BPA - 200 questions(newbie)

[groyne]

hi, i know these questions must be getting really old, but please bear with me, i'm a little new to all of this...
some questions:
1.i checked dxvideo's pcb layouts for the BPA - 200 and he hadn't used the servo circuits.really, what goes wrong if you don't include them???(makes the Pcb layout sooo much easier. )
2.the AN - 1192 says use a 60vrms transformer.that means a 30 - 0 - 30 right??...so that means the final dc o/p should be around 27v. but how do they show 42v at the o/p, and that too unregulated...(as an off question, isin't unregulated inherently unstable??)
and finally, going out on a limb here, but what are the chanches any of you have some pcb layouts of the circuit? i'm finding the going pretty hard.using trax maker 2000.
thanks a lot.

[officeboy]

1- I would forget about the servo's, they are not required and the stability they add is probably traded off by degradation in the sound. Just make sure your gain setting parts are well matched, and have some output resistors and i think you will be fine.

2- 30ACV x 1.4 is 42DCV. I would say this is pushing it a bit to high for power. You have no margin for error with main fluctuations, and your chips will be clipping/overheating straight away too. A 25V transformer would seem to be a much better choice, it still can’t quite handle a 4ohm load (requires .90c/w heat sink) but it’s better then the 30V. of course if you only plan on running this into an 8 ohm load, then you may be ok with something a bit higher.

As for PCB layout, PCB’s can be handy, but for something simple like this. I don't think you can beat nice point to point setup or just use some vario board.

[groyne]

but for point to point setup, say using a general purpose board, aren't the tracks too thin?
as far as matching resistors go, that'll be fine, i guess...and i can use the 0.1 ohms that are used in the app notees right? or do i need to add some more?
and thanks a ton for the help

[UrSv]

200 watts Gäjnklon (Gainclone) - BPA200

[tlmadsen]

I don't want to be rude, but Officeboy are not correkt in most of his statements.

Have a look at

Can you tell me how to beef up PA100?

before you start paralling.

Have fun

Thomas

[carlmart]

Originally posted by tlmadsen
I don't want to be rude, but Officeboy are not correkt in most of his statements.

Have a look at

Can you tell me how to beef up PA100?

before you start paralling.

Sorry to disagree, but Officeboy is correct in most of his claims, as I see it.

The way National suggests to go about the chip paralleling is interesting, but servos seem to have an effect on audio quality.

If the parts are matched, to within 0.1% on the gain resistors, and use slightly larger 5% resistors than those suggested by National for the output, things should be fine. Rowland used 0.200 ohm resistors on his first amp with the 3886 chips, and 0.500 ohms in the next.

The voltage suggested by Groyne (+/- 42VDC) is certainly high and can compromise the chip response. Paralleling chips is good for lower impedance speakers, but they may demand too much at that voltage going into protection or heat too much.

The only think I do not agree, for single or multiple chips, is doing p2p wiring. It is messy, can be prone to accidents and shouldn't improve on a properly made pcb. You can do short paths with a pcb too.


Carlos

[groyne]

thanks carlmart,

however, won't using higher valued resistors cause a much higher drop at the same power o/p.

if i wanted to reduce power dissipation across the resistors, i'd then have to reduce my o/p power, would'nt i?

and i think i'm gonna go the pcb way anyway.seems nicer, and troubleshooting would be much easier, i think( though ideally i should'nt need it. )

[tlmadsen]

Hmmmmmmmmmmmmmm

I think that we all are somehow on the same pace here, but I would like to make a few points clear.

When Officeboy say that the DC-servos are not needed he is basically right, BUT you have to deal with the DC-offset when ýou are paralling chips. This can be done by DC-servos, capacitors, multiturn-offset adjustment, bigger output resistors and so on.

However:

"Just make sure your gain setting parts are well matched, and have some output resistors and i think you will be fine"

will NOT do the job. First of all the "matching" of the gain setting parts have nothing to do with the DC-offset and making then "match" by 0.000001 % will NOT remove the DC-offset.
It is correct as carlmart states that making the output-resitors "big" can to some extent solve the problem of DC flowing between the chips. To make this a "complete" cure you have to make the output-resistor bigger that 0.5 ohm witch I think the the upper limit, otherwise you start to loose to much on efficiency and so on.

I don't what to start the old discussion about DC-servos or not, but I think officeboy is a little bit to fast stating that they degrading the sound without ever have tried them. There are actually difference between how DC-servos are implementet and the one used in BPA-200 are not bad.

The +/- 42 V DC is correctly on the high side, but unlees you have a BIIIIIIIIIG transformer (+500 VA) the voltage will drop when you start pushing current through your BPA-200. (try to calculate the current you need at max. outpout) With no signal thay can actyally take +/- 47 V DC.................and yes it is true, I have tried is.
If you have the need for the high power, use 8 ohm speakers and have plenty of cooling you will be fine with your 2 x 30 V AC transformer.

I think p2p wiring of a BPA-200 is possible, but it will fore sure not be better than a PCB and the changes for some going wrong are quit high.

Have fun


Thomas

[peranders]

Originally posted by groyne
1.i checked dxvideo's pcb layouts for the BPA - 200 and he hadn't used the servo circuits.really, what goes wrong if you don't include them???(makes the Pcb layout sooo much easier. )

If you have them you can remove the offset and also the temperature drift plus you can also have smaller output resistors (creating a higher damping factor) plus a bass response to 1 Hz or below.

If you choose good opamps for the servo I'd say that they won't degrade the sound quality very much.

[Elroy]

Ok, I'm a new guy here, but I've been in audio for 30+ years.
Can some one explain to me how a lowpass circuit that starts to roll off at around 6 Hz could affect the sound of an amplifier when used for negative feedback? Do not the blocking caps do the same thing, but with out the advantage of the servo effect?