motorboating audio amplifier

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• Solid State
• cure for motorboating sound
• Thread starter timhum
• Start date 2012-07-17 8:28 pm
• 2012-07-17 8:28 pm

I have just finished making a amp for my son to busk with, it has a couple of mic inputs and two guitar inputs into a 20 watt BTL amp all powered from a lead acid gel battery and the modules were from CPC. It sounds well enough but when the volume is turned up and before distortion sets in there is a Da da da da sound which continues until the volume is backed off again. I think it is loud enough before this tedious interference but to get the full value from the system I wonder what can be done to stop this noise. Any ideas? Thanks in anticipation, Tim  

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• 2012-07-18 4:24 am

I guess that the obvious solution would be to lower the gain or increase the input attenuation until the motorboating doesn't happen with the volume control at max. If you want to try to make the amplifier stable at the maximum volume control setting, with the present gain and input attenuation, then you will probably need to provide a schematic and some photos of the circuitry. Is it AC coupled? Is there an input filter? Is there any capacitor in or near a feedback loop? Maybe there's a "response speed" (i.e. RC time-constant) mismatch between a filter upstream and something downstream or in a feedback loop.  

• 2012-07-18 5:19 am

This can also happen due to a ground loop or if output circuit is powered at the end of the connecting wire. Gajanan Phadte  

• 2012-07-18 6:48 am

timhum said: I have just finished making a amp for my son to busk with, it has a couple of mic inputs and two guitar inputs into a 20 watt BTL amp all powered from a lead acid gel battery and the modules were from CPC. Click to expand...

• 2012-07-18 8:07 am

Thanks very much for all your helpful replies guys. In spite of my scanty detail you have all given me things to act on. First I will decouple the supply to the mic and guitar input modules in the manner Dave Zan describes since he has come up against this problem before. I shall certainly post the results! I have looked at the circuit for the power amp and it is the BTL version of the standard implementation of the TDL2005 amp, on this link Power audio amplifier schematics with Tda2005 The only difference is that the zobel networks are missing on the version I bought as a kit. The inputs are AC coupled and the outputs from the input modules pass through a 10 K pot and 47K resistor where they join at the input to the master volume pot thence to the power amp. Again, my grateful thanks to all who posted, thank you for your time and helpful attitude. Tim  

• 2012-07-20 8:18 pm

So I did as David suggested, did a few sums and found the parts in the oddments box. It worked like a charm and I have a very happy son. Thanks again for all your replies, Cheers, Tim  

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• Motorboating (electronics)

Motorboating (electronics) explained

In electronics , motorboating is a type of low frequency parasitic oscillation (unwanted cyclic variation of the output voltage) that sometimes occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put", in audio output from speakers or earphones. [1] [2] [3] [4] It is a problem encountered particularly in radio transceivers and older vacuum tube audio systems, guitar amplifier s, PA systems and is caused by some type of unwanted feedback in the circuit. The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output. The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses. Sometimes the oscillations can even be seen visually as the woofer cones in speakers slowly moving in and out.

Besides sounding annoying, motorboating can cause clipping of the audio output waveform, and thus distortion in the output.

Although low frequency parasitic oscillations in audio equipment may be due to a range of causes, there are a few types of equipment in which it is frequently seen:

