My first solid state amp

[AAK]

Hey everyone. I've been DIY Audio member for about 10 months, more as a lurker, and I decided that today is great day to put in my two cents. I've worked on numerous audio projects but never a solid state amplifier. I’m hoping that my efforts on this amplifier will be a spring board to my own kit operation which I’ve been thinking about for sometime now.

So to get started instead of trying to reinvent the wheel I decided after doing some research to go with a classic three-stage topology biased in Class AB. A differential stage with a current mirror for the input stage, voltage amplification stage, and EF output stage. By far the most challenging part was the board layout. But after several layout iterations, prototypes, and many hours of tweaking and listening, I'm very pleased with the results. The prototype boards have been in my system since November and sound incredible so I had Advanced Circuits make 25 professionally made boards with 2oz copper traces. I assembled and tested eleven boards, I kept a pair for myself, and the other four pairs are available to anybody who wants one at near cost. For more info email me at alsaudiokits@yahoo.com.

The amplifier which I’ll call the RTS100 delivers 105 watts into 8 ohms, 210 watts into 4 ohms. Has a frequency response of 2.5Hz to 205Khz, with a damping factor greater than 500 into an 8 ohm load after the output relay, and over 1500 before the relay at 1Khz. The relay has terminal lugs for easy bypass for those who hate relays. The amplifier is incredibly stable. I did put in provisions to add a zoble network but after some testing with an 8 ohm load in parallel with a variety of capacitors values at different power levels I found it’s not necessary. Signal to noise ratio measures at greater than 100db. There's also provisions for short circuit and thermal protection. Anyway, enough said. I’d like to here from you all. I’ve posted the schematic and will follow up with some pics.

[AAK]

Assembled RTS100 pic.

[AAK]

My assembly plant.

[AAK]

Here's square wave output at 2.83Vrms at 1Khz into 8 ohms.

[AAK]

Here's a square wave output at 2.83Vrms at 10Khz into 8 ohms.

[AAK]

Here's a square wave output at 2.83Vrms at 5KHhz with an 8 ohm load in parallel with 1uF cap.

[John Hope]

AAK: A very neat job, congratulations! The squarewave scopeshots look extremely good, but then you do have a 200kHz bandwidth. If I may suggest, limit the bandwidth deliberately to around 50-75kHz with an input RC to reduce the effects of stray pickup and RFI.

What made you decide on bipolar output devices instead of MOSFETs?

You mention short circuit protection, but unless I missed something, I don't see any actual overcurrent protection mechanism on the output stage. What will prevent if one were to short the speaker terminals? If there's a possibility of damage to the amp with a short across the LS, Murphy's Law guarantees that such a short will happen. Sooner or later. . . Robustness is all.

Regards

John H

[AAK]

Hi John, thanks for the reply and the suggestions. Actually the first RTS100 prototype used mosfets for the OS. I switched to BJTs primarily for economic reason. Lateral mosfets aren't cheap and unless you buy many there difficult to match. But If I where to build a more powerful amplifier 250 watt and up i would go with lateral mosfets.

I've attached a revised schematic that includes short circuit protection that's implemented on the RTS100.

Best regards,

Al

[fab]

Nice work

I have a question

Originally posted by AAK
...
The amplifier which I’ll call the RTS100 delivers 105 watts into 8 ohms, 210 watts into 4 ohms....

210W into 4 ohms that is more than 10A. Unless the output transistors have a higher gain than about 25, it means a base current of 400 ma! Are Q7 and Q8 mounted on heatsink too (20W dissipation)? How the driver stage (Q7 , Q8) can work in class A with only about 10 ma DC?

[amplifierguru]

Hello AAK,

Good work - you've certainly swung into production! Square waves look good and ringing seems fairly well damped, but you need to do the C tests with at least 10Vrms AND larger C's AT 10KHz - someone else will so it's better you know if it's stable there. You may just need that zobel.

A few small criticisms -

1) Your THD and distortion character is not mentioned

2) Conventional miller C will load your 6mA Vas and will degrade -ve supply rejection. Two pole or feedback comp would be much better. Throws the onus for performance onto the PS.

3) Given min beta output devices your driver will need to deliver twice the standing current for rated power into 4 ohm to the output stage - so is not in class A.

4) Your short circuit protection appears to limit output current at zero volts to around 5A PER DEVICE but they are only 150W fragile 30MHz devices with a 150C max so derate heavily to around 100W or 2A at 50V each reliably.

5) The protection circuit has it's operating transistor biassed at 0.4V at zero crossing and almost on for the whole 'off' half cycle. This could affect the operation of the Vas /amp in normal operation.

Hope this is some help.