simple preamp and mixer for basement recording

[blandman74]

Although this was really an excuse to try my hand at some solid-state design, ostensibly I built this preamp-and-mixer combo for a friend who needs to record synth/drum machine, live drums using one or two overheads, and an instrument input, all into a laptop's Line input. So, um, not exactly high fidelity. I decided to record L and R channels as mono channels 1 and 2. Since he owns nothing but dynamic mics :roll: I left out phantom power. This is what I came up with (and it works!).

Note that the synth/drum machine parts are run through a guitar amp blasting toward the drummer (so he can hear those parts while he plays); I thought putting a polarity switch into line input might help me try to cancel out that amp, assuming mics are placed properly, and get a slightly cleaner drum sound.

I'd like suggestions on where I went wrong ... not so much to rebuild but just to learn where I could've done better.

Haven't yet tried it out with live drums - I'll be heading to NYC (where the friend happens to reside) to try it out this weekend.

 

[pstamler]

There are quite a few spots here that need tweaking.

First off, the two mic inputs will, if the polarity switches are set identically, have opposite polarity at the outputs, due to the inverting summing amplifier in channel 2. You can fix that, of course, by wiring one of the polarity switched opposite from the other. The instrument input will have the same problem, with no polarity switch to fix it.

The outputs are unbuffered 10k pots, feeding what will probably be a 10k input on the laptop. That's not a problem for a 5532, but it will load the pot in such as way that the taper will be screwy. It also requires you to use short cables to the laptop, so as not to roll off the high frequencies with cable capacitance.

The instrument input impedance is 100k; this is fine for synths and the like, but low for something like a passive electric guitar or bass. Ideally this should be 1 meg. But even with 100k, you're going to have a voltage-offset problem if the IC on the instrument input is a 5532. With an input bias voltage of 0.5uA, you'd get about 50mV of offset voltage on the input of the IC, or 150mV in low-gain position, over 600mV in high-gain (I'm too pooped to go drag out the calculator for more accurate numbers, but these are in the ballpark). More if your 5532 happens to have higher bias voltage. Bad news with no coupling capacitor to the pot.

One of the 5532s in channel 3 has 1k running from its output to ground for no apparent reason. This will unduly load that IC, and increase the distortion.

The 10uF tantalum coupling caps are a bad idea; there's no polarizing voltage on them. Better you should have some non-polarized electrolytics, and bigger if you can fit them in.

The ceramic disc coupling caps are also a bad idea; they'd be much better as film caps or non-polarized electrolytics.

Finally, I'd suggest using better regulators than the fixed-15V units you've included. At the very least, something like LM317/337 chips will provide better regulation in nearly all parameters, and 5532s really like better regulation.

Peace,
Paul

 

[blandman74]

Thanks for that! Actually, the unmarked opamps are from a TLE2074. Shouldn’t have any input offset voltage problems ....

Luckily I had anticipated your point about the instrument input impedance – when I put it together I used 1 Meg resistor at pin 10 of the ‘074, using that same reasoning. I just forgot to correct that in the schematic.

The outputs are unbuffered 10k pots, feeding what will probably be a 10k input on the laptop. That's not a problem for a 5532, but it will load the pot in such as way that the taper will be screwy. It also requires you to use short cables to the laptop, so as not to roll off the high frequencies with cable capacitance.

I guess you’re saying I should use a lower value, like 5k? Both a 5532 and ‘074 should be able to handle loads as low as 3.3k, right?

One of the 5532s in channel 3 has 1k running from its output to ground for no apparent reason. This will unduly load that IC, and increase the distortion.

Which one? There’s no channel 3. You mean on pin 7 of the NE5532 (next to the 100p bypass cap)?

The ceramic disc coupling caps are also a bad idea; they'd be much better as film caps or non-polarized electrolytics.

Is this a noise issue? If so, then I probably will leave them, since this thing is surprisingly quiet (way under the noise floor for laptop anyway).

 

[bcarso]

Is the T1 center tap actually floating?

 

[blandman74]

No, you're right, it's grounded.

I have edited the scheme to fix the errors, FWIW.

 

[Samuel Groner]

Which one?

Well, three of them... As you don't show all designators (any reason for that?) it's somewhat hard to help, but those of the line in (probably channel 3 from Paul) and the second mic pre channel are the ones.

Samuel

 

[Nishmaster]

You may want to add a buffer stage after the pots to reduce taper weirdness. The taper will also change depending on what the input impedance of the device you plug into is, which might not be very fun.

Also the 2.2u ceramic coupling in channel 2 aren't really the best choice, as previously noted. A better bet would be a high quality film electrolytic of a higher value as well. I'd ditch the tants also and use lytics.

3 of your 5532s in channel 2 have 1k going to ground. Any reason for that?

Also, as Paul said, a coupling cap is going to be needed before the instrument gain pot or the pot is going to be seeing a lot of DC.

I'm in a pre-coffee state (so I may be missing something fairly obvious), but it looks like you have the two opamps in the line input section offering different gains. Is your gain changing depending on whether the polarity switch is up or down?

Like I said though, I've got about 7 or 8 neurons firing, so take all the aforementioned with the appropriate salty grains.

 

[pstamler]

[quote author="blandman74"]

The outputs are unbuffered 10k pots, feeding what will probably be a 10k input on the laptop. That's not a problem for a 5532, but it will load the pot in such as way that the taper will be screwy. It also requires you to use short cables to the laptop, so as not to roll off the high frequencies with cable capacitance.

I guess you’re saying I should use a lower value, like 5k? Both a 5532 and ‘074 should be able to handle loads as low as 3.3k, right?[/quote]

A 5532 will; an 074 will really be happier with loads of 10k and up. But you'd be way better off buffering the outputs of the pots with something like a 2604 (Fet input, so it won't run current through the pot, and strong output stage).

One of the 5532s in channel 3 has 1k running from its output to ground for no apparent reason. This will unduly load that IC, and increase the distortion.

Which one? There’s no channel 3. You mean on pin 7 of the NE5532 (next to the 100p bypass cap)?

Yep.

The ceramic disc coupling caps are also a bad idea; they'd be much better as film caps or non-polarized electrolytics.

Is this a noise issue? If so, then I probably will leave them, since this thing is surprisingly quiet (way under the noise floor for laptop anyway).

No, this is a bad-sound issue and a microphonics issue. There are a few ceramics which are reputed to be decent (NP0, C0G) but they don't come in 2uF size anyway.

There's still the problem of Mic channel 2 being inverting and Mic channel 1 being not.

Peace,
Paul