• Older audio amplifiers with capacitive (RC) or inductive (transformer) coupling between stages. [5] [6] [7] [8] This design is mostly used in vacuum tube (valve) equipment. Motorboating was a problem throughout the era of vacuum tube electronics but became rare as vacuum tube gear was replaced in the 1970s with modern solid state designs, which are direct-coupled. The recent resurgence in popularity of traditional tube-type audio equipment in guitar amplifier s and home audio systems has led to a reappearance of motorboating problems. The problem is sometimes caused in older equipment by the evaporation of the electrolyte from old-style "wet" electrolytic capacitor s used in the power circuits of legacy equipment, or in equipment of any age where an amplifier stage is sensitive to feedback via power supply rails , and can be remedied by replacing/upgrading the capacitors.
• In both old and new designs, even mostly directly-coupled operational amplifier circuits, feedback through the power supply rails can generate ultrasonic oscillations that vary in amplitude at a low frequency ( squegging ) due to the power supply voltage sagging as oscillations build up (the long time constant coming from the power supply reservoir capacitor) in such a way that the low frequency is audible even though the high frequency fundamental is not. Such problems can be difficult to diagnose. [9] [10]
• Audio equipment associated with radio transmitters , particularly transceiver s in two way radios, such as Citizens band , FRS , which have automatic gain control (AGC) or squelch noise control. Malfunctions in the AGC or squelch circuits, which have long time constant s, can cause low frequency oscillation. Another possible cause, sometimes in combination with the first, is leakage of the strong radio frequency (RF) signal from the transmitter into the receiver audio sections, which can cause quenching oscillations. This is a RFI problem, caused by inadequate shielding or filtering to keep the RF out.

As with all electronic oscillation , motorboating occurs when some of the output energy from an amplifying device like a transistor or vacuum tube gets coupled back into the input circuit of the device (or possibly into an earlier stage of the amplifier circuit) with the proper phase for positive feedback . This indicates there is an unwanted feedback path through the circuit from output to input of an amplifying stage. The technical conditions for oscillation, given by the Barkhausen stability criterion , are that the total gain around the feedback loop (comprising the amplifying device and the feedback path) at the oscillation frequency must be one (0 dB), and that the phase shift must be a multiple of 360° (2π radian s). Since most amplifying devices, transistors and tubes, are inverting, with the output signal 180° opposite in phase from the input, the feedback path must contribute the other 180° of shift.

Many types of parasitic oscillation are caused by small interelectrode capacitance s ( parasitic capacitance ) or mutual inductance between adjacent wires or electronic components on the circuit board, which create an inadvertent feedback path. However these usually cause oscillations of high frequency , at the upper end of or above the passband of the equipment. This is because the phase shift of the small reactances in the feedback path, which increases with frequency, only become significant at high frequencies. Low frequency oscillations like motorboating indicate that some device or circuit with a large time constant is involved, such as the interstage coupling capacitors or transformers, or the filter capacitors and supply transformer winding.

In vacuum tube circuits, a common cause is feedback through the plate power supply circuit. The power supply provides DC current to each tube's plate circuit, so the power supply wiring (power busses) can be an inadvertent feedback path between stages. The increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages, making the system a subaudio oscillator. This is caused by inadequate power supply filtering or decoupling. The electrolytic capacitors used in equipment of 1960s vintage contained liquid electrolyte, which dried out over decades, decreasing the capacitance and increasing the leakage current, and these are often the cause.

One solution suggested is a "capacitor job", replacing all the old electrolytic capacitors. [11] A more radical but comprehensive solution is to add modern IC voltage regulator s, or replace the entire power supply with a modern regulated one.

In radio equipment

In equipment that includes radio transmitter s, motorboating can be caused by radio frequency interference (RFI), the strong radio signal from the transmitter getting into audio or receiver circuits. Receiver audio circuits with automatic gain control (AGC) have a long time constant feedback loop which adjusts the gain of the audio stage to compensate for differences in audio level from causes like different speaking voices. Squelch circuits used in two-way radio s to cut out noise similarly have a feedback loop which turns off the audio when high frequency noise is detected.

If the inaudible radio frequency (RF) transmitter signal is inadvertently coupled into the receiver's audio signal path, it can trigger the AGC or squelch circuit to reduce the gain. Then, after a delay time set by the circuit's time constant, the circuit increases the gain again until the amplitude of the radio signal triggers another gain reduction. This repetitive cycle is heard as motorboating.

An example might be a 27 MHz Citizen's band radio in a car, connected to the car's 12 volt DC supply. If the decoupling capacitor s which bypass radio noise from the power supply wires are missing or inadequate, or the long power leads pick up excessive RF from the antenna then it is possible for the RF transmitter signal to enter the radio's receiving circuits through the supply wires. This then causes the motorboating to occur.

