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Old 12th February 2012, 05:02 PM   #1
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Default Some help on oscillation of pre amp

Hey there forum,

i'd like some advice and some help on this. I have a fair experience of messing around with electronics, but not that much with audio... So, i'm having a hard time wrapping my head around this.

My dad recently bought pcb's from Rod Elliot's project 27 (100W Guitar Amplifier (Mk II)) which in turn, gave us full access to the information relative to the schematics and what not. So i decided to build one for myself for fun (I'm not posting the full schematic here as respect to copyright and to Rod).

So i gathered a few spare parts and finished the whole thing. There is however an issue. I'm getting a screeching loud frequency sound when i get to higher levels on the Master control. Something i could describe as loud microphony.

I tried reducing the gain, since i thought it was reproducing some kind of oscillation in the pre amp. I raised R11 (refer to the schematic on the Rod Elliot webpage) up to 10k with no success. I raised the c8 cap that goes directly to the Master control pinheads but also just a change in sound characteristics but if any, only a small improvement on the screech. I disconnected the clipping diode distortion (mainly because i found it disgusting, but i thought was worth mentioning) and decreased R13. Still nothing better.

I've thought about changing the pot itself, but right now i dont have one at hand. What do you guys think? I've though maybe the wire leading to the pot is problematic since its too long and goes next to other wires that may interfere (i dont think so since i decided to put a shielded wire on that).

I've mainly run out of ideas. Can you guys help me?

I'm posting some images of the construction. The arrows (sorry but using GIMP image editor is not exactly my strongest of attributes) i drew are to point out the Master Control pot, the master control pinheads and the wire that leads up to the pot.

the building is not good at all, but its just for the moment. the wires to the MID, BASS and TRB are extended just for practical uses right now.

Thanks in advance.

obs: i dont know if this post belongs to the Musical Instruments section of the forum. If it does, im sorry to post it here.




indicating master pinheads and to master pot wire


side view of Pre indicating wire extensions


showing the pot.
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Old 12th February 2012, 05:13 PM   #2
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The images didn't go up... I'll try this again (i dont really know how to do this)

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Old 12th February 2012, 05:27 PM   #3
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The schematic shows a common ground connection to VR3, VR4, VR5. If this is not actually connected to ground for some reason, you'd have a fine oscillator there.
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Old 12th February 2012, 05:38 PM   #4
Jebem is offline Jebem  Portugal
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It seems to be oscillating. Do You have an oscilloscope to check it?
Make sure the stages are properly decoupled (capacitors close to the active gain devices) to start with.
Good luck.
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Old 12th February 2012, 11:36 PM   #5
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Not being able to post any pics now... i'll try again some other time as i'm thinking its not all that important for now.

sgrossklass, all the pots are connected to a single thick copper wire that connects to the pcb in a single point. So, no luck from there.

Jebem, unfortunately no. I'm starting to think its something important to get since i'm thinking on investing more on this in the nearby future. But, yeah, i took care with arranging caps in that way.

Any other ideas?

thanks
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Old 13th February 2012, 01:52 AM   #6
PRR is offline PRR  United States
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It's two stages with gain of 15 each stage. If the output at U1B can get anywhere near the input of U1A or the tonestack in the middle, it WILL oscillate. Leakage is usually capacitive, so oscillation happens at high frequency, sometimes supersonic.

The fact that Master damps it suggest the sneakage starts over by Q1 or its OUT wire.

With a "mere gain of 239", you'd almost have to have output leads laying on input circuits. "Real" gitar amps have two stages gain=50, 2,500 end to end (minus tone/mix losses). These MUST be laid out like a radar IF strip, input TO output without any backtracking.
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Old 13th February 2012, 02:23 AM   #7
gootee is offline gootee  United States
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What are the inputs connected to? Have you tried shorting them, or putting a resistor across them (to ground)?

Have you measured the rail voltages at the opamps? Do you have a scope?

Are the PCBs mounted or laying on a work surface?

Have you tightly twisted together ALL wire pairs?

Does anything change, at all, when you relocate or move anything (wires, etc)?

Are all jacks insulated from the chassis?

Is the 0.1 Ohm speaker feedback resistor connected correctly?

For a pot, you could try a couple of resistors in its place, just for diagnotic purposes.

Check for wiring errors.

Long wires to each pot should be shortened as much as possible, and twisted together tightly.

Last edited by gootee; 13th February 2012 at 02:29 AM.
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Old 13th February 2012, 10:29 AM   #8
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PRR, i kept the gain and post-gain phases (can you call it such? its how i learnt it in portuguese) of the pre away enough from each other to prevent some kind of interference between them. Your suggestion on the care on capacitors (in the search for leakage) made me think about the last cap (on the OUT). Is that what your suggesting?

gootee, i connected the inputs directly to guitar jacks (is this what you mean by your question?). I havent tried that but will do so.

If you mean if i measured a +15 and -15 at the opamps, yes and that was ok.
The PCB's are mounted and isolated from chassis. No worries there.

I get no significant changes in moving wires around except for picking up some kind of interference. Jacks are insulated. As for the speaker feedback resistor, i'll check later today.

Will try changing the pot today and just changing it to resistors too...

I'll post a response to these later on today.
Thanks to all and for your patience.
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Old 13th February 2012, 10:04 PM   #9
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So,

Got my hands on a new pot for the Master and... nothing. Still the unbearable sound. Did everything and nothing solved it... then it hit me.

i was using an old guitar (the first one i ever owned), that i keep at the shack, to test out the amp. i just suddenly had the idea to grab another guitar to test it out and... it was ok. I reckon some very nasty thing with the pickup that was feeding some signal when i raised the gain. Sorry i didnt think of this before.

As to what exactly happened there, i dont know. If anyone cares to hypothesize as to why, i'd like to hear it. in any other case, its alright, i can live without it...

Thanks for your guys' attention.
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Old 14th February 2012, 01:36 AM   #10
gootee is offline gootee  United States
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Glad you got it solved.
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