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Hey guys,
I built my Kookaburra last night, and finally got to attach it to power today. I was delighted to see that it worked, and I didn't mess anything up in the process.
I've not had a chance to hook it up to an amp yet (I managed to blow up my gainclone when I finally hooked up the Kookaburra -- can't figure out why), so I've only been able to watch the preamplified waveform on my scope.
It would appear that for the first 1/4 of the pot's turn, there's nearly no signal. Then, the 1/4 to 3/4 range seems to be doing what's expected. Finally, once you go past 3/4 (it may be sooner), the waveform is clipping pretty substantially according to the scope.
Is this to be expected?
Chris
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It depends voltage of the input signal into the kookaburra. The kookaburra can only swing about 4.5VPP, which for a preamp, is generally plenty. Edited by user Wednesday, October 25, 2006 7:15:47 PM(UTC)
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Rank: Member
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Russ,
I've verified on a previous occasion that the signal from my portable test MP3 CD player is 1.5V P-P when measured on my scope. That's the standard line-level voltage, from what I remember.
I'm supplying the Kookaburra with a Hammond MFG 182P22, which has 22V/rail, in case that could possibly affect anything.
In the past, I've hooked my gainclone amp (while it was alive...*sniff*) to the same scope, and never saw clipping when running a full-gain 1.5V P-P signal into it. This is the same program material, too (That CD has not left that CD player for years now), so I don't imagine the problem's caused on that end.
Any other ideas/tips for diagnosing?
Chris
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liscio wrote:Russ,
I've verified on a previous occasion that the signal from my portable test MP3 CD player is 1.5V P-P when measured on my scope. That's the standard line-level voltage, from what I remember.
Chris Sure, yes it is. But what you have to remember is that the PGA2311 is operating from 5V supplies but lower distortion then the PGA2310(up to 15V rails). So, you cannot feed it a full scale 3V(1.5VPP) sine wave and then turn it to full gain without expecting clipping. The max gain of the PGA2311 is 31.5db that works out to a voltage gain of 37.6. So your 1.5VPP would become 56.4VPP. Really its just a function of scale. The truth is, almost any preamp would clip at that gain with that input signal. If you have some other preamp to test that has 31.5db gain (something withe less then 50V rails) you would see exactly the same thing. There are a couple of thing you can do if you like. 1) Don't turn your preamp up too loud on music which will swing full 1.5VPP. :) 2) Or, if you really want to you could add a voltage divider on the input to throw away some of the input signal. Say 10K with 3.3K, which would reduce the input signal to 1/3. Just a note, I have used Kookaburra with normal consumer sources for quite some time, and in fact we are using it just as many commercial preamps do. What you are seeing is perfectly normal. Cheers! Russ Edited by user Thursday, October 26, 2006 6:37:32 AM(UTC)
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Thanks for the quick response, Russ.
I really wasn't expecting to play my source really much louder than 1/2 way anyway, but after arc-welding my gainclone while the Kookaburra was attached to the same trafo and power regulator boards, I feared that I might have done some damage.
Now I have to decide whether or not I can salvage the gainclones I have, or break down and order a RevC kit (you'll see my post in that forum, too).
Thanks again,
Chris
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liscio wrote:after arc-welding my gainclone while the Kookaburra was attached to the same trafo and power regulator boards, I feared that I might have done some damage.
You are certainly welcome. If your kooka is giving you good sine wave oin the scope it is likely just fine. Check for DC on output. If there is none you can feel confident about using it. I very much doubt that the problem was with the signal coming from the Kookaburra, it was likely some other wiring issue. A functional kookaburra should never have more than 5V on the output (that is the max rail for PGA). A normal gain clone would not do anything but possibly clip with such an input. Very sorry about your bad luck on that amp. As for RevC it is a very good match for the kookaburra. Cheers! Russ Edited by user Thursday, October 26, 2006 6:45:34 AM(UTC)
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Oh, I'm certain the problem that caused the light show was not originating from the Kookaburra. I think that the problem was likely a short that was caused by the way I tapped the Kookaburra's AC input off of the transformer.
At least, that's the only explanation I could come up with. The only physically damaged part was one out of two of my power regulator boards, and it's quite possible that the gainclones survived just fine.
And to verify what you said about the 5V, the clipping signal was clipping at *exactly* 5V.
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