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Old 22nd March 2013, 07:03 PM   #1
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Default Why is my amp oscillating ?

Hi. I try to build a 100 W guitar amp. I purchased the Project 27 board from Rod Elliot which works fine when I use a frequency generator or an iPod as input. But when I connect the preamp to it, it starts oscillating. First I thougth the preamp layout is bad, but the funny thing is that the preamp works fine on its own. So when no load is connected to the preamp it works fine, no oscillation. I measured it with an oscilloscope and it does exactly what it should. But as I said the whole thing starts oscillating when the pre- and the poweramp are connected. I used shieled leads as input, as connection between pre and power, and also as output to the speaker jack.

Its my first DIY amp and I really dont know what to do. Could you help me ? Should I post pictures from the whole thing ?
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Old 22nd March 2013, 09:17 PM   #2
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Link to project: 100W Guitar Amplifier (Mk II)

What is the frequency of the oscillation?

Did you follow figure 1B showing how the preamp, power amp and power supply are connected together? Does the circuit ground connect to the chassis only where a ground symbol is shown?
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Old 22nd March 2013, 09:48 PM   #3
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I wired the jacks and the pots as shown in figure 1B, except i connected the preamp direct to the poweramp without the effects loop. I did not have the connect the circuit ground to the chassis. I know that this is dangerous and also illegal I guess, but for testing purposes I deciced to try it without.

I dont know what exactly the frequency is, because the scope is in my school and I did not write it down. But it was around 3-4 kHz
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Old 22nd March 2013, 09:57 PM   #4
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The frequency was about 3-4kHz, I cant tell you exactly because the scope is in my school. I wired the pots and the jacks exactly like shown i figure 1B, except I did not use an FX-Loop, so i went directly into the poweramp from the pre-out. To prevent hum I did not connect the circuit ground to chassis in the test phase. I know that this is dangerous and maybe illegal but I thought i would better when testing.

Edit: Sry for the double post, but I got a lag or something, somehow I did not see my previous post.

Last edited by CaptainMorgan; 22nd March 2013 at 10:11 PM.
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Old 22nd March 2013, 10:38 PM   #5
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Since you are new to the forum, your post will be reviewed by a moderator before it will show until you get 5 posts.

Do the Volume or tone controls affect the oscillation?
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Old 24th March 2013, 02:41 PM   #6
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Since I dont have anything to measure at home I can tell you tommorow what exactly affects the oscillation. Here are some pics, maybe there is an obvious reason which I just dont know/see or whatever. First I tried it like this (with shorter cables of course)
Click the image to open in full size.

Then I tried it like this:
Click the image to open in full size.
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Old 24th March 2013, 06:33 PM   #7
jaycee is offline jaycee  United Kingdom
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Don't loop power supply wiring like that, especially the ground. Run wires directly back to the power supply capacitors for both the preamp and power amp. Grounds should be connected at one point. Also, shorten the wiring to the output transistors.
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Old 24th March 2013, 06:55 PM   #8
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So the power supply for the poweramp should not come from the preamp ? I will try that. Should i twist all the power supply wires with the ground ? Or should be only the positive and negative voltage supply twisted ? And should the transistor wires also be twisted ?
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Old 25th March 2013, 01:43 PM   #9
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Ok, I solved the problem. It was either the master volume pot which had a ground connection to much. Or it was because I did not connect ground to the chassis. Anyways it works now I am happy.
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