Guitar Sustain Circuit

There is alternate for the design of the Guitar Sustain. This version is designed specifically for operation from ±15V supplies (±12V supplies can also be used with no changes).  This version can be used as a full range limiter for music, and might typically be applied in an amp rack to prevent amps from being overdriven (and distorting).  While the Figure 1 version can also be used, it is less convenient because of the single supply, and the rectifier is not quite as good as the unit shown here. This is the figure of the circuit.

Car Audio Power Bridging Terms

Not only getting married is there a condition, bridging also has conditions. Because if you do not comply with these conditions then your power can be broken down in just a few hours or just a few songs with no maximum. Then what are the conditions? These conditions have been written by the manufacturer, so you should read the manual that indicates the condition if the power is in the cartridge. 

Then what are the conditions? These conditions have been written by the manufacturer, so you should read the manual that indicates the condition if the power is in the cartridge. In refilling the car audio power, we should not arbitrarily connect the cable, it must be right between the pluses and minuses - that's the first condition.

Intercom Preamp Circuit Using Transistor

There are thousands of pre-amplifier circuits. These are three which have interested me and are a little different. This is a simple design circuit of intercom pre-amplifier. This circuit is built by transistor and several low cost components. Tr1 is operated in grounded base mode with input to its emitter to give low impedance input. The values shown give correct operation from 9v. This is the figure of the circuit.


High quality microphones also tend to be low impedance, typically around 600 ohms. This is low input impedance, high quality pre-amplifier of the sort that could be used in a stage mixing desk. The circuit uses a dual rail power supply - convenient because there were many op-amps in the machine. Note that Tr1 is a PNP transistor. Theoretically PNP transistors can have lower noise level than NPNs. Tr2 amplifies Tr1's output. Tr3 is simply a constant current collector load for Tr2, with its current controlled by the 180R emitter resistor. This can be altered to give more current to feed lower impedance output loads.

Intercom Preamp Circuit Using Transistor

There are thousands of pre-amplifier circuits. These are three which have interested me and are a little different. This is a simple design circuit of intercom pre-amplifier. This circuit is built by transistor and several low cost components. Tr1 is operated in grounded base mode with input to its emitter to give low impedance input. The values shown give correct operation from 9v. This is the figure of the circuit.


High quality microphones also tend to be low impedance, typically around 600 ohms. This is low input impedance, high quality pre-amplifier of the sort that could be used in a stage mixing desk. The circuit uses a dual rail power supply - convenient because there were many op-amps in the machine. Note that Tr1 is a PNP transistor. Theoretically PNP transistors can have lower noise level than NPNs. Tr2 amplifies Tr1's output. Tr3 is simply a constant current collector load for Tr2, with its current controlled by the 180R emitter resistor. This can be altered to give more current to feed lower impedance output loads.

Low Noise Balanced Microphone Preamp Using TL071 IC

This is a simple design circuit which has very low noise, close to the theoretical minimum, high hum rejection and variable gain with a single rotary pot. The circuit design consists of differential compound pairs of transistors with a common mode (floating) gain control connecting the emitters of the pair. The compound pairs of 2N4403 and BC549s are far more linear than any single transistor. The circuit is differential in and out and therefore requires a balanced to unbalanced buffer to give suitable output for the next signal stages of a channel in a mixing desk. This is provided by a high performance op-amp differential gain stage, which can be a TL071 or similar IC of your choice. The stage has a gain of six or 15 dB and that sets the maximum input level at about 1.5 volts rms before clipping. This equals an SPL of over 150dB with a typical microphone. This is a figure of complete design for the circuit.


The operation of the circuit is input stage is configured for least noise and this has meant a non IC approach. There are some special ICs that can be used for microphone pre-amps, they contain a circuit like this one except fabricated on one chip. Components should all be readily available except for the 10 k ohm pot for the gain control. This needs to be a reverse log taper - or else use a multi-position switch with 6 dB gain steps covering the 60 dB range of the circuit. Make sure it is make before break. The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the ICs.

