DIY audio blog for projects, schematics, PCB's, designs, electronics, mods


Category: Transistors

Power Amplifier Design by P. Baxandall Part 2. Negative-feedback Concepts.

In the January issue the concept, and possible consequences, of slew-rate limitation were discussed, with particular reference to one cause, in which the first stage of an amplifier is unable to supply the current demanded by the collector-to-base feedback-stabilization capacitor in the second stage. With suitably modified circuit designs such effects may be made insignificant. Before specific circuits are discussed in later articles, the present article will deal with some basic ideas about negative feedback and transfer functions.

Baxandall power amp design part2 – >>>LINK<<<

uPC1237 Amplifier/Speaker Protection Module

uPC1237 is a well known IC used for protecting the speakers form DC as well as amplifiers from over current. Almost any Sony amplifier starting from the lower range and right up to the higher-end ES series are using this chip.

So I thought was a good idea to make a stand alone module which could be used by the DIY community.

To start of, here is the datasheet of the uPC1237:

One important aspect is that this IC could be used to prevent the output devices from over current. This happens when a load too low is introduced at the amplifier’s output or a short circuit is present. Most power amplifiers have a dedicated circuit built in them, to monitor the current flow trough the power transistors. This part of the schematic is forcing the current to stay within the recommended SOA of the devices.

It is a common situation where in case of a short circuit at the output, the output transistors get damaged “to save the fuses” which is absolutely unwanted fact.

Using uPC1237 for over current protection allows the designer to drop the SOA limiting elements in the power amplifier. This will save space on the PCB, lower the cost of the project and increase the reliability.

Due to some patent issues one can not find a recommended schematic to do the current limiting job in the uPC1237’s datasheet. There is no built-in circuit either. However all you need is to monitor the current flow trough one of the emitter resistors at the output of the power amplifier. According to the datasheet pin 1 must be driven above 0.67V. This is the threshold. Rising the voltage above 0.67V will trigger the relay opened. Using a current source will require the pin 1 driven with current above 110uA.

This is what I use in my amp to do the job:


Now the schematic of the module itself. It can be downloaded HERE


A few words about the PCB. Now I wanted to have a rather non standard approach here. I needed a module which could be mounted directly on the rear panel and bolted through the speaker terminals. I’ve seen this type of modules around the eBay. However those were using small current relays not suitable for more powerful amplifiers. For my project I wanted to have a good quality DPST relay with at least 10A rated current. Those can be quite bulky so the PCB is quite large. However, reliability is more important than the cost of the PCB. The devices protected by this module can be quite expensive so no cheap tricks here.

Here is the relay – OMRON – G4W PCB Power Relay

Those are available for purchase from Farnell in Europe:

This is what the PCB looks like:


The relay is located between the mounting pads to keep the paths equal and at the same length. The mounting pads are of 15mm diameter. The mounting holes are at 5.5mm. Those can be drilled to a larger diameter if necessary. My speaker terminals have 5mm threads and fit just fine.

A PDF file of the PCB is available for download here:

PLEASE NOTE – this project is provided to the DIY community for free. It’s free for personal use. Any commercial usage is prohibited.


LME49810 + uPC1237 Audio Amplifier

This is a complete schematic showing a semi discrete audio amplifier based on the LME49810 driver chip by TI. It was actually developed by National Semiconductor. Now it is a part of TI’s portfolio after the acquisition of NS. The schematic uses an old IC to handle the offset detection, speaker protection and the delay.

Audio amplifier based on LME49810 and uPC1237 – >>>LINK<<<

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