Lightspeed Attenuator a new passive preamp

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john65b

john65b

Member
Joined 2005
Paid Member
At the moment I am using a series resistor/shunt LDR configuration for my volume control. The LEDs are driven by infra red remotely controlled voltage controlled current sources and the LDR is buffered by either a simple fet circuit or a 300B valve, depending on my mood, which is then followed by a source follower MOSFET driving the loudspeakers. I don't need much gain as the loudspeakers give 105 dB for 1 watt of power with hardly any voltage swing required. The whole lot is very simple, very transparent and very musical.
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How about a schematic downstream of the LS (from buffer/driver to MOSFET output)? Sounds interesting...
 
wlowes

wlowes

Member
Joined 2007
john65b said:
George and Maximus,


I always felt the low impedance of the Krell was keeping the LS from really shining, but was confused by it sounding very good on one of my equally low impedance (22k) Gainclones.


Now I am curious about the impedance matching. I am running LS LDR/LDR design in front of a Peter Daniel Chip Amp that has the 22k loading resistor. I think it sounds wonderful, but I am wondering if I have never heard the full potential. I have a basic walwort power supply on the LS. It has a 50k log pot for volume and a 1k linear for balance as mikelm posted well back in this thread.

What is the difference in sound I can expect if I change the loading resistor in the chip amp?
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georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
wlowes said:
john65b said:
George and Maximus,


I always felt the low impedance of the Krell was keeping the LS from really shining, but was confused by it sounding very good on one of my equally low impedance (22k) Gainclones.


Now I am curious about the impedance matching. I am running LS LDR/LDR design in front of a Peter Daniel Chip Amp that has the 22k loading resistor. I think it sounds wonderful, but I am wondering if I have never heard the full potential. I have a basic walwort power supply on the LS. It has a 50k log pot for volume and a 1k linear for balance as mikelm posted well back in this thread.

What is the difference in sound I can expect if I change the loading resistor in the chip amp?
Click to expand...

Hi, it will still sound very good this way, but it's not at it's full potential though, if you can raise the input of the amp to over 50k or 100k you will notice a huge increase in dynamic swing and a much more open transparent relaxed sound. It will make you realize that the other way sounded a little pinched and constricted.

Cheers George
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john65b

john65b

Member
Joined 2005
Paid Member
Now I am curious about the impedance matching. I am running LS LDR/LDR design in front of a Peter Daniel Chip Amp that has the 22k loading resistor. I think it sounds wonderful, but I am wondering if I have never heard the full potential. I have a basic walwort power supply on the LS. It has a 50k log pot for volume and a 1k linear for balance as mikelm posted well back in this thread.
Click to expand...

If you have a voltage regulator (LM317 or LM7812), also try two 9V batteries in series for a clean 18VDC supply too.

This alone surprised me as I was also using a wallwart, then a Laptop SMPS. I thought Current was the end-all, but a nice clean power supply is more important....
 
M

maximus

Member
Joined 2004
paulhynesdesign.com
FET buffer/MOSFET current amplifier

Hi John65b,

I am preparing a DIY project based on this simple voltage gain stage and simple class A current buffer for posting on the Audiocircle Forum shortly. I originally designed it as a pure single ended system to drive eight 8” drive units in parallel (0.75 ohm load) for my line source loudspeakers a couple of years ago. It replaced a similarly simple “Circlotron” push pull class AB output stage with balanced driver, which I have been using for around six years. I still use both amplifiers. The single ended class A amp suits high efficiency loudspeakers and the “Circlotron” type amp is more of a muscle amp for less efficient loudspeakers.

I haven’t finished writing the application details yet but I will let you know when the project is posted on the Audiocircle Forum.

Regards
Paul
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
Paul hi, just thinking for the future about your remote control module, you should try to make it so it runs of either 12+ regulated or 5+ double regulated, this is what's available inside my production Lightspeed Attenuator. And there's room in it for your module to be retro fitted if my customers want to do it, I have sold many, so this way you could get a quite few extra orders of your module. The pot hole size on the front of the Lightspeed Attenuator is 10mm, your remote receiver could be made to utilize this hole after the pot is removed? Just a thought


Cheers George
 
G

gootee

Member
Joined 2006
fullnet.com/~tomg/index.html
Yes. A 6v supply and a 5v regulator would probably be a bad idea. With standard regulators, allowing AT LEAST 3v of headroom seems to be a good idea, to make sure that it always stays in regulation. Otherwise, any input noise or disturbance with amplitude greater than headroom minus dropout voltage just goes right through to the output, and to some lesser extent, usually, for somwhat lower disturbance amplitudes. Allow plenty of headroom. (The same thing can happen if the output voltage spikes or bounces upward. i.e. It could let input disturbances or noise through during the output's upward transients.)
 
wlowes

wlowes

Member
Joined 2007
gootee said:
Yes. A 6v supply and a 5v regulator would probably be a bad idea. With standard regulators, allowing AT LEAST 3v of headroom seems to be a good idea, to make sure that it always stays in regulation. Otherwise, any input noise or disturbance with amplitude greater than headroom minus dropout voltage just goes right through to the output, and to some lesser extent, usually, for somwhat lower disturbance amplitudes. Allow plenty of headroom. (The same thing can happen if the output voltage spikes or bounces upward. i.e. It could let input disturbances or noise through during the output's upward transients.)
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Thank you both... this is very interesting. I am using a 5.5v wallwart. I likely have some improvements coming. I'll report back on the weekend when I have time to make the adjustments.
I also have a very good regulated power supply but need to find a decent transformer to put in front of it. At least I know to leave head room there as well.
 
P

Paul Ebert

Member
Joined 2003
What am I doing wrong?

OK, so I finally got some time to measure my opto's. I put 1.2 volts across the LED (short leads with the dot side on negative) and measured resistance across the cell. For the eight or so I measured, the resistance was 'OL' with voltage and without.

Am I doing something wrong or they all bad? I imagine I'm doing something simple wrong - hope so, actually.
 
B

Badge

Member
Joined 2004
I did my measuring with the same volume pot and bias resistor recommended for the build. I provided a regulated 5 volts DC through the pot to the circuit. Dot side is negative and measure resistance at four different volume positions across the long leads. Let them warm up at least five minutes before doing the measurements
 
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