LM833-N NAB Pre-amp Circuit

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,

Apologies in advance if this has been covered elsewhere. I couldn't find the info I'm looking for, so I figured I'd just go ahead and ask. I'm new to this forum though, so I hope I'm not violating etiquette here.

I have what I think may be one of those simple questions that turns out to not be all that simple. I'm building a small instrument that incorporates a variable speed portable cassette player with a tape loop — changing the speed of the motor changes the pitch of the loop. It's been done before. It occured to me that rather than using the cassette player's pre-amp, it might be worth building my own and connecting it directly to the play head in order to streamline the overall build. I have some experience building circuits, but I'm admittedly a beginner (so apologies if I sound like an idiot), and I've been made to understand that tape pre-amps in particular can be pretty fussy.

With that said, I've been snooping around for a relatively simple NAB (I think that's what I need?) tape pre-amp, and there's one on the LM833n datasheet that seems pretty straightforward. (The datasheet is here: https://www.ti.com/lit/ds/symlink/lm833-n.pdf and I've included an image of the schematic). I also happen to have a couple of these ICs lying around. So I'm wondering if anyone has had any luck using this circuit for a similar application (I couldn't find any documentation online of anyone building it) OR if someone who knows more about electronics can eyeball it and tell me if it seems like it would work.

Alternatively, are there other relatively simple, OK-sounding tape pre-amp circuits that sound that I should look into? Or would I be better off using the factory pre that is built into the tape player? I suppose that might be the case, but I can't help but think it would be a cooler final product with a customized pre-amp circuitry.

Thoughts? I'll be grateful for any advice anyone is willing to toss my way.
 

Attachments

  • NAB.jpg
    NAB.jpg
    79 KB · Views: 260
Administrator
Joined 2007
Paid Member
Welcome to diyAudio :)

These are text book circuits and should work well. Remember the opamp needs a split supply (plus and minus rails... at least as it is drawn, you could make it single rail though).

The tape head itself is DC coupled to the opamp input and that is needed to provide a DC reference point for the opamp.
 
Thank you!! I'm learning more and more each day — but with every new piece of info comes a million new questions. This is quickly turning into an obsession lol.

OK — I'm going to give it a shot. I guess I just needed a little affirmation that it wouldn't be a total waste of time.

As for the split supply — I figured I'd cross that bridge when I got to it, as I know there's info out there about this. It seems like the supply voltage cannot exceed more than ±15v, so could I just supply +12v and leave GND at 0v? Or is it going to need more juice than that? (Sorry, this seems like an extremely stupid question too — I won't be offended if you tell me to go Google it!)
 
Administrator
Joined 2007
Paid Member
It is not the so much the voltages that matter but the fact that there is a 'zero volt' ground reference (the 0V) and then a plus and minus supply.

If we used that circuit as drawn on a single rail supply then the opamp would not function because the input (the tape head) would be creating a zero voltage condition on the opamp input.

The opamp (for single rail) needs to be biased to to around half the supply voltage so that the output can swing equally up and down.

So a 12 volt supply would see the output pin settle at 6 volts DC and with the ability to go higher or lower from that point according to the signal.

To do this the input needs to be artificially biased to 6 volts DC and the tape head could then be AC coupled to the opamp.

The opamp output would also need to be AC coupled to remove the 6 volts DC.

(Exactly the same scenario exists if the circuit were just discrete transistors)
 
Administrator
Joined 2007
Paid Member
I just happen to have a simulation of a very similar circuit to yours and for single rail.

These are your values plugged into it. You can see the response and gain and also the DC voltages on the chip.

R4 and R5 generate a basic midpoint reference and R3 applies that to the chip. That area would need to be a bit cleaner for your use with some decoupling.

The signal is AC coupled as is the output.
 

Attachments

  • SR1.jpg
    SR1.jpg
    237.9 KB · Views: 206
  • SR2.jpg
    SR2.jpg
    354.4 KB · Views: 201
Wow — thank you! That is insanely helpful. All of this more or less makes sense to me, and that schematic is so useful. I guess maybe I'm a visual learner. With this and a little bit of research, I think I'll be in pretty great shape. Now just waiting for the workday to end so I can go try it. Thank you again!!
 
Account Closed
Joined 2010
You can try the pioneer ct-777 circuit first for the lowest noise possible if the LM833 can take it(2kohm at high frequency shouldn't be a probem though), if not you can use it with op2227 or op2228 which will be even lower noise .anyway the tape noise will be much higher
 

Attachments

  • pioneerct777.png
    pioneerct777.png
    66.2 KB · Views: 181
Administrator
Joined 2007
Paid Member
Wow — thank you! That is insanely helpful. All of this more or less makes sense to me, and that schematic is so useful. I guess maybe I'm a visual learner. With this and a little bit of research, I think I'll be in pretty great shape. Now just waiting for the workday to end so I can go try it. Thank you again!!

You're welcome :)

That midpoint reference voltage needs to be very clean for a front end preamp and so you have options there. You could for example use a decoupled Zener diode reference. It doesn't have to be exactly 6 volts (for a 12 volt rail). If the midpoint is off by some amount the only difference it makes is that the opamp output is 'off' by the same amount.

So a 9 volt reference with a 12 volt rail would still work but would mean the circuit clipped asymmetrically... the opamp output would only be able to swing 3 volts positive but 9 volts negative.

With a good noise free reference that was well decoupled you would then set R3 (in my example) to give the input impedance you wanted.

Have fun, I'll look in tomorrow :)
 
Account Closed
Joined 2010
The opamp (for single rail) needs to be biased to to around half the supply voltage so that the output can swing equally up and down.

So a 12 volt supply would see the output pin settle at 6 volts DC and with the ability to go higher or lower from that point according to the signal.

To do this the input needs to be artificially biased to 6 volts DC and the tape head could then be AC coupled to the opamp.

The opamp output would also need to be AC coupled to remove the 6 volts DC.

(Exactly the same scenario exists if the circuit were just discrete transistors)
he could go the Nakamichi BX-300 way...
 

Attachments

  • nakbx300.png
    nakbx300.png
    265.9 KB · Views: 202
  • nakbx300a.png
    nakbx300a.png
    215.2 KB · Views: 147
Administrator
Joined 2007
Paid Member
The two opamps show a way of constructing a low impedance 6 volt supply... the reasons why Nakamichi do this seems to be related to the use of the NE562 chip as far as I can tell.

Split supplies are already available on that diagram.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.