Engineers Needed! How do you reduce THD in audio, how to get more gain, and more

[ski]

EDIT: If you think you would be able to help me tonight, PM me, I will pay your for your tutoring!

I'm down to the wire on this project, and I'm hoping that some of our resident experts can help me with this :cheers:

I posted a few weeks ago about the preamp for the tape head I'm working on. I solved for all of my resistors and capacitor to get the right frequency response, and below is the theoretical frequency response, followed by what I actually get:
http://www.littleboyinc.com/uploader/uploads/graph3.JPG
http://www.littleboyinc.com/uploader/uploads/graph1.JPG

Here is the schematic of the circuit:

http://www.littleboyinc.com/uploader/uploads/preamp2.JPG

And the preamp WITH the Class AB power amp. I need 400mA at the output to drive the 8 Ohm speaker so I can get at least 1 Watt... I have 1.4 W at the moment, so it's all good.

http://www.littleboyinc.com/uploader/uploads/ampsmall.JPG

Link to the enlarged version of the whole circuit (http://www.littleboyinc.com/uploader/uploads/ampbig.JPG)

Questions (if you can help me with even one, I will be very grateful!)

1. I'm only getting 30 dB relative difference on my preamp at 50Hz and 2kHz. I have tried different values for R1, R2, R3, and C1. Is there something I can add to get better gain, or other specific values I should try?

2. I need less than 0.1% THD at 1Khz for the preamp. I'm getting upwards or 2% and 3%!!! Yikes. What are some ways to reduce THD in the preamp. Also, I'm getting 2000% THD at the load (end of the power amp), what type of feedback should I try to add to reduce this?

3. Simple one -- I need to split the 13.6V car battery into +6.8 and -6.8. I did that in the circuit above, but when I connect it to the class AB power amp, the top voltage line drops from +6.8 to +1.5, and the bottom line goes from -6.8 to -12.1. I'd like to run this off of just the car battery, so is there any way I can rig this so I can hook up the voltage lines to the power amp without them changing? You can see I just put separate DC sources in for now.

Thanks... I know this is long, but hopefully I have described everything okay! :)

[ski]

UPDATE: take a step back... I had 400mA DC current going through the speaker. Apparently that will blow out the speaker! Can only have AC current, an I'm in the micro-amp range... f**k.

Oh, and if you think you can help and have some time tonight or today, PM me, and I'll pay for tutoring :)

[dsuds]

My electronics is rusty, so the advice is free.

On #3, could you use a pair of 6V zener diodes in series for your voltage taps? Use the center tap for "ground". This should stabilize your incoming voltage as a zener diode will try to present a fixed voltage regardless of current. This should also help greatly with THD since the transistors can the be properly biased.

On the DC through the speaker. I think that a simple capacitor in series with the output should stop the DC component.

Hope this helps.

[sizemic1]

To get more gain, just turn it up to 11. :hihi:

[ski]

dsuds -- I put a 100 microfarad capacitor at the load and that blocked the DC current.

See, they way at least 1 WATT to the 8 Ohm speaker, so I thought 400mA of DC would do it, but I don't ever remember calculating power with AC current.... I'm getting nano-amperes in AC :eek:

Again, just PM me if you want to something to do tonight and I'll treat you with something from Amazon or some other place :cheers:

[dsuds]

Ski, you have a pm.

[ski]

This is a VERY elementary question, but I'll ask anyway to make sure:

If you have voltages VDC and VAC (a sine wave offset by a DC voltage), how can you get rid of the DC offset to make the sine wave (AC) centered about zero?

[ski]

I've got the whole 2nd half of the circuit working! Now this is the preamp (first) part. I tested the 2nd half of the circuit assuming that the output of the first part was a 1kHz sine wave centered at zero and about 50mV (what I measured).

http://www.littleboyinc.com/uploader/uploads/preamp4.JPG


I want my distortion (THD) to be low, like 0.05% or 0.10%, but I'm getting at best 2.00% by messing with the input capacitor that blocks DC from damaging the tape player head. I need at least 40kOhms input resistance one way or another

http://www.littleboyinc.com/uploader/uploads/graph4.GIF

As you can see, the output (in red) is very choppy! Also, there is a DC offset on the output... I want it to sit on the x axis at 0V with the input. What configuration of capacitor and how big should I try to get just AC out?

So close to being done, anyone that can take a shot, please do -- due at 5pm (I've been working on this for several weeks... the first time I didn't procrastinate, thank GOODNESS)!

[dsuds]

I know we discussed several things at length yesterday, but the general (ie basic) way to eliminate DC in an AC circuit is to isolate the DC out with a capacitor in series with VO. I'm not sure how big a cap would be needed, too many dusty brain cells.

I was also thinking that the input cap C6, 47uf is probably too big considering the input voltage. This may cause what you may be seeing in the output. Try dropping it into the pf range. The effect may be the capacitor sucking/feeding the input signal (a capacitive "bounce"). If you are showing this at the input to the op amp (magnify the scope setting), then it definitely is the case. It'd be wourth a look anyway.