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Haz 17th February 2012 05:57 PM

What about this coil in parallel with the resistor? Is it needed? whats its purpose and does
it effect performance without it? Sorry for all the questions

pacificblue 18th February 2012 05:47 AM

That R//L assembly is there to protect the amplifier, when highly capacitive loads are connected. If you look at the impedance curve of a capacitor, you see that the impedance becomes very low at high frequencies. An amplifier that reproduces such a high frequency, would effectively work into a short-circuit. To avoid that it gets damaged, you put a current limiting resistor in series, usually 10 Ohms.

When you leave it at that, you loose more than half of the amplifier power in that resistor with a typical 4 or 8 speaker connected. Now the impedance curve of the coil comes into play, which falls to very low values for low frequencies. Connected in parallel to the resistor, the coil shorts it out for all audio frequencies. Typical values are in the range of single-digit H, and the absolute value is not critical, because either way its value is so small that the audible frequency range will be unaffected.

The resistor has no influence on the sound, because it is effectively shorted out for our hearing range.

The coil can have effects. People use air-core coils to avoid distortion from core saturation. Due to the small size of the coil, the copper resistance is negligible. That leaves us with the undesired effect of electro-magnetic fields. Such a coil can induce a signal into other parts of the circuit, so it is a good idea to place it in a way that keeps the effect small. E.g. place it far away from line level wires, traces and components. Put it so that the coil runs in parallel to the signal wires or traces, to keep the inductive coupling low. If you put it at a right angle, the coupling is high.

You may now wonder when you need those components. That is the case each time you cannot be sure, which load is connected, and also when you know that the load can be highly capacitive. E.g. in an active speaker with nothing but a short piece of wire in between the amplifier and an inductive speaker, there will be no need for the R//L circuit. For everything else you should provide it however, because there is always the possibility that an electrostatic speaker is connected to the amplifier or that very long runs of cables go from the amp to the speaker or that the speaker's cross-over poses a highly capacitive load or that your children play around with the components you have lying around, and they check what happens, when they stick the pins of those big capacitors into the speaker terminals.

When the designer put 0,1 Ohm in that place, he may have thought about providing a place for a load sharing resistor in case you want to connect amps in parallel.

Haz 19th February 2012 11:10 PM

Thanks for taking the time to post such detailed information, its much appreciated.

The amplifier will connect to the speakers with short cables, But I will add the coil in there anyway. Should the coil be made from enameled copper wire? I guess so otherwise the short coil would short out.

I have some enameled copper wire I could use form some old core inductors. Or even use some insulated solid core wire and make a coil from that?

Does the gauge of the wire used matter?, I herd you should use 1mm I have a capacitance and indutor meter coming soon so can experiment with different wire and the number of turns eg.

pacificblue 20th February 2012 05:47 AM

Enameled wire is the most practical, if you have it. You can also use insulated solid core wire, but the thicker insulation compared to enamel will increase the coil length and thus reduce the inductance. You will need more turns than with enameled wire.

The wire need not be very thick, because the effective copper resistance is small, and the resistor you wind the coil around will help with heatsinking. 1 mm is OK.

axa 24th March 2012 03:32 PM

yuanjing tda7293 amp 350W

This seems to be the place to discuss / warn people about these cheap amps that you get on ebay. I wasn't using it as a hi fi amp, but I'm sure this will help people considering buying one of these.

My experience with the yuanjing 4 x tda7293 btl amp module

I was using it as a replacement power amp in a 70's Yamaha B100 bass amp. The B100 has a stunning discrete transistor preamp, great eq, beautiful. Unfortunately the triple bias diode in the power amp is very hard to obtain and very expensive if you can get it .. which is why I decided to give the tda7293 a go.
It's now used as the power stage in a peavey heritage.

The information with this product is very sketchy. One of the important things to note is that the you don't earth one side of the output. The output goes straight to the +ve and -ve termainals of the speakers.

Mine had a slight hum on it, acceptable for my purposes, but not hi-fi. It wasn't in a grounded enclosure, so that could have been improved upon, maybe.

The biggest problem with these is heat, I put mine on an ample 35 x 8 cm finned aluminium heatsink, I reckon @ 100W output, the sound starts cracking up in under 10 minutes - I can't see that you're ever going to get the rated 350W out of one, even with fans & a larger heatsink.

My impression of these amps is that they are a bit of an experiment - certainly not a tried and tested item from a reputable company, but for 25, what was I expecting? They do sound pretty good though and it works well enough at low volumes. They do have thermal shutdown and are forgiving to work with, so there is room for a bit of experimentation. If anyone else has had better luck with these, or if I can help them with their project, then please let me know.

goody75 31st March 2012 06:34 PM

Soldering Question
I am just starting on my empty Yuanjing board from Jim's Audio. I've been reading this thread and have found a lot of great mods from linuxguru and others. I'm very exited to get this thing up and running.

I have two soldering questions related to this board:
1) I see that it's a double sided board but can anyone confirm if the Jim's Audio board has plated thru holes? It looks like it but I'm not certain.
2) If they are plated thru holes, does it make a difference if I solder the component on the top side of the board (to shorten the trace)? I feel kind of silly asking this question because the trace would only be shortened by the thickness of the board but I am a novice and who knows...

AndrewT 31st March 2012 07:18 PM

Solder on the bottom side of the PCB.
Due to the extra heat flow through to the top side, you need a bit more temperature. Either hold the iron on for longer or increase the bit temperature.
Problem !!!
Don't overheat the components.
Try heating the pad only and then roll the tip across the pad to the component leg. Takes a bit of practice but helps with getting the PCB up to temp before overheating the component. This particularly suits the oval pads and oversize pads. Small pads don't allow for much rolling into contact room.

goody75 31st March 2012 07:28 PM

Thanks AndrewT.

I have another question - this time about the voltage rating for the smoothing caps. The schematic from Jim's Audio uses 10,000 uF caps rated at 63V. I already have some decent smoothing caps but they are only rated for 35V. I am using a 300VA transformer with 24-0-24 taps (Antek AS-3224). Since 35V>24V, can I use the 35V soothing caps?

AndrewT 31st March 2012 07:33 PM

A 24Vac transformer only gives out 24Vac when the mains input voltage is at rated voltage and the output current is at rated current.
At all other combinations of mains voltage and output currents the output voltage varies, a lot.

The output voltage is AC, you have to convert that to DC for the amplifier.
When you use a capacitor input filter (smoothing caps and rectifier) the Vdc~1.4*Vac

Expect anywhere between 30Vdc and 45Vdc from a 24Vac transformer.

You should buy 50V caps for a 24V transformer.

goody75 31st March 2012 09:06 PM

Thanks again, Andrew T.

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