80~88.

Is this measurement ok?


PCB_front_MX50X2_1.jpg

PCB_back_MX50X2_2.jpg
 
#1,079
Could be slight variation in resistors . 1% resistors could could make a difference . Take reading again , noting position of each reading , then swop the 2 resistors of highest and lowest reading and see if your measurement is affected . Can you post a schematic ?

Matching the 669's and 649's to eachother , respectively , should also even things out .
 
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