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Class D Switching Power Amplifiers and Power D/A conversion

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Old 19th July 2011, 01:56 AM   #21
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No, I'm afraid you've got it backward. If the output impedance of the preamp was 50k and the input of the power amp also 50k, then one half of the voltage would be dropped in the peramp output and one half at the powr amp input. When the output impedance of the preamp is much lower (typically around 100 ohms or less for solid state) than the input of the power amp (typically 10k or more), then almost all of the voltage is delivered to the power amp, which is what you want, and very little lost at the output of the preamp. Another way of seeing it is to realize that with lower resistance it is the more like a dead short, and with higher resistance it is more like an open circuit. It's all about Ohms Law and voltage drops.

Mike
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Old 19th July 2011, 02:29 AM   #22
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You lost me. WHAT DO YOU THINK OF THAT PREAMP? Too Gaudy???

Tanks for all your help. I think I will be doing some more research.
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Old 19th July 2011, 03:18 AM   #23
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I don't know, that tube seems to be a Chinese copy of the Russian 6N23P but I'm not sure. If it is, it's being run at a lower plate voltage than normal, that power supply shows 24volts DC, and that tube should be at 90volts plate. Maybe it's being run at low voltage to give the 'tube sound" some people like. It doesn't cost much, but then again, you usually get what you pay for. One issue is it's not really a preamp, it's a headphone amp, and the 30dB gain (about 30X) is way more than what is required, it could lead to noise issues and wonky volume control where you would have to keep it at a really low setting to aviod overdriving your amps. What you need would have somewhwere around 10dB gain (about 3X) at most.

Mike
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Old 19th July 2011, 07:10 PM   #24
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Start with example 1 from post #15. It is not expensive and appears to fit in with your project. It also looks like a good basis for tweaking, e.g. replace the NE5532N that is in there with an NE5532A or something even better, if you wish to. You can easily replace a few resistors to adjust the gain to your needs, although the 6 dB that the kit is set to will probably be OK for your setup.

Don't forget that you need a regulated split supply for it.
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Old 20th July 2011, 04:00 AM   #25
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Quote:
Originally Posted by pacificblue View Post
Start with example 1 from post #15. It is not expensive and appears to fit in with your project. It also looks like a good basis for tweaking, e.g. replace the NE5532N that is in there with an NE5532A or something even better, if you wish to. You can easily replace a few resistors to adjust the gain to your needs, although the 6 dB that the kit is set to will probably be OK for your setup.
Thanks for your advice! Do you think a balanced output is a good idea, since I am bi-amplifiying. I could use something like this? Example

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Originally Posted by pacificblue View Post
Don't forget that you need a regulated split supply for it.
Is this what you mean for a power supply? Example
Then I would have to hook it up to a transformer? This is more that I expected to get into with amps, but I think I could put together one of these kits.
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Old 20th July 2011, 09:54 AM   #26
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The Hifimediy amps aren't balanced so you do not want a balanced output, so avoid that.

I wonder if something like this 3-band tone control preamplifier low noise opamp ! | eBay might appeal to you more? It provides tone and volume controls and having an on-board power supply you would just need to hook up a transformer.

Note - I haven't used this myself, but Jim's audio on ebay has a pretty good reputation.
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Old 20th July 2011, 06:02 PM   #27
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[QUOTE=birdyfoot;2644358]Is this what you mean for a power supply? Example
That is one example. Versions with the LM78/79 should be available at lower prices.

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Then I would have to hook it up to a transformer?
Yes.
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