Well Behaved Amps

[richie00boy]

I've made loads of amps in loads of different ways of doing input stages, VAS and output stages, and never yet found one that was perfectly well behaved at turn on or turn off. I can get them to be silent, but there is always a bit of slow movement of the cone in and out. So has anybody come across any amp schematics that they built and there was no movement?

 

[ostripper]

I've made loads of amps in loads of different ways of doing input stages, VAS and output stages, and never yet found one that was perfectly well behaved at turn on or turn off. I can get them to be silent, but there is always a bit of slow movement of the cone in and out. So has anybody come across any amp schematics that they built and there was no movement?

Just about all my amps have no thump or movement except my luxman clone. I traced this to the cascode capacitor. On powering down most will never exceed 20mV offset , this has to do with the LTP match mostly. I did build a current source free design once (like original DX amp) ... nice thump .

My blameless's (AX/BX) are so quick to start up you don't even know you turned them on .. not even a barely audible "tick" with your ear on the midrange .. much less any movement. The only way to see this millivolt turn-on is with a scope set to mV/div.

This is with these front ends hooked to a 200w - 8 output monster EF2.
What type of designs are you getting this cone movement on anyways ???

OS

 

[infinia]

Yeah usually amps that use resistors and boostraped VAS instead of constant current sources behave badly on turn on /off. Symmetrical topologies help here as well.

 

[richie00boy]

Well I've tried bootstrapped VAS and CCS VAS, resistive tail LTP, CCS tail LTP, current mirror or not, singleton input, practically everything except cascodes and symetrical topology. Some amps were better than others, but none have been zero movement.

 

[ostripper]

Do you have a softstart ? My 60w amp without one (same blameless module) has more "tick" than my 1KVA 75-0-75v PS with one. (inrush differences)
OS

 

[richie00boy]

I have a soft start of about 0.25 second on the primary of the toroid, but the problem is there with and without that and also from a bench DC supply. I don't think this is the issue, the issue should be solved in the amp.

Can you post schematics of your amps that you say have miniscule cone movement, I can't (easily) find anything looking like them on your hosted content nor in threads started by you.

 

[sregor]

There is a mention in the Leach amplifier page about using resistors to supply the current for the ltp input stage (instead of a current source), which as far as i can tell, combine with caps on the zeners to slow the turn on of the input stage. I believe some other amplifiers use similar practices. YMMV.

 

[Michael Chua]

Originally posted by richie00boy
I can get them to be silent, but there is always a bit of slow movement of the cone in and out.

Are you using a very large negative feedback cap to ground?

 

[ostripper]

I have a soft start of about 0.25 second on the primary of the toroid, but the problem is there with and without that and also from a bench DC supply. I don't think this is the issue, the issue should be solved in the amp.

Can you post schematics of your amps that you say have miniscule cone movement, I can't (easily) find anything looking like them on your hosted content nor in threads started by you.

I am revamping the PCB's and schematics now , so my AX , which I base all the others on , is temporarily gone (It is below). The AX (true blameless) , has NO thump or bass driver movement. I use the same NFB - DC referance ratio on all my amps (33k/820 w- 220uf NP cap). This gives me 38DB gain and no thump (except my luxman - I solved that.) Worst I have ever experienced is a small "tick" (sound) during turn on. Some don't even do that. A cheap sherwood discrete receiver is louder than my worst and it is OEM. The "tick" is 100mv max and lasts about 10uS on the scope.

On shutdown , no woofer movement either ... just a slight distortion as the rails collapse to around 8-0-8V.
The BX is just like the AX , but with a bootstrap on the voltage stage. Just a little louder click on startup (200mv/10uS) , still no woofer movement. My DC cap and bootstrap is this series by Nichicon .. (220u and 100u @50V)UVP1H221MHD Nichicon Aluminum Electrolytic Capacitors - Leaded

The topology I would want in a room full of nitroglycerin is the CX (below2) .. it has NO tick,click or anything. Must be the 5 device balanced VAS that does this.

OS

 

[richie00boy]

Are you using a very large negative feedback cap to ground?

100uF. And quite spookily the same resistors as ostripper, 33k and 820R.