Good quality components should be used with metal film resistors in the collectors and emitters of the input pairs for least noise. Where a resistor has significant DC voltage imposed on it in high gain circuits always use low noise types. Metal film resistors are about the best only bettered by wire wound which is a bit impractical. Avoid metal glaze, and very old carbon composition types. Also avoid bead tantalum capacitors, as they go leaky and crackle. They are just about the most fragile electronic components made. The 100nF capacitor (C6) should be mounted as close as possible to the op-amp supply pins - a ceramic cap is recommended for best bypass performance at high frequencies.

The design source is by Phil Allison

15dB UHF Antenna Preamp Circuit

This is an UHF band TV antenna preamp circuit with 15dB gain and build by a transistor. It is formed based on BF180 UHF Transistor. This circuit is a simple circuit. This is a figure of the circuit.


The principle operation of the circuit is two stages. The first stage is an band pass filter constructed by the C1, CV1, L1, L4, C7 and C3, the second stage is a base-common voltage amplifier with low input impedance to match. Build the L1 ~ L4 as air core coil to obtain high Q-Factor. You can find out about air core coil construction and calculation here. After assembling, pack it into a proper metallic box and connect the ground of the circuit to the box to reduce noise effect.

Guitar Pre Amp Using TL072

The preamp circuit is shown in the figure has a few interesting characteristics that separate it from the "normal" assuming that there is such a thing. The circuit is built by TL072 as amp. This is simple design and provides excellent tonal range. The gain structure is designed to provide a huge amount of gain, which is ideal for those guitarists who like to get that fully distorted "fat" sound. The circuit is show in this figure;


However, with a couple of simple changes, the preamp can be tamed to suit just about any style of playing. Likewise, the tone controls as shown have sufficient range to cover almost anything from an electrified violin to a bass guitar. The response can be limited if you wish (by experimenting with the tone control capacitor values).

The preamp uses a dual op amp as its only amplification. The lone transistor is an emitter follower, and maintains low output impedance after the master volume control. As shown in the figure, with a typical guitar input, it is possible to get a very fat overdrive sound by winding up the volume, and then setting the master for a suitable level. The overall frequency response is deliberately limited to prevent extreme low-end waffle, and to cut the extreme highs to help reduce noise and to limit the response to the normal requirements for guitar. If you use the TL072 op amp as shown, you may find that noise is a problem, especially at high gain with lots of treble boost. I strongly suggest that you use an OPA2134 a premium audio op, you will then find this quite possibly the quietest guitar amp you have ever heard. At any gain setting, there is more pickup noise from the guitar than circuit noise and for the prototype pre amp used carbon resistors.

Low Noise Microphone Preamplifier

This is a design for a low noise microphone preamplifier, which is ideally suited to low impedance (600 Ohm nominal) microphones. One limitation is that it is not balanced, which is not a problem in a home recording environment, but will allow the mic lead (and case) to pick up noise with long cable runs or in a hostile environment.

As shown, it is not really suitable for professional work (although it has been used on stage in its unbalanced form with good results), but the addition of a 1:1 microphone transformer on the input will convert it into a balanced preamp with very high performance. In many cases, a transformer will actually outperform active balancing circuits, because there is (or should be) no ground reference. The shield of the balanced cable must be earthed of course, but in my experience with live music and studio work, less noise is picked up if the internal wiring is floating.

It is most regrettable that good mic transformers are rather hard to come by, and are expensive. If you happen to have a suitable one in your junk box, give it a try with this circuit - I doubt that you will be disappointed with the result. I have used this circuit in Front-of-House, fold back (monitor) and studio mixers, and managed to obtain excellent results - I still have a little 6 channel mixer (which I use only occasionally now) using this circuit, and have never been even slightly tempted to replace it with even the best of op amps

This is the figure;

It requires a well regulated (or extremely well smoothed) supply voltage of 30V, and will typically be able to supply a maximum output level of around 7V RMS allowing for typical component spreads. With the component values shown, impedance matching is correct for a 600 Ohm mic, and the gain is about 40. Note that this is far too high to use with any microphone for close-miked vocals or instrument amplifiers, but is suitable for normal speech.

