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Old 3rd September 2016, 12:41 PM   #41
Mark Johnson is offline Mark Johnson  United States
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Quote:
Originally Posted by Mark Johnson View Post
If you turn Bongiorno's preamp upside down, and power it from +30V instead of -43V, you get something similar to the NS-10 preamp that Nelson Pass designed for Threshold in the late 70s.
The Threshold NS-10's product name came from the amazing fact that its signal propagation delay, from preamp input to preamp output, was ten nanoseconds: NS-10. So I thought, why not fire up the simulator and try it on the Bongiorno/NS-10 circuit in post #29? Let's measure its input-to-output prop delay.

The simulator's answer is: 9.32 nanoseconds (blue highlighted text). Well done chaps.

(you can make it faster still by decreasing the compensation capacitor C3; I've set it at 15pF which gives zero dB of gain-peaking. If you're comfortable with, oh let's just say, +2dB of gain peaking, reduce C3 and get higher bandwidth and lower propagation delay.)

_
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Last edited by Mark Johnson; 3rd September 2016 at 12:48 PM.
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Old 3rd September 2016, 12:55 PM   #42
Bigun is offline Bigun  Canada
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The Apex preamp isn't too complicated, you could make up a layout on strip-board. A good preamp also needs a clean supply although the Apex design does look like it would have good PSRR.

If you really want something simple, with less DIY element, then look to eBay. For example: Preamp 9 Single Ended Pure Class A 20Hz 20kHz Transistor Preamplifier Board | eBay
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Old 3rd September 2016, 03:42 PM   #43
Jrp27 is offline Jrp27  India
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My power amp when connected directly to an ipod works fine . However the amp sounds distorted/ metallic even ipod is turned full.

Do i am assuming a small 2-4db gain preamp should do. Idea is that the amp should reach its full capacity when ipod volume is at 50%.

Wouldn't mind the controls too. Any suggestions.
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Old 3rd September 2016, 05:50 PM   #44
Mosquito is offline Mosquito  Argentina
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Quote:
Originally Posted by Mark Johnson View Post
(you can make it faster still by decreasing the compensation capacitor C3; I've set it at 15pF which gives zero dB of gain-peaking. If you're comfortable with, oh let's just say, +2dB of gain peaking, reduce C3 and get higher bandwidth and lower propagation delay.)
Hi,
I simulated this nice circuit, and LTSpice said it can put 4V pk pk over a 10K load even at 250Khz, with minimum distortion.
Nevertheless, using exactly the same models as You (Cordell), I can t get away with less than 27p for C3, or there is gain peaking (+2dB with 15p).
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Old 3rd September 2016, 08:18 PM   #45
Mark Johnson is offline Mark Johnson  United States
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Yes Mosquito you're right! At the last minute I increased the first stage's bias current to 300uA but forgot to also resize C3. Oops.

C3=15pF is sized for a 200uA first stage. To see that 15pF does indeed give 0dB of gain-peaking in that case, simulate with
  • R4 = 3K (first stage collector load resistor)
  • R6 = 75K (first stage emitter bias resitor)
  • R5 = R7 = 680R (feedback gain setting resistors)
You win the eagle-eyed reader award. Congratulations!

I think you'll probably decide you prefer the 300uA design with resized C3.
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Old 3rd September 2016, 08:33 PM   #46
xrk971 is offline xrk971  United States
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Can I get 12 dB out of this circuit?
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Old 3rd September 2016, 09:00 PM   #47
Mark Johnson is offline Mark Johnson  United States
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Originally Posted by xrk971 View Post
Can I get 12 dB out of this circuit?
James Bongiorno did (in 1972). Nelson Pass did (in 1977).

But today, I'd suggest you buy an LM7171 for $2.81 and do it that way instead: (link). Unless you agree with the statements
I really want a transistor based preamp. I just like the warm fuzzy sound. Chips sound shrill imho.
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Old 3rd September 2016, 09:18 PM   #48
Mosquito is offline Mosquito  Argentina
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Quote:
Originally Posted by Mark Johnson View Post
I think you'll probably decide you prefer the 300uA design with resized C3.
Indeed.
As for the chip, sure it can outperform the diy efforts, but are also 0 fun, as are the chip amps. Build a thingy that works correctly, and sometimes is top performer, out of discrete components, is a source of inmense satisfaction, imho...
Cheers
J.
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Old 3rd September 2016, 11:44 PM   #49
xrk971 is offline xrk971  United States
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I might give this a go on my veroboard and see how it sounds. To get 10dB what R values should I change and anything else such as compensation caps?
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Old 4th September 2016, 02:27 AM   #50
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Jrp27 View Post
My power amp when connected directly to an ipod works fine . However the amp sounds distorted/ metallic even ipod is turned full.

Do i am assuming a small 2-4db gain preamp should do. Idea is that the amp should reach its full capacity when ipod volume is at 50%.

Wouldn't mind the controls too. Any suggestions.
You are assuming that the ipod output distorts when running at full volume due to a limitation in the ipod. OK - do you know, but trying a different music source, if the amplifier still sounds clean at these high volumes to at least eliminate the possibility that the problem is with your amplifier and not the ipod ? could it be that you are listening to MP3 lossy-compression which will tend to show it's warts when played louder ? are you putting a big load on the ipod output but having a long cable and/or your amplifier has a relatively low input impedance ?

For 3dB of gain, it really is likely that you can adjust a couple of resistors in the feedback network of your amplifier and avoid a pre-amp altogether.
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