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Old 30th August 2003, 10:19 PM   #1
sobazz is offline sobazz  Denmark
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Default CDP output signal to high for P3A?

A month ago I built some low wattage P3A mono blocks. They are powered by 18-0-18v transformers giving approx. 30w per channel.

When I connect the amps through the passive preamp to the line out of my HK amp they are absolutely lovely. But when I connect them directly to my el cheapo HK cd player the output from the amps is heavily distorted. The signal level from the CDP is 2v RMS, while it is only around 1v RMS from my HK amp.

What can I do to cure this problem? Is it caused by too low rail voltage?
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Old 31st August 2003, 06:36 AM   #2
paulb is offline paulb  Canada
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Default Re: CDP output signal to high for P3A?

Quote:
Originally posted by sobazz
When I connect the amps through the passive preamp to the line out of my HK amp
I don't quite get what the two arrangements are you're describing. OK, I get that the second arrangement is CD player directly to P3As. But does the first arrangement use the same CD player?
CD players can clip. I've seen it on a scope. Maybe the input impedance of the P3A is dragging it down, versus the HK that presents a higher impedance and therefore doesn't drive the CD's output opamps into distortion.
Anyway, I'm running my P3As from an 18-0-18 transformer and have not observed this problem.
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Old 31st August 2003, 11:35 AM   #3
sobazz is offline sobazz  Denmark
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Okay, sorry. I'll try to explain it again.

There's no problem when the cd player is connected to the input of my HK680 and the passive preamp for my P3A amps is connected to a line out (tape out).

The problem appears when I connect the cdp directly to the passive preamp. At VERY low levels there's no audible distortion but as I turn the pot the distortion gets very clear. In fact I don't have to turn the pot more than 10-15 percent before the output is *pure* distortion.

When I connect the power amps to the Pre Out of the HM680 there's no problem either.

Would active circutry in the preamp help?

EDIT: The pot used in the preamp is a Black Beauty
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Old 31st August 2003, 01:38 PM   #4
paulb is offline paulb  Canada
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The input impedance of the P3A or its passive preamp is the only explanation I can think of. The HK680's line out or preamp out seems to have no problem driving it, but the CD player struggles with it. If you have a scope it would be interesting to see what happens as you turn up the volume (you'll be able to see clipping if that's what's happening).
One possibility is that something is wrong with the P3A or the passive preamp that gives it an unusually low input impedance. For instance, if R2 was 2K2 instead of 22K, or if the passive preamp used a 2K pot, or something like that.
Assuming there is no error, it sounds like you need a buffer in the preamp. A unity gain opamp or a single JFET (see the passdiy.com ZenV4 article) would do.
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Old 31st August 2003, 03:40 PM   #5
sobazz is offline sobazz  Denmark
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Unfortunately, I don't have a scope. But the same happens when I connect my META42 style headphone amp directly the the CDP. The headphone amp uses a 50kohm mini potentiometer and 1kohm input resistors. It is powered by 2 9v batteries and has a gain of 3.

I'm pretty sure the amps are clipping.

I forgot to mention that the ALPS pot is 10kohm, and I'm almost certain that R2 is 22kohm. But I'll check it soon.
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Old 31st August 2003, 04:20 PM   #6
sobazz is offline sobazz  Denmark
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I would like to create a unity gain circuit. But I'd prefer an op-amp based one. I guess I could use a minimalistic OPA134 circuit. Any ideas?

In order to avoid misunderstandings I'd like to use this schematic as reference. It's the schematic for Chu Moy Pocket headphone amplifier, which can be found here.

Click the image to open in full size.

The resistors in the feedback loops should of course be omitted, and I guess C1 is unnescessary. What values for R1, R2 and R5 should I use if any at all and should I connect the pot before or after this circuit?

I hope you can help me.
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Old 31st August 2003, 06:55 PM   #7
paulb is offline paulb  Canada
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Yes, that will work fine. For unity gain, you can leave out R3, and replace R1, R4 and R5 with a straight connection (0 ohms). I would leave the C1 cap in, since that is coming from an external source that may have a DC offset. The input impedance of this amp is equal to R2, and 100K should be high enough to avoid loading down the CD player.
There should be bypass capacitors on both the V+ and V- connections, but I know they're shown in a different schematic in Chu's article. Use at least +/- 9V to run the circuit - that is, don't use a single 9V battery with the resistive splitter.
Use this to drive the pot of your passive preamp.
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Old 10th September 2003, 04:38 PM   #8
sobazz is offline sobazz  Denmark
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Well, I tried the above circuit but with limited succes.

When I connect the circuit (C1, R1, R3, R4 and R5 are left out) between my CDP and the pot and the pot to my power amps there's a signal coming through even when the pot is at minimum position. The signal is heavily distorted.

When I turn the pot the distortion level decreases but only marginaly and the signal amplitude increases heavily - why?

There's no audible distortion when I connect my headphones (32ohm) after the pot (instead of the power amps).

Even when using a low level source such as my compact disc player which has an output of 0,7v RMS the output of the power amps are distorted.
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Old 10th September 2003, 06:02 PM   #9
sobazz is offline sobazz  Denmark
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Let's say the pot is at minimum throughput - that is maximum resistance between viper and output equals minimum resistance between viper and ground.

The opamp will not see any load, will it? Or in any case a minimum load. If I connect a resistor in series with the opamp output the problem should be cured. Or? If yes, should the resistor be connected outside the feedback loop?
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Old 10th September 2003, 06:19 PM   #10
paulb is offline paulb  Canada
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Something is wrong here. At minimum, you should have no signal whatsoever. Check the resistance from wiper to ground with the pot at minimum, it should be 0 or very close to it.
At minimum volume, the opamp will see the total resistance of the pot as its load. The P3A will be driven from a 0-ohm source, so should be dead quiet.
The opamp can easily drive the 10K pot, you don't need to add a resistor to pad it.

Something other things to try:
- You do have the wiper of the pot connected to the amp, right, and the non-ground end to the source? Just thought I should check.
- I assume you have an input capacitor on the P3A. If not, put one in.
- Try putting the opamp buffer after the pot.
- You say you can drive your headphones directly from the pot. OK, try driving both your headphones and the amp at the same time. Is the headphone sound distorted? Is the amp sound distorted? This should give us a clue.
- Check the input wiring on the P3A very carefully.
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