[Tomo]

Hi,

I am thinking about launching another adventure. This time with some power opamps like LM1875 or LM675. I believe if driving at low power, these chips will operate at Class-A.

The subject will only drive low impedance headphones. Thus, the gain can be very low.

(Input offset current is large, but I can connect something like 1kohm bypass. So I will forget about that.)

The problem with these opamps is that they are not unity gain stable. And, with some gain, these can blow the heck out of headphones. I found this as alternative to Multi-Loop.

Tomo

P.S. Can you post the Multi-Loop Schematics? I could use one for the record. Thanks.

 

[Tomo]

Anyways,

I was wondering if I can add a resistor to ground to establish some gain. I think the RC filter that connects two inputs prevents any signal beyond 500kHz (in this case).

Am I correct?

T

 

[Budgie]

Multiloop schematic-
http://www.elecdesign.com/Globals/Pl...tent/1518.html

I started out using LM1875's for a headphone amp, adjusted for minimum recommended gain and minimum power supply voltage. (don't remember what gain/voltage I used) It worked reasonably well, but still had a bit too much gain for low impedance/efficient phones. I had not found Headwise or Head-Fi at that time, so I had no idea that the chips could be forced into class A operation. It sounds like a very interesting project and I hope you go through with it. I would be interesting.

I still use a class A amp (Sekeres) with the current source you suggested, but with a split supply to eliminate the output cap. Sounds great!
Thanks for that current source!

I wonder if that current source would be a good way to pull an LM1875 into class A?

 

[Tomo]

Hi,

Supposedly pulling to class-A might not be necessary. According to the GC bunch, the chip might well be operating in Class-A at ear splitting volume of Gradoes ... (100~300mV).

I intend to use +-15V~+-20V unregulated powersupply which should induce 10-20W dissipation. This should give me 50-100mA supply current. It will be nice and warm like Szekeres amp.

I will reduce the gain to something less than 3. Note also that DC offset will become critical factor in safty. I believe, however, by reducing bypassing resistance to 10k or even 1k should reduce the offset to manageable level.

Tomo