It sounds very good for the class of amplifier. That is, a high quality small PA amp. It certainly isn't a polite little black box for domestic level, audiophile listening but on the other hand, it's wasted as a general purpose amp that might be destined for small discos, clubs or band gigs.
When the project was introduced, the magazine published a pic or two of how some constructors finished it, which was clearly for PA, but seemingly a cut above head-banging noise quality, as promoted. The design is unashamedly the same "balanced VAS" design derived from Hitachi's Mosfet app. note dating back to 1979 and used in many EA and Silicon Chip DIY projects until more recent times, showing how adaptable and stable it is.
Speculating, I made just one of these amplifiers and it sold quickly after a demo to a guy who wanted something very good for use in movie sound post-production. I thought about building more but people look at larger amps mainly on a bangs/buck comparison and a nominal rating of 200W/8R can be done cheaper now, in several ways.
I would be looking to use it in large rooms with big speakers that may not be very efficient. I haven't tried it of course, but awful loads like Magneplanar speakers and similar low impedance drivers might be on the list of applications. Just an opinion, though
Nice one .. thanks for taking the time to reply
Hello Andrew
I have started to use 2uH in my amps, and all sound good to me
Thanks for the feedback
Nick
6.8 uH is the "one size fits all" inductance value suggested long ago by Theile, as rendering a solid state amp. "unconditionally stable". Amplifier design has changed a bit, the concept of unconditional stability has been questioned and now amplifiers have increased bandwidth, since the early 1970s when the value was proposed.
However, this amp was designed around it and a recent (Mk3) version of Silicon Chip's ULD amplifier uses even 10uH! The last, Mk4 version,shows a return to lower numbers and I'm sure Andrew will get a kick out of seeing load lines again for the single pair of output devices. Ultra-LD Mk.4 Power Amplifier, Pt.3: 110W Version - October 2015 - Silicon Chip Online
My suggestion is to build it as specified - it's not expensive or difficult to fit on the PCB whether you construct it as detailed or on a length of 13mm dowel or whatever is convenient. Otherwise, get hold of an oscilloscope and run some 10KHz square waves through it at decent power levels into a resistive load before committing to untried values.
However, this amp was designed around it and a recent (Mk3) version of Silicon Chip's ULD amplifier uses even 10uH! The last, Mk4 version,shows a return to lower numbers and I'm sure Andrew will get a kick out of seeing load lines again for the single pair of output devices. Ultra-LD Mk.4 Power Amplifier, Pt.3: 110W Version - October 2015 - Silicon Chip Online
My suggestion is to build it as specified - it's not expensive or difficult to fit on the PCB whether you construct it as detailed or on a length of 13mm dowel or whatever is convenient. Otherwise, get hold of an oscilloscope and run some 10KHz square waves through it at decent power levels into a resistive load before committing to untried values.
I built this studio 350 amp using to3 type transistors as I had a few in a box. After adding a few small changes like 220 ohm preset in the tail pair and same in the bias circuit.
Offset is 20mv (well that suits me). I even changed out the zobel circuit as I did not like the idea of the resistor, coil then cap to ground..mine uses coil then resistor and a 100nf to ground.
Psu caps are a 15,000uf per rail at 100v.
Also fitted v limiting circuit that gives the power transistors a life line instead of bang pop with a shorted output if that happens.
Even a soft start/dc fail safe of my own design..sorry will not pass pass that circuit on..
Offset is 20mv (well that suits me). I even changed out the zobel circuit as I did not like the idea of the resistor, coil then cap to ground..mine uses coil then resistor and a 100nf to ground.
Psu caps are a 15,000uf per rail at 100v.
Also fitted v limiting circuit that gives the power transistors a life line instead of bang pop with a shorted output if that happens.
Even a soft start/dc fail safe of my own design..sorry will not pass pass that circuit on..
It's a standard circuit rearrangement of the Zobel and output coil filters - It was commonly used by Neville Theile, noted for his loudspeaker design parameter analyses and it's a clever saving on the need for 2 resistors. There's no need for concern that there may be dire consequences there.
Ohhhh....you have piqued our imagination. Do tell
This is off- topic but you don't need to change anything for a lower power requirement than the specified maximum power rating of any amplifier. The volume control is what limits power, right up to the clipping level where distortion occurs. This design is mono, not stereo so you need two complete amplifiers with their power supplies for a stereo build. Each amplifier can then be used for any requirement ≤ 200W @ 8 ohms or 350W @ 4 ohms, right down to a few Watts, as required.
If your motivation is to save a little money, you could reduce the number of output transistor pairs by one and also reduce the power transformer's AC output voltages by 5V per winding. That is, 2 x 45AC windings rather than 2 x 50VAC. The power rating of the transformer will depend on the application but for professional use, you may still require a 250-300VA rated transformer for each amp. It is unlikely that you can use cheaper (lower voltage rated) electrolytics in the power supply as their voltage rating needs to be at least 75V and there is not much choice there.
350 Studio Amplifier Circuit Scheme and PCB Layout - Amplifier Circuit Design
no problem ... I will dig out some pictures of the construction when I made the amplifier, just need to find them 1st
regards
Hello .. I did add protection, it was the circuit / PCB that was covered in the original silicon chip magazine article, I will upload a pic later - very poweramp easy to make but can be overall expensive as I made 2 mono blocks
but one of my favorite bipolar amps - I think I have 10 spare PCB,s inthe workshop ?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.