Weird audio Amp problem

Thread Starter

Joh Drinda

Joined Jul 30, 2015
17
I built a busking Amp for the demonstration of my whistling to good music. It consists of:
1) electret Mic PreAmp,
2) ETI-486 Howl-round Stabilizer,
3) Pedal Reverb and a
4) 100W (class D) Amp running on a 11.1V LiPo.

Problem: When I turn the system on, the whistling volume is moderate, yet then when I turn it briefly off/on, the volume goes suddenly quite high and then gradually weakens.
I don't know what is causing it gradual drop? Maybe it has something to do with an old electrolytic? - Thank you in advance for any suggestions, Jo

Here's a sample of my inherited whistling, which I care to maintain (even now at age 77):
https://app.box.com/s/u68q3fn26jtwb9673xbfc751mef4wq6e
 

wayneh

Joined Sep 9, 2010
17,153
I learn something every day around here!

Any change if you eliminate 2 & 3 from the chain? Are you hoping to achieve and hold the louder volume?
 

MisterBill2

Joined Jan 23, 2018
9,468
The system includes 4 devices that affect the amplitude of the sound by means of their gain. When the volume "weakens", is it returning to what it had been originally? Or to a different level? IS there a start-up delay when you switch the system on the first time?
Next question, when you switch the system off and then back on, is that one switch or four switches, since you have described the system as having four blocks.
My guess is that when it is switched off and then back on quickly, some of the blocks stay fairly close to their "on" state for a while, and other blocks go through a start up sequence that involves adjusting to a specific gain value..
If you can answer those questions then we can do some thinking and come up with an answer that is correct, otherwise we are just guessing, which is OK if it does not matter if the answer is correct or not.
 

Thread Starter

Joh Drinda

Joined Jul 30, 2015
17
After turning it on, I usually turn it off and on again to get the higher level of volume, which then gradually weakens until I give it another jolt (= turn it on & off again). I even added a PB switch to the Mic so, that I can "revive it" again, but it cause a nasty switching noise. If I can filter this switching noise out that'll OK too, because I can always find a dead point in the melody, where I can "revive it" the volume loss again.
Of course it would be better to find the problem and finish the madness once and for all.
Maybe I have to pay for an electronics technician to locate the flaw.
The Pic shows the Amp I use in this chain.
 

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MisterBill2

Joined Jan 23, 2018
9,468
The first step will be to determine which device of the four is changing. If you can switch one element off and then back on you will be able to determine which has the problem. An alternative would be bypassing the anti feedback thing and seeing if that fixed the problem.
 

Thread Starter

Joh Drinda

Joined Jul 30, 2015
17
Could the mic preamp have any auto level control built in?
SG
No, it's just a simple 2 transistor electret Mic PreAmp. I now changed it for a China built electret PreAmp and the problem is still the same. Thus, the problem has to do with the 100W Class D Amp. I'm going to try a voltage step-up circuit to see if this will keep the volume steady.
Thanks anyway for the good guess, Jo
 

MisterBill2

Joined Jan 23, 2018
9,468
I see the text on the amplifier states "for whistle tune only" and I wonder is that your words or did it come with the amp? If you are powering it with a battery that may be the problem. Can you put a voltmeter on the battery and watch the voltage as things change? That will provide a lot of insight. In addition a 100W amplifier will need a bit of heat sinking, so feeling the amp to see ifit is getting hot will also provide information.
 

AnalogKid

Joined Aug 1, 2013
9,383
2) ETI-486 Howl-round Stabilizer,
Did a quick search, didn't find anything on this. Manufacturer / vendor / datasheet - ???
If this is an automatic volume control that turns down the system gain when acoustic feedback increases the signal amplitude out of the mic above a threshold value, it's attack/release time constant capacitors are suspects.

ak
 

LesJones

Joined Jan 8, 2017
3,506
Hi AK,
I think the ETI design will be the standard frequency shifting system. From memory it uses two balanced mixers to shift all the audio frequencies by a small amount (About 12 Hz) In the first mixer the input is mixed with some non audible signal. (Let's say 30 kHz) It is then mixed in the second balanced mixer with 29988 Hz . The output of that will be the same as the original audio but all frequencies will be shifted up by 12 Hz. So in a PA system each time round the feedback loop it will have 12 Hz added so it will be moved out of any peaks in the frequency response of the system which will help to reduce howl round.

Les.
 
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