OPA541 Gain and schematic

[Rulek]

Hello.

I'm preparing myself to build a OPA541AP Gainclone.

I found a schematic that you can find on the picture.

I would like to drive OPA with tube stage (I'll try some 6sn7, 6N6P, e88cc, ecc99) I would like a tube stage to gain about 20x (I have a glassware software so it's not that difficult : )

1. What should be a gain of opa for it to be reasonable?

2. I would like to eliminate I-limitation resistor, should I expect any issues with that?

3. How to calculate resistors value for OPA I would like to understand that not only apply a resistor?

4. On attached schematic there is a V- and V+ connected to the output. Will it work?

5. What is the gain of this values in schematic?

6. I have a 2x 300W toroids with 2x22V secondary voltage and that should give about 2x31V DC. I have OPA541AP so maximum voltage is +- 35 but how will the amplifier work with 4 Ohm load. As far as I know lower voltage is better for lower load. If so can someone explain why?

Thank you very much for all the answers.

[AndrewT]

The maximum output from the chipamp is ~28Vpk.
With a chipamp gain of 10times (+20dB) it requires a maximum of 2.8Vpk to drive the chipamp to clipping.

Now add in a preamp with 20times gain (+26dB) and the preamp can accept a signal of <=140mVpk to send the chipamp to near maximum output.

Do you have sources that need all that gain, i.e. from 140mVpk to 28Vpk?
100mVpk ~ 100mVrms = 100mVac

[jackinnj]

Full load current flows through the current sense resistor.

[Rulek]

Thanks for answers : )

I would like to build a system of Tube output dac like CS4397 with a SRPP and than have a passive resistor ladder and next stage would be this OPA+Tube Pre. I'm planning to use it with a 20cm fullrange + 15 bass in sealed enclosure.

So I have a 4 Ohm load, but despite of 95 dB of the system I would like to have a universal amplifier that can drive whatever I'll figure out in the future.

Can I do a attenuation of preamp with a resistors or something?
I will for sure have a output higher than 140mV from a Tube load dac : )

God I need to buy an oscilloscope : )

[AndrewT]

build a buffer instead of a high gain pre-amp. Valves can be made into good buffers.

If a few sources need extra gain, then have a switchable or add-on gain stage for those sources only. Switchable from +0dB to +6dB and +12dB, covers most users needs.
That give an input sensitivity of 2.8Vpk, 1.4Vpk and 0.7Vpk for 28Vpk output.
In rms terms that is normal sensitivity of 2Vac and with gain, either 1Vac or 500mVac.
Into 4ohm speaker load, the sensitivity is even higher by slightly more than sqrt(2) or about 3dB to 4dB. i.e.normal = 1.2Vac to 1.4Vac

[Rulek]

Thanks for advices. I still have a question about the schematic. Can I short the V+ and V- to output pin like on schematic. That seems a little bit odd for me.

On the application note there are 2 diodes in there.

Are there any disadvantages of full load current flowing through the current sense resistor?

[AndrewT]

Those diodes do not short across the supply pins. Note they both point "up" towards +ve. They are reverse biased.

They are there to protect the output stage of the chipamp from back emf if the amp suddenly limits for any reason. Without the diodes the amplifier can limit to protect itself and the back emf from the load can then destroy the output stage due to excess reverse voltage. The diodes limit that reverse voltage to supply rail+Diode Vdrop.

What current sense resistor? Is it referred to in the datasheet?

[Rulek]

Yes 8-th leg of the chip is "current sense" leg. there is this resistor between the diodes from the output to current sense. Putting this resistor makes the limitation of current. with R=0,1 ohm Imax=5A.

My question is if there will be any danger for a OPA or speakers if I don't use it and instead of that have "unlimited" current?

Andrew T thanks for your patience : )