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ski
12-02-2004, 12:15 PM
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:

Here is the schematic of the circuit:

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.

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
12-02-2004, 01:57 PM
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
12-02-2004, 04:53 PM
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
12-02-2004, 05:46 PM
To get more gain, just turn it up to 11. :hihi:

ski
12-02-2004, 05:56 PM
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
12-02-2004, 06:28 PM
Ski, you have a pm.

ski
12-03-2004, 05:41 AM
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
12-03-2004, 07:10 AM
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).

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