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birdyfoot 10th July 2011 05:04 PM

Need help designing preamp + amplifier
I would like to use a preamplifier to boost and regulate the input signal of two class-d amplifiers (simultaneously), the T1 and T2 from HifimeDIY. The input sensitivity of these amps is 1.5Vrms at 50K impedence, which is as I understand it, is typical for low impedance headphones. I was thinking of using the sure electronics 2x2w board, but it is designed for a 4ohms load. Is there a way to make it work, i.e. via some combination of resistors? I Don't know much about this stuff so If you could just point me in the right direction I would appreciate it.

Also, I would like to use a VU analog panel meter to measure the input signal in volts RMS. I found this one at amazon for only five dollars, but have been cautioned that these kind are infact only 5% accurate, but I think that that might be good enough. What do you think? Would something like this be a useful measurement tool to be sure that I was giving my amplifiers sufficient, but not too high of an input signal.



birdyfoot 13th July 2011 08:36 PM

How's this for an idea...?

What ft I took a 4ohm, 2W power resistor and connected it in-line with the 50 K input of the class-d amplifier. That way the pre-amplifier would see a 4 ohm + 50K load, and would work well.

Remaining question: where would you put the vu meter, and the resistor, and the amplifier?

pacificblue 17th July 2011 06:37 AM

50k is not a typical impedance for any headphones. Low impedance headphones have 8-32 Ohms. High impedance headphones have 600-2k Ohms.

The sure 2x2 W amp is a class D power amp. If your sources don't give enough output voltage to drive your other power amps to acceptable levels, you should rather build or buy a dedicated pre-amp.

Forget about that VU meter. Sufiicient input signal is, when it is loud enough for you. Too high of an input signal is, when the sound starts to become distorted or the amplifier protection kicks in.

By the way that VU meter is class 2,5 which means it has 2,5 % deviation. For 2 V measuring range that means 0,05 V deviation which would absolutely be good enough for this application.

birdyfoot 17th July 2011 02:49 PM

Yes, the more I research the more I understand. Let me verify... 50K is 5% of 1Ohm?

I have decided to put two 10W 4ohm resistors in line between the sure 2x2W board (acting as a preamplifier) and the two HifimeIDY boards (main amplifiers), one resistor for each channel. I beleive that this will bring the total load impedance up to 4.05 ohms per channel, which I hope will allow the preamplifier to function nominally. I have gone ahead and purchased the VU meter, which scales from zero to two volts, and is a class 2.5 meter. I think that I will place it on the returns of each channel (bridged) going back to the preamplifier. In any case I will learn soon enough if these components work as I am imagining, through experiment. So far I have only invested about $30 USD in this project.

pacificblue 17th July 2011 04:18 PM

50k = 50000 Ohm.

A VU meter measures voltage, not current. It must be connected parallel to the output, not in series.

Michael Bean 17th July 2011 04:30 PM

That's not a very good way to solve your problem, using a power amp with resistors on the output to emulate a real preamp will almost certainly make the final output to the speakers noisy and more distorted. I strongly suggest using a real preamp that is intended for that purpose instead, it will work much better, you'll be a lot happier in the end.


birdyfoot 18th July 2011 12:03 AM

Okay, well...

The sure 2x2W board seems like a very nice board, and of sufficient Class-D quality to serve as a preamplifier, and 2W seems like enough power to pre-amplify other Class-D amplifiers that require only line level input of 1.5Vrms. The sure amplifier however works best with a 4 ohm load, so a 10W 4 ohm resistor, for a 2W amplifier should work perfectly. Right?

What confuses me is the 50K impedance input of the main amplifiers. I supose that is because they are engineered with a 50K pot at the input. What will this "look" like in term of impedance to my preamplifier? These amplifiers don't have the same quality sound at all different impedance. Does a potentometer not apply the same kind of line resistence to the signal as a speaker or a resister?

Michael Bean 18th July 2011 02:33 AM

OK, here's the math: Power required to drive 50k to 1.5volts is 1.5 squared / 50k = .000045watts, 2watts is 44444 times the power required. Seems a little bit excessive don't you think? And as I already said, any decent quality preamp will have much lower noise and distortion than the power amp your thinking of adapting for the purpose, and that noise and distortion will be amplified by the power amps. Beyond all that, have you tried it without a preamp? Most CD players have a maximum output of 2volts, DVD players and many other sources will put out nearly that much, and all of them will drive a 50k amplifier input, it may work just fine without a preamp at all. I don't want to discourage you from trying new things, but doing this in a more conventional way will bring you much better results. Just trying to be helpful, but of course you're free to do as you wish.


birdyfoot 18th July 2011 03:21 AM

I see, so you would expect for the power amp that I am using to be too powerful for my purposes, 4 ohm resistor set aside, what I am building would be several thousand times too powerful, I would have to use it at very low volume, and then the signal to noise ratio would be much higher?

Using your formula (Mike, thank you) I find that the sure 2x2W amplifier might work for 1.5 Vrms at 4 oms impedance because it would require as much as .5 watts to accomplish this. I would be splitting that between two main amplifiers, and in my mind that means I would need perhaps twice as much wattage, and thus at 1 watt which is half the output of the sure 2x2w amp, I think it would work, and that it is not overkill. I also believe the T1 and T2 can be ordered without volume pots.

Just out of curiosity...

50K impedance? How? That is because of the pot that comes on the T1 right, which ranges in the amount of resistance it produces? How does a real preamplifier cope with the changing resistance of a potentiometer on the main power amps?

birdyfoot 18th July 2011 03:24 AM

Also, if you would, what is an example of a DIY preamplifier board that would work?

Thank you for for responses!

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