TDA7250 Stereo Amplifier Brand New 2 x 100W 4R, 2 x 75W 8R, 200W 8R Mono Bridged

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Seller: keepworking ✉️ (2,055) 99.6%, Location: Stourbridge, GB, Ships to: WORLDWIDE, Item: 166571674608 TDA7250 Stereo Amplifier Brand New 2 x 100W 4R, 2 x 75W 8R, 200W 8R Mono Bridged. TDA 7250 Stereo Amplifier based on the circuit of Velleman's K4005B rev. 3 amplifier., which has long since been discontinued. One of the few high-power amplifier designs that can be assembled/used as a stereo unit and connected to a single dual-rail supply and power ground without it suffering from any ground loop induced hum. This does of course depend on the person wiring up this board not creating any ground loop or other hum-generating problems (such as a loose / open circuit / non-existent signal ground for example) themselves due to inappropriate wiring. The Velleman circuit is basically the full 2-channel application circuit for the TDA7250 but with 1K resistors between the speaker positive terminals and ground, and with the bias-setting current sense resistors reduced to 0.1R. My board layout has a star earth/ground and star supply topology. Double-sided 2oz copper, 1um ENIG. 1.6mm board, FR150 material. Black solder resist, white legend layer. Size is 111.6mm by 128.8mm.  The longer side is the one along which the output power transistors are fitted. Total board height from the tips of the solder joints on the bottom of the board to the tops of the power transistors is between 33mm and 33.5mm approx. The tallest components are the power transistors and these stand between 29 and 29.5mm above the board surface, approx. Mounting centres are 84.25mm by 119.25mm approx. The holes in the top and bottom left corners are 2.75mm in from the left-hand edge and 3.25mm down (and up) from the top/bottom edges. Mounting holes are 3mm with 6.5mm track boss top and bottom (approx.). The board has at least 1mm of fibreglass material track border all round. The power transistors have formed legs so they will stand against a heatsink with the board long-edge butted up against it. The quiescent bias current is between 200mA and 400mA per channel depending on supply voltage, so choosing an approximate average of 300mA at +/-35v supply, the power transistors are dissipating approx. 20 watts (combined total) whilst the amplifier is idling.  You'll therefore need a large heatsink with a large surface area exposed to free moving air (either a vertical face or a large horizontal surface area facing upwards and not covered by anything). All high-current paths are duplicated top and bottom (aside from a few small areas of 1cm or less). The top and bottom side high-current tracks are joined at periodic intervals with 1.25mm clear through-holes filled with solder and domed top and bottom. The layout is vastly superior to the original amplifier design which was a long, thin 1oz copper single-sided board designed to sit inside a heatsink extrusion. Hand soldered by me using Loctite Multicore 96SC lead-free solder with almost 4% silver content. Of note in the layout is the extremely short high-current track lengths, with the speaker connector blocks between the output transistors and the power connector and transistors all in-line which means everything that carries any current including the ground rail are as close together as possible. The speaker (and power) terminal blocks will accept 2.5mm2 CSA wire, i.e. QED 79-strand speaker cable, etc. The signal input terminal block is smaller and will accept 1.5mm2 wire. The amplifier is over-current protected by two 6.3A slow-blow fuses. It is also protected from reverse-polarity as the two centrally located power diodes connected in reverse across the fuse outputs will cause the fuses to blow. Specifications from the Velleman rev. 3 manual: Rms output power: 2 x 100W / 4ohm; 2 x 75W / 8ohm. Rms mono-bridged power: 200W / 8ohm. Total music output: 400W. Harmonic distortion: 0.003% at 1kHz. Signal-to-noise ratio: 96dB (A-weighted). Stereo channel separation: 76dB. Damping factor (at 100Hz): >2000. Input impedance: 22kohm. Input sensitivity: 150mV, 300mV or 950mV switchable Supply voltage for 8ohm: +35 to 40VDC and -35 to 40VDC / 2.5A. Supply voltage for 4 ohm or mono: +25 to 30VDC and -25 to 30VDC / 5A. What you can see from the specification is that this amplifier is firmly in the hi-fi category. Whilst the 96dBA SNR can only be considered adequate, I'm personally not able to hear any hiss or noise from speakers driven by this amplifier unless I put my ear right up to the speaker cone and even then, what I can hear is marginal (so low I can barely make out whether there is really noise/hiss there or not). The 0.003% THD figure is very good of course, but the outstanding credential this amplifier has is its damping factor of 2000 quoted at 100Hz. Compare this to any class-D amplifier that most likely gets to 150, or an LM3886 amplifier for example that might get to 300. An exceptionally high damping factor does 2 things. Firstly, it maximises the speaker's compliance with where the output of the amplifier says it should be. If you've ever listened to the Quad 405 or mk.2 version (the current-dumping amplifier) then by comparison that amplifier has an incredibly loose, splodgy bass frequency response that just sounds more like some heavy plant driving down the road outside when it goes off. It may be original and vintage, but it has absolutely no bass control at all. More correctly, it has no high-energy signal control which is where the high power transistor fill-in is operating.  