e100. Under-coupling is the very worse thing to do to any amplifier because failures can occur in a matter of seconds! Monitors, d… Author Topic: Symptoms of an aging amplifier circuit? Thoughts on what that could be? Sansui AU-9500- leaky tantalum coupling caps, dirty controls, bad bias pot. The potential divider network R 1 and R 2 and the resistor R e together form the biasing and stabilization network. Capacitor Coupled Class AB Output Stage: The basic circuit of a Class-AB amplifier using a complementary emitter follower output stage and a Capacitor Coupled Class AB Output Stage load is shown in Fig. Fault Symptoms Arising from Bad, Leaking or Blown Capacitors The diagnosis for any faulty apparatus will be different in each case. The electrolyte from bad caps spills over the board and make damages. Capacitor reviews being really subjective, the score bands are wide. The capacitors we’re talking about are aluminum electrolytic caps, which are used to block direct current (DC) but let alternating current (AC) through. Resistor R-26 and capacitor C-26 act as a filter to remove the RF component of the signal, leaving only the audio portion (see illustration). I have an old audio amplifier (JVC A-GX2B) that sometimes produces crackling noises on one of the outputs. ; The circuit is very compact and extremely light. So, I opened it and discovered that its two largest capacitors seem to be leaking : Can I replace it with any capacitor with the same characteristics (4700uF 50V), or should I instead take specific capacitors for audio? Coupling cap size can have a profound impact on an amp's overdrive tone because a smaller coupling cap can reduce the severity of blocking distortion. This makes the coupling capacitors far less critical than they are in a conventional amplifier design. Carefully choosing the right type of replacement capacitors results in a remarkable improvement in sound quality, even when the original caps were in perfect condition. The reactance of a capacitor is greatest at low frequencies, therefore the choice of coupling capacitor values must allow for a low reactance at the lowest frequencies the amplifier is designed to amplify. 58 ohms? It’s not that old capacitors “go bad.” They do— But it’s that the original capacitors were not “audiophile grade” types to begin with. Amplify the audio signal and pass it on to the audio output stage - This audio signal appears across the volume control R-27 where it is coupled to the grid of the triode section of the tube through coupling capacitor C-31 . Coupling caps and caps used in tone circuits are a different story. In the above schematic, stage coupling capacitors are outlined in red. This voltage could interfere with the bias of the stage or damage its parts. A capacitors job is to block dc, allow ac currents to flow and provide filtering. A quick check of the fuse should be the first step, but if the fuse is good, then the problem lies elsewhere. That is, when driven should be wearing the lower level, they are very quiet. While many people don't like using electros, far more serious problems will occur if the feedback cap were to be a film type. Fisher, Stromberg-Carlson and Bogen console amps- selenium rectifier replacement, full recap and bad … Always suspect a bad tube as they are the most common failure point in a tube amplifier. Replacing them will not cause a loss of tone, or make the amp sound less "vintage." There are some advantages of RC coupled amplifier which are given below,. Sansui AU-919- needed infamous "black flag" capacitors changed, also had a bad regulated supply and broken connectors. You may see buck boost coils there, a diode bridge, heavy switching MOSFETs, and things like that. As you can imagine, a bad capacitor is going to have other measurable characteristics that are suboptimal. All of those items are a bad influence on impressionable caps, and can make them go bad. Often the oxide layer is in such bad shape in older equipment that it must be reformed or else the capacitor will fail catastrophically. Their job is to pass ac signals from one amplifier stage to the next while blocking any direct current. Repair Your Electronics by Replacing Blown Capacitors: Checking for blown capacitors in your malfunctioning electronics is fast and easy if know what you're looking for. A bad tube can cause lots of different symptoms including everything from complete signal loss, hum, hiss, static to something that sounds like whale sounds. Instead, it means that reasonable people could probably argue over which is better, though they may have very different characteristics. It’s been an unfortunate tendency among circuit designers to use polar electrolytic capacitors as coupling caps in audio circuits, even audio circuits that have no DC on either side of the capacitor. The other type of capacitor is the called coupling cap or signal cap. A coupling cap that leaks direct current creates a dc voltage on the input of the next stage. Typically there are few bad electrolytic capacitors on the input board. Instead of coupling capacitor like in RC coupled amplifier, a transformer is used for coupling any two stages, in the transformer coupled amplifier circuit. There are op-amps that use AC-coupling, they're called "chopper-stabilized" op amps. So power supply filter caps go bad often. Within an electronic circuit, active components like transistors behave differently than passive components like resistors. Because often you want to use an op-amp to amplify DC voltages or currents. Coupling capacitor value tube amplifier Coupling capacitor value tube