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Old 19th March 2013, 01:35 PM   #1
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Default Amp Camp Amp Mod

Hi!

Here is a mod I've made to the excellent Amp Camp Amp by Nelson Pass....
Now it is fully DC coupled (as long as your source is also DC coupled), running on symmetrical supplies. Don't ask for pictures, because it still exists in its ugly
"air wired form".... But: It works perfect, rock stable for two months, DC offset
on the output is around +/- 15 mV, Io (with the values on the schematic) is around 0.8A, and the sound is,..... well, fabulous. I've tried about a dozen similar designs, but this one beats them all - as long as you can live with about 5-6 Watts of output power. I'm running mine from a 200VA toroidal transfo, 2X13V AC, 2x33000 uF filter caps, + a simple darlington based capacitance multiplier filter.
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Old 19th March 2013, 07:14 PM   #2
routhun is offline routhun  United States
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Dragonweed,

This is great. I don't like to use capacitor coupling with speakers. Can I use 2n5462 in place of 2n5461?

Thanks
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Old 19th March 2013, 07:16 PM   #3
routhun is offline routhun  United States
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Also, can I increase supply voltage to +/-15v ? What needs to be changed for this voltage increase?
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Old 19th March 2013, 08:18 PM   #4
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Well, about the FET-s, as you can see on the datasheet, the three types have different Vgs valuees, as well as different Forward Transfer Conductances. So far I've tried it with only the 5461, but it might work with the 5462 too. Try it, you may have to adjust the value of R9 in order to get it working correctly. The limiting value for max. power supply voltage, is the Vds of the FET-s, which is 40 Volts, so theoretically you can increase power rails to +/- 18-19 Volts, but then the max. dissipation values will increase too, so be careful.
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Old 21st March 2013, 12:17 AM   #5
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Kind of reminds me of NOStalgia. Link: NOStalgia - the all-FET JLH/PLH Class A amplifier

The JFET increases the open loop gain and that will change the distortion profile. Did you measure the gain or distortion? Got any scope photos? I've found it really hard to drive Q1 in this configuration when the output swings close to the rail. I added a PNP emitter follower to drive Q1.
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Old 21st March 2013, 12:40 AM   #6
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If you are not going to use a source follower to drive Q1, why not use LTP input?
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Old 21st March 2013, 12:41 AM   #7
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might you share your PS schematic???
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Old 21st March 2013, 01:14 AM   #8
flg is offline flg  United States
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Yes buzz, the way I see it, it is another amp, not a modification to the original topology. Before I say something less than tactful, I don't really understand the new topology so much. The F.E. went from a common Drain/Source Follower to a common Source. But, there is no Iq current in Q4? Is there? There is a Cap blocking current to ground through the Rs, except when there is signal present, so Q4 must draw current throught he output?
Maybe some explanation would help me out?
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Old 21st March 2013, 01:27 AM   #9
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Quote:
Originally Posted by flg View Post
Yes buzz, the way I see it, it is another amp, not a modification to the original topology. Before I say something less than tactful, I don't really understand the new topology so much. The F.E. went from a common Drain/Source Follower to a common Source. But, there is no Iq current in Q4? Is there? There is a Cap blocking current to ground through the Rs, except when there is signal present, so Q4 must draw current throught he output?
Maybe some explanation would help me out?
I agree. It is a different amp. Beauty of the ACA is that you basically have very nice SE amp, showing only character of output stage. There is no real advantage to this choice other than higher OLG, allowing more feedback. Didnt really look closely at implementation, but Loadthud is probably right about drive issues. You would probably want pretty high idss input transistor.
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Old 21st March 2013, 01:30 AM   #10
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I cant say that i understand C1 either. Bootstrapping of some sort. Bypassing the resistor with cap might bemore helpful, as it would increase gain.
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