In sharp contrast, the TDA7250 amplifier couldn't possibly sound tighter than it is. You can really hear it. Secondly, as the output resistance/impedance of the amplifier forms a potential divider with the speaker impedance, the effects of speaker impedance variation with frequency is absolutely minimised. As you may know, an impedance such as 4 ohm or 8 ohm quoted on the back of a speaker is nominal. Due to parasitic components in the driver unit(s) and in addition the complex impedance presented by the crossover network, an 8-ohm speaker could actually present an impedance to the amplifier of much less than 1 ohm or much greater than 30 ohms at any given frequency or band of frequencies.  Neither will it be consistent, meaning the perfect audio you were expecting will sound overtly boosted at some frequencies and cut at others.  Moreover, where attenuation occurs in a complex filter network, phase changes also occur, meaning a loss or significant confusion of the stereo image. To hear this amplifier's huge damping factor, you have to choose some good quality speakers with a hi-fi pedigree. And you must use appropriate speaker cables. It's not snake oil.  Personally I use QED 79 strand as it's the industry standard and can be had for £2 per metre. I have been known to double up as well, i.e. use both conductors in the twin cable twisted together at both ends to connect between one terminal on the amplifier and one on the speaker, meaning I need two twin figure of 8 cable runs per speaker. You will notice the difference, particularly if you've been running a class-D amplifier or an old Quad 405, or to be honest just about every other commercially available amplifier - vintage or new.  QED used to publish a graph showing the effect of different speaker cables with cross-sectional areas lower than 79-strand and the cable run length versus effective damping factor. Using 13-strand bell-wire or even 42-strand cable won't cut it and will undo all the good work this amplifier is doing to drive your speakers accurately. Choose 2.5mm2 conductor CSA minimum (i.e. QED 79-strand or a higher CSA, higher strand count).  I'm not sure about the OFC helping but you can't order 79-strand without it being OFC so hey-ho. I've changed the design very slightly, but as supplied the amplifier is identical to the Velleman 4005B rev. 3. Firstly, I've added a 2 x 3 row header with jumpers fitted.  This header breaks the link between the op-amp pre-amp outputs and the TDA7250 application circuit inputs. With Pre-out, amp-in and ground present for both left and right channels, the header provides the following options: 1. Coupling the pre-amplifier to the amplifier (Velleman mode - as supplied with 2 jumpers fitted). 2. Grounding the amplifier inputs with the jumpers between the amp-in and 0v pins for both left and right channels. 3. Direct drive of the TDA7250 amplifier circuit between the amp-in and 0v pins for both channels, completely bypassing the pre-amp stage. 4. Fitting of a dual-gang log volume control between the pre-amp and amplifier. Each gang would have its top/far track end connected to pre-out, its track start connected to 0v and its wiper connected to amp-in.  The TDA7250 circuit section has input resistors to ground of 22K, so you should choose a suitably low value of potentiometer such as 10K or even 4K7.  You may even be able to squeeze in a balance pot if you choose your values correctly. Secondly, there's a +15, 0v, -15v 3 x 1 pin 0.1" header outline on the board which can be soldered to or have a header fitted. It's only a feed from the Zener-regulated and 100uF/0.1uF decoupled op-amp supplies but it would neaten up your build not having to worry about where to get some additional wide operating supply rails from if you were thinking of adding active tone controls, an RIAA pre-amp, etc.  It runs at about 10mA as supplied (closer to 20mA with a board supply of +/-40V) but this could be upped by changing the feed resistors (currently the 1W 1K resistors close to the LEDs) as the Zener diodes are 1.3W. By the way, the LEDs are in the feed paths to the Zener diodes to indicate positive and negative supply health. If a higher auxiliary +/-15 volt current and therefore Zener current is required, there are two unmarked LED bypass resistor fitting positions top and bottom of the LEDs (0.4" spacing) which would allow a correctly-calculated small-valued 1/4-watt or better resistor to bypass all but 20mA of the Zener currents from the LEDs.  You'd base the calculation on a nominal 2V voltage drop across the LEDs and a maximum allowed current of 20mA subtracted from the Zener current to give you the required bypass current. Thirdly, Stereo/mono (bridged) mode and the pre-amp gain are set by two headers with jumpers supplied fitted. As supplied, the amplifier is set up in stereo mode with the lowest pre-amp gain. Fourthly, I've added resistors to the input signal path such that the pre-amp gain can be halved in stereo mode or thirded in bridge mode. This was for my particular application, as I had very high-level inputs to deal with and wanted the board to have this capability so the build looked neat.  Nonetheless, this feature may be useful to you. Fifthly, I've left 2 clear holes between the power diodes and fuses marked on the legend layer with a square and +35 and -35.  The holes are 1.25mm clear as are the 4 clear ground/0v holes at the centre of the board. These make ideal points for 1.25mm solid-core wire connections to reservoir capacitors as they are directly on the supply rails and high-power tracks, and are just a few centimetres from the power resistors that feed the power transistors.  