amplifier Power amplifier coupling caps will generally be electrolytic types, because the values involved are large and film capacitors are simply too bulky and expensive. In my MJR6 and MJR7 amplifier designs the speaker coupling capacitor was included in the overall feedback loop, primarily to improve the low frequency damping factor, but this will also minimise distortion, and the low measured distortion figures included the effect of this and all the other capacitors. Coupling capacitors are used to block D.C. (D.C. = bad Ju-ju) , and pass A.C. (A.C. = the music signal). The fix was to replace the filter capacitors. « previous next » Print; Search; Pages: [1] Go Down. The figure below shows the circuit diagram of transformer coupled amplifier. If it is connected by either a wire or a resistor to the driving circuit, it is direct coupled and the high grid voltage is not a defect. For all stages that are not cathode followers, the voltage on the socket pin for the grid should be dead zero, not positive even a fraction of a volt. It was the filter capacitors but not what you're thinking. There are several signs of under-coupling in a grid-driven tetrode or grounded-grid amplifier. Capacitor C1 is a coupling cap. You name it and a bad tube can cause it. If yes, which ones ? This is because active components are designed to be subjected to a range of voltages and to perform a variety of functions. Frequent Contributor; Posts: 329; Symptoms of an aging amplifier circuit? Symptoms of an aging amplifier circuit? Just because two caps are both rated "3" does not mean they are the same. In comparison, the new capacitors have such a low resistance that they are almost unmeasurable by this meter. It blocks the high voltage DC on V1's plate (pin 1) and keeps it out of the volume control. It is still an important capacitor and a low quality capacitor will still have a detrimental effect on the sound. Filter caps certainly can go bad, and I replace lots of them, but I don't consider it a "must," especially on amps from the eighties or newer. Replacing one part at a couple dollars a piece is much cheaper than replacing an entire monitor for hundreds of dollars! If there is a capacitor between it and it's driving circuit, the coupling capacitor is bad. ; It offers a … FWIW, I have seen/heard a hi-fi amplifier with the same symptom. First, the electrolyte is conductive liquid which causes servo DC and differential circuits to malfunction. When the output capacitor (load capacitor) is meshed too far (too much capacitance), especially at high drive power levels, the amplifier will be under-coupled. The point at which the frequency rolloff is down -3db (corner frequency) is dependent on the input impedance of the component it will be feeding (the load). Bad Transistor Symptoms. The bad capacitors have extremely high resistances. Interestingly, it has the same symptoms as one of the bad woofers. Capacitors Tube amp capacitors or guitar amp capacitors, our selection currently covers the most commonly used types in vacuum tube amplifier applications. That’s going to restrict current and consume power. Capacitor manufacturers use proprietary mixes of chemicals and DC electricity to create this insulating layer, which deteriorates with time and idleness. This is a bad idea because you subject the capacitor to backwards voltages on half of every cycle of audio. Very mid range E, not a lot below 200 Hz The one speaker seems to be fine – no stripped wires, no seized cone. Amplifier current demand from capacitors might be constant (class A) or vary a lot with the music (class AB). It used the resistor and the capacitor which are not expensive so the cost is low. Capacitors that are placed under heavy stress are more likely to go bad than caps worked less hard. I usually suggest leaving them in a classic vintage amp as long as they test good. Your tech will give you the bad news: it needs a cap job, or a replacement of all of the amplifier’s power filter capacitors. The video shows the "bandpass" from different capacitor values used as coupling capacitor in audio circuits (20 Hz - 20 KHz). Very large electrolytic capacitors are characterized by capacitance (opposition to change in voltage), inductance (opposition to change in current) and ESR (effective series resistance) that represents pure resistance. I have allocated a large space for this capacitor since some of the boutique capacitors are quite large. The values measured alright but they had very high ESR and were letting through the switching noise from the high voltage rectifier. 18-18. Bad electrolytic caps in the preamp often cause muffled, muddy tone, unpleasant distortion and/or “motorboating”. However, a coupling capacitor acts as a high pass filter, meaning it will attenuate frequencies below a certain point. The frequency response of RC coupled amplifier is excellent. At this point the amplifier can not properly control DC voltage at the speaker output terminals, which has to be about 0 volts. Having a spare set of tubes to swap into the amp one at a time is a must for gigging tube amp users. A generally accepted value for coupling capacitors in an audio amplifier would be between 1µF and 10µF, (this can be changed later when the design is finalised). The circuit is termed a complementary symmetry amplifier. Electrolytic caps tend to need replacing more frequently than the other type of capacitor, the signal cap. (Read 1191 times) 0 Members and 1 Guest are viewing this topic. The power supply is often where the power comes into equipment. 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