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AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 13th November 2017, 01:11 AM   #41
AKSA is offline AKSA  Australia
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Dan,
If you wanted low impedance but wanted to preserve the existing harmonics, you could use single ended (say a mosfet with a largish 100mH inductor like a tube to double the efficiency) or even Pass SE amps. Otherwise, you'd use a PP output stage, like any of the amps on this forum (Honey Badger, X hex, etc) with a choice to use Class AB or Class A.
Then you'd have a full on audio power amplifier, taking the brief to its conclusion. But this was simply an attempt to set up a gain block for use as a part of a whole amp.

Kean, thanks a lot, I understand now....... good points, all of them, but I've not found CCS impedance issues limiting open loop gain. In truth, I've always tried to keep loop gain down so compensation is a bit easier and sound is more natural.

Bimo,

sama sama....... terima kasih atas Emprit, yang terbaik.......



Ciao,

Hugh
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Last edited by AKSA; 13th November 2017 at 01:13 AM.
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Old 13th November 2017, 01:38 AM   #42
Bigun is offline Bigun  Canada
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AKSA's Lender Preamp with 40Vpp Output
Quote:
Originally Posted by Max Headroom View Post
Yes, most certainly....who needs op amps ?
yeah ! AKSA buries Burson

I suspect High has many more tricks up his sleeve....
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Old 13th November 2017, 01:57 AM   #43
Max Headroom is online now Max Headroom  Australia
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Quote:
Originally Posted by xrk971 View Post
This was designed to be a preamp - so not for low impedance loads. Otherwise it would be a headphone amp. Are you talking 1kohm or 120ohms?
For lowish I mean 1k or so, certainly 5k.


Quote:
Although adding a pair of power transistors either in push pull, or SE Class A could make this a low impedance amp.
Yes.

Quote:
Originally Posted by AKSA View Post
Dan,
If you wanted low impedance but wanted to preserve the existing harmonics, you could use single ended (say a mosfet with a largish 100mH inductor like a tube to double the efficiency) or even Pass SE amps. Otherwise, you'd use a PP output stage, like any of the amps on this forum (Honey Badger, X hex, etc) with a choice to use Class AB or Class A.
Yes.

Quote:
Then you'd have a full on audio power amplifier, taking the brief to its conclusion. But this was simply an attempt to set up a gain block for use as a part of a whole amp.
Understood.

Quote:
Kean,......good points, all of them, but I've not found CCS impedance issues limiting open loop gain. In truth, I've always tried to keep loop gain down so compensation is a bit easier and sound is more natural.
Heresy....according to the opamp camp.

Dan.
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Old 13th November 2017, 02:23 AM   #44
AKSA is offline AKSA  Australia
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Yes, heresy, and I've never understood why every one believes the THD wars.......

Some say a very good SET amp does up to 1-2% and yet it's the best......
Others say a JLH SS Class A in quasi and 0.05% THD is very good too......
But I say we are chasing the wrong numbers based entirely of a real world, subjective listening experience. How do we measure depth of image, and engagement of the music? Bah, can't measure it, doesn't exist!! Hmmmm.........

We need to look forward beyond the THD, a gross sum of tiny misdemeanors, and examine the proportions of the harmonics...... and further more, how the amp manages intermodulation........

Nasty 30C day in Melbourne, not enjoying it.

HD
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Old 13th November 2017, 02:24 AM   #45
Bigun is offline Bigun  Canada
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AKSA's Lender Preamp with 40Vpp Output
You dont know when you have it good, its cold and snowy here!
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Old 13th November 2017, 02:24 AM   #46
xrk971 is online now xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
@Max Headroom: Hang tight and I will measure against 4.7k load. I think 1k is too low.

@Aksa: what frequencies should I measure intermodulation distortion at? 100Hz and 5kHz?

Btw, 92F is nothing. It's a swamp here in the summer in DC, sometimes 98F (38C) and high humidity. Why the Founding Fathers would choose a swamp to put the Nation's Capitol in - I don't know. It's 41F (5C) now and falling.
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Last edited by xrk971; 13th November 2017 at 02:33 AM.
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Old 13th November 2017, 02:33 AM   #47
AKSA is offline AKSA  Australia
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Yes, Bigun, I would LOVE the cold right now...... you are a Brit, with all the madness and walking out into the bright mid-afternoon Sun!

The preamp would do about 44Vpp, so assuming 22Vp and 2.5k load we have 8.8mA load current which is reasonably within the remit of the 12mA CCS, leaving 3.2mA, more than to supply the VAS 1.5mA. So that would be about the max.

If you want to run a heavier load, say 32R, a typical value for a low Z headphone, you'd probably replace the output buffer with a mosfet, and run it at an undistorted 4Vpeak, which would be only 125mA. So you'd run the mosfet at around 130mA for this, and since the buffer is driving a gate, it's easy to drive. In fact, you could drop the buffer, directly connect it to the gate. I'd suggest something like a IRFP610.

Cheers,

HD
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Old 13th November 2017, 02:37 AM   #48
xrk971 is online now xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
That BSP129 can handle 300mA so in theory, could we out a 2SA1837 and run bias at 125mA through it? Keeping the buffer and this might be a slick headphone amp? Would need some big heatsinks though, or drop the Vcc otherwise that's 3w dissipation per active. Probably Vcc of 24v would be more than sufficient. But resistor network may need to be revamped.
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Old 13th November 2017, 02:49 AM   #49
AKSA is offline AKSA  Australia
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Yes, it would be fine X, but with Vcc of 48V it's more suitable to a front end of a small Class A power amp.

For a headphone amp, to limit dissipation I'd use little more than 20V because at low Z headphones 8Vpp levels would torpedo your ear drums!

HD
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Old 13th November 2017, 04:34 AM   #50
xrk971 is online now xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Default Lower impedance and IMD measurements

I installed a 4k7 resistor, and that, coupled with a voltage divider for the input resulted in a net 3.4kohm load.

Here is 4vpp into 3.4kohms, I adjusted the gain on the audio interface a bit but distortion is still pretty low, around noise floor of measurement setup:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-4-0vpp-3-4k-test-2-png

Here is 20vpp into 3.4kohms, it seems 3rd order harmonic distortion increases relative to 2nd order at this loading, but still a decent result that would probably sound quite good still:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-20vpp-3-4k-test-2-png


Here is 36vpp into 3.4kohms, 2nd and 3rd order about the same (some people like this sound as it has more bite), and any higher in amplitude, and it starts clipping:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-36vpp-3-4k-test-2-png

Here is IMD using DIN standard 250Hz and 8kHz at 4:1 ratio with 20vpp into 3.4kohms, IMD=0.02%:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-imd-20vpp-3-4k-test-2-png

Here is IMD with same DIN standard but for 20vpp into 25kohm, IMD=0.01%:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-imd-20vpp-3-4k-test-2-png

So, I would say that 4.7k ohm load can work, just be careful of clipping at higher drive levels.
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Last edited by xrk971; 13th November 2017 at 04:37 AM.
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