Audio sector gainclone.

dome406

Member
2004-03-28 10:49 am
uk
Have just recieved my lm3875 from audiosector (very fast sevice) and am very pleased however have a few questions, sorry again for all my previous posts :).

Can the power rectifier boards be used with just the 8 diodes and two 10uf 50v capacitors, or do you have to fill the whole board? if so do you use values fom the briangt circuit layout rev3 from chipamp?

If the board can be used with just 8 diodes and two capacitors can you use the led holes or is this only when the board is fully populated.

If you use the snubber capacitors do you just ignore the places to put the smaller capacitors as they go to the same place.

What is the advantage of using a fully populated board e.g. snubberise and zobel.

Does it make a huge difference of deteriorate the sound.

I noticed that in the rev 3 brian gt uses 1kohm instead of 220ohm (supplied)where as rev2 uses 220ohm, for r1 does this make a difference should I use 1k.

Sorry if this is somwhere else but I am just a bit unclear. Thanks for any help.
 
yes u only need 8 diodes, 2 caps for working power supply. snubber components are optionally and the values found on brian's site are good.

yes you dont use smaller cap holes when using snubber

advantage? there are pages of discussion about this :dead:
long story short. if u have hard to drive speakers, snubber may help. zobel is a safety for amps that osscilate, use only when necessary is what i think

for r1, no difference really, both values serve their purpose
 
There should be 2 rectifier boards, right? If I'm correct each hold 8 diodes and the 2 caps. I used 1 board with 8 rectifier diodes for my stereo amp. 4 diodes for v+ and 4 for v-. I think it may be a good idea to use the 1k input resistor if you are just using an attenuator. It gives the amp a slighty friendlier input impedance. My amp sounds better with that resistor for sure so it's definitely doing something right. I haven't tried the zobel or snubberized diodes but I did remove the 2 10uf caps from the rectifier board which for me was an improvement .
 

dome406

Member
2004-03-28 10:49 am
uk
half way there

I have just finished assembling the main part of the pcbs from audiosector, and am now awaiting my transformers from farnell.

I am just a bit confused about how to wire them as there is nothing on the pcb to mark the live and neutral ac connections, so I have made a diagram on how I think it should go. Could you tell me if this is correct?

http://img244.imageshack.us/img244/4342/wiring1ra.jpg

Also could you tell me if the amp in the picture would work e.g. the minimum amount of components to work. Is the RF and Rz capacitor optional.

One last quiestion how fragile are the lm3876 do they withstand a fair amount of heat and static before they break?

thanks very much and look forward to listening to my new amp :).
 
Re: half way there

dome406 said:
I am just a bit confused about how to wire them as there is nothing on the pcb to mark the live and neutral ac connections, so I have made a diagram on how I think it should go. Could you tell me if this is correct?

There is absolutly no difference how you connect secondaries to the board, by that I mean polarity does not matter, as you have two sets of diodes connected to the same point.

That's why, on my boards, the connections are marked by AC1/AC1 and AC2/AC2. The previous versions, with dash lines, were unneccessary confusing.
 
dome406 said:
Can the power rectifier boards be used with just the 8 diodes and two 10uf 50v capacitors, or do you have to fill the whole board? if so do you use values fom the briangt circuit layout rev3 from chipamp?

If the board can be used with just 8 diodes and two capacitors can you use the led holes or is this only when the board is fully populated.

If you use the snubber capacitors do you just ignore the places to put the smaller capacitors as they go to the same place.

What is the advantage of using a fully populated board e.g. snubberise and zobel.

Does it make a huge difference of deteriorate the sound.

I noticed that in the rev 3 brian gt uses 1kohm instead of 220ohm (supplied)where as rev2 uses 220ohm, for r1 does this make a difference should I use 1k.

1 You don’t need to fill the whole board; 8 rectifiers and 10u caps are minimum required to run the amp. Additional parts are for snubber option, and the best is to follow Carlos' latest combination http://www.diyaudio.com/forums/showthread.php?postid=703003#post703003

I’d like to remind everybody that snubber idea was introduce into chip amp circles by CarlosFM. There is also the spot for bigger caps, that are used when snubber is involved (see picture below).

2 You can use LED holes even when not all components are mounted on the board. With LED connected, R3 has to be on board as well, as it’s in series with LED and provides drop from full supply voltage value.

3 If you use the snubber capacitors, C1 and C3 cannot be installed, as this place is used by R1, R2 which are part of the snubber. However, the bypass cap can be installed directly across the big cap, and the value recommended by Carlos should be used.

4 The advantage of using fully populated board will depend on your system and taste. I don’t like neither the snubber (not anymore at least) and I don’t like the Zobel, some people like it though and they claim it’s the biggest improvement to chip amps. Well, that’s why I created that option on my boards and you are free to experiment and make your own decisions.

Snubber seemingly increases dynamics, the bass response, and even resolution, but at the same time introduces some colorations that may not be accepteable by people who prefer more natural presentation. One musician who was listening to my amp with a snubber described it as synthesized sound with unnatural tonal balance.

Zobel is useful when your speakers or cables present unfriendly load (to the amp). It some instances the interferences occur and Zobel may cure it, but more than often, does not, and different measures have to be taken (like replacing speaker cables). Zobel also changes tonal balance, and some people prefer it this way.

5 It doesn’t really matter if you use 220R or you use 1K series resistor, you can also use anything in between. I find that lower value brings less sonic signature, but presently, I’m not using any series resistors at all, and the sound is even more transparent.

6 You need to install Rf, which is a feedback resistor.
 

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dome406

Member
2004-03-28 10:49 am
uk
Sorry if this is a stupid question but is the Rf resistor the 22.1k resistor soldered between pin 3 and 8 of the lm3875 or is this another one, because I have soldered it below the the board if it is. The only reason I ask this is because I got 4 22.1k resistors and had none left by the time I got to the Rf one. :D and that would make sense.

And another quick question if you have time is what do the cz and rz capacitor and resistor do.

Sorry for all the questions I just want to understand all the options rather than just mess around with everthing and nothing work :).

Thanks very much.
 
Can't you get some burning alcohol or propanol?

You can also take a stuff brush short hair but soft, brush dry first then brush wet with plain water and dish detergent like "Yes". Rinse with water and blow it dry with compressed air or let it dry in 40-50 deg for an hour or so.

I use first propanol and then Yes and water. I dry with compressed air.
 
Here's the preferred way to mount feedback resistor. Although it is intended for Caddock mostly, it should also work with regular resistors. There is a small hole right beside pin 8 for one resistor's end, the other end solders directly to pin 3 on the chip. This method eliminates the chip pins length (when mounted underneath) and allows shortes feedback loop, at the same time being easy to implement, without need for elaborate resistor pins bending. ;)
 

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