• Book: Amos , S. W. . Roger Amos . Newnes Dictionary of Electronics, 4th Ed. . Newnes . 2002 . 205 . 0080524052.
• Book: Van der Veen , Menno . Modern High-end Valve Amplifiers: Based on Toroidal Output Transformers . Elektor International Media . 1999 . 15 . 0905705637.
• Book: Dailey , Denton J. . Electronics for Guitarists, 2nd Ed. . Springer . 2013 . 163 . 978-1461440871.
• Book: Jones , Morgan . Valve Amplifiers, 4th Ed. . Elsevier . 2011 . 467 . 978-0080966403.
• The problem was common in the early days of radio: Westcott . O. D. . "Motorboating" - What it is, and why . Radio News . 149 . Experimenter Publishing Co. . August 1927 . July 7, 2013.
• Bouck . Zeh . Motorboating Cured . Radio Engineering . 7 . 3 . 614–615 . Radio Engineering Publishing Co. . New York . March 1927 . January 9, 2015.
• Book: Duncan , Ben . High Performance Audio Power Amplifiers . Newnes . 1996 . 191 . registration . motorboating. . 0080508049 .
• Book: Self , Douglas . Audio Power Amplifier Design Handbook, 4th Ed. . CRC Press . 2012 . 7.42 . 978-1136123733.
• http://www.antiqueradios.com/forums/viewtopic.php?f=5&t=180838 Help Troubleshooting Old Transmitter
• http://www.diyaudio.com/forums/tubes-valves/150659-el34-pp-motorboating-questions.html EL34 PP motorboating questions
• Web site: Keen . R. G. . Motorboating . Tube Amplifier Debugging Page . GEO Guitar Effects Oriented web page . 1997 . July 3, 2013.

This article is licensed under the GNU Free Documentation License . It uses material from the Wikipedia article " Motorboating (electronics) ".

Except where otherwise indicated, Everything.Explained.Today is © Copyright 2009-2024, A B Cryer, All Rights Reserved. Cookie policy .

An amplifier is only an oscillator waiting for the chance

Definitions of amplifier stability are the inverse of conditions for oscillation. Oscillations can take the form of sustained audible signals, very low frequency 'motorboating', or supersonic 'thud-hiss (distortion)'. These are frequently sensitive to control settings, such as motorboating to the bass setting, and supersonic to the treble.

Oscillation will result if: the phase shift (time delay) around the feedback loop is in-phase (positive or assisting), and the gain around the loop is greater than unity, that is greater than the total loop losses

If you have oscillation the problem is how to get the feedback around the loop to a low level when it is in phase; conversely how to keep it sufficiently out of phase (negative or opposing) when the loop gain is high. Motorboating This is very low frequency instability at around vibrato frequencies, 1 to 10Hz. In older amps this is almost always due to large value electrolytic capacitors 'drying out' or basically losing capacitance with age. Sometimes these can be 're-formed' but replacement is often a better option. Other causes can be due to where, exactly, the various filter capacitors are 'grounded'. In one case intractable low frequency hum and instability was traced to a filter cap being grounded to the signal shields. Single point earthing at the main filter capacitor isn't a new idea but it still works. Many guitar amps ignore this with many grounds to chassis all over the place, inputs, output, power supply, and others. Daryl Mills of Logic Research wrote (23/7/04)... I did a repair on an old Jansen lately... You mention the capacitors dry out, very true as I measured some that the value had shifted more than 100%. You don't mention what I found was also a culprit (the amp was motorboating and oscillating), many of the old resistors although they looked fine had also shifted in value of 50% or more and were really doing a lot of damage. I've even kept a perfect looking 10k that measures 15% just as a 'talking point'. The older the resistor, the more suspect it must be. Very old 'dumbell' resistors (see Amplivox chassis pic ) with wire connections wrapped around the end, using body-end-dot coding, must be checked. These are now so old that, like waxed paper caps, you should consider replacing them on sight. Higher values, 1M and up are always suspect as these are particularly prone to drifting high. Daryl's experience shows this is not always the case, a low value going really low. Decoupling This is a fragment of a Goldentone 1755 power supply showing the decouping networks, so-called because they are intended to provide DC power while de-coupling the stages from each other for AC signals. There are a couple of different ways you can look at decoupling networks depending on what you're doing. The simplest way is that the shunt cap has to have a low reactance to signal frequencies or the stages will have a load in common to impress signals across. Another way is as a set of low-pass CR networks which are tuned to a very low frequency well below the amplifier passband (so that when the frequency is low enough to pass the decoupling network, the amplifier gain has already dropped to a low value below its bass cut-off frequency. If either the cap or series resistance feeding it should go low in value the filter frequency will rise, f=1/wCR, into the passband of the amplifier (where it now has gain) and a low-frequency oscillation may result. But it may produce other odd effects if the unwanted coupling is of opposing phase (see below). A similar supply decoupling network is frequently used for the screen of pentodes in low-level stages (rare in guitar amps except possibly as the reverb driver stage) and similar considerations apply. It is not unknown for screen by-pass caps to go low or open and cause problems. “By-pass” because they shunt any signal to ground. Supersonic Supersonic instability can be recognised by a characteristic “thump, hiss”, and a loss of level and/or fidelity when a control, normally treble, is advanced. Below this setting everything may appear to be fine. Amps with this kind of fault, particularly solid-state ones, should not be run in this condition for more than a few seconds. You may not be able to hear it but assume that it's close to full available output power and flogging the guts out of your amp - you may even see the pilot light dim. It is particularly deadly for piezo tweeters. This may come in the form of a solid oscillation, or more insidiously as a parasitic oscillation. This is a burst of oscillation that only occurs when the amplifier is driven, over only part of each input cycle, and possibly only at high levels. This generally makes a subtle mess of the sound and flogs the output stage and speakers. The only real way to investigate this is with an oscilloscope where the oscillations may appear as fuzz on signal peaks. Keep in mind that these may be high enough in frequency to be outside the passband of many cheaper “audio” CRO's. The workshop AM radio tuned off a station can sometimes give an indication of RF-oscillation in an amp. Most valve guitar amp output stages are fitted with small RF-stopper resistors right on the grid pin, VHF-style, typically 1k5. The object is to kill the Q of the hidden VHF oscillator lurking in the black heart of each output valve. With 807's in particular the anode has to get the same “stopper” treatment or sometimes a lossy Radio Frequency Choke at the top cap. Apart from self-oscillation within a stage, HF instability can be caused by direct capacitive coupling back to an early stage from one of the later high level stages. After eliminating broken grounds and all by-pass caps as okay, the cure is generally adding shielding, such as (restoring lost) preamp valve cans, re-dressing underchassis wiring, changing open signal wires for shielded, even installing tinplate shielding. Look out for added crossover networks possibly in multi-way cabs matched to a head later, or retro-fit tweeters. Few early amps were tested on other than their intended load, generally a group of identical speakers in a direct-radiator cab. Stability may be degraded by a passive crossover. Audio frequency This is rare and generally due to a microphonic preamp valve (but I have just replaced a microphonic 6CA7/EL34, one of the worst cases of full-on audio feedback microphony I've seen). It is not unknown for 'Foo' to have rewired the output connector with the feedback reversed. After replacement the phasing of an output transformer must be checked, it's not enough to assume that because it doesn't take off the NFB is corrrect. Many valve guitar amps have such low NFB that reversing it won't always result in oscillations. The correct way to check NFB phasing is to disconnect the NFB, drive the amp very lightly, then re-connect the NFB and observe if the signal level rises (wrong) or falls (right). If you measure this reduction and express it in dB, that's your amps NFB level. The negative feedback line often consists of a resistor with a parallel phasing capacitor. If this cap happened to fail open the amplifier stability would be reduced.

Electronics for Guitarists pp 159–189 Cite as

Solid-State Power Amplifiers

• Denton J. Dailey 2  
• First Online: 19 October 2022

344 Accesses

In this chapter, we will examine the basic operation of some common power amplifier circuits. Recall that the capacitively coupled, class A amplifiers covered in the last chapter have a maximum theoretical efficiency of 25%. This means that if you were to design a perfect class A amplifier that delivered 10 W to a load, the amplifier itself would dissipate 30 W. Nearly all of this power would be dissipated by the output transistor(s), which would require a large heat sink. Recall also that most of the discrete transistor amplifiers examined in Chap. 3 were common emitter (CE) amplifiers. The output resistance of the CE tends to be high—typically thousands to tens of thousands of ohms. This makes the CE amplifier unsuited for driving a low-resistance load, like a loudspeaker, which will typically have a resistance of 8 Ω or less.

• Barrier potential
• Complementary symmetry
• Composite transistor
• Compound transistor
• Coupling capacitor
• Crossover distortion
• Current mirror
• Darlington pair (Darlington transistor)
• Decoupling capacitor
• Emitter follower
• Ground loop
• Motorboating
• Parallel connected transistors
• Parasitic oscillation
• Power transistor
• Rail splitter
• Root-mean-squared (RMS)
• Star ground
• Swamping resistor
• Sziklai pair
• Thermal resistance
• Thermal runaway
• Tone control
• V BE multiplier
• Zener diode
• Zobel network

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WiiM Amp Streaming Amplifier Review

• Thread starter amirm
• Start date Feb 20, 2024

Rate this streaming amplifier:

1. poor (headless panther), 2. not terrible (postman panther), 3. fine (happy panther), 4. great (golfing panther).

• Total voters 351

Founder/Admin

• Feb 20, 2024

• Thread Starter

Major Contributor

Both power sweep graph are labeled for 8 ohms. Typo?  

CleanSound said: Both power sweep graph are labeled for 8 ohms. Typo? Click to expand...

Wild that this only hits 90 SINAD. It would be better to use a chromecast audio with a real DAC.. Edit: I realized this is a speaker amp and that is what was tested since there wasn't a line out. The performance is actually good for a speaker amp considering the price of this entire box.  

Master Contributor

Just confirmation to what I've been reading generally....great value with good feature set (would have loved to see some analysis of the eq but...). If only I needed one (or could think of even a weak excuse), I'd have already ordered it.  

Is 21 dB max gain? I’ve seen other tests that show it at 27 dB and am curious if they changed the design. Michael  

Deleted member 48726

Voted poor. I do for all amps that will alter the frequency response according to which speaker is connected. Many speakers have high impedance much higher than 8 ohm in mid to high frequencies. There is a chance this amplifier from 2023 will change how these speakers sound. I think this is more important than how much below threshold the THD+N is. Also no power cube tests.  

Something with about 1 ohm output impedance at 20kHz is really nothing to call home about.  

mdsimon2 said: Is 21 dB max gain? I’ve seen other tests that show it at 27 dB and am curious if they changed the design. Click to expand...

The Sonos Amp is a competitive option if you want wireless rear speaker support and the extra power of its amp. The analog in is worse on the Sonos, but for digital, it probably doesn’t matter. The Sonos wireless sub is nicely designed for the size as it has ex-Velodyne engineers. Where the WiiM Amp wins is the availability of 4-band PEQ… @amirm , if you have time, can you see if enabling PEQ affects noise? The limited power does make me with for a future WiiM Preamp with HDMI ARC support.  

It's crazy to be in the HiFi hobby today. This guy at $300 + a pair of Ascend Acoustics CBM-170SE at $368, total $668 and you have a very high fidelity system. That's a streamer, DAC, looks to be a very basic DSP in there as there is EQ, integrated amp, and a pair of excellent speakers. This is a system a college student can afford.  

