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Old 18th July 2011, 03:55 AM   #11
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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
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Old 18th July 2011, 04:30 AM   #12
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GOOD
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Old 18th July 2011, 04:13 PM   #13
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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?
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Old 18th July 2011, 05:16 PM   #14
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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
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Old 18th July 2011, 07:37 PM   #15
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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?

Quote:
Originally Posted by Michael Bean View Post
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.
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Old 18th July 2011, 08:48 PM   #16
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Quote:
Originally Posted by birdyfoot View Post
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.

Quote:
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?

Quote:
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?

Quote:
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.
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Old 18th July 2011, 09:50 PM   #17
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Before I go any further with this I need more details. What kind of satellites and sub woofer do you have? What features do you need the preamp to have?
After you answer those questions I'll probably have more.

Mike
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Old 19th July 2011, 12:01 AM   #18
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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.

Last edited by birdyfoot; 19th July 2011 at 12:11 AM.
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Old 19th July 2011, 12:19 AM   #19
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The output impedance of the preamp should be as low as possible, the lower the better, you do not want it to match or even come close to the 50k input impedance of the amps.

Mike
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Old 19th July 2011, 01:26 AM   #20
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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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