Reducing gain on a minimalist OPA541 gainclone

[Alcaid]

I'm going to use the schematic under to build an OPA541 gainclone, and I wondered how to lower the gain to 10 (Because of the low bandwith of the opa541). What is the best combination concerning DC offset and sound quality:

1) 1K (-in to GND) and 10K (feedback),
2) 2K (-in to GND) and (20K feedback),
3) or any other solution?

Do I have to use another resistor (+in to GND) than the 22k used in the schematic?

[Mad_K]

I would use 1K and 10K for all resistors (or 2K/20K or some other combination if you require higher input Z). 1K in series with the input and 10K to gnd; 10K feedback and 1K to gnd. I've tried it and I had about 1mV DC-offset

[Matttcattt]

what would the safe/sensible minimum and maximum value for the gain be? most people use 22, and some use 10.

[macboy]

Most of National's chip amps are stable only for gains of 10 or higher, so that is the minimum. The maximum depends largely on the gain-bandwidth product. That limits the bandwidth at any specific gain. For the LM3875 and LM3886, this is typcially 8 MHz, but can be as low as 2 MHz. That means that at a gain of 20, the worst-case bandwidth (-3 dB) is 2 MHz / 20 = 100 kHz, but it is typically 8 MHz / 20 = 400 kHz. I would suggest that you try to keep the -3 dB bandwidth to at least 100 kHz in order to avoid any rolloff effects in the upper part of the audible range. A gain of 10 to 25 is probably a good range to stay within.

[Mad_K]

Originally posted by Matttcattt
what would the safe/sensible minimum and maximum value for the gain be? most people use 22, and some use 10.

I wouldn't go above 22x. I have run opa541's stable at 2,4x at work...

[Shoog]

All the OPA chips are supposedly unity gain stable, but I haven't tested that. I would definately run them at lower Gains than the LM chips because their open loop gain is much lower.

Shoog

[Alcaid]

Originally posted by Mad_K
I would use 1K and 10K for all resistors (or 2K/20K or some other combination if you require higher input Z). 1K in series with the input and 10K to gnd; 10K feedback and 1K to gnd. I've tried it and I had about 1mV DC-offset

If I use the 1k/10k combo, What pot do you suggest? Is 50k log ok?

[metal]

Hello guys

I have used OPA541 and OPA549 after I read the datasheets thoroughly,and noticed that the full output voltage swing is equal in both directions,the -ve and +ve for the OPA541 .In contrary to OPA549,which suffers from inequal full output voltage swing in the -ve and +ve direction

Another thing to mention is that I used 24-0-24 volt 5 Amp transformer for OP541,and it worked very fine with 4 Ohm speaker,when I connected the OP549 to this supply,it shut it self down.This means that I had to use 18-0-18 volt transformer,and I did and connected 4 Ohm speaker also,but the output power was not as high as OPA541,and using a lower impedance speaker in not a practical solution because the distortion will increase,even connecting two 4 Ohm speakers in parallel so as to obtain more power won't be helpful as this will draw more current.

Not forgetting that the protection circuit is much better in OPA549 as it senses the load indirectly.In contrary to OPA541 which uses a power resistor for protection purposes.But I had to consider the +/- 40 volt of the OPA541 and forget about the OPA549 option.

I used 1 K Ohm to ground and 15 K Ohm as the feedback resistor,the input resistor is 15 K Ohm also.

[Alcaid]

I thought you said you read the datasheets thoroughly.
The OPA541AP is max +-35V and the OPA549T (and S) is only max +-30V. No wonder it shut down with your dual 24V supply.

[till]

I have no problems of that kind with an OPA549 amplifier at +-20V.

Post you excat schematic to see whats the mistake.