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Old 11th February 2008, 07:56 PM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by johan851
So what you're saying is that the higher the impedance seen by the preamp is (Zin') the more ........... But I also need to make sure that Zin' doesn't go so low that my preamp can't drive it. Correct?

I'm saying that since my preamp is designed to drive loads as low as around 20r0, it makes sense to have lower resistor values in the attenuator so that .............. by the attenuator. That doesn't mean I want extremely low values...maybe R1 = 5k1, R2 = 10k.

The point I'm still missing, I think, is why Rs' matters. Does the impedance at the amp input / preamp output as seen by the power amp actually matter?
it has very little to do with noise at these impedance and voltage levels.
It is much more related to what range of Rs the power amp will tolerate and what the Rs will do with the RF filter that is likely to be built into the power amp front end.
Some power amp manufacturers specify the optimum Rs.
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Old 11th February 2008, 09:36 PM   #22
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Ah, I think I get it now...we're worried about frequency response as a result of the filter created at the input of the gainclone? Or is it just that the power amp is expecting a preamp with very low output impedance, and we're messing with that? Is there an optimum rs for the gainclone?

*Edit* Specifications indicate that my preamp has an output impedance of less than 0.05Ω. That's pretty low...
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Old 12th February 2008, 08:04 AM   #23
AndrewT is offline AndrewT  Scotland
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I don't recall National suggesting an optimum Rs for the basic chipamp.
Most builders report improved results from using a low source impedance.
The range of optimum Rs will depend on the external components around the chipamp but mostly it will be determined by the Input stage of the chipamp and what current National have chosen to pass that stage.

I suggest you add a second output to your headphone amp and insert a 10r in the outlet line.
Use this to feed any power amp with Zin>=600ohm.
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Old 14th February 2008, 04:30 PM   #24
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Quote:
Originally posted by johan851
So looks like a gain of 30 is the best choice. I get the feeling, though, that the amp is going to be extremely loud when used with my preamp. I guess that's something that I'll just have to deal with?
Since your preamp/headphone amp puts out a relatively high voltage for a preamp, you may want to set the chipamp's gain to somewhere in the range of 13--16 (22.5--24dB), which is typical of smaller amps anyway. At this gain, only 2v in will give you over 60w into 8 ohms with 36-volt rails. The chip sounds fine at this gain level, and there's plenty of feedback margin, and the M3 is probably more comfortable at this level anyway.

Craig
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Old 14th February 2008, 04:37 PM   #25
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Quote:
At this gain, only 2v in will give you over 60w into 8 ohms with 36-volt rails.
At which gain... 30ish or 13-16?
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Old 14th February 2008, 05:29 PM   #26
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Quote:
Originally posted by johan851
At which gain... 30ish or 13-16?
I would suggest you to try 13-16 and 30 with input attenuator and decide which sounds best to you.
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Old 14th February 2008, 07:33 PM   #27
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Well, the amp is supposed to sound better at a gain of around 30, and the kit comes with parts to set the gain at 33, so I'll just stick with that. As I look at the numbers, I'm going to need some kind of input attenuator no matter what I do.

Someone can double check my math here in case I've screwed it all up...

13Vpp (from preamp) = 6.5VAC = 4.6V RMS

Using a transformer with 18VAC secondaries, I get about +/- 25V volt rails. I don't know quite where this guy will start clipping, but the gain of the supplied parts in the kit is about 33. Even if I adjust the gain down to 13, my output signal with 4.6V RMS input will be 59.8V RMS.

59.8V RMS = 84.6VAC = ~170Vpp. That's obviously not going to work.

So if I leave the gain at 33, and attenuate the input signal by a factor of 10:

460mV RMS in gives me 15.2V RMS out, which is 43Vpp.

43Vpp is about right, and corresponds to an output power of about 30w RMS into 8 ohms, 58w RMS into 4 ohms.

So I'll just stick an attenuator pot in there and dial it in until I have it about where I want.
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Old 14th February 2008, 08:17 PM   #28
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Default Gainclone gain

At a gain of 16, you'll get 32v out (64w into 8R) from 2 volts in.

Suggestion: Download the design spreadsheet from http://www.national.com/appinfo/audi...gn_Guide15.xls
(it works fine in the free OpenOffice, by the way) and play with parameters until you find something you like. This also amounts to an interesting basic course in op-amp design.

Remember also, as someone pointed out above, that the National chips are not guaranteed to be stable at gains less than 10.
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Old 14th February 2008, 08:30 PM   #29
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Quote:
Originally posted by johan851

59.8V RMS = 84.6VAC = ~170Vpp. That's obviously not going to work.
I think you've multiplied something twice. The 84.6v is already peak-to-peak (each way). And it's way more than the National chip can deliver; as I say above, 2v in will be very close to maxing you out, no matter what PS you use. Remember that nearly all discussion is in terms of either RMS AC voltage or peak-to-peak AC voltage. A 50v rms AC transformer gives you a single-ended 70v Vcc, and opamp specs are in terms of |Vcc|+|Vee| -- it's easy to get lost in all this... Just keep one foot on the floor and your eye on the square root of 2 ...
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Old 15th February 2008, 01:08 AM   #30
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I've taken several courses in opamp design, all the way down to the gate level (analog EE major) and I think I know the basics work. (emphasis on "I think", not "I know" ) The one thing I'm messing up is what figures we're throwing out that are voltage peak to peak and voltage in terms of wave amplitude. One is half the other. I'm not a power guy.

Looking at Wikipedia, I think that whenever I said VAC, I meant Vpeak, and when I said my transformer voltages were VAC, they should be VRMS. I'll edit it and fix it up. I think the results are similar, though, if not the same.

http://en.wikipedia.org/wiki/Alternating_current

I think it should be this:
Quote:
13Vpp (from preamp) = 6.5Vpeak = 4.6V RMS

Using a transformer with 18Vrms secondaries, I get about +/- 25V volt rails. I don't know quite where this guy will start clipping, but the gain of the supplied parts in the kit is about 33. Even if I adjust the gain down to 13, my output signal with 4.6V RMS input will be 59.8V RMS.

59.8V RMS = 84.6Vpeak = ~170Vpp. That's obviously not going to work.

So if I leave the gain at 33, and attenuate the input signal by a factor of 10:

460mV RMS in gives me 15.2V RMS out, which is 43Vpp.

43Vpp is about right, and corresponds to an output power of about 30w RMS into 8 ohms, 58w RMS into 4 ohms.
So aside from my labeling I'm pretty sure that's correct. And I realize that 84.6Vpeak or 170Vpp is way (way) more than the chip could deliver...that's why I'm saying that even with a gain of 13, I'm going to need some kind of input attenuation.
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