Crest Audio CPX 1500 Bias

[coper]

Hi,

Does anyone know the value of the Bias adjustment for the CPX 1500 ?
I already have the schematic, but there is no info about the bias adjustment.

Thanks for any help.

 

[sajti]

Hi,

Does anyone know the value of the Bias adjustment for the CPX 1500 ?
I already have the schematic, but there is no info about the bias adjustment.

Thanks for any help.

I have no service manual for this amplifier, but the PRO series manual says something about 5-7mV on the 0R33 emitter resistor. I would set something like this.

Sajti

 

[JonSnell Electronic]

I would think that 5 - 7mV is correct. Ensure that all resistors have at least 2mV across them to avoid crossover distortion.

 

[coper]

Hi,

But 7mv only gives about 21ma of idle current! Not a bit to low for these kind of transistors ?

Thanks.

 

[JonSnell Electronic]

It runs in class B which is just on.

 

[sajti]

Hi,

But 7mv only gives about 21ma of idle current! Not a bit to low for these kind of transistors ?

Thanks.

Yes it's low, however this amplifier not highend type. Some manufaturer using even lower current.

Sajti

 

[coper]

Thanks all for the useful info.

Best regards.

 

[nigelwright7557]

Hi,

But 7mv only gives about 21ma of idle current! Not a bit to low for these kind of transistors ?

Thanks.

I design my own amps and didn't have a spec for setting up bias so I devised my own procedure.
Turn bias right down. Apply a 1KHz sine wave to input. Monitor output on scope. Increase bias until crossover distortion just goes.
Its often as low as 10mA.
Done....

 

[Jakers Lloydi]

Copy of schematic

Hi,

Does anyone know the value of the Bias adjustment for the CPX 1500 ?
I already have the schematic, but there is no info about the bias adjustment.

Thanks for any help.

Hello, will it be possible to lend me a copy 🙂) I have same unit that need attention. No transformer and heat sink but there's an available online so, if i got a copy of schematic there's a chance to restore it. Owe you that if you could. Many thanks. 🙂)

 

[sgrossklass]

Just a note on this:

I design my own amps and didn't have a spec for setting up bias so I devised my own procedure.
Turn bias right down. Apply a 1KHz sine wave to input. Monitor output on scope. Increase bias until crossover distortion just goes.
Its often as low as 10mA.
Done....

IMHO this would tend to give overly optimistic results - 1 kHz is easy. I'd rather be using 10 or 20 kHz at least. Depends on what your fidelity requirements are, of course - and how slow your output transistors are to begin with. I'd try to stay near the optimum of about 13 mV per emitter resistor for an EF output stage if possible (cooling, SOA etc. permitting).

 

[coper]

CPX 1500 Schematic

Hello, will it be possible to lend me a copy 🙂) I have same unit that need attention. No transformer and heat sink but there's an available online so, if i got a copy of schematic there's a chance to restore it. Owe you that if you could. Many thanks. 🙂)

Hi, I only have this. Best regards.