[P68] Balanced input???

[Tazzy]

Is it possiblie to add a Balanced input to P68?

[carlmart]

Originally posted by Tazzy
Is it possiblie to add a Balanced input to P68?

Yes, it is possible. But some parts rearrangement should be in order.

Why don't you ask Rod about that?


Carlos

[Tazzy]

I asked him after I posted the first message

[carlmart]

What did Rod suggest?

The next question is why do you want to use a balanced connection.

Do you plan to put your sub amp close to the speaker?

My first idea to make a balanced version of the 300W, is doing something like what Nelson Pass did on the Aleph 4. As I said some parts should be added or cut, like R3 on the 300W amp.

Perhaps R3 could be replaced with 220 ohms, and a similar one added on the other end, between the feedback and Q3's base.

R1 and R9 should be the same too, as well as capacitors C1 and C4.

In any case I would suggest these changes to Rod and listen to what he says.


Carlos

[dwjj]

Couldn't you just put a balanced receiver in front of the normal circuit? Rod's simple op-amp solution, driving where C1 connects?

http://www.sound.westhost.com/project51.htm

[dwjj]

Figure 1 is another circuit

http://www.sound.westhost.com/balance.htm

[carlmart]

All this question is interesting if you're going to use a long cable between preamp and power amp. Are you?

The idea of making the balanced input directly on the power amp is to eliminate stages. If you add a balanced to unbalanced stage before the amp you will be adding distortion. noise, etc.

Project 51 is reasonable for the balanced output stage, which is similar to that used by Mackie. But the unbalanced to balanced circuit is not good.

The other project is interesting but very complicated. It's an instrumentation amplifier, which also needs 0.1% resistors to justify using it.

If you need balanced stages, use balancing or unbalancing chips like SSM2142 and DRV134.

Can you explain more on your intended application?


Carlos

[Tazzy]

The reason I'm interested in converting the P68 to balanced is a the balanced crossover I've got.

It can operate in normal mode so if the conversion isn't a good idea it won't be a problem.

[carlmart]

Originally posted by Tazzy
The reason I'm interested in converting the P68 to balanced is a the balanced crossover I've got.

It can operate in normal mode so if the conversion isn't a good idea it won't be a problem.

OK. If the crossover converts the signal to balanced and you can avoid it, because the cables are short, bypass it.

As I said, if connections are short you don't need to go balance. Except if you are working from the source with a balanced signal, feeding balanced pots and all that.

Don't believe it if you read it otherwise. The only good thing you will have for balanced connections is the XLR connector you will be probably using.


Carlos

[Tazzy]

What Rod told me:
Dennis,
It is possible, but requires an outboard balance circuit. You could
cheat and use the SIM outputs as inputs - common mode rejection is not
that woderful, but it should be reasonable. You would need to reduce
the resistance values and experiment with the inverting input to get it
right, but an outboard circuit is probably easier.

Cheers, Rod

I've got the feeling that the road to a balanced p68 isn't such a good choice...