Thump when I switch on.

[andrew ivimey]

Some people might remember with nostalgia switching on a (SS - grrrrr) amplifier and having an accompanying and worrying thump go through teh loudspeakers.

any simple solutions? I thought of adding a switched anything above say 47 ohm resistor in series with the amp and the loudspeaker which having switched on the resistor can be switched out for listening; sort of what some sophisticates would do with a relay.

I only have this problem because having been delighted with an Inca Tech Mk2 hamplifier for £35, the only weak point is this thump. It has an excellent MC/MM phon stage, eight inputs, line out and really does sound good for an SS amplifier - I suppose some SS amps sound okay

[Ianm2]

hi Andrew, its dc offset, its quite common on some amps, the japanese use a relay to delay switch on until all has settled down. That's the click you hear.

Perhaps if you can find the schematic, there is a capacitor in the feedback loop, attention here MAY prove fruitful, but its the limits of my knowledge, a biggish electrolytic.

It depends how bad it is, it is normal for some amps to have it, especially simple british ones.

here is the circuit, you want to play with c11a

http://i20.photobucket.com/albums/b2...ower_AMP_2.jpg

[andrew ivimey]

Hello Ian, - thanks for the circuit diagram - I'd have thought a cap would affect the tone considerably, or do you mean switch in switch out as I was suggesting with a resistor ?

[soulminer]

I had a similar problem with the motor on my Lenco thumping as it turned on and off. I just put a 0.047uF X2 type cap across the switch contacts. That silenced it completely. I think this was the actual original method of controlling switch arcing due to reactive components (motors, transformers) in the circuit.
I also get a slight thump now and again from my monoblocs too, but I am not sure if the cap across the switch will work in the amp itself. I don't see any reason for it not to though. HTH.

[alnewall]

I've just fitted one to the T amp. A 290ohm resistor and a changeover switch after the 12 supply. Doesn't stop it altogether but it's a big improvement and prevents undesirable cone movement at switch on.

[acorn]

Hi Andrew

Have you tried bleeder resistors across your power caps you can use a 330 K 2 watt for 300 volts and a 470k 2 watt for 400v to 500 vlots It works for me it takes the surge from the HT on switch on

Regards Acorn

[Ianm2]

hi Andrew, I am not sure about a relay and resistor, no idea really, give it a go, I don't know that much, just that in solid state commercial amps there are about 3 ways to try to get rid of dc offset, aside from a delayed start.

Its caused I think by the input stage differences in transistors, speculation, but perhaps like different valve pairs having different characteristics, I never did understand electronics properly.

but the 3 ways are

1/ a potentiometer across the input pairs...

2/ this cap listed, most common now, virtually all amps have this method

3/ an active dc servo..chip with feedback, by people who don't like caps. some pro audio amps.

non are ideal. remember your amp probably will have a cap there anyway, so perhaps use something like a nice big starget by elna, at least 220 uF 25volts

If it is dc, then I doubt switch contacts or bleeder resistors will aide, there may be not a lot you can do about it as its inherent in the design. But by all means play.

if its oldish, >5 yrs, perhaps the reservoir caps would benefit from being replaced? and whilst you are at it, caps in the signal path. you must check tho that if you do change this, its listed the same on your amp, with all these mk1s and 2s around I don't know what is what.

[freefallrob]

My little old Creek CAS4040 makes a 'little bump' when you switch it on not bad though, I always have the volume turned right down to begin with, not that it probably doen't make much difference........super phono stage though too !

[John Caswell]

Hi Andrew,
It is pretty well bound to be O/P stages charging up and settling and possibly the pre stages as well, depending how much is DC coupled prior to the o/p. The big danger is driving the bass unit against its end stops
The best solution is a relay in the o/p feed driven by something like a 555 chip with about a 30 second delay, or a small transistor with a large capacitor in its base circuit causing it to take a long time to turn on.

Regards

John aka Dr John

[John Caswell]

Hi Andrew,
Have a look at the White Noise site www.wnaudio.com he does something that I think will suit your application.

John aka Dr John