Very Simple Single Stage Op Amp Preamp

Karl71 · 2017-03-27 1:34 am

I've been a very occasional member, but now I need some help...

I'm driving a Class D Audio Mini amp with a Mass Audio Relay Bluetooth receiver. I run out of gas a little, the input sensitivity of the amp being 1V. I think I need no more than an extra 3dB to drive the Mini satisfactorily.

I like simplicity, so a very quite and low distortion single op amp stage seems in order. I've seen a number of references to the idea of doing such, but finding a schematic has been impossible. Stunningly, app notes have been useless. I have read of the Cmoy headphone amp used for this purpose. Anything else out there? Battery powered, please. The Mini is on an SLA, and I love the quiet!

Thanks very much.

Karl

scottjoplin · 2017-03-27 1:59 am

Hi. You could use one gain section of this preamp to give you 6dB or you can reduce the gain if necessary High Quality Audio Preamp

Karl71 · 2017-03-27 2:10 am

Thanks very much. I've looked at that one, and I've considered using the second stage alone since it already has output caps.

scottjoplin · 2017-03-27 2:20 am

Yep, nothing much to it, just standard non-inverting op amp circuit, does the job perfectly well particularly using the LM4562. I suggest you use the input capacitor coupled circuit of the first stage circuit with the second stage circuit

Karl71 · 2017-03-27 2:39 am

Would the gain of each stage add, being 12 dB, because I would only use the volume control on my device? Wireless device -->Bluetooth-->amp. There would be no insertion loss in this case since I would not use any controls between the stages.

I do want something much simpler. 3dB is probably it. I don't want to throw away gain, and it doesn't make sense. What makes even less sense is the dearth of schematics For what I need. Seacrch terms?

I will stick with full range drivers and leave AF design to others!

Griping aside, I do appreciate your help!

scottjoplin · 2017-03-27 2:48 am

I ment just use one gain stage but with the capacitor and resistor network of fig2 on the input of the circuit in fig 4

Karl71 · 2017-03-27 3:06 am

Ah, got it!

I don't understand why this information isn't available online. Where is it in app notes for devices? Too rudimentary? Where do I go?

scottjoplin · 2017-03-27 3:17 am

I know what you mean, just had a quick scout around, google images is usually useful but you do have to have an idea what you are looking for looks like, which kinda defeats the object ha! It is a basic op amp circuit, so if you just read up about op amps you'd find it easy to design your own circuit. In fact that ESP site you've just looked at is very good. This is a link to the first section on op amps, but if you go to main index and then articles there is a lot of information Audio Designs With Opamps

sgrossklass · 2017-03-27 4:24 am

It does get a bit trickier because when running stuff on a 12V battery, one needs the single supply version of any such circuit. The opamp should also perform well within the constraints of the supply available, TL07x may not be first choice here.

Running multiple things on a single battery also screams ground loop, grounding would have to accomodate for that.

Karl71 · 2017-03-27 4:28 am

I would use a separate SLA battery.

To power the op amps, I would use the method Chu Moy implemented for his cmoy amp. Look it up; I don't know what it's called!

blakkshepeaudio · 2017-03-27 6:12 am

If you are not too sure about what to do, you could try a couple of these or similar. Gain is adjustable and runs happily off 12V.
I built a couple into a box with some batteries and a pot for up to 20dB for general purpose use. Sounds good.

edit - new link

http://www.ebay.com.au/itm/2PCS-LM3...nal-ModuleT-/301924581974?hash=item464c1b8256

georgehifi · 2017-03-27 7:39 am

scottjoplin said:
Hi. You could use one gain section of this preamp to give you 6dB or you can reduce the gain if necessary High Quality Audio Preamp

If you use a OPA134 for each channel, you can utilize the dc offset trim pins 1 and 8 and have it direct coupled, no output cap, wouldn't that be lovely. Best cap is no cap.

Cheers George

AndrewT · 2017-03-27 11:12 am

Karl71 said:
.................
I'm driving a Class D Audio Mini amp with a Mass Audio Relay Bluetooth receiver. I run out of gas a little, the input sensitivity of the amp being 1V. I think I need no more than an extra 3dB to drive the Mini satisfactorily.
.............

Karl71 said:
Would the gain of each stage add, being 12 dB, because I would only use the volume control on my device? Wireless device -->Bluetooth-->amp. There would be no insertion loss in this case since I would not use any controls between the stages.

I do want something much simpler. 3dB is probably it. I don't want to throw away gain, and it doesn't make sense..............

+3dB is just an extra gain of 1.4times.
If you already notice that the music/audio is too quiet then i don't think that adding a gain of 1.4times is going to make you happier.
For your low level sources I suggest you have a switchable +6dB and +12dB gain stage.
But only apply it to your low level sources. Save the direct input for all the normal source levels.
Many (particularly digital) do between 1500mVac and 2400mVac.

Altecboy · 2017-03-28 11:12 pm

a really simple board

If you look up:

Dual OP Amp Preamp DC Amplification Empty Board for NE5532,OPA2134,OPA2604,AD826

on ebay, you will find a board which is as simple as you can get.

Karl71 · 2017-03-30 2:56 am

I'm ordering the parts for this:

[wiki=http://www.diyaudio.com/forums/analog-line-level/268772-looking-cheap-simple-opamp-gain-stage.html]%[/wiki].

Too much gain, but I'll learn a lot from it. I'll play with the gain to get it where I need it for my purpose.

Thanks to everyone for help! I'll report back.

russc · 2017-04-09 2:05 pm

ref - Dual OP Amp Preamp DC Amplification Empty Board for NE5532,OPA2134,OPA2604,AD826
Thought this might be ideal for my next project.
However, should it have space for supply bypass caps? I can see no provision for this.

Pirmin · 2017-04-09 2:16 pm

Can you make this 0 gain/unity?
Dual OP Amp Preamp DC Amplification Empty Board for NE5532,OPA2134,OPA2604,AD826 | eBay

scottjoplin · 2017-04-09 2:20 pm

A buffer? Yes, connect the output to the inverting input

Pirmin · 2017-04-09 2:27 pm

So if I use NE5532, this board can be used as a buffer Simply by adding a short between pin1 and pin?

russc · 2017-04-09 2:37 pm

On Ebay, search for "NE5532 board", you will find umpteen different boards.
Many completely populated. With/without power supply, with/without supply decoupling caps.
Just this morning I was planning on stripboard or even my first attempt at PCB fabrication. Now I won't bother - cheaper to buy a populated board even if intending to swap out parts.
Unity gain - pin 1 to 2 and pin 7 to 6 for the other channel.
Just take care when selecting boards whether you are intending to use dual or single op-amps. NE5532 is dual.

Very Simple Single Stage Op Amp Preamp

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