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Old 25th September 2012, 11:58 AM   #21
TV7 is offline TV7  Australia
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Originally Posted by HJWeedon View Post
Hi again TV7

After further thought I believe that putting a 0.001uF cap inside the RIAA feedback loop is a very bad idea. This is how good designs gets ruined and propagate into the world of circuits. Please find a proper way to stop the 1.8MHz oscillation, my bet is on the power-rails not being bypassed properly. By loading the loop-gain with a 1000pF there will be nothing left to track the RIAA components.

Hans J Weedon
Thanks Hans, please elaborate on the power rails not being bypassed properly, not sure if I follow you.

thanks
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Old 25th September 2012, 05:56 PM   #22
tomchr is offline tomchr  United States
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+1 on the grid stoppers and supply bypassing. Another way to approach this would be with an "anode stopper". I.e. a small RF choke or resistor in series with the anode. Same can be applied on the cathode. Also consider coupling the heater circuit to the chassis ground with a 1 nF ceramic cap from each heater pin to the chassis right at the tube socket.

~Tom
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Old 25th September 2012, 11:02 PM   #23
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have you tried to bandpass out everything above human hearing to give your tweeters an easy time of it....
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Old 26th September 2012, 01:26 AM   #24
tomchr is offline tomchr  United States
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Originally Posted by pointy View Post
have you tried to bandpass out everything above human hearing to give your tweeters an easy time of it....
Addressing the problem at the source would probably be a better solution...

~Tom
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Old 26th September 2012, 09:15 AM   #25
DF96 is offline DF96  England
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I doubt if tweeters are too bothered by 1.8MHz. You would need professional ultrasonic transducers to deal with that!!
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Old 26th September 2012, 10:25 AM   #26
palmas is offline palmas  Portugal
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I doubt if tweeters are too bothered by 1.8MHz. You would need professional ultrasonic transducers to deal with that!!
Tweeters would not reproduce that, but would heat up!
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Old 26th September 2012, 10:40 AM   #27
SY is offline SY  United States
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What's the impedance of a tweeter at 1.8 MHz?
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Old 26th September 2012, 12:56 PM   #28
TheGimp is offline TheGimp  United States
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Originally Posted by TV7 View Post
Will try the solutions and the 12AU7 Hans, although how would a series signal resistor of 10K to 47K affect the CF bias?, possibly some. Maybe not at all? Thanks
Since the grid does not conduct current, the GS resistor only functions with the miller capacitance to form a low pass filter far above the audio range.

Try a smaller cap for your suppression cap instead of 0.001uF.

I've found in some cases as little as 47pf g-k can be effective in guitar amps.

I'd go for the stop resistors on all grids first though.
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Old 26th September 2012, 03:21 PM   #29
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Hey there,

good to hear that the problem could be located - although the cure currently in place is a little drastic...

Apart from that the description of the whole event (red-plating power amp and such) makes me think it would make sense to take precautions for sources or preamps behaving bad.

Even if a bad source / preamp feeds a high-amplitude 1,8MHz signal into the input of the power amp, it should be taken care that this RF is *not* being amplified, driving the O/P stage to its dissipation limits and beyond. I think proper limiting of the power amp frequency response below the RF range won't hurt...

Greetings,
Andreas
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Old 26th September 2012, 04:37 PM   #30
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hI tv7.

This morning I did a computer simulation of the RIAA preamp as drawn. My simulation shows that without the 10k/1nF capacitor you put in, the circuit does indeed want to oscillate. My simulation wanted to oscillate at 8MHz, but that is sort of arbitrary and depends heavily on layout and such. The important point is that 10k/1nF does stop the loop oscillation. It also shows that it is not the classical VHF parasitic oscillations that tubes often show, but the actual feedback loop that oscillates.

The loop gain is a little low at low frequency, about 5dB, but you can easily boost that to 10dB giving a little better tracking to the RIAA curve by changing the plate resistor in the first 12AX7 stage to 47k. since that stage is AC coupled, you do not need to do anything else. With that gain change, you should also double the oscillation taming capacitor from 0.001uF or 1nF to 2.2nF.

Good luck and have a good day.

Hans J Weedon.

PS: The surprise to me was that what I thought was a drastic change was actually required for stabilizing the feedback loop.

Actually with the gain-increase a little bit better loop compensation would be to use the 1nF capacitor, but with a 470 Ohm resistor in series to ground. Resistors do good things at times. This is called pole/zero compensation

HJW.

Last edited by HJWeedon; 26th September 2012 at 04:49 PM.
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