Amp design for subwoofer and wideband duty - Page 2 - diyAudio
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Old 12th October 2011, 11:22 AM   #11
Bensen is offline Bensen  Belgium
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Oops,
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Old 12th October 2011, 11:27 AM   #12
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Got it, thanks.
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Old 12th October 2011, 02:58 PM   #13
Ron AKA is offline Ron AKA  Canada
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You may find This Article of interest.
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Old 12th October 2011, 04:04 PM   #14
Bensen is offline Bensen  Belgium
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Default it's alive

Thanks Ron,

I know the circuits of Rod Elliot. But it's more fun when it comes entirely out of my own hand this time.


I got the diff vas circuit running on vero board without the OPS. I used a headphone with a 2k2 resistor in series, connected at the output of the VAS.

I've played some music and heard nothing strange. I hooked it on to my signal generator with a sinus of 1KHz,10Khz and a square wave of 1kHz and 10kHz. The 10kHz square looked not that good, could it be that the load of 2K2 of my headphone is to big?

What else should be interesting to test to evaluate this circuit?
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Last edited by Bensen; 12th October 2011 at 04:18 PM.
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Old 12th October 2011, 04:48 PM   #15
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The solution to the DC offset problem is very simple. A 100uF capacitor between the 430 ohm resistor in the feedback network (R25) and ground brings the DC offset right down to 230 microvolt.

The capacitor uncouples the resistor from the DC path. If you want a more detailed explanation, I will have to look a few things up to make sure that I've got it right.
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Old 13th October 2011, 12:05 PM   #16
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Those differential pairs were just crying out for current mirror loads, so I added them. The THD at 20kHz is about a quarter of what it was before. The frequency response is fine for a sub amp, but I opened it up a bit for general purpose use. With some tweaking and optimization of values there might be some more improvements forthcoming.

Another thing - it might be better to use a MOSFET for thermal tracking. I'm not sure that a BJT Vbe multiplier goes well with MOSFETs. Does anybody have an opinion on this?
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Last edited by ingenieus; 13th October 2011 at 12:16 PM.
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Old 13th October 2011, 04:40 PM   #17
Bensen is offline Bensen  Belgium
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Interesting Ingenieus,

How did you conclude that the diff pair needed the current mirrors? Do you compare the collector current in both legs, or what do you use as trigger to go for a current mirror?
I've never used current mirrors befors.
With the optimised current mirror (as Bob Cordell calls these in his book) the symmetrical topology should work.

Concerning the vbe multiplier, as I explained in post #1 ,I will be using a 2SK2733 mosfet, but due to the lack of the spice model for this,...

Thanks for the input!
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Old 13th October 2011, 04:46 PM   #18
AndrewT is offline AndrewT  Scotland
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I believe the current mirror only becomes effective, if both sides of the LTP are fed and loaded symmetrically.
I further believe that this may be the reason that resistor loading is preferred to mirror loading when sound quality is uppermost in the priority list.
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Old 13th October 2011, 06:14 PM   #19
Bensen is offline Bensen  Belgium
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Andrew,

I don't fully understand your explanation? Are you saying that a current mirror is effective when, for example a differential VAS is used ( symmetrically loaded).

Is your experience that an amp with resistor loaded LTP sounds better than one with current mirrors?

Greetz
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Old 13th October 2011, 06:35 PM   #20
AndrewT is offline AndrewT  Scotland
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I have one family of amps that I particularly like the sound of. It is resistor loaded.
The Leach is also resistor loaded but that didn't seem quite as nice.

On this Forum there seems to be a significant minority that believe the resistor loading is better than the mirror loading.
Recently there have been a few mirror loaded designs with symmetrical loading that are reported to sound good.

I think you will have to assess that for yourself.
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