Bridge-Parallel with LM3875

[Will]

The national application notes described the use of LM3886 in this schematic. But I have a handful of 3875 instead, so will it be ok to use them instead? I'd like to make an amp ala "Jeff Rowland". Inputs appreciated....

regards,
will.

[tlmadsen]

HI Will

Just go ahead with your LM3875, you just get a little less output compared to the LM3886.

Have a look at

200 watts Gäjnklon (Gainclone) - BPA200


and my replay in

Can you tell me how to beef up PA100?

Have fun

Thomas

[Will]

Thanks Thomas.

The other intriguing thing I have been thinking is, does it make sense to reduce the gain - i.e. by lowering the 20.5K resistor to , say about 15K ? Will it sound better to have a lower gain amp (trying to emulate a tube amp).. So essentially this is a high power low gain amp.

regards,
will.

[tlmadsen]

Hi Will

In short; No. You will then have to get the "gain" from your pre-amp to be able to drive your speakers. If I remember right the LM38xx series of chip amps are only stable at some amount of gain. Check the datasheet.

The only really quick "fix" that I can think of, that would "emulate" a tube amp is to put a reletively large (several ohms) power-resistor at the output of your amp. (I will now be crucifyed by all the tube-people )

Have fun

Thomas

[arniel]

Please excuse my ignorance on this subject, but for op-amps in general, where they are not stable at unity gain... does the cap in the feedback path (designed to reduce gain to unity at DC) cause stability problems?
If not, at what frequency does the feedback start to cause instability?

And will a DC servo remove the need for any capacitors in the signal path, from the beginning of an input buffer IC to the speaker terminals?

Cheers,

Arnie

[peranders]

Originally posted by arniel
Please excuse my ignorance on this subject, ...

Your ignorance is excused

Some opamps/power opamps are designed for gain of 1 or higher, some are gain or 5 or higher, some for gain 10 or higher. This is clearly stated in the datasheet. Depending of your application you must choose the type which is right for you.

Question 2: yes.
http://www.national.com/an/AN/AN-1192.pdf
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf

After Ohm's law , this is basic knowledge, how to read Bode plots and understand how much feedback you can apply.

You could pick up some good book and a very good one is for free.

Opamps for everyone. Warning: Very thick book, 464 pages!.
http://www-s.ti.com/sc/psheets/slod006b/slod006b.pdf

[Will]

Where's that note that says the unity gain stablility of LM3875 and LM3886 in the application notes ?

I'd assume LM3886 is unity gain stable and LM3875 not, cos' the bridge-parallel circuit is designed for 3886 where the input opamp U5 is a 3886 and running at unity gain.

regards,
will.

[peranders]

Originally posted by Will
Where's that note that says the unity gain stablility of LM3875 and LM3886 in the application notes ?

Neither of those IC's are unity gain stable.

The datasheets does tell this only in a diagram.

Please look at page 10 and "open loop frequency response", the last one on this page.

Draw a vertical line at 135 deg phase response. When you cross the gain line, draw a horisontal line. This gives you gain 10 dB, at 1.8 MHz

If you have gain more than 20 dB you have a very clean phase curve, near -90 deg which is ideal.

[officeboy]

Description;
LF412ACN, Dual JFET Input Op Amp

Designator;
U5, U6

Manufacturer’s Part Number;
NSC, LF412ACN

Page 16
AN-1192.pdf

[cdoggy81]

:tu: