Building a Gainclone with LM3886 General Direction and PSU

[star882]

A friend of mine actually built a hybrid digital amplifier that is powered by an old laptop power supply. I have seen it on display at a university engineering fair and it works very well. I even borrowed it for a few days for testing and I was very impressed at its quality and especially its very high efficiency.
Amanda Harris: the Prius of home audio - EcoRenovator

[bmwpowere36m3]

Originally Posted by Pentode

Trouble is with regulated supplies for PAs... won't they have some hysteresis, meaning will the be slow to regulate say... hf signals above a few hundred hertz, just thinking this would add distortion under certain musical conditions and power levels.... another reason why some don't use Fuses in speakers and HT lines to help keep resistances down to a bare minimum when passing 'peaky' amperes - just my thoughts.

Dave

Originally Posted by star882

A good voltage regulator should be able to respond to transient loads very quickly. Test it with a load resistor and a MOSFET gated by a signal generator.

Power supply circuits for modern CPUs have to supply 10s of amps at a low voltage and respond very, very quickly to transient loads. Many even need to dynamically change the output voltage for energy saving features.

Originally Posted by danielwritesbac

Oh yes. Decibel Dungeon has a briefing on how to use computer power supplies with a chip amp. The technology does seem to work well.

Originally Posted by star882

A friend of mine actually built a hybrid digital amplifier that is powered by an old laptop power supply. I have seen it on display at a university engineering fair and it works very well. I even borrowed it for a few days for testing and I was very impressed at its quality and especially its very high efficiency.
Amanda Harris: the Prius of home audio - EcoRenovator

What the hell... a little off-topic are we?

[danielwritesbac]

A summary of all that: Some sort of regulated power supply can make for nice results.

Also, if you build a class AB amplifier with the same care and attention to detail that is normally used in class D amps then the sound will be approximately as clear and the efficiency will be great, for either.

Additionally, the power supply has the largest effect on sound, not a cap, not a coil, not a resistor, not a heatsink, not a potentiometer. . . Much attention to the power circuit makes for excellent sound. Starting out with excellent power makes the rest so much easier.

[jnewbold]

Originally Posted by bmwpowere36m3

II think I found my amp design, taken from the same site:



I'm also going to get a couple of pots (10k & 50k) to experiment with. Components include: Rf 22k, Rg 680R, Rm 10k, and Cs 2200uF.

Hi bmwpowere36m3.

I am in the process of building Mick Feuerbacher's 2 resistor LM3886 and just came across this thread. Have you completed yours yet? Did you try the two different pots and did you detect a worthwhile difference?

I have a 10K stepped attenuator to try and also the "Optivol - Simple LDR volume control" from Greg Ball (SKA amps) [URL="http://www.ska-audio.com/Forum/YaBB.pl?num=1178438672"].

The Optivol is very simple and, for me, is an improvement on the attenuator. However, it likes a high input impedance so it remains to be seen whether it will suit Mick's simplified LM3886.

Joe

[bmwpowere36m3]

Originally Posted by jnewbold

Hi bmwpowere36m3.

I am in the process of building Mick Feuerbacher's 2 resistor LM3886 and just came across this thread. Have you completed yours yet? Did you try the two different pots and did you detect a worthwhile difference?

I have a 10K stepped attenuator to try and also the "Optivol - Simple LDR volume control" from Greg Ball (SKA amps) [URL="http://www.ska-audio.com/Forum/YaBB.pl?num=1178438672"].

The Optivol is very simple and, for me, is an improvement on the attenuator. However, it likes a high input impedance so it remains to be seen whether it will suit Mick's simplified LM3886.

Joe

Yes, I've built it and it sounds great. So far I've only used it with my 10k pot... used Vishay RN resistors and Panasonic FC caps.

[jnewbold]

Originally Posted by bmwpowere36m3

Yes, I've built it and it sounds great. So far I've only used it with my 10k pot... used Vishay RN resistors and Panasonic FC caps.

I just finished mine yesterday. Not PTP but on a PCB. I have tried it with both a 10k stepped attenuator (Dale resistors) and the LDR volume control.

Without any type of input connected, I got a savage hum. With the 10k stepped attenuator, as in Mick Feuerbacher's schematic, hum disappeared and it sounds very nice. DC offset is about 10.5Mv on each channel.

However, with LDR volume control in place of the stepped attenuator, the hum returned. So I soldered a 22K resistor across the input and hum is gone. The LDR volume control sounds clearer ad more dynamic and is an improvement over the stepped attenuator.

However, I need to experiment a bit with the LDR input resistor to see if the value makes a difference.

The LDR volume control is powered from the +V/0V of the Gainclone PSU through a (LM7815) 15V regulator. It is on a very small PCB (50mm X 12mm) so can be mounted near the back of the amp box for short input wire lengths, and the control pot mounted on the front panel.

[danielwritesbac]

Originally Posted by jnewbold

. . .
Without any type of input connected, I got a savage hum. With the 10k stepped attenuator, as in Mick Feuerbacher's schematic, hum disappeared and it sounds very nice. DC offset is about 10.5Mv on each channel.

However, with LDR volume control in place of the stepped attenuator, the hum returned. So I soldered a 22K resistor across the input and hum is gone. The LDR volume control sounds clearer ad more dynamic and is an improvement over the stepped attenuator.
. . .

The amplifier needs an approximately 12k input load in order to be able to reference for minimized dc offset and minimized noise.

Typical datasheet designs of 20k pot in parallel with 22k resistor can make enough load.

Maybe the LDR makes only a 50k or 100k load, and that's why you had to "add to it" with a resistor?

Reference point:
input load 10k~15k

Interesting point:
Any bit of padding or losses at the input circuit, such as a series resistor, may decrease the load and it also may decrease the drive force of the source, whereupon the end result is not as clear at a distance but probably more pleasant if/when operated at closer range.
This sort of thing may account for the difference between your two volume controls. A small, creative loss, such as an input transformer, passive match with caps, LDR, and some preamp designs, can help serve as an "un-compressor" in making clearer sound at a shorter distance (oh, say about where you're sitting when you listen to your speakers). The method does decrease the range that the sound will carry clearly, so its not for outdoor use.

P.S.
Thank you for the story and application note on the LDR.

[mr.duck]

I'm thinking of making an attenuator for a gainclone that is a low value like 1K. The idea being to use a value so low that it wont require a preamp/buffer to sound great.

Do you think 1K is too low and do you think a low value like this would help the SQ? Maybe I will try 500 Ohm instead.

Also, has anyone compared different pot values like 5k vs 50k with gainclone?

[AndrewT]

it's not the chipamp after the volume pot that will be the problem.
It is whether the source can drive the load that the volume pot presents.

[mr.duck]

Originally Posted by AndrewT

it's not the chipamp after the volume pot that will be the problem.
It is whether the source can drive the load that the volume pot presents.

Hi Andrew. Yes, I'm sure that is correct. But do you think there is much benefit using as low a value as possible? Reason behind it being that some people like gainclones with preamps, a.k.a. buffered volume control. Therefore a 1K pot with a maximum output impedance of 250 Ohm may sound better than a 100K pot with 25K output impedance for example.

Low end source like computer or ipod could drive a 1K load so I don't see that load as much of a problem.