CleanSound said: It's crazy to be in the HiFi hobby today. This guy at $300 + a pair of Ascend Acoustics CBM-170SE at $368, total $668 and you have a very high fidelity system. That's a streamer, DAC, looks to be a very basic DSP in there as there is EQ, integrated amp, and a pair of excellent speakers. This is a system a college student can afford. Click to expand...

CleanSound said: Ascend Acoustics CBM-170SE Click to expand...

phoenixdogfan

It's just an amazing bargain: DAC, Streamer, Bluetooth Receiver, 4 band EQ, and a 60 watt power amp for $300. Also looks really nice. Has a clean Bauhaus minimalistic "Apple made me" kind of look. For what it is, and what it costs not much of anything to complain about. Does it have a remote?  

Addicted to Fun and Learning

Not too bad, on par with the expectations. Thanks for getting this review out @amirm !  

So...when is Wiim going to release more than 4-band PEQ again?  

Active Member

If you have a couple of spare bookshelf speakers (and maybe a sub), this seems to be a fairly nice and affordable way to stream good quality stereo music and play better than TV quality audio to guest and secondary bedrooms, considering the Bluetooth remote, control app, and the HDMI ARC TV input. EDIT: And rumors are that it is headed for Roon Ready Certification; time will tell.  

The impedance dependency is sad, but for the money this thing is hard to beat, mainly because of everything that's included in the box. I can confirm that WiiM streaming capabilities are solid - solid enough that I have one unit (a mini) I keep turned off along with a Fosi amp on a smart switch in my shed. When it's time to listen, I power them both on at once. I usually keep them powered off for a week or more at a time (it's winter and I'm not outside much lately.) The WiiM is booted up, back on WiFi and ready to stream music in well under a minute every time. This is from inside a plastic box, inside the shed, in my backyard. I don't have to add it back to the network or any nonsense like that. In this day and age, that quality of software+hardware engineering is not universal in audio, and for the money it's unheard of. Add 120 semi-credible watts to that and you have a pretty tidy system on your hands. The high-quality sub-$1000 system is definitely becoming a reality here.  

Thanks @amirm . Wondering how did voice control work if you used during testing?  

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source of ''motorboating''?

• Thread starter ferninando
• Start date Sep 9, 2015

Lunatic Member

• Sep 9, 2015

What is generally considered to be the cause or source of motorboating? On my scope I see the outputs, one normal amplitude the other shorter and both traces are ''pulsing'' like motorboating. the short trace only wants to go positive and barely negative. any ideas? thanks  

junk junkie

Its a low frequency oscillation. Could be problems in the feedback circuit, could be too much cathode bypass cap that lets the unit attempt to reproduce frequencies that it simply can't, might be poor decoupling between power supply stages from weak/bad/too-small power supply caps. What amp are we talking about here? What has been done to it?  

AK Subscriber

Probably need to see the schematic to be sure. What is the frequency of the oscillation? True motor boating will be a few cycles per second. Is this a feedback amp? Sometimes it might appear to motor boat if the global feedback loop has applied so much feedback that the amp is at the brink of instability. It may sound like "put put put" but its actually caused by too much global feedback. To be sure, its best to disconnect the global feedback loop or at least lower it a good amount while testing for LF instability. If its still there after disconnecting or lowering the global feedback, then the cause is probably related to an implicit feedback loop around the power supply and audio circuit. This is more of an inherent loop, not directly added with a wire like global feedback. It can be caused by several things which may play off of each other, such as too much capacitance in the power supply filter caps, or several capicator coupled stages in the amp that are set to have the same or close to the same time constants. (Each time constant causes phase shift, and if you get additive phase shifts at a given frequency from multiple couplings that exceed 180 degrees, poof! Instability. At least in the cases where I've dealt with motorboating, I fixed it by ratcheting down the size of the filter caps in the power supply and also by moving those poles in capacitor coupled stages apart fron one another, like double or more. I think it's a bit tougher problem to nail as opposed to HF instability because its not obvious at first glance where the "loop" is.  

