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Old 28th March 2012, 08:09 PM   #391
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Default Since you are an experienced constructor...

...let me make you aware of a remarkably good active preamp kit that would take you an hour or two to build, can be powered by any 12V wall wart, has high gain, low noise and distortion and costs under $25 from a variety or sources. This is the Kits r Us K100 kit, based on a Philips all purpose line level preamp chip that uses voltage controlled amplifier circuitry for its controls. This can be modified to run flat (without tone control effects) but also includes one of the better Baxandall tone control implementations I have seen. This can be housed in any reasonable plastic enclosure, and has a high enough impedance input that it will work nicely with your DAC.

If you want an op amp solution, check out the Velleman K8084 kit. It is based on TL072 op amps. This is a high impedance circuit, through and through, and needs shielding, but as a discrete design, is easily modified to meet any reasonable requirement. You can also substitute NE5532 op amps in this circuit, for even better performance. This kit retails for about $36, also from multiple suppliers. Though the potentiometers supplied are consumer quality, it is very easy to substitute similar valued ALPS or other audiophile grade, as well as improved capacitors. If you are perfboard capable, the design is available on line and you can build your own, including power supply (but keep the AC well away from the preamp board, unlike the kit design, for best results). I have used both of these kit boards, and they are both very good to excellent, as well as inexpennsive.

Though the thought of a commercial active preamp might horrify you, it turns out that the Gemini PA7000 (about $160 from various sources) is a TL072 op amp design similar to the Velleman kit, actually has excellent sound quality, decent pots, and is also pretty nearly silent. You might want to hide it in a box somewhere to avoid looking at it and offending your high end sensibilities, but you might well find it easy to listen to.

There is actually a line level preamp out there on Amazon that looks intriguing but I have no direct experience. This also costs under $100.
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Old 28th March 2012, 08:18 PM   #392
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Quote:
Originally Posted by deChrLam View Post
So I finally found the input impedance of these amplifiers. It is 3.23 KOhm. This makes these amplifiers very unsuitable for "passive" preamplifiers.

For my needs I will have to spend a lot of money and get Hypex UCD HG with 100K input impedance. I would really prefer not spend so much money, but the passive solution is the best for me. The extra expense of the Hypex is compensated by the fact that I do not have to buy an expensive active preamplifier. Or does anybody know what I can do to make the L15D/L25D work for me? Are the hypexes passive friendly because of the opamp on board? Can I modify the L15D/L25D to take an opamp? Questions about questions, sorry guys about the long post, just trying to get a good system together without giving "the man" too much of my hard earned money.

My setup consists of just one source, no record player, no headphones.
DAC 50 Ohm Output Impedance >
10K Potentiometer with maximum 2,.5 KOhm output impedance >
Amp >
3 way 200 Watt 4 Ohm 84 dB/W speakers in a large room
What's wrong with a pot buffered by a good opamp? Perhaps I don't have golden ears but I have been satisfied for years with a Panasonic ECG stereo log pot and an OPA2132 chip.
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Old 28th March 2012, 09:07 PM   #393
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Default Yes, I like op amp designs....

Absolutely nothing wrong with this. This is pretty much the same as you would get with a modified Velleman board. Oh yes, I forgot that the AC transformer for the Vellleman board costs about $14. I recommend moving the bridge rectifier/regulator components OFF the Velleman board for decreased noise.