By making the SET GAIN resistor a 50k linear pot, the gain can be varied from virtually 0 up to a maximum of 40 (32dB), with low noise and distortion at all settings. The output level from a well known brand of vocal mic has been measured at over 1 Volt peak-to-peak with loud singers, so the "conventional wisdom" of mics having low output is clearly wrong. For this reason, making the preamp with variable gain is almost an essential requirement.

The open-loop gain of this little circuit is about 3,400 - this is obtained by disconnecting the feedback resistor, and bypassing R5 with a suitably large value capacitor. All this from a single amplifying transistor !

Measurements taken when I was building lots of these show that the equivalent input noise was about -127dBm, so with a gain of 40dB, signal to noise ratio should be about 87dB relative to an output of 0dBm (approx 775mV). This is completely unattainable in practice, because of the noise from the microphone itself as well as other extraneous noises which cannot be eliminated.

Needless to say, the use of metal film resistors is a must to get the best possible noise performance. I'm not completely happy with the requirement for electrolytic capacitors, but for the impedances involved relatively large value caps are a must. Use of Low Leakage electros may be worth the effort, but I have not experimented with this option. I have used solid "tag" tanalum caps in this circuit, but they are (or were) revoltingly unreliable and I stopped using them after it was necessary to change every tantalum cap in a batch of about 20 small 8 channel stage mixers. I was not impressed !

Increasing Open-Loop Gain and Reducing Output Impedance

If you think it is worth the extra effort (which quite frankly I don't), the next version is interesting. The open loop gain of this configuration is now an astonishing 1,200,000 - or over 120dB from a single amplifying transistor. The op-amp acts as a unity gain buffer - basically a "high tech" version of the emitter follower in the previous circuit.

Although open loop gain, as well as output impedance and output drive capability are all improved, the noise figure can be expected to be slightly worse. It is also likely that although measurable distortion may be reduced, the preamp may lack musicality - that undefinable something that no-one has actually been able to quantify. IMHO this is very unlikely with this circuit, but one never knows.

Gain with the values shown remains at 32dB, and the op-amp should be powered from the +30V and 0V rails (i.e. NOT with a split supply - the voltage will be too high for the op-amp). The second version has not actually been built and used in earnest, but has been simulated and is a viable proposition - it can be expected to work as described without problems.

This is the figure:

12V Version of Preamplifier

This is a very quiet preamplifier, but is only suited to low impedance inputs - the noise figure degrades rapidly as the input impedance is increased. The design - in particular the collector current for Q1 - was based on the noise / current / impedance graphs for the Philips version of the BC549 - minor variations are likely with different transistors, or BC549 devices from other manufacturers.

The entire circuit is naturally Class-A, and with a gain of 32dB, has an output impedance of less than 100 Ohms. The recommended load impedance is 22k or greater, so it is quite capable of driving a set of tone controls or a fader. Buffering with a good quality opamp will naturally reduce output impedance (and also increases output drive capability and open-loop gain) as shown in the example in the figure.

The exact same design has also been used as a virtual earth mixer for the mix bus in mixers from 6 to 24 channels. The only change is to remove the 1k2 resistor at the input, and connect the mix bus directly. The optimum impedance must be retained for low noise, so for a 10 channel mixer, each channel should have an output resistance of 12k to 20k to the bus. Fewer channels require lower resistance and vice versa.

This is the figure;

The circuit seems to discourage people because of the single +30V supply. The up figure shows the resistor values needed to run the preamp from a 12V supply, but naturally the output voltage is dramatically reduced before clipping. I have included this version to demonstrate that lower voltage operation is possible, as this seems to be something that people want.