Audio Note M Zero, anybody?

[RochaCullen]

Hi Fran,

"It went on to say how you could change the gain on the boards by changing the value of one resistor."

Could you point me to where this is mentioned. I did make them myself. I'll post some photos when I get a chance as they are great bits of kit. What you say makes sense. I built these amps using a kit and following instructions so I am not an electrical guru by any means.

If you can send on that link I will see about isolating that resistor and replacing it on the board, and if that is a bit fiddly I can do what you say on the input.

Thanks again,

Nathan

PS: I hear plenty of folks use the preamp I have with Quad 405s to great effect. If our theory is correct and it is the levels of gain that is causing a problem that would point to the Quads having a very low, or lower, gain. I must see about digging up their specs.

Put 100k in series with the hot, and then put one end of the 10k just after the 100k, with the other end of it to ground. You could do it all right on the input jack.

I'm saying this because I'm thinking you bought the kits and assembled yourself - if not do you have anyone near you who could solder them in without wrecking the amp?

Doesn't have to be exactly 100k and 10k either - probably the series one in the range 50-150K and the shunt from 5 to 30k would do AOK.


I went off and had a look at the LM3886. I didn't find the manual etc for your exact one, but did find one for another set of boards (the BrianGT ones also on chipamp.com - http://chipamp.com/docs/lm3886-manual.pdf). As they are all reasonably similar, the specs are undoubtedly similar. Anyway, it gives the gain on it as 35dB - a massive amount, and clearly meant to be made as an integrated or with a passive pre. It went on to say how you could change the gain on the boards by changing the value of one resistor.

***************************
Lads - very funny!!


Fran

[RochaCullen]

Just spotted that on a page discussing the quad 405 that it has a gain of "15x or 23.5dB from the amplifier input"

http://www.keith-snook.info/QUAD-405-mods.html

Which is some 10db less than what you mentioned my gainclones would be kicking out.

Nathan

[RochaCullen]

I found that pdf that you were speaking of, which contains t he directions to change the resistor.

It states that for the amplifer Gain = Rf/R3. And the current gain with the 660ohm resistor is 33db. So by playing with the mentioned formula, you can get a gain of 23db by chaning R3 to be a 1k 0hm resistor.

Maplin here I come. Lets hope this sorts it, otherwise I might have to offload this lovely preamp.

Nathan

[Fran]

If you can't find one in Maplin, I'd have some here I could send down to you. If it was me, I'd go lower - about 15dB would be more than enough....

Fran

[RochaCullen]

Good thinking,

I'll try both and see how it comes out. I've calculated to get 15db I will need a 1571 ohm resistor, so a 1600 ohm resistor.

I'll get all of these today. I would assume metal film is the best type to get??

Thanks for all your help fran. Lets hope this works.

If you can't find one in Maplin, I'd have some here I could send down to you. If it was me, I'd go lower - about 15dB would be more than enough....

Fran

[Fran]

Yeah, metal films usually have lower self-noise but in this case they are most likely passing very low current so it will hardly matter. What might be worthwhile is to buy 10 of a couple fo values (they're cheap anyway) and then measure each one. Use the closest matching pair for each monoblock. Those maplin resistors are usually 10% tolerance, so if you were unlucky you could get one at 1600R and another at 1440R (although thats unlikely). You might be able to hear a difference as much as that.

Don't forget you can also parallel/series different values to get new ones, plus don't go less than 10dB gain.
***********************

Prompted by this thread, I've ordered up a LM3875 kit from audiosector to try them out!! I have one if his nosDACs and find it a superb performer.

Fran

[RochaCullen]

You won't be disappointed the LM38-- gainlcone kits are great. I got the dual mono set and built a set of monoblocks, they cost me almost 400 quid to make as I was just getting into DIY and had to buy everything, but the quality is there.

I got in very late so only managed to mod one of the monoblocks last night and while I did not get to do too much listening, the results seem favourable. The noise while not completely gone is vastly reduced. And fair play, Fran, you were right I got the best results when I put in the 1600 ohm resistor. I will mod the other monoblock tonight and let you know how I get on.

I am hearing a bit of static on both channels, but I've heard a bit of noise is normal with valve amps, and hopefully the levels will go down once I have both monos modded.

I'll keep you posted,

And thanks again for you help.

A mate on another forum recommended one of the following to complete get rid of any residual hum:

http://www.ebay.co.uk/itm/120785936469? ... 1423.l2649

I may consider that further down the road. I would however wonder what the performance implications are from using such a piece of kit.

Nathan

Yeah, metal films usually have lower self-noise but in this case they are most likely passing very low current so it will hardly matter. What might be worthwhile is to buy 10 of a couple fo values (they're cheap anyway) and then measure each one. Use the closest matching pair for each monoblock. Those maplin resistors are usually 10% tolerance, so if you were unlucky you could get one at 1600R and another at 1440R (although thats unlikely). You might be able to hear a difference as much as that.

Don't forget you can also parallel/series different values to get new ones, plus don't go less than 10dB gain.
***********************

Prompted by this thread, I've ordered up a LM3875 kit from audiosector to try them out!! I have one if his nosDACs and find it a superb performer.

Fran

[Fran]

Good stuff.

the thing is that the preamp still has very high gain at 21dB. Don't forget that dB is a log scale so every one dB counts!!

Bringing down the gain in the other monoblock will probably help with the overall room noise too. The line isolation thing might work alright but you would want to make sure its not detrimental to the signal. I've seen people use step down transformers as well to dump some gain.

I'm facing the very same problem with a valve amp I made. Sound is glorious, but the gain is quite high. I could increase the feedback in the amp which would drop the gain, but too much and it kills some of the SQ. Worse than that, my preamp has a stepped pot and there is too big a gap between the bottom couple of steps!!

Looking forward to trying out the chipamps now. I ordered up 2 x 225VA transformers so I can make them as monoblocks.

Fran

[RochaCullen]

Hey,

Well I finished the mod on the second monoblock and the noise floor lowered dramatically. I do get a bit of static on both channels, but I assume this is just a valve thing, and to be expected? Sorry I am a valve newbie.

Despite dropping the gain on the monoblocks that much, there is still plenty of volume and grunt. The drop has slighly mellowed the sound if my ears are not mistaking me, which is not a bad thing.

Thanks for all your help Fran.

I will see about taking some pics of my mono's, which might give you some ideas as to final construction. I've set them up so they look a little like some meridian 205s, with the heatsink out front. The 225va trannys should be plenty, I went a bit overboard and put a pair of 300va trannys in mine.

Nathan

I'm facing the very same problem with a valve amp I made. Sound is glorious, but the gain is quite high. I could increase the feedback in the amp which would drop the gain, but too much and it kills some of the SQ. Worse than that, my preamp has a stepped pot and there is too big a gap between the bottom couple of steps!!

Looking forward to trying out the chipamps now. I ordered up 2 x 225VA transformers so I can make them as monoblocks.

Fran

[Fran]

Shouldn't necessarily be a valve thing. Can you definitely isolate it down to being the preamp?


Second thing.... it would be wise to check the amp for DC offset after changing the gain. I've read in preparation for doing my own (which I want with less gain) that ideally you would try combinations of feedback resistor and input resistor to get the gain you want but also the lowest DC offset possible.

Anything less than 100mV should be AOK.

I'm interested to see how these compare to the likes of the Pass F5, various valve amps etc!! Will they drive my stacked quads!?!?!?


Fran