How to choose?

[shmulik]

Hi All,

Reading posts on this forum made my fingers itch. I own a very sensitive loudspeaker driven by 45 type tube amplifier, but realizing how simple it is to build an amplifier that, at least by the posts, sounds very good made me want one. the problem is I don't know how to choose the right chip for my needs. I read many of the posts and simply got lost.
let me list some facts:[list=1][*]I can solder OK[*]quality parts are not cheap in Israel so buying a kit with parts is a viable option[*]my current amp is 2W per channel so I don't need a powerhouse for an amplifier[*]sensitive speakers means dead quite amplifier or the hiss drives you mad[/list=1]

Please advise regarding the type of chip to use (please explain why if possible) and if a kit exist, point me to it. I understand that it may be a matter of taste but in our hobby taste counts. Although this is the chip amp forum, if someone thinks I'm better off with a D type amp, please give your reasoning - I have no prejudices.

Shmulik

[AndrewT]

Hi,
a chipamp with near lowest acceptable operating voltage may be too noisy for very sensitive speakers.
Using very low resistances around the feedback loop and at the input may help keep the noise low, but I suspect the integrated nature of chipamps will result in more noise than a discrete circuit optimised for low noise.

[dfdye]

This sounds like a job for a low gain circuit built around a couple of OPA549's. The unity gain stability of these chips would make a nice little amp with very little gain that would meet the "low noise floor" requirement. I don't know of any kits, so another option may be to get a chipamp kit and just drop the gain substantially (I think the min stable gain for the LM1875 is 10X, but I would shoot for ~15X to be on the safe side--don't hold me to that number, I'm not looking at the data sheet!).

Regardless of the chip, I would try a strategy of using a lower gain circuit to compliment the lower rail voltages associated with the amp you are shooting for. That would do worlds for lowering the noise floor in an amp that will only need to deliver a few watts.

Just food for thought at this point--I am sure we can get into specifics a little later.

David

[Nordic]

Andrew would you agree with me that a 2W tube amp can drive better than an equivalent powered discrete amp...?

I suspect you my be a little dissapointed with the step to chipamps.... I wish I had drivers that could live off 2W...


I would build the Lm1875, needs no fancy parts, you should be able to get everything for a good channel for about $5, excluding heatsink and PSU.

I think it can be quiet enough, my experience with chips is that the circuit is either pretty noisy, or near dead quiet.

[ionomolo]

Originally posted by dfdye
This sounds like a job for a low gain circuit built around a couple of OPA549's. The unity gain stability of these chips would make a nice little amp with very little gain that would meet the "low noise floor" requirement. I don't know of any kits, so another option may be to get a chipamp kit and just drop the gain substantially (I think the min stable gain for the LM1875 is 10X, but I would shoot for ~15X to be on the safe side--don't hold me to that number, I'm not looking at the data sheet!).

I agree completely with you. I'm working with an LM4562 driving a pair of (medium) power bjt's and it's noiseless. Would the line driver from the LM4562 (page 27) datasheet do just fine up to 5W or so if the output transistors are beef enough? I wonder if it could make a great 5W amp with a couple of BD139/140 and a BC139 for the Vbe multiplier. You Andrew must know it for sure.

With respect to the original question i would suggest to build a plain LM1875 amp kit bought online to begin, but then take some time to play with immense amount of possibilities that such speakers give. The line driver used as small power amp will be my next project for sure!

[Zigis]

Hi Shmulik,
if you mention D class, I can advice this:
sureelectronics Tripath boards?

after small modification it sound really good and quiet ,enough headroom for your loudspeakers.

Zigis.

[shmulik]

someone suggested trying an earphone amp utilizing op amps. I don't know what the exact ones but I'll give it a try.
BTW, I connected a 100W amp (EAR509) and using the pre I can get listenable levels (I think this amp gets to full power at ~0.7V but not sure). I don't know if this fact is relevant but maybe it relaxes the low gain restriction.

[seventenths]

Chipamps can be VERY quiet.
With 94db fostex 166's in BiBs, I have yet to find an adult that can tell me if the amp is on or off with the source connected but not playing and the volume control wide open. Actually touching ones ear to the cone means disqualification Current implementation is a simple, inverted 3875 with a big PSU (400va - ~20000uF per rail @ 29volts)
Anyone close who doubts is more than welcome to come see or hear for themselves...


7/10

[ttan98]

I drive a compression driver with 110dB spl with a LM3875 amp, I put my ear next to the horn, very quiet, and it has no resistor attenuation/L-pad applied.

I use 680ohm and 22kohm combo for feedback.

You should not have any problem. I major culprit would be grounding loop, you should take care of it.

[casiomax]

A bridge LM386 could drive 2W, do a search the circuit somewhere on the internet. I found to remember it was called a Little Rebel.