Mofo

[simon]

Sorry Phil, I really didn't get that.

Okay, so does the Chinese preamp alone give you enough gain?

[Nick]

You need to measure the gain of the two preamps when set in the "center" position you use them in. And while you are at it, measure the gain of each preamp when set to 100%, that would give actual information to allow you to work out what gain you actually need.

What loudspeakers are you using? I would have thought with a 2v source, the most you could use would be 20dB (*10) of gain before the mofo hits the stops.

[Nick]

The comments about the cap and split rails interested me, being a SS newb. If I use the existing 19V LED SMPS I'm using for the MoFos can I just apply 19V to the V+ pin and 0V to the V- pin? (Plus the local decoupling caps.) Or do I need something further?

Yes, but you will need the blocking caps and use the op amp set up for single rail use.

https://mil.ufl.edu/4924/docs/TI_SingleSupply_OpAmp.pdf

[ed]

Simon, my first attempt..single supply....fwiw I much preferred the dual power supply

mofo proto.jpg (96.75 KiB) Viewed 2992 times

[simon]

Ah, yes I see that now. Dual rail supply has it's advantages.

With a dual rail supply Cin isn't necessary then? But perhaps makes some sense to block any DC on the input?

R1 and R5 set the gain to 2x. What's the purpose of C1 and C3? Decoupling/noise suppression? How are they sized? I haven't seen them shown on the datasheets I've looked at.

[Nick]

C1 allows the feedback loop to float at half rail voltage while still providing the AC path to allow the feedback network to set the gain. C3 is I assume about making sure the amp is stable by reducing gain at HF.

[simon]

Right so C1 forms a divider with C2 and isn't required with a split rail supply?

[simon]

The three legged zener X1 was a new one on me but looking at the datasheet I can see it's not really a zener though it can be used as one. It looks like it's being used as a shunt reg, which presumably isn't as important with a dual rail supply? A well filtered CRC type supply should be adequate here?

[Nick]

Well, from my reading the shunt reg is just there to set a value for the sort of half vcc voltage. Not sure if its any better or not than a PD with filtering, but they are simple sources of low noise voltages. Nor sure if the pot is needed to set the voltage or not, fixed resistor would be ok. But if you use a split rail supply all that goes away.

[simon]

Makes sense.

Well, whadaya know? I bought an extra OPA2134 when I was making the active crossovers in case of magic smoke type incidents.

[simon]

So something like this for a simple 2x gain?

20221103_174705.jpg (121.67 KiB) Viewed 2949 times

The 100R resistor on the output is because Rod Elliott shows it in his buffers - fig 3 about half way down this page
https://sound-au.com/project09.htm
I presume it's for stability on the output or something similar?

[Nick]

Yep

[simon]

How does the dual rail PS become dual rail? Does the PS gnd need tieing to the -ve IN? Otherwise it's just a single rail supply as far as I can see.

I've only shown the decoupling caps on V+ as shown in the LME49720 datasheet, but that shows what looks like a single rail supply I think. Do I need decoupling caps on V- too?

Screenshot_20221103-175543_Adobe Acrobat.jpg (173.91 KiB) Viewed 2946 times

[simon]

BTW this MoFo seems to take a good hour to warm up and then becomes much nicer. Anyone else noticed this?

[Ray P]

Here are my two approaches to pre-amps for a MoFo like use-case (actually, https://www.audio-talk.co.uk/phpBB3/vie ... php?t=8189). Both will give circa 20dB of gain.

https://www.audio-talk.co.uk/phpBB3/vie ... php?t=8496

https://www.audio-talk.co.uk/phpBB3/vie ... php?t=8147

Should it be of interest to anyone, I also have the 'gyrator' boards and some other parts, to build this 3A5 DHT pre-amp - similar to the 2P29L preamp above and also giving around 20dB of gain. I'll never get around to building it so happy to trade or sell?

https://www.bartola.co.uk/valves/2016/0 ... #more-5565