alot to think about. its a fisher 460 consol pull. had a new opt made by heyboer and installed it. sat for 6 mos while doing other things but it didnt work rite then and it didnt do this motorboating or whatever it is on the scope with 8ohm dummyloads and 1k input. that just started today. all parts are orig. and tested good for values both Rs and Cs  

I think I'd try some new power supply caps and see what that gets you. If you didn't check them for ESR, that could explain it. If the caps aren't working effectively, they won't decouple the amplifier sections from one another. Old timey cap testers did a "power factor" test, which is essentially ESR at 60 or 120 hz. Works the same in a power supply setting at least.  

Could also be a rectifier diode dead. That causes what you describe.  

RIP 1961-2018

Power supply is where I'd start of course.  

This ain't no picnic...

There's a risque joke here somewhere... -D p.s. What is it with Fisher 460 amps and blown OPTs? I have one sitting here with the exact same problem (although I have not rewound it yet)  

I will thoroughly re-examine all the PS and other voltages and especially the PS cap values and esr. Ive done it all many times before without sucess so may as well do it again. eventually something mite show up. This is getting to be one of those "wish I never bought it" things. hasnt worked from day 1 and was supposed to.  

Active Member

ferninando said: What is generally considered to be the cause or source of motorboating? Click to expand...

• Sep 10, 2015

George W said: Every new born baby since the beginning of time? Click to expand...

larryderouin

I'm vertical and breathing...most of the time..

larryderouin said: Owners who are too lazy to learn to SAIL. Click to expand...

Records & Coffee

"Was someone looking for me?" - Scarlett Johansson  

Feedback through the power supply is a major cause. Look for bad filtering caps in the power supply. If you're careful, you can jumper new filters into the circuit with it on, you will instantly know when the motorboating goes away.  

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The authorities were prepared to suppress Saturday’s rally and its leaders.

Aleksei A. Navalny, the opposition leader who had called the demonstration, was arrested on Wednesday and sentenced to 30 days in jail. Other prominent opposition politicians — including Ilya V. Yashin, Dmitry G. Gudkov and Ivan Y. Zhdanov — were also rounded up before the event and released only late in the evening.

A post on the Facebook page of Mr. Yashin, a street activist and one of the politicians who was barred from running, said 10 masked police officers had removed him from his apartment in Moscow overnight before the Saturday demonstration.

“It is horrible. My feeling is that we live under an occupation,” said Nadezhda Pilinskaya, 59, referring to the heavy police presence in the city center. “They fear that the end is coming, the end of this regime.”

Ms. Pilinskaya, a retired entrepreneur, added, “We will have nobody to choose from on Election Day.”

At stake in the Sept. 8 election are the 45 seats on the Moscow City Council, which is responsible for a large municipal budget and is controlled by the pro-Kremlin United Russia party. Election officials have so far registered nearly 200 candidates, most of whom are largely supportive of Mr. Putin.

Protesters say that without opposition candidates, the coming city election is rigged. Some protesters chanted, “Where is my signature?” Others yelled, “Where is my candidate?”

The police could be seen spraying some demonstrators with a chemical irritant. One woman, Aleksandra Y. Parushina, bled from a blow to the head with a nightstick.

“None of us was breaking the law. This situation was provoked by the police,” said Ms. Parushina, her head wrapped in a blood-soaked bandage. “I even lost consciousness for a minute,” she said while awaiting an ambulance.

Even before the election dispute, protests had broken out in provincial cities as Russia’s economy swoons under Western sanctions. Street actions began over bread-and-butter issues such as the placement of garbage dumps and the dismal wages of medical workers, which highlight growing frustration over gloomy standards of living.

While the near-weekly demonstrations in the capital and other cities have pierced the image of unified support for Mr. Putin, the scale of support for such rallies is unclear.

The Moscow police said that 3,500 people came out for Saturday’s rally, including about 700 journalists and bloggers who had registered beforehand. The number could not be independently verified.

Andrew E. Kramer contributed reporting.