There are also runs of +35 and -35v supply holes either side of the board's power terminal block if you prefer your reservoir caps on the input sides of the fuses. Something to watch for ================== I fit R23 which is a 470K resistor from the TDA7250 mute input to ground. You'll see the board as supplied has an outline for a 2-pin temperature sensor connector (I was intending a 2-pin screw terminal block similar to the one used for connection of the input signals but I don't fit or supply it). The Velleman amplifier kits generally use Airpax normally open temperature switches with 2 pins in a TO-220 case, with the tab to be mounted on the heatsink close to the power transistors. The one for this kit had a rating of 90 degrees C. I believe Farnell or RS sell these if you want to fit one. Now the Velleman kit never included the temperature switch or the 470K resistor even though they are shown in the circuit and in the product shots on the box. They just weren't supplied.  As far as I know, R23 was probably designed-in to increase the rapidity and/or reliability of the mute in an over-temperature condition.  It changes the voltage of the MUTE input pin on the TDA7250. The MUTE input is triple-function depending on the voltage applied to it with reference to the NEGATIVE supply rail, not 0v.  The 470K resistor means the TDA7250 is in operating (non-muted) mode but only if the positive/negative supply voltages are 28 to 29V or higher.  Since the original kit and rev. 3 manual mentioned +/-25v rails as the lowest supply voltage when driving 4R speakers, you can see why the part was omitted. This will mean that if your supplies are soft, as the audio peaks the amplifier will cut in and out as the increased current draw momentarily drags down the +/- voltage rails.  If you are using regulated supplies with a safety current limit, they will similarly fold if they aren't capable of providing the 5 to 6A per rail this amp needs for driving 4R speakers or 2.5A to 3A for 8R at the rated power. Although I haven't tried it (as my application uses +/-35V), I believe removing R23 will allow the supply voltage to drop below 28V (but I can't be certain without trying it - so ask me before buying if you want this feature).  Personally, having the amplifier mute when the supply rails drop is a safety feature for my application as it detects supply failure and protects the speakers from any DC offset that might occur as a result. You may *want* to power this amplifier from lower voltages. The TDA7250 itself will go as low as +/-15v but you may need to address quiescent biasing to ensure you get the same amplifier performance albeit with a much reduced headroom. The datasheet suggests that the TDA7250 offers greater overall conversion efficiency with supplies of +/-25V (as much as 70%) but again with a lower achievable RMS power.  To address quiescent bias current, the easiest way would be to change all four 7-watt 0.1R power resistors that feed the collectors of each of the power transistors. Soldering 0.1R resistors in parallel across the existing 0.1R 7W resistors would halve their value to 0.05R and the TDA720 would maintain a bias current with 15V supplies comparable with the amplifier as it stands powered from 30V supplies (all the TDA720 "knows" about biasing is by sampling the voltage drop across the collector resistors). Bear in mind the original ST data sheet application circuit shows 0.15R resistors and Vellman reduced these to 0.1R for this design. So Perhaps aiming for 0.05R with 15V supplies is too low and you may like to aim for 0.075R. The kit recommended a 30VAC dual secondary transformer, which of course meant 40-41v once full-wave rectified and subject to an appropriate reservoir and smoothing capacitor bank. === End of note Built here in the UK by me.  All components sourced from Farnell, RS or Rapid. Fully soak tested - I clamp the power transistors to a test heatsink with insulating washers and run the board from regulated supplies with a high-level input signal. All low-power resistors are the vastly superior 0.6W 1% 50ppm/C metal film type made by TE connectivity. Lower noise than carbon / carbon film alternatives. The 1W resistors are a 1% metal film type from Royal Ohm. The 7W 0R1 bias current sensing resistors are TT/Welwyn 5%. Overrated compared to the original design which only specified 5W. All non-polarised capacitors are Kemet apart from the two blue ones (!!) which are TDK. All 5% MLCC apart from the input AC coupling capacitors which are polyester. All polarised capacitors are Wurth (German) 20% electrolytics and rated 50v DC or higher. The TIP142 and 147 power transistors are ST branded, not generic parts. The fuse holders are Wurth. The screw-terminal board connectors are all Camden Boss. The headers are all Harwin. All components are brand new and purchased just for this build. If you've been mooching around various amplifier boards trying to decide which one to buy, there really is no other choice than this one if you want an incredibly competent, reliable, unfussy, powerful, versatile, low noise, low distortion, hard-driving class AB design that's been tried and tested for about 3 decades and change. The amplifier will be sent to you built and configured exactly as shown in the photos, securely boxed to prevent damage and fully insured in transit.
  • Condition: New
  • Supported Modes: Stereo, Mono-bridged
  • Year Manufactured: 2023
  • Audio Outputs: Terminal block
  • Number of Outputs: 2
  • Colour: Black
  • Amplifier Class: AB
  • Brand: Unbranded
  • Type: Power Amplifier
  • Audio Inputs: Pre Out
  • Unit Type: Unit
  • Number of Channels: 2
  • Number of Inputs: 2
  • Model: TDA7250
  • Features: Analog
  • Unit Quantity: 1
  • Country/Region of Manufacture: United Kingdom
  • Power: 100 W

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