Bridging circuit

[ThSpeakerDude88]

I was looking for a low-pass filter schematic online and stumbled across this:



http://sound.westhost.com/project14.htm


now I have a question, is this really safe to do?

[richie00boy]

Of course it is. Rod is an experienced guy and this sort of thing goes on all the time in the pro audio and car audio worlds. You just have to be sure your amp is suitable -- most are, and stay within the impedance limits.

[Netlist]

There should be no problem as long as you respect the 'Pitfalls'
listed above the schematic. I build this simple schematic a while back to experiment with the AlephX in balanced mode.

/Hugo

[ThSpeakerDude88]

okie dokie, I figured as much but just wanted to make sure

so its ok to, say, bridge my Fisher ca857 with this circuit?

[richie00boy]

I don't know what your Fisher is like, but examine the issues as listed with the schematic and make sure your Fisher amp uses a single power supply shared between both channels, and that the amp is not already internally bridged (pretty rare) by seeing if the ground/black terminals on each channel are not carrying voltage and are 'common' to each channel.

[anatech]

Hi ThSpeakerDude88,
Your Fisher has a small heatsink, transformer, outputs for the output power. It does have a common ground. Keep in mind that an 8 ohm load "looks" like a 4 ohm load to the amp when it is bridged.

Keep this in mind if you bridge the amp. Normally, you want to do this with an amp that would be considered to be "overdesigned" for the consumer market. Think heavy, unless it's a Carver ot the like.

-Chris

[ThSpeakerDude88]

alright. Well I forgot to mention the mod I have done also. I cut square holes out of the back panel and installed dual 80mm antec fans. The output transistors are pretty heafty, I forget the exact number but their by toshiba and have a max dissipation of 120w, each and there are four. ( Two npn and two pnp darlingtons.) Hmm...I'm not sure what you mean by a "small" transformer, what is big to you? Mine's huge, and weighs a ton. Its every bit as big as a good tube transformer.

Its rated power is 100 watts RMS per channel @ less than .09% THD, although the meter only reads to 120. I have run speaker sets A/B at the same time and can crank reserve power out of it into my four speakers, so it seems to do fine on a 4 ohm load.


Thanks for that little bit of info about impeadance, I never would have thought of that but it makes sence. That must be how my TDA 7265 is able to deliver 50 watts in bridge mode on only a +/- 16v power supply, because amps can deliver more power into 4 ohms, if it is seeing the 8 ohm as a 4 ohm , it makes sense.

[anatech]

Hi ThSpeakerDude88,
Just watch the temperatures. I don't think you could crank this into a 4 ohm bridged load. SOA on the outputs might be exceeded. Also, Mr. Transformer might be "big", but not compared to a comercial 100W / CH amp. And Mr. Transformer has Mr. Thermal fuse in him. Bummer.

Anyhow, the transformer is bigger than the heatsink would suggest. I guess Fisher was watching when Yamaha went through boatloads of power transformers.

-Chris

[ThSpeakerDude88]

I see your point. The heat sink isn't too small, rather the fins are too wimpy. Some nice, large, thick fins on that heat sink would be nice. Oh , hey I have a question. The amp was given to me because one channel was bad. I poked around in it and found two transistors loose in the pre stages, cold solder joints I guess. Resoldered them and it has been working fine ever since. Only problem is ( this is my question) when the previous owner had it, it seemed to get a little too hot than you would think an amp should get. I remember being at his house and he said "oh thats just normal." I got it and I could sit by it during the winter months and keep warm- litterally. Thats why I installed the fans, because if you left something on top of it and left it on for a few hours you might risk damaging it. It stays perfectly cool now even when you really crank it. Is there something wrong with it causing it to get hot or is this normal?

Just curious, where are you getting your information about my amp, do you have one like it?

[anatech]

Hi ThSpeakerDude88,
My info is coming from my memory, I've repaired a few over the years and my comments are general to the line of that era.

Your bias current is too high, turn it down. If you can, measure across the emitter resistors on idle with no load or signal. Post the figures and the resistor values. I like to measure across both, so let me know if it's across one or two resistors. Your reading across two resistors is more accurate. I like a max current of 20 mA with sets that have smallish heatsinks.

In general, there should be a minimum of four inches above normal receivers, Nakamichi runs warm (Harmon Kardon too) so it's 6 inches for them. I have seen people blow up there set with the owners manual sitting on top (just takes one sheet of paper). At least they intended to read the manual!

-Chris