Need help designing preamp + amplifier

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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.

Thanks,


Marc
 
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?
 
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.
 
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.
 
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.

Mike
 
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?
 
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.

Mike
 
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?
 
I took a look at the HifimeDIY site. The input impedance (AC resistance) of those amps is 50,000 ohms (50k), with an input sensitivity of 1.5volts for maximum power output, the math I provided works, it doesn't need anywhere near the wattage you believe it does, like I said, 0.000045 watts - practically nothing - as is typical for most power amps (actually the voltage is all you need to look at, not wattage). Agian, most normal signal sources, CD players and such, will have plenty of drive voltage for these amps. Unless you're planning to drive it with something that has a really low output voltage, you probably won't need a preamp at all. And the volume control is already provided on the power amp board, so you don't have to worry about that either.

Mike
 
Thank you for your response. I understand what would be simpler, but I have several design constraints/objectives that I am trying to build around. What I am trying to build is a 2.1 channel system to work with MP3 players or computer headphone jack input. As I understand it MP3 player devices vary in their output but that it is generally weak I will also be splitting this signal between two amplifiers. I don't really know, but I would assume that this would further weaken the signal. I have decided to use a VU meter to make sure that the input signal is optimal. While each of the two power amps has it's own volume and can be set to a safe balanced output with each other, I would like a separate master volume to attenuate both amps at the same time.

What kind of preamplifier would you suggest?
 
How about this; Hifimediy|VC23 Preamp or this; Velleman Electronic Kits VEL K2572 Universal Stereo Pre-Amplifier<br><a href="http://www.arcade-electronics.com/velleman_kits.aspx">Electronic Kits Selection Page</a> - Audio/HiFi Applications - Arcade-Electronics.Com - Detail or this; Stereo Pre Amp with Tone Control. Just a few I found by Googling "stereo preamp kits", literally dozens available at all different quality and performance levels.
Another question, I assume that by "2.1 system" you mean two speakers and a subwoofer, do you have something in mind for a line level or active or passive crossover to divide the signal between the speakers and the sub? If you don't use one it probably won't sound very good.

Mike
 
Yes I have looked a the Vellemen kits, but you have to watch out, some of them only range from 40Hz on up, or in some cases only up to 5kHz. Also I have noticed that the input of some of these preamps is very sensitive, less than 5mVrms. Correct me if I'm wrong but that doesn't seem like it would work.

I was thinking of something more audiophile grade that would compliment the HifimeDIY amps, but not as sophisicated that the preamp form them. I was wondering about units like these I found on ebay: example 1, example 2 example 3. I guess if I used a tube-preamp the whole system would benefit from that effect too (not sure if it's that desirable to me...). These amps don't feature very explicit specifications. I'm not sure if they would work any better. This one is more money but it looks like a better product, how can I tell?

What specific specifications should I be looking for in my preamp? Are there only a few parameters that I need to worry about, or that I should focus on?

do you have something in mind for a line level or active or passive crossover to divide the signal between the speakers and the sub? If you don't use one it probably won't sound very good.

Mike

Thanks for your concern. Yes the stereo will feature two small satellite monitors and a sub-woofer, and I will be building passive crossovers for all the drivers. Much of they system is almost completely designed except for the amplifier. I think I can handle most of that pretty well. I have done my homework as far as driver selection and box building too.

Most of the amplifier I expect to house in a separate partition of the sub-woofer as a plate amplifier. The preamplifier, however, and the VU meter I would like in a little wired remote-box to keep on the desk. This is also where the computer or MP3 player would plug in.

Thanks again for all of your input. The amplifier/preamplifier has been the real crux of my design process. Obviously there is no clear solution in sight yet.
 
Also I have noticed that the input of some of these preamps is very sensitive, less than 5mVrms. Correct me if I'm wrong but that doesn't seem like it would work.

Please check the output voltage from the sources (MP3 player and computer and anything else?). This will tell you what sensitivity/gain you need in your pre-amp. That said most decent active pre-amps will supply sufficient gain to work with most power amps for most normal source voltages.

I was thinking of something more audiophile grade that would compliment the HifimeDIY amps, but not as sophisicated that the preamp form them.

How much do you want to spend? Do you require a ready made module or a kit of parts or just a pcb and design?

What specific specifications should I be looking for in my preamp? Are there only a few parameters that I need to worry about, or that I should focus on?

First considerations are how many inputs does the pre-amp have to handle? How much (if any) gain does it need (power amp input sensitivity/lowest source input voltage)? Is it a stereo pre-amp or multichannel needed? Will your sub require line level or speaker level input? And again, how much do you want to spend?

Thanks for your concern. Yes the stereo will feature two small satellite monitors and a sub-woofer, and I will be building passive crossovers for all the drivers. Much of they system is almost completely designed except for the amplifier. I think I can handle most of that pretty well. I have done my homework as far as driver selection and box building too.

Most of the amplifier I expect to house in a separate partition of the sub-woofer as a plate amplifier. The preamplifier, however, and the VU meter I would like in a little wired remote-box to keep on the desk. This is also where the computer or MP3 player would plug in.

Thanks again for all of your input. The amplifier/preamplifier has been the real crux of my design process. Obviously there is no clear solution in sight yet.
 
Well, the T1 from HifimeDIY is 2x80W at 8 ohms, and is for the satellites which are about 60watts RMS each. The T2 is 2x100W at 4 ohms and is for the sub-woofer which will have two 80watt RMS drivers on it. Thus while I am making a "2.1" channel system, the schematic is much like a bi-amplified 3-way system, however, with both of the "sub"-woofers working as a mono channel.

The pre-amplifier only needs one input, which I would fit with a 2.5mm headphone jack. It will typically have to boost the signal of an MP3 player or computer. The signal must be split between two, nearly identical, class-d amplifiers that have the same 50K, 1.5Vrms input.

I think you must be right and that I will have to buy another dedicated pre-amplifier to work with it. I think I like the idea of using a tube pre-amplifier, like this one. The price is right, and it looks great, but I believe it is more of a headphone pre-amplifier, and the output impedance is 600 ohms. Haven't found one that says 50,000 ohms yet.
 
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I think I get it. If the output impedance of the preamp were 50K it would overpower the resistance of the input of the main amps (also 50k) and damage them. There is most likely a 50K pot on the T1 and T2 amplifiers. This also means that the input resistance of the T1 and T2 is variable with a maximum of 50K but ranges much lower. Please correct me if I'm wrong. This raises another question, however. Won't a variable resistance affect the performance of the preamplifier? I believe it would have with the Sure amplifier that I was intending to use.

So do you think that this tube amplifier would work at 600 ohms output?

TUBE PREAMLIFIER

It boasts some very high audio quality.


I guess this is my central question. Assuming that bought a pre-amplifier, what impact would placing a 50K pot between it and the amplifier have on the sound quality and the function of the preamplifier?
 
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