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Amp Camp Amp - ACA
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Old Today, 09:28 AM   #6881
mikelm is offline mikelm  England
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Quote:
Originally Posted by indra1 View Post
Yes, I think the IPP600 is special in the ACA, but you are right at the frontier there. To avoid fatal problems, usual prevention steps are :
1. use appropriate gate stopper resistor, in your case I think should be at least 680 for R15, M3 should be more stable due to R10 shunting from drain to gate.
2. operate at reasonable temperature
3. use RC shunt as per Paul Bysouth on M4 if the above steps are deemed insufficient
But there is also a chance that the IPP600 is inherently stable as is.

There is also other options such as a CFP gain stage that I try in simulation attached that also needs >820 ohm gate resistor R3 to counter a peak at ~2MHz. However, mosfets used are regulars of Pass Labs production, no unexpected nasties.
Hi Indra1, thanks for your post. It raises some very interesting points.

I have been playing with the circuit in spice with all the measures you suggest and with 330p + 1000ohms gate to drain both o/p MFs & 470R gate stopper for M3 & a 4.7K/470p i/p filter this circuit is indeed very stable even with a 1uH choke on the o/p & 1uF across the 8ohm load.

Trouble is, now the -3db HF roll off is now in the sub 60Khz region and I know from experience that I much prefer this to be in the 120Khz region.

For me this raises a very interesting question: Do DIY amplifiers, built for our own use, connected to known loads, have to be stable into any load ? Making amps that are stable into any load is important if selling them to the unknowing public and professional designers correctly regard this as a condition of a finished design but do we think that this also applies to us ?

I think it is important that home made amps are stable enough so they do not spontaneously self destruct but beyond this I think we have the freedom to design our amps so that they are stable with the leads / speakers they are intended for but not necessarily for hypothetical extremely reactive loads. With this & other amps I think this gives us DIYers an advantage over commercially available gear.

I have lots of work to do on my amps before I will regard them at optimised - but even at present to my ears they are the most honest sounding amps I have heard and give insights to the tonal texture of the music that I have not heard before.

For me it would be a pity to lose this transparency for the sake of unnecessary unconditional stability.

mike

Last edited by mikelm; Today at 09:33 AM.
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Old Today, 09:40 AM   #6882
mikelm is offline mikelm  England
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Quote:
Originally Posted by aljordan View Post
I am amazed at how quiet and excellent the ACAs sound with the SMPS. I was looking into building a linear supply for them a while back, but when listening, I can't discern anything wrong that would justify the work and expense.
On the scope I could just notice that linear supplies were slightly cleaner that the supplied SMPSs so I suspect that a well designed, regulated supply will be marginally better - but I do agree that the supplied SMPSs are very good.
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Old Today, 11:49 AM   #6883
indra1 is online now indra1  Indonesia
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Quote:
Originally Posted by mikelm View Post
... For me it would be a pity to lose this transparency for the sake of unnecessary unconditional stability.
Agreed, and it is difficult to predict what will happen as you are truly at the frontier there. As I said, the IPP600 could be good as it is. And your speakers should survive in the event that the IPP600 self destruct protected by the output capacitor. Please do as you see fit.
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Old Today, 01:12 PM   #6884
mikelm is offline mikelm  England
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Quote:
Originally Posted by indra1 View Post
Agreed, and it is difficult to predict what will happen as you are truly at the frontier there. As I said, the IPP600 could be good as it is. And your speakers should survive in the event that the IPP600 self destruct protected by the output capacitor. Please do as you see fit.
Yes, But just to make it clear about the IPP600, I think they do need some extra protection. I will experiment and post back here when I know more.
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