Rod Elliot also has an excellent all purpose voltage doubler PS design for op amp boards on his web site. Inexpensive to implement, parts widely available.
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Old 28th March 2012, 09:26 PM   #394
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Originally Posted by stujonesmd View Post
I recommend moving the bridge rectifier/regulator components OFF the Velleman board for decreased noise.
Its easy to get a ground loop on a pcb. I found I got better results keeping the power supply seperate on my pcb's.
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http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software.
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Old 30th March 2012, 10:19 PM   #395
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The Velleman PCB is well laid out. If this preamp is properly shielded (inside a small metal case) it is practically silent. This is in fact a very good design, very similar to those used in a number of expensive audiophile preamps. However, having the board fed by AC means that it MUST be well shielded from the electromagnetic field created by the wires between the transformer and the boardl
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Old 30th March 2012, 10:35 PM   #396
MrSlim is offline MrSlim  Canada
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A few more buffer alternatives, if you are inclined to go discrete are the JISBOS or it's replacement the Alpha 20 from AMB or even a B1 Buffer from Mr Pass. I have a DCB1 in the works that will be the front end for L19D's, but you could build a basic B1 pretty cheaply. The B1 design includes a volume pot, but if you want to use an existing Passive Pre, just build a couple of the JISBOS or Alpha20s to put in after it. Glass Jar has Alpha 20 kits Here for ~$25 each.
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Old 31st March 2012, 08:23 AM   #397
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Hey Guys 391-396, thanks for the responses. I reckon I will go for the Velleman shielded with the volume and transformer and rectifier and filter caps well away from it. I am very happy with the alternatives you have presented. Big thanks and big thumbs up.

One other question:
Saarmichel reported that the over current protection on the L15D activated a few times while using his homemade speakers which have a 2.5 Ohm minimum impedance in the bass. My planned speakers Visaton Vox 253 MTI 4Ohm have what looks like a 3 Ohm minimum impedance between 150 Hz and 200 Hz. Does anyone else have any problems with 4 Ohm speakers?

Here is the Frequency and Impedance chart for them:
http://www.visaton.de/bilder/frequen...x253mti_fs.gif

Should I just be safe and go with the L25D, because it has 2 big output chips instead of 1 like the L15D?
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Old 31st March 2012, 03:09 PM   #398
alkasar is offline alkasar  France
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@dechrlm : what is your source ? Most modern dac have an opamp output able to drive as low as 600hms. I used a simple stepped attenuator pot with L15D and my dac: result was just excellent. Maybe you could give it a try ?
Alternatively, if your dac output is weak, consider replace the output opamp in your dac. This might be the easiest and cheapest
Introduce an additional active stage can only result in degradation, not noticeable in the best case. If not necessary, i think it is better to avoid.

I played L15D on my MTM diy speakers, 3.5ohms @ 150Hz. Played quite loud a few hours with no issue. Power supply at the time was connex SMPS 500R +-50V. It was not my amp and don't have it anymore.
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Old 1st April 2012, 07:39 AM   #399
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@alaskar I have not purchased a DAC just yet, but I was thinking the Cambridge Audio Azur DacMagic. I think so too: less stuff in the signal path is best.

Do you have a link to your MTM drivers or design?

Which transformer should I use with this filter and bridge module 6x10000μF/80V ?
45 Volt x 1.414 = 63.64 Volt
50 Volt x 1.44. = 70.71 Volt

The power here is a clean 230 Volts in Switzerland, and not 250 Volts like some users in London report. The voltage saftey margin for the 80V capacitors is quite small, but also I do not need full 300 Watts power. My speakers only 200 Watts.

What power does the L25D amplifier provide with 62 Volts?
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Old 1st April 2012, 04:15 PM   #400
ljm_ljm is offline ljm_ljm  China
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Quote:
Originally Posted by deChrLam View Post
@alaskar I have not purchased a DAC just yet, but I was thinking the Cambridge Audio Azur DacMagic. I think so too: less stuff in the signal path is best.

Do you have a link to your MTM drivers or design?

Which transformer should I use with this filter and bridge module 6x10000μF/80V ?
45 Volt x 1.414 = 63.64 Volt
50 Volt x 1.44. = 70.71 Volt

The power here is a clean 230 Volts in Switzerland, and not 250 Volts like some users in London report. The voltage saftey margin for the 80V capacitors is quite small, but also I do not need full 300 Watts power. My speakers only 200 Watts.

What power does the L25D amplifier provide with 62 Volts?

I personally think that the use AC45 appropriate.

The same power transformer voltage is low, then the current margin will be larger.
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