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Old 15th June 2011, 10:18 PM   #1101
mikelm is offline mikelm  England
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Mmm - as I mentioned before, as DIYers, we have it much easier because we can make the amp tailor made for our own speakers - doesn't have to be stable into any load. My current version is still stable into a resistive load ( in spice ) with no compensation which swordfishy says sound amazing but manufacturers don't really have this option available because for them the load is an unknown.

Ringing may be the a natural function of a good amp but it can still roughen the sound. The ringing I am seeing with tricky loads is in the 100- 500khz range and amps don't have much PSRR at that frequency and once you start generating it - with ringing - it can get everywhere and really spoil the sound.

So I think it is best for us to make our amps at least stable with our own speakers and JLH would then suggest to add a little more compensation for good measure. This will probably make the sound a bit more smooth and a bit less exciting - but this comes down to a personal preference.

Last edited by mikelm; 15th June 2011 at 10:20 PM.
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Old 15th June 2011, 10:57 PM   #1102
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More compensation will not decrease the output inductance of the amplifier - in fact compensation itself increases the effective inductance. A more effective method would be to add series resistance to the amp output, which would dampen the resonance. That is what I think, anyways.

Regulating the input stage is one way of getting rid of major PSRR troubles, because it eliminates feedback loops on the rails. Probably something simple like my K-multiplier is sufficient for the vast majority of amps:

Keantoken's CFP cap multiplier

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Old 15th June 2011, 11:34 PM   #1103
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The discussion is becoming more speculative and academic. SWF is struggling valiantly to make it stable, and I don't think it's layout because he is using a ratsnest build which normally has very low parasitics. No, I think it's topology, component choice and dimensioning.

I would normally stabilise it with lag comp, then add some phase lead. More clearly is required, and this could compromise sound quality. Greg, if the issue is the highly capacitive gates on the outputs - unlikely in my view as I have a stable amp with a vertical mosfet gate and only running the VAS at 8mA - then you could always add 'drivers', EFs to drive the gates. You would insert a 'stay alive' resistor between the driver emitters, like Self's EFII.

Notable in all this discussion is the very tenuous link between sound quality and measurement. This is really important, and drives us to consider
the measurement system suggested by Earle Geddes on this forum.

Hope this helps,

Hugh
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Old 16th June 2011, 02:14 AM   #1104
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Quote:
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..... as I have a stable amp with a vertical mosfet gate and only running the VAS at 8mA - then you could always add 'drivers', EFs to drive the gates.
Bad .... Bad ... Boy... Driving mosfets with the VAS
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Old 16th June 2011, 03:54 AM   #1105
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Yes, Michael, I know, but what can I say??

It works, damn these bad habits.......

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Old 16th June 2011, 04:01 AM   #1106
mikelm is offline mikelm  England
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Quote:
Originally Posted by AKSA View Post
The discussion is becoming more speculative and academic. SWF is struggling valiantly to make it stable, and I don't think it's layout because he is using a ratsnest build which normally has very low parasitics. No, I think it's topology, component choice and dimensioning.
I am a little perplexed by this. Previously I was struck how closely Greg's real life square wave ringing matched what I saw in spice under similar loading.

Now in spice my 2x 3310 design is still stable without any compensation with a resistive load and also with 1nF in parallal and even without an o/p snubber so I can only conclude that either it is a necessary to have 2 3310's in parallel and 20mA, or that it is a layout issue.

sorry I can't help more. I might get one channel working today or tomorrow but my first build will be with 9610s.

mike
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Old 16th June 2011, 04:02 AM   #1107
GregH2 is offline GregH2  Australia
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Yes Hugh, I think you're right in that it is probably not the unequal gate capacitance - I have tried compensating for it using both asymmetrical gate resistors and adding capacitance to the N channel fet. Neither seemed to help.

My new depletion mode fets have arrived. I go on holiday early Sunday morning but I'm hoping very much to get a version working with them before then...

Going to stick with the ZVP3310A for at least one more attempt, as I am convinced it has sonic advantages. If all else fails it's back to old faithful - the 9610.
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Old 16th June 2011, 04:11 AM   #1108
GregH2 is offline GregH2  Australia
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Mikelm,

I think you're doing the right thing. The 9610 version seems rather bulletproof. It is tempting to stick it to a proper heatsink and try even more current in the VAS - like 100mA! The transconductance improves substantially with more current for that device - but then it might become unstable, who knows?

My layout is very bad and it is perfectly reasonable that it is the culprit. The signal stages are quite nicely tied together, but I do have long leads to the output fets and also the feedback path. That said, I do have it oscillating in spice with the same circuit. I really need to try it in spice with more current in the vas. I think there is something to that idea as all of my stable versions have had over 15mA.

Look forward to your impressions. My thoughts are that the 9610 is "valvey" sounding, but lacks the punch, drive and general dynamics of the ZVP devices.

Both sound good, but the flavour is slightly different!

Last edited by GregH2; 16th June 2011 at 04:13 AM.
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Old 16th June 2011, 04:13 AM   #1109
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MikelM, have you added parasitics to your simulation? Say 12nH for each film or ceramic, 10mR series resistance. Give 25nH for electrolytics, and give them no less than 50mR resistance.

This will help "reality-check" you simulation. Then try adding trace inductance in sensitive parts of the circuit.

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Old 16th June 2011, 04:15 AM   #1110
mikelm is offline mikelm  England
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Greg,

what i/p filter are you using ?
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