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Old 12th July 2009, 04:03 PM   #21
jaycee is offline jaycee  United Kingdom
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Yep. 1000VA total will be more than adequate. Most off the shelf recievers will probably only use 500VA total!

It depends what you're doing - I'd assumed 5.1. Personally I think with good front speakers, a centre speaker isnt neccesary, but thats just me You could easily make a small mixer circuit to mix a centre output into front left/right
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Old 12th July 2009, 04:45 PM   #22
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ideally I was thinking 7.1 but given space and cost I think that 4.1 will be more than adequate.

How are gain controls best implemented in amplifier circuitry?

thanks for all your help
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Old 12th July 2009, 06:49 PM   #23
jaycee is offline jaycee  United Kingdom
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Assuming you don't need any gain, just attenuation, then it's implemented as simply as a potentiometer wired up as a potential divider. Typically 47K log pots are used although everything from 10K-100K is used depending on the circuit.

In this particular application, I dont know what your source will be - presumably a DVD player with 6 channel outputs. You might want to use an opamp connected as a simple buffer before the pots.
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Old 12th July 2009, 07:13 PM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jaycee
a DVD player with 6 channel outputs. You might want to use an opamp connected as a simple buffer before the pots.
six buffers to feed the 6channel volume control and six buffers after the volume control to feed the six interconnects.
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Old 12th July 2009, 08:09 PM   #25
cbdb is offline cbdb  Canada
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Quote:
I think that 4.1 will be more than adequate.
I am assumeing you either have 6 (5.1)discrete outputs out of your DVD player or 5.1 decoder. Its best to have 5 seperate amps and speakers, but if you can only build 4 amps your surrounds will have to be a summed mono of the 2 surround outputs. The centre channel in the front is where most of the dialogue comes from so you cant just loose it.
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Old 13th July 2009, 06:02 AM   #26
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I see your point.

Okay then....slowly I will begin construction of 6 boards.

so for now the plan is like so:

1) 2 toroidals 500va

2) 6 33,000uf + *bucket* of 4700uf

3) 5.1 channels

4) no preamp

and for the tricky( for me ) bits

5) attenuation

6) motororised attenuation? startup?

7) a good softstart circuit.....before or after the 220v filter?

P.s. what are the exact dimensions of the quasi nmos 200? I usually check with the board desginer for the exact measurments so I can set them directly in the viewer software in order to print the master. Can you tell me what these measurements are?
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Old 13th July 2009, 11:45 AM   #27
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Quote:
Originally posted by Alexontherocks
7) a good softstart circuit.....before or after the 220v filter?
The filter should come first. If you buy an IEC connector with built-in filter, there is actually not even a choice. Look for a filter that matches the actual current consumption. Bigger is not better in that case.
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Old 13th July 2009, 11:55 AM   #28
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this is the project I intend to use. the toroid filter will be a current compensated epcos toroid good for 8A at 230v

Click the image to open in full size.
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Old 13th July 2009, 02:05 PM   #29
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When you design a filter there are two things to consider about current.

One is the current rating. E. g. a toroid like yours will not necessarily be damaged by a current that is higher than 8 A. It will saturate and produce harmonics instead of filtering them out. To avoid that, the highest current that is usually drawn, should remain below that rating.

The second is the filter curve. Mains filters are low-pass filters. If you design a filter, you usually know the load that will be present. E. g. if you design the input filter for an amplifier you know the input resistor size and can easily calculate the capacitor for a certain frequency. The current consumption of an amplifier is however not constant, so the filter frequency will be changing all the time. The corner frequency will be higher, when the current is high than when it is low. If you use a filter that is designed for too high a current, it is possible that its corner frequency falls too low, when little current is drawn. The filter will start to act on the 50 Hz or 60 Hz mains frequency and convert some of the energy into heat.

Commercial filters are usually 12 dB filters. If you design your own filter, go for a steeper one, e. g. 24 dB. Find out the lowest current the amplifier draws and set the filter so that it starts to act just above the mains frequency with that load.

E. g. your schematic shows a current compensated choke, but no value is given, so we do not know, at which frequency it acts. After the choke there is a 150 nF capacitor. You mentioned 8 A, which corresponds to 28,75 Ohms at 230 V. The corner frequency of that capacitor will be ~37 kHz. If the lowest current was 0,1 A, which would correspond to 2,3 kOhms, you get a corner frequency of 460 Hz and you could use a bigger capacitor to bring that further down, maybe to 100 Hz or so.

Of course you have to do the math for the entire filter including the choke. The above was just to make the idea clear, why a mains filter should be matched to the load, not oversized.

One remark to your schematic. You have drawn the windings of the choke connected in opposite directions. Although that is electrically correct, the windings of current compensated chokes are usually drawn in the same direction and the input sides are marked with a dot to show that they work in opposite directions.
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Old 14th July 2009, 07:48 AM   #30
quasi is offline quasi  Australia
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Quote:
Originally posted by Alexontherocks
P.s. what are the exact dimensions of the quasi nmos 200? I usually check with the board desginer for the exact measurments so I can set them directly in the viewer software in order to print the master. Can you tell me what these measurements are?

The Nmos200 PCB is 90mm long x 75mm wide. You will need to add about 3-5mm in the length for the heatsink bracket depending on what you will use.

Cheers
Quasi
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