Anyone have experience with diy amps?

[BobEllis]

You could use a CL-30 Negative Temperature Coefficient thermistor for soft start duties - Cold it has a high resistance that drops when warm. Some may argue that they impact the sound due to the resitance in the mains circuit. I use them in my Leach amps and A75s and cannot hear a differnce when they are in or out of the circuit. I stopped blowing fuses when I added them, though. A few bucks at Mouser, one extra postion on a terminal strip and you're in business.

[thylantyr]

You could use a CL-30 Negative Temperature Coefficient thermistor for soft start duties - Cold it has a high resistance that drops when warm. Some may argue that they impact the sound due to the resitance in the mains circuit. I use them in my Leach amps and A75s and cannot hear a differnce when they are in or out of the circuit. I stopped blowing fuses when I added them, though. A few bucks at Mouser, one extra postion on a terminal strip and you're in business.

Thermistors are good for class A amps due to
continous current draw which keeps the thermistor
hot so it keeps resistance low. But I wouldn't use them
otherwise. When they cool, lets say on a soft musical
passage, resistance rises current limiting the power
supply. Audible efffects will be subjective. 8)

[JonMarsh]

Many SMPS use NTC's for inrush current limiting, but then bypass them with a relay at the end of soft start.

This is getting WAY too close to my day job!

~Jon

[thylantyr]

Many SMPS use NTC's for inrush current limiting, but then bypass them with a relay at the end of soft start.

This is getting WAY too close to my day job!

~Jon

Seems like a retarded idea. NTC + Relay. My QSC amp
has this too. I added more rail capacitance on the input
side and the poor ole' SL32 thermistor went up in smoke.

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I found a bigger replacement, but cost $6 + shipping, etc.

Instead, I bought four 50 Ohm 5W resistors at the surplus
store for 25 cents each.

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and installed it in the amp;

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I'm testing the amp with two big caps -- for evil fun --

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For more fun + punishment {funishment<tm>},
I use the old school 'water bucket' load.

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[Brian Walter]

Dougie, I'm not exactly sure what the softstart ciruit is that you are refering to, is it an inrush current limiter or a circuit that turns on the amplifiers when is senses a signal? I guess it doesn't really matter because Rod Elliot has circuits for both on his website. The low signal turn on circuit can be found here http://sound.westhost.com/project38.htm, if you want to try using that. I'm sure you could make that for well under the $69.50 price. And if it was me, I would use power caps that are rated at least 15% over the rail voltage, if not more.

Brian Walter

[thylantyr]

And if it was me, I would use power caps that are rated at least 15% over the rail voltage, if not more.

I would also, but I was checking out my Crest 10k and
they are using 67V {95V surge} caps on 80V rails.. // lol //

Brave engineering at work

[BobEllis]

...a circuit that turns on the amplifiers when is senses a signal? ... The low signal turn on circuit can be found here http://sound.westhost.com/project38.htm, ...

I built this circuit for use with my computer driven bedroom system. Leave yourself plenty of room to adjust the sensitivity. I ended up doubling the gain of the input buffer and still find it turns off if the volume is at a low listening level, like background music or going to sleep to some new age stuff.

I also found that I got a lot of crosstalk with 10K input resistors. I just use the left channel now.

[Cowanrg]

there is more beneft to a soft-start than just NOT blowing fuses...

i use 1.7kva transformers on my UCD400's. and they are NOISY as hell if just started up cold after sitting overnight. however, if i use a softstart to start them up, they are very quiet. supposedly soft-starts can make components warm up more gracefully, thus prolonging component life, and possibly making things sound better.

i know that my transformers will almost always hum very loudly for the first hour or so that they are on unless i use a soft-start on them. then, they are quiet from turn on. plus, its just nice to be able to turn something on and not have to worry about things... think if your power goes out and comes back on again? or those times when it flickers on and off. if you dont have a softstart, your fuse or something else is going. its just a nice thing to have. just becuase its not absolutely necessary doesnt mean its not a good thing to have.

im also installing dc blockers for my toriods too. i like a dead-quiet amp.

[mikec]

IME a soft start circuit is not necessary with UcD amplifiers (unless you are craving added complexity and cost). The three-channel amps that I built have a total of 70,000uF of filtering and start up w/o any problem with a 5A fuse. The Hypex modules are normally on all the time and can be put into idle mode with the flip of a switch. So, the filter caps are always charged.

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